research paper on ground water

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A Comprehensive Review for Groundwater Contamination and Remediation: Occurrence, Migration and Adsorption Modelling

Osamah al-hashimi.

1 Babylon Water Directorate, Babylon 51001, Iraq

2 School of Civil Engineering and Built Environment, Liverpool John Moores University, Liverpool L3 3AF, UK; [email protected] (K.H.); [email protected] (E.L.); [email protected] (T.M.Č.)

Khalid Hashim

3 Department of Environmental Engineering, College of Engineering, University of Babylon, Babylon 51001, Iraq

Edward Loffill

Tina marolt Čebašek, ismini nakouti.

4 Built Environment and Sustainable Technology Research Institute, Liverpool John Moores University, Byrom Street, Liverpool L3 3AF, UK; [email protected]

Ayad A. H. Faisal

5 Department of Environmental Engineering, College of Engineering, University of Baghdad, Baghdad 10001, Iraq; moc.oohay@lasiafazmahladebadaya

Nadhir Al-Ansari

6 Department of Civil, Environmental and Natural Resources Engineering, Lulea University of Technology, 97187 Lulea, Sweden; [email protected]

Associated Data

The data presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

The provision of safe water for people is a human right; historically, a major number of people depend on groundwater as a source of water for their needs, such as agricultural, industrial or human activities. Water resources have recently been affected by organic and/or inorganic contaminants as a result of population growth and increased anthropogenic activity, soil leaching and pollution. Water resource remediation has become a serious environmental concern, since it has a direct impact on many aspects of people’s lives. For decades, the pump-and-treat method has been considered the predominant treatment process for the remediation of contaminated groundwater with organic and inorganic contaminants. On the other side, this technique missed sustainability and the new concept of using renewable energy. Permeable reactive barriers (PRBs) have been implemented as an alternative to conventional pump-and-treat systems for remediating polluted groundwater because of their effectiveness and ease of implementation. In this paper, a review of the importance of groundwater, contamination and biological, physical as well as chemical remediation techniques have been discussed. In this review, the principles of the permeable reactive barrier’s use as a remediation technique have been introduced along with commonly used reactive materials and the recent applications of the permeable reactive barrier in the remediation of different contaminants, such as heavy metals, chlorinated solvents and pesticides. This paper also discusses the characteristics of reactive media and contaminants’ uptake mechanisms. Finally, remediation isotherms, the breakthrough curves and kinetic sorption models are also being presented. It has been found that groundwater could be contaminated by different pollutants and must be remediated to fit human, agricultural and industrial needs. The PRB technique is an efficient treatment process that is an inexpensive alternative for the pump-and-treat procedure and represents a promising technique to treat groundwater pollution.

1. Introduction

Earth is known as the blue planet or the water planet because of the reality that most of its surface is covered by water, and it is the only planet in the solar system that has this huge quantity of water [ 1 , 2 ]. For various authorities and agencies dealing with water problems, the conservation of surface and groundwater purity without pollution is indeed an aim. In addition, groundwater is the main potable water supply used in many nations; this is also water for agriculture and industry [ 3 , 4 ]. The effect of global warming, climate change, the rise in weather temperature and evaporation increment, population growth, excessive use of fresh water in agriculture and industrial activities have all led to increasing reliance on groundwater [ 5 , 6 ]. Groundwater became fundamental for social and economic development. It is the sole source for drinking to about 2.5 billion people around the world [ 7 ]. There are many reasons to develop groundwater, but among the most important are [ 8 ]:

• (1) Groundwater usually lies in underground natural reservoirs. This promotes groundwater as a convenient source of water. Additionally, groundwater can be found in different quantities depending on aquifer capacity. Many times, aquifers detaining water larger than many human-made reservoirs; for example, the Ogalalla aquifer located in the United States produced up to 500 Km 3 of water for four decades, which is larger than Nasser lake in Egypt. The huge quantities of groundwater give an ability to pump water during the drought period, while surface water (in some places) is unable to be pumped in these quantities or at such high quality during such period.
• (2) In many cases, groundwater quality is better than surface water. This is due to the ability of aquifers to provide natural protection for groundwater from contamination.
• (3) Groundwater is a cheap, reliable source of water. It can be pumped out using small capital and can be drilled close to the location needed for water. Additionally, groundwater can be easily organized, managed and developed. For example, individuals can easily construct and operate their groundwater well on their land.

Pumping and treatment is a common technique used for groundwater treatment; however, the lack of groundwater quality restoration in the long term has been demonstrated in this method. An innovative approach to groundwater remediation is, therefore, necessary. The permeable reactive barrier (PRB) is proven as a promising technology for groundwater treatment by an interaction between the reactive material and the contaminant when the dissolved compounds migrate. In the permeable reactive barrier (PRB), water moves in a natural gradient, and no further energy is used to achieve the treatment [ 9 ]. The PRB is classified as in situ treatment, and the contaminant is transformed in the contaminated site into less toxic or immovable forms. The key benefits of the PRB innovation are minimal maintenance costs and long durability. However, the aim of this work is that future researchers will find a clear, in-depth and detailed explanation of groundwater contaminants, movement and detailed theoretical explanation for the fate of contaminants in the environment.

2. Groundwater Contamination

Groundwater is the global population’s main source of fresh water and is used for domestic, food production and industrial purposes. About a third of the world’s population depends on groundwater as the main water source for their drinking purposes [ 10 ]. According to the United Nations Environmental Agency (UNEP), there are 32 cities around the world with a population greater than 10 million known as “megacities”; about 16 of these cities majorly rely upon groundwater [ 8 ]. In China, there are 657 cities, and approximately 400 cities are using water from the ground as the main source for their water supplies [ 11 ]. It is without doubt that subsurface water/groundwater is an essential resource of water to humanity; furthermore, it is vital for the ecological system on earth. Keeping this water resource sustainable, accessible, effective and efficient is a major concern for scientists working in a related field. However, urbanization, farming, industry and climate change all pose significant threats to the quality of groundwater. Toxic metal, hydrocarbons, contaminants such as organic trace pollutants, pharmaceutical pollutants, pesticides and other contaminants are endangering human health, natural ecosystems and long-term socioeconomic development [ 12 , 13 ]. Chemical contamination has been a major subject in groundwater investigations in recent decades. While groundwater contamination poses a significant threat to human populations, it also provides a chance for researchers to learn more about how our underground aquifers have evolved, as well as for decision makers to understand how we might maintain the quality and quantity of these resources [ 14 ]. According to the Canadian government, the contamination of groundwater can be defined as the addition of undesired substances by human activities [ 15 ]. Chemicals, brines, microbes, viral infections, medications, fertilizers and petroleum can all contribute to groundwater contamination. However, groundwater contamination is differs from surface water contamination in that it is unseen, and recovery of the resource is difficult and expensive at the current technological level [ 16 ].

Due to human and natural activities, chemicals and pollutants may be found in groundwater. Metals such as arsenic, cadmium and iron could be dissolved in groundwater and may be found in high concentrations. Human activities such as industrial discharges, waste disposal and agriculture activities are the main cause of groundwater contamination. Furthermore, it could happen due to urban activities such as the excessive use of fertilizers, pesticides and chemicals in which pollutants migrate to groundwater and reach the water table. In any case, using groundwater for drinking, irrigation or industrial purposes requires different tests to ensure that it is suitable for these purposes.

The presence of inorganic contaminants in groundwater is a big concern especially when groundwater is used for drinking or agricultural purposes. If these contaminants are presented in the groundwater with levels higher than the permissible recommended concentration, they cause health problems throughout the food chain [ 17 ]. Table 1 presents different inorganic pollutants in groundwater, sources and health effects.

Inorganic pollutants presented in groundwater.

In addition, discharging organic pollutants into the environment and water resources represents a pressing concern for people’s health. The existence of organic contaminants in groundwater represents a crucial environmental problem, as it may affect the water supply reservoirs and people’s health [ 18 ]. Additionally, it can affect the ecological system [ 19 ]. Usually, groundwater contaminants come from two sources: (1) landfills, solid waste disposal lands, sewer leakage and storage tanks leakage and (2) agriculture and farmyard drainage [ 20 ]. Table 2 shows the most common organic pollutants usually found in the groundwater, the sources and the health effect.

Organic contaminants, source to groundwater and their effects.

In the environment, groundwater in shallow or deep aquifers is never found completely sterile [ 21 ]. Coliform organisms and bacteria are the main cause of the microbiological pollution of groundwater. When present, these pollutants need immediate attention to protect lives from outbreaks of pathogenic disease [ 22 ]. Microbiological contaminants naturally occur in the environment by the intestines of humans, warm-blooded animals and plants. These microorganisms could cause dysentery, typhoid fever and different diseases [ 21 ].

3. Groundwater Treatment Technologies

In recent decades, scientists developed sophisticated and highly successful techniques for the remediation of water from many contaminants. These techniques generally focused on the treatment of surface water resources such as a river, lakes and water reservoirs. However, in recent years, scientists and environmental researchers have become more aware of treating underground water, and groundwater has become an essential source of water in most places; it represents about 30% of the freshwater reserve in the world [ 29 , 32 , 37 , 38 ]. Groundwater is usually treated by drilling water wells, pumping the polluted water to ground facilities to perform different approaches of treatment such as air stripping and treatment tower and granular activated carbon (GAC). Pressurized air bubbles are also used to treat contaminated groundwater. The selection of the effective treatment/remediation procedure depends on the characteristics of contaminants and pollutants, in addition to the reactive media available [ 39 ].

3.1. Pump and Treat Method

One of the popular procedures to remediate contaminated groundwater is by dissolved chemicals, solvents, metals and fuel oil [ 40 ]. In this procedure, contaminated groundwater is piped to ground lagoons or directly to treatment units, which treat the groundwater using various methods such as activated carbon or air stripping. Finally, the treated water is to be discharged either to the nearest sewer system or re-pumped to the subsurface [ 37 ]. This technique can treat large volumes of contaminated groundwater but has many disadvantages, such as the high cost, spreading of contaminants into the ecosystem, as well as its long operation time; in addition, it may cause a reversal to the hydraulic gradient [ 41 , 42 , 43 ] as cited in [ 40 ].

3.2. Air Sparging Procedure and Soil Vapor Extraction

The procedure of air sparging and soil vapor extraction (SVE) is considered one of the most common techniques used in remediating groundwater contaminated by volatile organic contaminants (VOCs). It is considered efficient, fast and relatively economical [ 44 ]. This method involves the injection of pressurized air at the lowest point of the contaminated groundwater; this will clean up the groundwater by changing the state of volatile hydrocarbons to a vapor state. While pumping air under the saturated zone, pollutants are stripped out of the aquifer and oxygen is provided for the biodegradation of contaminants [ 45 ]. The extracted air is to be treated by vacuum extraction systems to remove any toxic contaminants [ 46 ]. The limitations for this method are the high cost when working in hard surface area and when many deep wells are required for the treatment. In addition, soil heterogeneity may lead to uneven treatment of the contaminated groundwater.

3.3. The Permeable Reactive Barriers (PRBs)

It is an innovative remediation technique [ 47 ]. Practically, it is in situ technology to remediate groundwater using reactive media designed to intercept a contaminated plume. Typically, reactive media is designed to degrade volatile organics, immobilizing metals. PRB media is placed with porous materials such as sand; this will enhance the hydraulic conductivity, so the plume of contaminants will pass through the PRB under a natural gradient descent [ 37 , 48 ].

In the treatment wall, contaminants are removed by adsorbing, transforming, degrading and precipitating the targeted pollutants during water flow through barrier trenches. PRBs are defined as an in situ remediation zone in which contaminants are passively captured, removed or broken down while it allows uncontaminated water to pass through. The primary removal method is either physical (sorption, precipitation), chemical (ion exchange) or biological [ 49 , 50 , 51 , 52 ].

There are many geometries for placing the permeable reactive barriers (PRBs): (1) A continuous wall that contains reactive media. This is the most common placement in which the reactive media is placed perpendicular to the contaminated plume of groundwater flow; (2) funnel and gate in which contaminant plume is directed to a treatment filtering gate by two-sided impermeable walls at sites in which the soil is very heterogeneous, placing the PRB in the most permeable portion of the soil. Furthermore, when the contaminant’s distribution is non-uniform, the pollutant’s concentration can be better homogenized when entering the PRB gate; (3) radial filtration/caisson configuration in which the filter is placed in a cylindrical shape of reactive media surrounded by coarse material with a core of course materials. Additionally, there must be a radial centripetal flow by applying a hydraulic gradient. The third type of PRB has a long lifespan and a better treatment efficiency by extending the contact time between the pollutant and the reactive barrier [ 47 , 53 , 54 ].

Different reactive materials can be used to remediate contaminants, for example, zero-valent iron (ZVI; Fe0), which is a mild reductant and can treat heavy metals. ZVI can de-halogenate may halogenate hydrocarbon derivatives [ 55 ]. Bio-sparging materials and slow oxygen releasing compounds have the ability to treat groundwater containing petroleum hydrocarbon plums such as nitrobenzene and aniline by utilizing the biodegradation of these pollutants in PRBs [ 56 ]. Vegetative materials could be used in PRBs such as mulch to remediate chlorinated solvents and perchlorates [ 57 ].

Contaminants can also be precipitated on chemical reactive materials in the PRBs, for example, fly ash, ferrous slats, lime, phosphates and zeolites, iron/sand, iron/gravel, iron/sponge, granular activated carbon, organic carbon, copper wool and steel wool [ 37 , 54 ].

3.3.1. Characteristics of the Reactive Medias

Choosing a good reactive media depends on the following characteristics [ 58 ]:

• 1. Reactivity: The ability of reactive media to react/remediate contaminants and the equilibrium constant. All these factors are necessary to determine the required time for the remediation, which is important to calculate the volume and size of the in situ reactive barriers.
• 2. Stability: It is required that any good reactive material is to be active for a long period to remediate groundwater. Additionally, it is also necessary that the reactive media stay under the surface as a secondary precipitate. Once the PRB is installed, it is very expensive to be excavated and replaced with a new PRB.
• 3. Cost and availability: it is very important that the reactive media be available and inexpensive.
• 4. Hydraulic conductivity: the PRB must have a permeability equal to or greater than the surrounding soil to ease the groundwater flow within the PRB and achieve the remediation.
• 5. Environmental compatibility: Reactive media need to be similar/match the surrounding subsurface soil by mean of grain size for the goal that there will be no change in the hydraulic conductivity of the soil. Additionally, it needs no unwanted by-products to be produced during the remediation.

3.3.2. Uptake Mechanism of Contaminants

In the remediation of groundwater from contaminants, four physical, chemical and biological uptake mechanisms are considered as uptake mechanisms [ 58 , 59 , 60 ], which are: (1) adsorption and ion exchange, (2) abiotic redaction, (3) biotic reduction and (4) chemical precipitation. Remediation of contaminants in groundwater can be achieved by two or more of these mechanisms [ 61 ].

• (1) Adsorption and Ion Exchange

The process in which species in an aqueous environment are attached to a solid surface is referred to as adsorption. Usually, adsorption interaction is considered a rapid and reversible phenomenon. Adsorbents such as zero-valent iron (ZVI), zeolite and amorphous ferric oxyhydroxide (AFO) are the most common adsorbents used in the adsorption of contaminants; most of the adsorbents have a large surface area per gram and could be used in a PRB. ZVI has the most adsorption rate, and it is the most popular reactive media used in PRBs. Adhesion of pollutant’s ions, atoms or molecules while it is in a liquid, gas or dissolved solid state is referred to as adsorption. It utilizes chemical forces to create a thin film of the adsorbate on the adsorbent’s surface. The adsorbent is any kind of material that can adsorb substances through its surface area characteristics. In the adsorption theory, the surface area of the adsorbent is predominant. The solid phase that provides a working adsorption area is the adsorbent, while the substances and species adsorbed on the adsorbent are referred to as the adsorbate. Adsorption efficiency depends on adsorbate concentration, liquid-phase temperature and pH [ 62 ].

Ion exchange is a process of remediation of inorganic chemicals and dissolved metals from liquids and groundwater. The ion exchange process is that the ion (a single atom or group of atoms) is either positively charged after its loss of electrons or negatively charged after gaining an electron. When liquids loaded by pollutants pass through the ion exchange resin, contaminated substances will be exchanged by the effect of metallic ions attraction by the resins. These resins can be re-generated after being exhausted, or it may be a single-use resin [ 63 , 64 ]. Ion exchange phenomena is a reversible reaction process in which a pollutant’s ion is replaced with an identical ion on the immobilizing barrier. Most ion exchangers are natural such as zeolite, but also, there are very good synthesized ion exchanger resins that can be used in specific needs, especially for the treatment of inorganic contaminants [ 58 , 60 ]. The ion exchange method is applicable to remediate heavy metals [ 65 ] and dissolved metals (chromium) from polluted liquids. Additionally, this method could be used to treat non-metallic pollutants such as nitrate and ammonia [ 63 ]. The limitation to the use of this method is that the oxidation of the soil will cause damage to the resin and will decrease remediation efficiency [ 66 , 67 ]. Another concern is that the contaminant has not been destroyed if treated by the ion exchange method; it is only transferred to another medium that needs to be disposed of. This method is not good if the groundwater contains oil or grease, as these pollutants may clog the exchange resin [ 67 ].

• (2) Abiotic Reduction

The chemical reactions that lead to the decomposition of contaminants in groundwater are referred to as abiotic remediation. In this technique, the harmful compounds are to be reduced either by immobilization in the treatment wall of the reactive barriers, or it is permitted to pass through the barrier in a harmless form. Zero-valent iron (ZVI) is the most popular reactive material used in the abiotic remediation of groundwater; after the reaction of ZVI with the contaminants, low solubility minerals will be precipitated, for example, the remediation of U and Cr from groundwater, which is removed by the precipitation of these contaminants by the abiotic process. Equation (1) shows the ability of ZVI to reduce U(VI) to U(IV) in groundwater with high carbonate and moderate pH via producing UO 2 (Uraninite), which is a solid, less crystalline product of uranium.

For the chromium (Cr), ZVI reducing Cr(VI) to Cr(IV) [ 58 , 60 ] as shown in Equation (2):

Cr(VI) could be reduced to Cr(III) by ferrous iron via introducing dissolved dithionite ions ( S 2 O 4 2 − ) to an aquifer, which can reduce the solid phase of ferric iron. Dithionite oxidizes to sulphite ( S O 3 2 − ) and F e 3 + is lowered to F e 2 + . Cr(III) is to be stalemated by precipitate in the solid form of Cr(III) and Fe(III) hydroxide along with the reduction in some halogenated organic compounds by the effect of F e 2 + as shown in Equations (3) and (4).

• (3) Biotic Redaction/Oxidation

When physical or chemical remediation of groundwater shows little or no degradation of contaminants, then degrading pollutants with a biological oxidation process may be helpful. Many pollutants such as chlorinated solvents tend to be easily reduced if oxidized; here, microorganisms will perform a reduction process by exploiting contaminants as their main source for energy and the required materials to synthesize their cells [ 49 ]. The bioremediation technique is a very effective remediation process based upon the degradation of contaminants by microorganisms; remediation efficiency in this process depends on the working environment, such as the temperature, pH, electron acceptors and the concentration of nutrients [ 68 ]. In biodegradation, it is necessary that germs use electron acceptors to accept any electrons liberated from pollutants; electrons transfer, releasing energy that is essential for microbes’ lives. In the presence of oxygen, under aerobic conditions (which is preferable), energy producing from this process is higher than that released without the presence of oxygen. Additionally, the oxidation rate of contaminants is higher. In the groundwater, the presence of oxygen is usually little; in this case, the anaerobic microbes electron acceptors is utilized. However, it is effective to remediate groundwater contaminated by monoaromatic hydrocarbons by using oxygen-releasing compounds in the PRBs [ 49 , 56 , 69 ].

The basic concept of biotic reduction, biotic oxidation, is to supply an electron donor along with nutrient materials to be used by microorganisms to break down the contaminants. Leaf mulch, wheat straw and sawdust can be used as electron donors, and municipal waste can be used as a nutrient material. Dissolved sulphate in the wastewater is a good electron acceptor, which can oxidate organic materials and can consume acidity coupling with metal reduction as shown in the below Equations (5) and (6):

• (4) Chemical Precipitation

This process consists of contaminants removal as hydroxides (Equation (7)) and carbonates (Equation (8)) via mineral precipitation resulting from increased pH. Firstly, contaminants are reduced to a less soluble species, and finally, they are retained as minerals in the barrier. Limestone (CaCo 3 ) and apatite [Ca 5 (PO 4 ) 3 (OH)] are commonly used in chemical precipitation

A summary of the available and common reactive media is presented in Table 3 ; the geochemical process, nature of contaminants, reactivity and availability are significant factors in the selection of the best convenient reactive media in remediating groundwater.

Reactive media for the remediation of groundwater contaminated by metals and radionuclides (Bronstein, 2005).

4. Modelling of Sorption Process

“Sorption” refers to the physical or chemical process in which a substance becomes in contact with another, which consists of two processes:

• (1) “Adsorption” is a surface process; substances transfer from their aqueous phase (liquid or gas) to the solid phase surface that provides a surface for adsorption known as “adsorbent”; the species transformed from the aqueous phase to the surface of the solid phase is called “adsorbate” [ 62 ]. The existence of nitro groups on the adsorbate stimulating adsorption, hydroxyl, azo groups increases the adsorption rate, while the presence of sulfonic acid groups decreases adsorption [ 70 ].
• (2) “Absorption” is defined as the whole transfer of substances from one phase to another without forces being applied to the molecules. The relationship governing the transfer of substances in aqueous porous media and the mobility of substances from liquid or gas states to the solid state is referred to as “isotherm” [ 71 ]. Adsorption isotherms is curvy relationships connecting the equilibrium concentration of a solute on the surface of an adsorbent ( q e ) to the concentration of solute in its aqueous state ( C e ); both phases should be in contact with each other [ 70 , 72 ].

4.1. Sorption Isotherm Models

Several isotherm models are used to describe sorption parameters and the adsorption of pollutants as follows:

4.1.1. Freundlich Model

In 1909, Freundlich gave an imperial relationship that describes the capability of a unit mass of solid to adsorb gas in the presence of pressure. The Freundlich adsorption isotherm is a curve correlation between a solute concentration on a solid’s interface and the solute concentration in the adjacent aqueous environment [ 73 ]. The Freundlich isotherm model describes absorption in the terms of adsorbate concentration as follows:

where K f   m g g   is the coefficient of the Freundlich isotherm, n < 1, which describes the empirical coefficient expresses the amount of sorption [ 72 , 74 , 75 ]. ( K f )   a n d   (n) can be calculated by solving equation xx logarithmically and plotting ln q e verses ln C e where K f = 10 y − i n t e r c e p t and the slop of ( 1 n ) as shown below:

According to the Freundlich isotherm, the sorbet contaminants is directly proportional to their concentration at a small amount and decreases when contaminants accumulate at the surface of the reactive media [ 76 ].

4.1.2. Langmuir Model

The theoretical Langmuir isotherm model has been derived to describe the physical besides the chemical adsorption, as well as quantifying and describing the sorption on sites located on the adsorbent. Langmuir assumes the following [ 70 , 71 , 76 ]:

• Each adsorbate molecule is to be adsorbed on a well-defined binding site on the adsorbent, and adsorption reaches saturation when all these sites are occupied.
• Each active binding site on the adsorbent interacts with one adsorbate molecule only.
• No interaction existed between adsorbed molecules. All sites are homogeneous (energetically equivalents).
• The surface is uniform, and monolayer adsorption occurs.

Accordingly, the equation of the Langmuir isotherm model is:

where C e (mg/L) represents the concentration of solute in the bulk solution at the equilibrium state. q m (mg/g) represents the maximum adsorption capacity. b is a constant that represents sorption free energy. q e (mg/g) represents the amount of the adsorbed solute by a unit weight of adsorbent within the equilibrium conditions. The Langmuir equation’s constant can be determined with the linearization of Equation (12) as follows:

This equation describes that C e q e is plotted as a function of C e , the parameters of q m and b   are determined from the slope ( 1 q m ) with y-intercept ( 1 q m b ) linear regression to Equation (12) [ 76 ].

4.1.3. Temkin Model

The Temkin isotherm assumes that heats of adsorption would more often decrease than increase with the increase in solid surface coverage. It takes into account the adsorbing species–adsorbent interaction. Temkin isotherm has the following formula:

where R represents gas universal constants (8.314 J/mol K). T is the absolute temperature (K). a T e and b T e are constants.

4.1.4. Brunauer–Emmett–Teller (BET) Model

The BET was developed based on the Langmuir model in an attempt to minimize the Langmuir isotherm restrictions. This isotherm assumes that more molecules can be adsorbed on the monolayer, and it is possible within this isotherm that bi-layer (multi-layer) adsorption will occur. This isotherm could be proclaimed as:

where q m is the maximum adoption capacity, b represents a dimensionless constant, and C s is the concentration in the case of saturated sites and homogenous surfaces.

4.2. Kinetic Models

Adsorption kinetic models are important to describe the solution uptake rate and adsorption required time [ 74 , 75 , 77 ]; these models providing a description for the sorption process onto the sorbents. The sorption mechanism occurs in three steps; the first one is the diffusion of adsorbate through the aqueous phase surrounding the adsorbent; secondly, the diffusion of adsorbate in the pore of the particle (intrapore diffusion); finally, the adsorption occurrence due to physical or chemical interaction between the adsorbate and adsorbent [ 75 , 78 , 79 ]. However, three kinetic models are used to describe the sorption mechanism and the predominated stage as follows:

4.2.1. Pseudo-First-Order Model

A model that is quantified according to Equation (15) below:

where q e is the contaminant’s amount sorbet in equilibrium conditions (mg/g), q t represents a contaminant’s quantity sorbet during any given time (t) (mg/g), k 1 is a constant rate of pseudo-first-order adsorption (min −1 ).

The pseudo-first-order equation has been integrated at boundary conditions of t = 0 to t = t and q t = 0 to q t = q e , then transferred to a linear form as shown in Equations (16) and (17) [ 80 ].

For this kinetic model, log q e − q t must be plotted against time interval; if the intercept of q e theoretical differs than q e experimental , then the reaction does not follow the model of the pseudo first order.

4.2.2. Pseudo-Second-Order Model

The kinetic model of pseudo-second-order adsorption is applicable for small initial concentrations to calculate the initial sorption rate [ 74 ]. The pseudo-second-order equation for the sorption rate has the following form:

where q t is the magnitude of adsorbate, which is adsorbed by an adsorbent (mg g −1 ) at a given time (min), q e represents the amount of adsorbate adsorbed (mg g −1 ) in equilibrium conditions. k 2 is a constant of the second-order sorption rate (mg (mg min) −1 ) [ 80 ].

4.2.3. Intra-Particle Diffusion Model

In 1962, Weber and Morris proposed the kinetic model of intra-particle diffusion, and it has been used for the analysis of adsorption kinetics of lead ions by adsorbent (CHAP) [ 76 , 80 , 81 ]. Based on this model, the uptake graph of ( q t ) versus the squared root of time ( t 0.5 ) must be linear in the overall adsorption process; in addition, if the line intersects with the origin, then the intra-particle diffusion is the predominant adsorption process. The k d represents the intra-particle diffusion initial rate (mg (mg min) −1 ), which could be calculated through the following formula:

where q t represents the amount of sorbate on the solid phase (surface of sorbent) at any time t (mg g −1 ), and t represents time (min).

5. Contaminant Transport Equation and Breakthrough Curves

Soil is a dynamic system in which toxic contaminants are used as a sink or a pathway. When contamination occurs on the surface soil, some of these contaminants will percolate under the water table and form a plume of contaminants. This plume will be developed over time ( t ), and contaminants will be driven downstream, as shown in Figure 1 . It is very important to understand how these contaminants will dissolve in the flow and how they will be carried out downstream; it is very important to discover the concentration of these contaminants as a function of time. The predominant mechanism for the attenuation and retardation of contaminants is sorption. Sorption phenomena will happen when the solid phase of the environment attenuate these contaminants, which will lead to contaminants being removed from the water, and the concentration of pollutants will be reduced downstream. The transport mechanism of pollutants in a saturated environment is the advection that carries contaminants without mixing. The hydrodynamic dispersion is driven by molecular diffusion and mechanical dispersion. If the hydraulic dispersion goes to zero, then the transport will be conservative, and there will be no retardation or any attenuation to the contaminants; on the contrary, if there is retardation to the contaminates, then the concentration of contaminants will be reduced at the downstream by the effect of sorption.

Contaminants concentration development in groundwater (“t1, t2–t6” are time intervals).

5.1. Modeling of Contaminants Transport

5.1.1. advection.

In the advection, contaminants transport downstream along with the flow with advective velocity. It is the physical transport of contaminants across the space:

where V x a is the linear advective velocity.

Darcy velocity is given by the meaning of Darcy law, which is:

where ( K ) is the hydraulic conductivity, ( K r ) is the relative conductivity, ( θ ) is the volumetric moisture content, and ( ∂ h ∂ x ) is the head gradient in the x-direction.

where (F x ) is the advective flux ( K g t . m 2 ), ( V x a ) is the advective liner velocity ( m s e c ), ( n ) is the effective porosity, and ( c ) is the concentration of contaminants ( k g m 3 )

Substitute   F x , F y   and F z   in the conservative equation:

In the saturated medium, ( n ) = 1.

5.1.2. Hydrodynamic Dispersion

Molecular diffusion.

In a stagnant fluid, diffusion is the process of molecules random movement. It is basically driven by the concentration gradient and occurs by the Brownian motion. Therefore, diffusion usually increases with the increment of entropy.

In general, diffusion follows Fick’s first law:

where ( F ) is the mass of solute per unit area per unit time ( M L 2 T ), ( D d ) is the diffusion coefficient ( L 2 T ) ≈ 10 − 9 ( m 2 s e c ), and ( ∂ c ∂ x ) is the concentration gradient ( M L 3 L ).

According to the mass conservation of dissolved contaminants:

The time-dependent concentration equation is:

n = 1 in a saturated medium.

Substituting Fick’s first law in Equation (28)

For a one dimensional flow:

The diffusion coefficient ( D d ) here is the free diffusion coefficient (i.e., in water); if the flow medium is porous, then the effective diffusion coefficient ( D * ) is used due to the effect of the tortuous flow path:

w is related to the tortuosity (T): T = l e l ≥ 1 as shown in the below Figure 2 ; laboratory studies showed that 0.01 > w   ≤ 0.5

Determination of tortuosity in a porous medium.

Mechanical Dispersion

There is a number of mechanisms that lead to the assurance of the mechanical mixing of contaminants in the aquifer as follows:

• (a) Mechanical dispersion due to pore size

When dissolved contaminants pass through a porous medium, pore size will affect the hydraulic conductivity of this media; when particles are fine, porosity will be below, and the advective velocity will be slow, as shown in Figure 3 .

Mechanical dispersion due to pore size.

• (b) Mechanical dispersion due to path length

If a pore is medium, the mechanical mixing may happen due to the effect of the length of the pathway, which will be passed by the dissolved contaminants. Each molecule of contaminants will pass through a different pathway that is unequal with the pathway of other particles, as illustrated in the below Figure 4 .

Mechanical dispersion due to path length.

• (c) Tylor dispersion

Taylor mechanical dispersion occurs when dissolved contaminants pass around the aquifer’s solid particles. Solids pass faster in a middle way between two particles than another pass near a solid particle, as shown in Figure 5 . This is because the linear velocity in the centre of pores is greater than that near the edge of solid particles.

Tylor mechanical dispersion.

All the above mechanisms lead to mechanical mixing for solute contaminants in both the longitudinal direction (with the main flow direction) and the transverse direction (out of the main flow direction).

The coefficient of mechanical dispersion ( D ) is related to aquifers’ dispersivity (α), which reflects the extent to which the aquifer is dispersive and the advective velocity of flow.

where ( D L ) and ( D T ) are the mechanical dispersion coefficient in the longitudinal and transverse directions (m 2 /sec), respectively. ( α L   ) and ( α T ) are the longitudinal and transverse dispersivity (m), respectively. ( V L a ) and ( V T a ) are the longitudinal and transverse advective velocity (m/sec).

In the low permeability medium, the permeability is near to zero; in this case, there will be no effect on the mechanical dispersion, and only the diffusion will be predominant.

5.1.3. Advection–Diffusion Equation

The theory of contaminants transport model in porous media is subjected to a partial differential equation governing space and time. The theory incorporates four different processes, all merged in one equation; one process is advection, which means that a substance follows the direction of water (driven by water flow) and itself moves with convection. The second process is dispersion, which is caused by the heterogeneity of pollutants, and a package of contaminants will move faster than the others. Then, there is a chemical reaction, which described by a kinetic equation. Finally, there is the adsorption to the soil, which means that the contaminant may spend some of its time tied to the solid phase and sometimes in the mobile water. The equation that describes all of this is the advection–dispersion equation, as follows:

In the above equation, the change in mass per unit volume ( m ) of the contaminants due to the reactions within the aquifer is referred to as ( r ).

where ( F x ) is the total flux in the ( X ) direction. ( V x n C ) is the addictive flux, and (– ( n D x ∂ C ∂ x )) is the dispersive flux.

Substituting ( F x ) in Equation (37) for (x, y and z) directions:

In the 1D flow, with a constant dispersion coefficient and constants porosity in space and time (=1 in a saturated medium), the equation of advection–dispersion can be written as:

The term ( r ) is considered an important factor in the attenuation of contaminants in a porous media, which is related to the sorption, the predominant process of contaminants attenuation in a permeable reactive barrier during contaminants’ mass transfer. Generally, ( r ) depends on the bulk density ( ρ b ) of the medium and the amount of contaminants sorbed ( q ) with time, thus:

By substituting the value of ( r ) (Equation (41)) in Equation (40), the advection–dispersion will be as follows:

The sorption process is represented in the above equation by the term ρ b n ∂ q ∂ t , ( q ) represents contaminants concentration that sorbed on the solid phase of the reactive media, which can be described by the Langmuir or Freundlich isotherm models as a function of concentration. Equation (42) can be rewritten as follows:

where ( R ) is the retardation factor, which reflects the effect of retardation of contaminants during its transport to the downstream.

The “breakthrough curve” describes the relationship between the concentration of contaminant vs. time, which is an important tool for design and optimizes the sorption in a field-scale PRB by relating the data obtained from laboratory columns to the field scale breakthrough curves. In a continuous constant influent of contaminants, the breakthrough curve will be shaped as (S); the best point on this curve is referred to as the breakthrough point, which has an outlet concentration of contaminants that matches the desired concentration in water. A summary of empirical and theoretical models used to predict the breakthrough curves are described below:

• Bohart–Adams model

The purpose of performing column experiments is to calculate the relationship between the concentration and time, the breakthrough curve in addition to calculate the maximum adsorbent capacity of adsorption. Results will be used to design a full-scale adsorption column. The Bohart–Adams model is one of the models that has been formulated to fulfil this purpose; it has been based on the rate of surface reaction theory [ 82 ]. This model has been built on the following assumptions [ 48 ]:

• 1. This model can describe the concentration at low levels ( C « C 0 ) ( C = 0.15   C 0 ).
• 2. When t → ∞ ; q 0 → N 0   with saturation concentration.
• 3. The external mass transfer is limiting adsorption speed.
• 4. The Bohart–Adams model has the following formula: C C 0 = 1 1 + exp K N 0 Z U − K C 0 t (44) where C 0   and C represents the instantaneous (initial) concentration of the pollutant in solution (mg/L). K is the kinetic constant (L/g/min). N 0 represents the congestion concentration (mg/L). Z represents column bed depth (cm). t represents the time of service (min), and U is the velocity of the flow (cm/min).
• Thomas model.

The Thomas model is widely used to calculate adsorbent maximum adsorption capacity. It uses data obtained from continuous column experiments. The Thomas adsorption column is given below:

where C 0   and C are the concentrations of influent (mg/L). K T is the constant rate (mL/mg/min), q represents the higher adsorption capacity (mg/g), M represents an adsorbent quantity in the column (g), t is the time of adsorption (min), and Q is the feed flow rate (mL/min). The Thomas model is based on the following assumption:

• 1. No dispersion is driven.
• 2. The Langmuir isotherm coincide with the equilibrium state.
• 3. Adsorption kinetics ( K ) should follow the rate of pseudo-second-order law.
• Yoon–Nelson model

In this model, the decreasing probability of each adsorbate is proportional to its breakthrough adsorption on the adsorbent. The following formula is a representation of this model:

where K Y N represents the Yoon–Nelson rate constant. The Yoon–Nelson model is limited by its rough form.

• Clark Model

Clark’s breakthrough curves were based on the mass transfer principle in conjunction with the Freundlich isotherm. Clark has developed his breakthrough curves as follows:

where n represents the exponent of the Freundlich isotherm,   A and r represents the parameters of the kinetic equation.

Wang et al. (2003) invented a new model based on the mass transfer model. It has been used as a solution of Co and Zn ions in a fixed bed under the following assumptions:

• 1. The adsorption mechanism is isothermal.
• 2. The mass transfer equation is as the following: − d y d t = K w x y (48) where K w represents the kinetic constant, the fraction of adsorbed metal ions is represented by y . (with x + y = 1 ) ,     x represents the fraction of metal ions moving through the fixed bed.
• 1. There is symmetry in the breakthrough curve.
• 2. The axial dispersion in the column is negligible.

By integrating the above equation and presuming that y = y w at t = t w . w = 0.5 , the entire breakthrough equation can be expressed as:

where ( x ) can be expressed as:

Finally, the Wand model, similar to the Yoon–Nelson model, cannot provide enough detail on the adsorption mechanism.

6. Review of Previous Research on the Use of PRBS

The first permeable reactive barrier was constructed at a Canadian air force base in (1991) [ 83 ]; since that date, many studies have been conducted to examine the PRB’s efficiency. There were 624 publications that discussed the permeable reactive barrier from 1999 to 2009 [ 84 , 85 ]. Previous research has been conducted to study the ability of different reactants to remediate different pollutants in the permeable reactive barrier. The following is a list of the most important scientific studies.

The remediation of groundwater contaminated by chlorinated ethenes such as vinyl chloride (VC), dichloroethene (DCE) and trichloroethene (TCE) was studied using in situ biodegradation with a special functional microorganism known as Burkholderia cepacia ENV435 [ 86 ]. The researchers chose these microorganisms for many important characteristics, such as their good adhesion ability to aquifers’ solids; in addition, these microorganisms can establish an organized existence without the need to induce co-substrates. Furthermore, these organisms can grow in a high density in fermenters (−100 g/L), and finally, they can accumulate high internal energy, which this microorganism can use to resist the effect of chlorinated solvents and survive. Results showed the concentrations of VC, DCE and TCE decreased by 78% after two days of organism injection.

The output of a pilot-scale PRB for the remediation of chlorinated volatile organic compound-contaminated groundwater (VOCs) has been investigated. This study used a granular zero-valent iron reactive barrier, which was mounted in a funnel with a gate mechanism. Results showed that consistent VOC degradation was observed over the research period. It is observed that the degradation mechanism is due to pH increment, which leads bicarbonate ( H C O 3 − ) to convert to carbonate ( C O 3 2 − ), the carbonate combines cations ( C a 2 + ,   F e 2 + ,   M g 2 + ,   etc . ) in solution, which form mineral precipitates. It is observed that mineral precipitates formed in the reactive media represented as an unconquerable limitation to the treatment process [ 87 ].

A zero-valent iron PRB’s effectiveness in eliminating chlorinated aliphatic hydrocarbons (CAHs) has been investigated. The contact of reactive media (ZVI) with the CAHs in an aqueous environment caused a rise in the pH; this resulted in the precipitation of carbonate minerals and a loss of 0.35% of the porosity in the reactive fraction of the PRB [ 88 ].

The rapid evolution of the PRB’s application from a full in situ implementation on a laboratory level to treat groundwater polluted by various types of inorganic and metals was assessed [ 89 ]. This study concluded that different reactive media can be used in the preamble reactive barrier to remove inorganic compounds, such as the use of zero-valent iron PRB to remove TC, U and Cr from groundwater. Furthermore, solid-state organic carbon may be used to extract dissolved solids associated with acid-mine drainage. According to this research, there are different mechanisms for the treatment of inorganic anions; for example, the rate of Cr(VI), TC (VII), U(VI) and NO 3 could be successfully decreased by the mean of reduction using zero-valent iron (Fe 0 ). According to a monitoring program for a Cr(VI)-contaminated area, the concentration of Cr(VI) has decreased from 8 mg L −1 to > 0.01 mg L −1 , owing to a decrease in Eh and an increase in pH.

At a former uranium production site in Monticello, Utah [ 90 ] investigated the design and efficiency of a PRB in extracting arsenic, uranium, selenium, vanadium, molybdenum and nitrate. In this study, field and laboratory column tests have been performed. The reactive media in PRB was the zero-valent iron. After one year from PRB installation, the performance of ZVI–PRB is described by the reduction in concentrations of elements up-gradient and down-gradient of the barrier. The inlet concentrations of arsenic, manganese, molybdenum, nitrate, selenium, uranium and vanadium were 10.3, 308, 62.8, 60.72, 18.2, 396 and 395 µg/L, respectively. These concentrations have reduced to be >0.2, 117, 17.5, >65.1, 0.1, >0.24 and 1.2 µg/L, respectively. The removal mechanism for these radionuclides is by reducing uranium to lower molecules along with precipitation. Additionally, adsorption is another chemical process that leads to a reduction in these elements.

The use of a reactive biological barrier to remove nitrate pollutants has been investigated. The autotrophic sulphur-oxidizing bacteria has been used as an electron donor, and sulphur granules have been used as a biological agent. Sulphur-oxidizing bacteria colonized the sulphur particles and removed nitrate, according to the findings. The best operation conditions have been investigated, and it was found that an environment near the neutral pH achieved 90% removal of nitrates [ 91 ].

The efficacy of a ZVI barrier mounted in the field in eliminating chromium solid-phase association has been studied, and the removal efficiency after 8 years of operation has been investigated. Results showed that ZVI has the ability to reduce the concentration of Cr from an average <1500 µg/L to about >1 µg/L. The reduction in Cr(VI) to Cr(III) along with the oxidation of Fe(0) to Fe(II) and Fe(III), resulting in Fe(III)-Cr(III) precipitating as oxyhydroxides and hydroxides, has been discovered to be the most common Cr removal mechanism. It was also discovered that the reacted iron produced a coating of goethite (α-FeOOH) with Cr, resulting in precipitation [ 92 ].

Experiments have been performed to discover the efficiency of seven selected organic substrates in removing inorganic nitrogen in the form of NO 3 − , NO 2 − and/or NH 4 + in a denitrification PRB in batch scale experiments. Softwood, hardwood, coniferous, mulch, willow, compost and leaves were all reactive materials. The softwood was found to be suitable for use as a reactive medium in PRB due to its very good ability to denitrify nitrogen. Reduction in nitrate was due to the effect of denitrification (which represents 90% of the nitrate removal of which the dissimilatory nitrate reduction to ammonia (DNRA) represents 10% of the removal process [ 93 ].

The efficacy of activated carbon PRB for removing cadmium from contaminated groundwater has been investigated. The original cadmium concentration was 0.020 mg/L, but after it passed through a PRB of activated carbon, the polluted plume was adsorbed, and the cadmium concentration was nearly zero for the first three months. After that, the barrier became saturated, but the effluent cadmium concentration remained below the quality limit of 0.005 mg/L for more than seven months [ 94 ].

The use of polyvinylpyrrolidone (PVP-K30)-modified nanoscale ZVI in removing tetracycline from liquid has been investigated. Tests revealed that PVP-nZVI consists of Fe(0) in the core and ferric oxides on the shell. PVP-nZVI will adsorb tetracycline and its degradation products, according to the findings. It is also observed that the adsorption of tetracycline has been reduced with time due to the formation of H 2 PO 4 − , which has a strong tendency to react with the mineral surface [ 95 ].

Tetracycline adsorption using graphene oxide (GO) as a reactive media has been investigated. Results showed that tetracycline formed a π–π interaction and cation–π bonds with the surface of GO, with the Langmuir and Temkin models providing the best fit isotherms for adsorption and the Langmuir model calculating a maximum adsorption capacity of 313 mg g −1 . The kinetics of the adsorption model are also equipped with a pseudo-second-order model with a better sorption constant ( k ), 0.065 g mg −1 h −1 than other adsorbents, according to the results [ 96 ].

The design, construction and testing of a permeable barrier at the Casey station in Antarctica to remediate and avoid the spread of an old diesel fuel spill have been discovered. Five segments of a bio-reactive barrier were allocated and installed in the funnel and gate configuration, each segment divided into three zones; the first one is a slow-release fertilizer zone to enhance the biodegradation, the second zone is responsible for hydrocarbon and nutrient capture and degradation, while the third zone is responsible for cation capture and access to nutrients produced by the first zone. The first zone’s reactive media was a nutrient source, followed by hydrocarbon sorption materials (granular activated carbon plus zeolite); to extract cations nutrient released and accessed from the first region, sodium activated clinoptilolite zeolite is used. Oxygen delivery to the system was applied to enhance the microbial reactions. The function of each zone is the first zone to provide nutrients such as phosphorate to the microorganism. Due to its high surface area and microporous surface (500–1500) m 2 /kg, granulated activated carbon can adsorb hydrocarbon pollutants in the second zone. In the third zone, the Australian sodium zeolite is placed to capture any accessed ammonium cation from the solution due to its high ability to exchange ions with ammonium. Tests and results showed that the ion exchange of zeolite best-controlled nutrient concentration, while the sodium zeolite captured any migrated ammonia from the groundwater. Additionally, results showed that the fuel is degraded in the PRB faster than in the hydrocarbon spill area field. In the cold world, activated carbon–PRB is a strong technology for removing hydrocarbons.

In batch and fixed-bed column experiments, the adsorption of tetracycline (TC) and chloramphenicol (CAP) was investigated by [ 97 ] using bamboo charcoal (BC) as a reactive medium. The predominant mechanism of TC and CAP adsorption on BC is π – π   electron-donor–acceptor (EDA), cation–π bond in combination with H-bond interaction, while the hydrophobic and electrostatic interaction has a minor effect on the adsorption. Results showed that BC has a strong adsorption capacity to TC and CAP; with increasing influent concentration and flow rate, adsorption efficiency improves. Surface diffusion was the most common mass transfer mechanism for antibiotic adsorption [ 98 , 99 ].

An overview of the use of PRBs in the remediation of a broad range of pollutants, demonstrating that it is a viable alternative to the pump-and-treat process, has been discussed by [ 85 ]. The most popular PRB reactive media, according to this study, is zero-valent iron (ZVI). Efficient PRB architecture requires accurate site characterization, groundwater flow and flow conditions requirements and ground flow modelling.

The potential efficiency of a microscale zero-valent iron PRB in removing tetracycline (TC) and oxytetracycline (OTC) with the formation of transformation products during the remediation have been discovered. To investigate the effect of solution pH, a series of batch experiments were carried out, including iron dose and environment temperature. Results showed that pH has a key factor controlling the efficiency of removal; increasing iron dose and working temperature also increased the removal efficiency. Pseudo-second-order model and Langmuir isotherm were found to be most fitted to adsorption kinetics and removal isotherms [ 100 ].

The effectiveness of removing copper ions Cu(II) and zinc ions Zn (II) heavy metals from groundwater using cement kiln dust and a sand PRB was investigated by [ 48 ]. In this research, the re-use of a very fine by-product powder resulted from the cement industry known as cement kiln dust (CKD) has been investigated to remove appointed heavy metals instead of throwing this CKD into the environment. The optimum weight ratio of CKD/sand, which provides the best remediation, has been investigated in column tests from 99 days of operation time. The remediation mechanisms were the adsorption/desorption, precipitation/dissolution and adsorption/desorption of the pollutants. Contaminant transport in porous media, as well as breakthrough curves, are also explored. Breakthrough curves refer to the relationship between the concentration of the contaminants at any time in any position in the domain. Results showed that the best CKD/sand ratio was (10:90 and 20:80) because other ratios showed a loss in the hydraulic conductivity and loss in groundwater flow due to the accumulation of contaminants mass in the voids between the sand causing clogging and flow loss.

The mechanism of remediating pharmaceutical pollutants (tetracycline) from groundwater using zero-valent iron coupled with microorganisms as reactive media has been investigated by [ 55 ]. In this research, three PRB columns have been studied, beginning with columns filled by zero-valent iron, the second with zero-valent iron and microorganisms and, finally, the third one with microorganisms. Results revealed that zero-valent iron has the best effect on removing tetracycline. Removal efficiency reaches 50% while it was 40% with zero-valent iron and microorganisms’ PRB and 10% by the effect of microorganisms’ PRB. The mechanism of this reaction is that the zero-valent iron (Fe 0 ) has been adsorbed and reduced tetracycline, Fe 0 converted to Fe +2 and Fe +3 , and the tetracycline has been degraded.

The use of a bio-PRB coupled with a good aeration system to remediate groundwater polluted with nitrobenzene and aniline have been studied. To degrade the NB and AN, suspension-free cells of the degrading consortium and the immobilized consortium were used in this study. Results showed that both AN and NB were completely degraded within 3 days in the immobilized consortium, while it needs 3–5 days to degrade using the free cells. It was also discovered that in the presence of oxygen, the removal efficiency of NB and AN was increased [ 56 ].

In a permeable reactive barrier, [ 101 ] investigated the effect of MnO 2 and its mechanism of tetracycline elimination. The zero-valent iron serves as the reactive media in this PRB. In this research, three PRB columns were studied, the first one with ZVI, the second had ZVI-MnO 2 , while the third consisted of MnO 2 only. Results show that the ZVI in the presence of MnO 2 is the most effective material in removing TC. Its removal efficiency reached 85%, while the ZVI removed about 65% and the MnO 2 removed 50% of TC. This research revealed that MnO 2 accelerated the transformation of Fe 2+ to Fe 3+ , then the Fe 3+ degraded tetracycline. The functional group that played the predominant role in this reaction is the hydroxyl radical produced in this process.

A series of laboratory and field studies in the Ukrainian city of Zhovty Vody has been performed to assess the reliability of a reactive barrier made up of zero-valent iron and organic carbon mixtures to remediate uranium-contaminated groundwater. In these studies conducted by [ 102 ], three reactive media were examined. The first was zero-valent iron, which was used to study the sorption, reduction and precipitation of redox oxyanions; the second was the phosphorate materials, which has been used to transfer the dissolved materials to other phases; the third was bioremediation materials and organic carbon substrates. The study revealed that the treatment mechanism of the uranium is sorption by the ZV, and it also observed that the microbes have the ability to sorb the uranium U(VI) to the bacterial cell walls. Due to the effect of enzymatic production, dissolved oxygen reduced first, then due to the effect of denitrification, UO 2 CO 3 reduced to uranite and sulphate reduced to sulphide; finally, amorphous uranium oxide will be formed on the microorganism surfaces. In this research, new placement of the reactive media has been used in which rows of cylinders with iron reactive media have been placed instead of the regular funnel and gate placement; this placement reduced the in situ installation cost.

The effectivity of PRB made from sodium alginate/graphene oxide hydrogel beds (GSA) for the remediation of ciprofloxacin (CPX) antibiotic contaminating the groundwater has been investigated. In this research, the key factors affecting the performance have been studied, and longevity and the cost of PRB have been discussed, and a proper design for the PRB has been proposed. Results show that the adsorption capacity of CPX on the GSA was 100 mg for each gram of GSA at pH 7.0; the leading mechanism in the adsorption process was the pore filling, H-bonding, ion exchange, electrostatic interaction and hydrophobic interaction. The results indicate that the GSA’s ability to remove CPX from groundwater when used in a PRB is concrete evidence that GSA is a good option for removing CPX from groundwater [ 103 ].

The removal of tetracycline from aqueous solutions using binary nickel/nano zero-valent iron (NiFe) reactive media in column reactors has been studied. Results show that if a mixture of 20 mg/L of TC plus 120 mg/L of NiFe in a 90 min time of interaction, TC will be removed by 99.43%. In this research, sand particles loaded with reactive media (NiFe) have been used. Electrostatic interaction has been used to load the reactive media on sand particles. A Tc removal mechanism was investigated using UV-Visible spectroscopy, TOC, FTIR and SEM analysis [ 104 ].

The use of the PRB system in preventing the migration of radiocesium into groundwater using natural zeolite and sepiolite has been investigated. These reactive media are natural, low-cost materials. Two-dimensional bench-scale prototypes at the steady flow conditions have been used in the experiment. Information on the transport behaviour of radiocesium and changes in hydraulic conductivity were investigated in this study. It has been determined that the remediation phase would reduce hydraulic conductivity over time. As a result, by combining sand with reactive media, the PRB has been modified to achieve steady-state operating conditions of flow [ 105 ].

The effectivity of the use of PRB of cement kiln dust as a reactive media in an acidic environment (pH 3) to remediate groundwater contaminated with dissolved benzene has been studied by [ 9 ]. Experiments were performed for 60 days with batch and column tests. Results showed that benzine removing efficiency reached more than 90%, and the best CKD/sand ratio was 5/95, 10/90 and 15/85, which achieved the best hydraulic conductivity. Results also show that barrier longevity reached (half a year) when CKD was about 15%. FTIR test results showed that adsorption happened due to the formation of H bonding and cation.

The removal of meropenem antibiotic with a cement kiln dust (CKD) PRB through batch and continuous column experiments have been studied by [ 106 ]. Results showed that pH 7.0 had a 60 mg adsorption potential for every 1 g of CKD, according to the findings. Initial concentration, flow rate and influence have all had an impact on CKD efficiency. Meropenem adsorption occurred due to the O-containing functional group’s effect on the surface of CKD, which leads to an H-bonding and π – π   a n d   n – π EDA interaction (donor–acceptor) between the CKD and the meropenem, which all lead to the adsorption.

The sustained treatment of a bio-wall and its effectivity in remediating groundwater contaminated by chlorinated volatile organic compounds (TCE) after 10 years of bio-wall installation has been studied by [ 107 ]. The reactive medium used in this barrier was mulch, utilizing the benefit of its high cellulose content (<79%). This research investigates a reactive barrier of mulch (1615 m long × 10.7 m depth × 0.6 m thickness). This bio-barrier consisted of 42% mulch, 11% cotton, 32% sand and 15% rock to increase the permeability. It is estimated that groundwater retention time within the barrier is 2–50 days, while groundwater speed was (0.002–0.3 m/day). Contaminants were trichloroethene (TCE), tetrachloroethene (PCE), dichloroethene (DCE) and vinyl chloride (VC). After 10 years of the bio-wall installation, results showed that mulch bio-wall effectively degrades TCE from groundwater to daughter products, TCE concentrations remained below the USEPA maximum levels, while it was over these levels in the up-gradient side of the bio-wall. The microbial population, geochemical environment of the barrier was still active. Investigating the concentration patterns, microbial community and the geochemical environment of the bio-wall demonstrates that the bio-wall is an effective reductive to the volatile organic contaminants.

The effectiveness of a horizontal PRB with a reactive media of zero-valent iron to prevent the scattering of chlorinated solvent vapour in the unsaturated region was investigated by [ 108 ]. In this research, the potential feasibility of using PRBs placed in a horizontal direction was investigated. The reactive medium in this study was the zero-valent iron (ZVI) powder mixed with sand, and the TCE was tested as a model for the (VOCs). Tests were performed in batch reactors. Results showed after 3 weeks of treatment and based on the type of ZVI powder, the concentration of TCE vapour was reduced in a range of 35–99%. The ZVI’s best output is determined by the particular surface area.

The use of sewage sludge and cement kiln dust to produce hydroxyapatite nanoparticles has been investigated. The removal of tetracycline using the new formed hydroxyapatite were examined and the best operation conditions were 2 h contact time, dosage 0.4 g/50 mL, agitation speed 200 rpm with a mixture molar ratio Ca/P = 1.662, the removal efficiency reached 90% with a TC maximum adsorption capacity of 43.534 mg for each gram of hydroxyapatite filter cake. Results show that adding 10% sand (to enhance the hydraulic conductivity of the PRB) to the hydroxyapatite reduced the adsorption capacity to be 41.510 mg/g. XRD, FTIR and SEM analytical tests proved that the predominant mechanism for the remediation of TC is due to the adaptation on the hydroxyapatite surface. During the process, two functional groups, (-OPO3H-) and (CaOH2+), were formed, both of which are positively charged with the ammonium functional group and negatively charged with the phenolic diketone moiety of TC species. The removal of TC was also aided by the effect of hydrogen bonding and surface complexes formed between TC and Ca [ 109 ].

7. Conclusions and Perspective

In recent decades, there has been an increment in the dependence on groundwater as a major source of freshwater for daily human needs, but in many places, groundwater is being polluted by organic and inorganic contaminants. It is very important to remediate groundwater before use to prevent the spread of contaminants to the neighbour environment. Many techniques and reactive materials have been used in the remediation of contaminated groundwater; one of the most popular technologies is PRBs, which is considered an affordable technology. It allows the treatment of multiple pollutants if a multi-barrier is being used. In PRB technology, there is no adverse contamination that may happen, as contaminants will not be brought out to the surface. On the other hand, this technology may have some limitations, such as the difficulty of detailed site characterization required prior to the design of PRB, and only contaminants passing the PRB could be treated in addition to the limited field data for the longevity of the PRB, so the prospective tendency is to use new by-product materials to improve PRB performance. In this way, the environment will be saved by the disposal of these unwanted by-products and will be considered a (green) refreshment to the environment.

Groundwater contamination is now a global issue; solving this problem involves close coordination between scientists at universities and government agencies, as well as the industry and decision makers at all levels. The way ahead for solving this problem must include addressing the levels of groundwater contamination in different countries by using developed measures, techniques and policies. In addition, the variation of the influence of groundwater contamination in different countries must be well studied, including the effect on climatic regions and geological features. To study groundwater contamination in the future, groundwater scientists will need to adopt and apply new technologies such as artificial intelligence, “big data” analysis, drone surveys and molecular and stable isotope analysis technologies. Finally, governments, especially those with developing economies, need to invest more in groundwater and encourage researchers, training and research in this important, valuable field.

Author Contributions

O.A.-H. and K.H. organized the conceptualization of the idea and the methodology employed in this paper. Following that, E.L., T.M.Č., I.N., A.A.H.F. and N.A.-A. worked on the critical evaluation of the existing techniques. All authors have read and agreed to the published version of the manuscript.

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Data availability statement, conflicts of interest.

The authors declare no conflict of interest.

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• Published: 28 March 2024

New water accounting reveals why the Colorado River no longer reaches the sea

• Brian D. Richter   ORCID: orcid.org/0000-0001-7216-1397 1 , 2 ,
• Gambhir Lamsal   ORCID: orcid.org/0000-0002-2593-8949 3 ,
• Landon Marston   ORCID: orcid.org/0000-0001-9116-1691 3 ,
• Sameer Dhakal   ORCID: orcid.org/0000-0003-4941-1559 3 ,
• Laljeet Singh Sangha   ORCID: orcid.org/0000-0002-0986-1785 4 ,
• Richard R. Rushforth 4 ,
• Dongyang Wei   ORCID: orcid.org/0000-0003-0384-4340 5 ,
• Benjamin L. Ruddell 4 ,
• Kyle Frankel Davis   ORCID: orcid.org/0000-0003-4504-1407 5 , 6 ,
• Astrid Hernandez-Cruz   ORCID: orcid.org/0000-0003-0776-5105 7 ,
• Samuel Sandoval-Solis 8 &
• John C. Schmidt 9  

Communications Earth & Environment volume  5 , Article number:  134 ( 2024 ) Cite this article

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• Water resources

Persistent overuse of water supplies from the Colorado River during recent decades has substantially depleted large storage reservoirs and triggered mandatory cutbacks in water use. The river holds critical importance to more than 40 million people and more than two million hectares of cropland. Therefore, a full accounting of where the river’s water goes en route to its delta is necessary. Detailed knowledge of how and where the river’s water is used can aid design of strategies and plans for bringing water use into balance with available supplies. Here we apply authoritative primary data sources and modeled crop and riparian/wetland evapotranspiration estimates to compile a water budget based on average consumptive water use during 2000–2019. Overall water consumption includes both direct human uses in the municipal, commercial, industrial, and agricultural sectors, as well as indirect water losses to reservoir evaporation and water consumed through riparian/wetland evapotranspiration. Irrigated agriculture is responsible for 74% of direct human uses and 52% of overall water consumption. Water consumed for agriculture amounts to three times all other direct uses combined. Cattle feed crops including alfalfa and other grass hays account for 46% of all direct water consumption.

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Introduction

Barely a trickle of water is left of the iconic Colorado River of the American Southwest as it approaches its outlet in the Gulf of California in Mexico after watering many cities and farms along its 2330-kilometer course. There were a few years in the 1980s in which enormous snowfall in the Rocky Mountains produced a deluge of spring snowmelt runoff capable of escaping full capture for human uses, but for most of the past 60 years the river’s water has been fully consumed before reaching its delta 1 , 2 . In fact, the river was overconsumed (i.e., total annual water consumption exceeding runoff supplies) in 16 of 21 years during 2000–2020 3 , requiring large withdrawals of water stored in Lake Mead and Lake Powell to accommodate the deficits. An average annual overdraft of 10% during this period 2 caused these reservoirs– the two largest in the US – to drop to three-quarters empty by the end of 2022 4 , triggering urgent policy decisions on where to cut consumption.

Despite the river’s importance to more than 40 million people and more than two million hectares (>5 million acres) of cropland—producing most of the vegetable produce for American and Canadian plates in wintertime and also feeding many additional people worldwide via exports—a full sectoral and crop-specific accounting of where all that water goes en route to its delta has never been attempted, until now. Detailed knowledge of how and where the river’s water is used can aid design of strategies and plans for bringing water use into balance with available supplies.

There are interesting historical reasons to explain why this full water budget accounting has not been accomplished previously, beginning a full century ago when the apportionment of rights to use the river’s water within the United States was inscribed into the Colorado River Compact of 1922 5 . That Compact was ambiguous and confusing in its allocation of water inflowing to the Colorado River from the Gila River basin in New Mexico and Arizona 6 , even though it accounts for 24% of the drainage area of the Colorado River Basin (Fig.  1 ). Because of intense disagreements over the rights to the Gila and other tributaries entering the Colorado River downstream of the Grand Canyon, the Compact negotiators decided to leave the allocation of those waters rights to a later time so that the Compact could proceed 6 . Arizona’s formal rights to the Gila and other Arizona tributaries were finally affirmed in a US Supreme Court decision in 1963 that also specified the volumes of Colorado River water allocated to California, Arizona, and Nevada 7 . Because the rights to the Gila’s waters lie outside of the Compact allocations, the Gila has not been included in formal accounting of the Colorado River Basin water budget to date 8 . Additionally, the Compact did not specify how much water Mexico—at the river’s downstream end—should receive. Mexico’s share of the river was not formalized until 22 years later, in the 1944 international treaty on “Utilization of the Waters of the Colorado and Tijuana Rivers and of the Rio Grande” (1944 Water Treaty) 9 . As a result of these political circumstances, full accounting for direct water consumption at the sectoral level—in which water use is accounted according to categories such as municipal, industrial, commercial, or agricultural uses—has not previously been compiled for the Gila River basin’s water, and sectoral accounting for Mexico was not published until 2023 10 .

The physical boundary of the Colorado River Basin is outlined in black. Hatched areas outside of the basin boundary receive Colorado River water via inter-basin transfers (also known as ‘exports’). The Gila River basin is situated in the far southern portion of the CRB in Arizona, New Mexico, and Mexico. Map courtesy of Center for Colorado River Studies, Utah State University.

The US Bureau of Reclamation (“Reclamation”)—which owns and operates massive water infrastructure in the Colorado River Basin—has served as the primary accountant of Colorado River water. In 2012, the agency produced a “Colorado River Basin Water Supply and Demand Study” 8 that accounted for both the sectoral uses of water within the basin’s physical boundaries within the US as well as river water exported outside of the basin (Fig.  1 ). But Reclamation did not attempt to account for water generated from the Gila River basin because of that sub-basin’s exclusion from the Colorado River Compact, and it did not attempt to explain how water crossing the border into Mexico is used. The agency estimated riparian vegetation evapotranspiration for the lower Colorado River but not the remainder of the extensive river system. Richter et al. 11 published a water budget for the Colorado River that included sectoral and crop-specific water consumption but it too did not include water used in Mexico, nor reservoir evaporation or riparian evapotranspiration, and it did not account for water exported outside of the Colorado River Basin’s physical boundary as illustrated in Fig.  1 . Given that nearly one-fifth (19%) of the river’s water is exported from the basin or used in Mexico, and that the Gila is a major tributary to the Colorado, this incomplete accounting has led to inaccuracies and misinterpretations of “where the Colorado River’s water goes” and has created uncertainty in discussions based on the numbers. This paper provides fuller accounting of the fate of all river water during 2000–2019, including averaged annual consumption in each of the sub-basins including exports, consumption in major sectors of the economy, consumption in the production of specific types of crops, and water consumed by reservoir evaporation and riparian/wetland evapotranspiration.

Rising awareness of water overuse and prolonged drought has driven intensifying dialog among the seven US states sharing the basin’s waters as well as between the United States, Mexico, and 30 tribal nations within the US. Since 2000, six legal agreements affecting the US states and two international agreements with Mexico have had the effect of reducing water use from the Colorado River 7 :

In 2001, the US Secretary of the Interior issued a set of “Interim Surplus Guidelines” to reduce California’s water use by 14% to bring the state within its allocation as determined in the 1963 US Supreme Court case mentioned previously. A subsequent “Quantification Settlement Agreement” executed in 2003 spelled out details about how California was going to achieve the targeted reduction.

In 2007, the US Secretary of the Interior adopted a set of “Colorado River Interim Guidelines for Lower Basin Shortages and the Coordinated Operations for Lake Powell and Lake Mead” that reduced water deliveries to Arizona and Nevada when Lake Mead drops to specified levels, with increasing cutbacks as levels decline.

In 2012, the US and Mexican federal governments signed an addendum to the 1944 Water Treaty known as Minute 319 that reduced deliveries to Mexico as Lake Mead elevations fall.

In 2017, the US and Mexican federal governments established a “Binational Water Scarcity Contingency Plan” as part of Minute 323 that provides for deeper cuts in deliveries to Mexico under specified low reservoir elevations in Lake Mead.i

In 2019, the three Lower Basin states and the US Secretary of the Interior agreed to commitments under the “Lower Basin Drought Contingency Plan” that further reduced water deliveries beyond the levels set in 2007 and added specifications for deeper cuts as Lake Mead drops to levels lower than anticipated in the 2007 Guidelines.

In 2023, the states of California, Arizona and Nevada committed to further reductions in water use through the year 2026 12 .

With each of the above agreements, overall water consumption has been reduced but many scientists assert that these reductions still fall substantially short of balancing consumptive use with 21st century water supplies 2 , 13 . With all of these agreements—excepting the Interim Surplus Guidelines of 2001—set to expire in 2026, management of the Colorado River’s binational water supply is now at a crucial point, emphasizing the need for comprehensive water budget accounting.

Our tabulation of the Colorado River’s full water consumption budget (Table  1 ) provides accounting for all direct human uses of water as either agricultural or MCI (municipal, commercial, industrial), as well as indirect losses of water to reservoir evaporation and evapotranspiration from riparian or wetland vegetation including in the Salton Sea and in a wetland in Mexico (Cienega de Santa Clara) that receives agricultural return flows from irrigated areas in Arizona. We explicitly note that all estimates represent consumptive use , resulting from the subtraction of return flows from total water withdrawals. Table  2 provides a summary based only on direct human uses and does not include indirect consumption of water. We have provided Tables  1 and 2 in English units in our Supplementary Information as Tables SI-1 and SI-2 . We have lumped municipal, commercial, and industrial (MCI) uses together because these sub-categories of consumption are not consistently differentiated within official water delivery data for cities utilizing Colorado River water. More detail on urban water use by cities dependent on the river is available in Richter 14 , among other studies.

We differentiated water consumption geographically using the ‘accounting units’ mapped in Fig.  2 , which are based on the Colorado River Basin map as revised by Schmidt 15 ; importantly, these accounting units align spatially with Reclamation’s accounting systems for the Upper Basin and Lower Basin as described in our Methods, thereby enabling readers accustomed to Reclamation’s water-use reports to easily comprehend our accounting. We have also accounted for all water consumed within the Colorado River Basin boundaries as well as water exported via inter-basin transfers. Water exported outside of the basin includes 47 individual inter-basin transfer systems (i.e., canals, pipelines, pumps) that in aggregate export ~12% of the river’s water. We note that the Imperial Irrigation District of southern California is often counted as a recipient of exported water, but we have followed the rationale of Schmidt 15 by including it as an interior part of the Lower Basin even though it receives its Colorado River water via the All American Canal (Fig.  2 ).

The water budget estimates presented in Tables  1 and 2 are summarized for each of the seven “accounting units” displayed here.

These results confirm previous findings that irrigated agriculture is the dominant consumer of Colorado River water. Irrigated agriculture accounts for 52% of overall consumption (Table  1 ; Figs.  3 and 4 ) and 74% of direct human consumption (Table  2 ) of water from the Colorado River Basin. As highlighted in Richter et al. 11 , cattle-feed crops (alfalfa and other hay) are the dominant water-consuming crops dependent upon irrigation water from the basin (Tables  1 and 2 ; Figs.  3 and 4 ). Those crops account for 32% of all water consumed from the basin, 46% of all direct water consumption, and 62% of all agricultural water consumed (Table  1 ; Fig.  3 ). The percentage of water consumed by irrigated crops is greatest in Mexico, where they account for 86% of all direct human uses (Table  2 ) and 80% of total water consumed (Table  1 ). Cattle-feed crops consume 90% of all water used by irrigated agriculture within the Upper Basin, where the consumed volume associated with these cattle-feed crops amounts to more than three times what is consumed for municipal, commercial, or industrial uses combined.

All estimates based on 2000–2019 averages. Both agriculture and MCI (municipal, commercial, and industrial) uses are herein referred to as “direct human uses.” “Indirect uses” include both reservoir evaporation as well as evapotranspiration by riparian/wetland vegetation.

Water consumed by each sector in the Colorado River Basin and sub-basins (including exports), based on 2000–2019 averages.

Another important finding is that a substantial volume of water (19%) is consumed in supporting the natural environment through riparian and wetland vegetation evapotranspiration along river courses. This analysis—made possible because of recent mapping of riparian vegetation in the Colorado River Basin 16 —is an important addition to the water budget of the Colorado River Basin, given that the only previous accounting for riparian vegetation consumption has limited to the mainstem of the Colorado River below Hoover Dam and does not include vegetation upstream of Hoover Dam nor vegetation along tributary rivers 17 . Given that many of these habitats and associated species have been lost or became imperiled due to river flow depletion 18 —including the river’s vast delta ecosystem in Mexico—an ecologically sustainable approach to water management would need to allow more water to remain in the river system to support riparian and aquatic ecosystems. Additionally, 11% of all water consumed in the Colorado River Basin is lost through evaporation from reservoirs.

It is also important to note a fairly high degree of inter-annual variability in each sector of water use; for example, the range of values portrayed for the four water budget sectors shown in Fig.  5 equates to 24–47% of their 20-year averages. Also notable is a decrease in water consumed in the Lower Basin between the years 2000 and 2019 for both the MCI (−38%) and agricultural sectors (−15%), which can in part be attributed to the policy agreements summarized previously that have mandated water-use reductions.

Inter-annual variability of water consumption within the Lower and Upper Basins, including water exported from these basins. The average (AVG) values shown are used in the water budgets detailed in Tables  1 and 2 .

The water accounting in Richter et al. 11 received a great deal of media attention including a front-page story in the New York Times 19 . These stories focused primarily on our conclusion that more than half (53%) of water consumed in the Colorado River Basin was attributable to cattle-feed crops (alfalfa and other hays) supporting beef and dairy production. However, that tabulation of the river’s water budget had notable shortcomings, as discussed previously. In this more complete accounting that includes Colorado River water exported outside of the basin’s physical boundary as well as indirect water consumption, we find that irrigated agriculture consumes half (52%) of all Colorado River Basin water, and the portion of direct consumption going to cattle-feed crops dropped from 53% as reported in Richter et al. 11 to 46% in this revised analysis.

These differences are explained by the fact that we now account for all exported water and also include indirect losses of water to reservoir evaporation and riparian/wetland evapotranspiration in our revised accounting, as well as improvements in our estimation of crop-water consumption. However, the punch line of our 2020 paper does not change fundamentally. Irrigated agriculture is the dominant consumer of water from the Colorado River, and 62% of agricultural water consumption goes to alfalfa and grass hay production.

Richter et al. 20 found that alfalfa and grass hay were the largest water consumers in 57% of all sub-basins across the western US, and their production is increasing in many western regions. Alfalfa is favored for its ability to tolerate variable climate conditions, especially its ability to persist under greatly reduced irrigation during droughts and its ability to recover production quickly after full irrigation is resumed, acting as a “shock absorber” for agricultural production under unpredictable drought conditions. The plant is also valued for fixing nitrogen in soils, reducing fertilizer costs. Perhaps most importantly, labor costs are comparatively low because alfalfa is mechanically harvested. Alfalfa is increasing in demand and price as a feed crop in the growing dairy industry of the region 21 . Any efforts to reduce water consumed by alfalfa—either through shifting to alternative lower-water crops or through compensated fallowing 20 —will need to compete with these attributes.

This new accounting provides a more comprehensive and complete understanding of how the Colorado River Basin’s water is consumed. During our study period of 2000–2019, an estimated average of 23.7 billion cubic meters (19.3 million acre-feet) of water was consumed each year before reaching its now-dry delta in Mexico. Schmidt et al. 2 have estimated that a reduction in consumptive use in the Upper and Lower Basins of 3–4 billion cubic meters (2.4–3.2 million acre-feet) per year—equivalent to 22–29% of direct use in those basins—will be necessary to stabilize reservoir levels, and an additional reduction of 1–3 billion cubic meters (~811,000–2.4 million acre-feet) per year will likely be needed by 2050 as climate warming continues to reduce runoff in the Colorado River Basin.

We hope that this new accounting will add clarity and a useful informational foundation to the public dialog and political negotiations over Colorado River Basin water allocations and cutbacks that are presently underway 2 . Because a persistent drought and intensifying aridification in the region has placed both people and river ecosystems in danger of water shortages in recent decades, knowledge of where the water goes will be essential in the design of policies for bringing the basin into a sustainable water supply-demand balance.

The data sources and analytical approaches used in this study are summarized below. Unless otherwise noted, all data were assembled for each year from 2000–2019 and then averaged. We acknowledge some inconsistency in the manner in which water consumption is measured or estimated across the various data sources and sectors used in this study, as discussed below, and each of these different approaches entail some degree of inaccuracy or uncertainty. We also note that technical measurement or estimation approaches change over time, and new approaches can yield differing results. For instance, the Upper Colorado River Commission is exploring new approaches for estimating crop evapotranspiration in the Upper Basin 22 . When new estimates become available we will update our water budget accordingly.

MCI and agricultural water consumption

The primary source of data on aggregate MCI (municipal, commercial, and industrial) and agricultural water consumption from the Upper and Lower Basins was the US Bureau of Reclamation. Water consumed from the Upper Basin is published in Reclamation’s five-year reports entitled “Colorado River—Upper Basin Consumptive Uses and Losses.” 23 These annual data have been compiled into a single spreadsheet used for this study 24 . Because measurements of agricultural diversions and return flows in the Upper Basin are not sufficiently complete to allow direct calculation of consumptive use, theoretical and indirect methods are used as described in the Consumptive Uses and Losses reports 25 . Reclamation performs these estimates for Colorado, Wyoming, and Utah, but the State of New Mexico provides its own estimates that are collaboratively reviewed with Reclamation staff. The consumptive use of water in thermoelectric power generation in the Upper Basin is provided to Reclamation by the power companies managing each generation facility. Reclamation derives estimates of consumptive use for municipal and industrial purposes from the US Geological Survey’s reporting series (published every 5 years) titled “Estimated Use of Water in the United States” at an 8-digit watershed scale 26 .

Use of shallow alluvial groundwater is included in the water accounting compiled by Reclamation but use of deeper groundwater sources—such as in Mexico and the Gila River Basin—is explicitly excluded in their accounting, and in ours. Reclamation staff involved with water accounting for the Upper and Lower Basins assume that groundwater use counted in their data reports is sourced from aquifers that are hydraulically connected to rivers and streams in the CRB (James Prairie, US Bureau of Reclamation, personal communication, 2023); because of this high connectivity, much of the groundwater being consumed is likely being sourced from river capture as discussed in Jasechko et al. 27 and Wiele et al. 28 and is soon recharged during higher river flows.

Water consumed from the Lower Basin (excluding water supplied by the Gila River Basin) is published in Reclamation’s annual reports entitled “Colorado River Accounting and Water Use Report: Arizona, California, and Nevada.” 3 These consumptive use data are based on measured deliveries and return flows for each individual water user. These data are either measured by Reclamation or provided to the agency by individual water users, tribes, states, and federal agencies 29 . When not explicitly stated in Reclamation reports, attribution of water volumes to MCI or agricultural uses was based on information obtained from each water user’s website, information provided directly by the water user, or information on export water use provided in Siddik et al. 30 . Water use by entities using less than 1.23 million cubic meters (1000 acre-feet) per year on average was allocated to MCI and agricultural uses according to the overall MCI-agricultural percentages calculated within each sub-basin indicated in Tables  1 and 2 for users of greater than 1.23 million cubic meters/year.

Disaggregation of water consumption by sector was particularly important and challenging for the Central Arizona Project given that this canal accounts for 21% of all direct water consumption in the Lower Basin. Reclamation accounts for the volumes of annual diversions into the Central Arizona Project canal but the structure serves 1071 water delivery subcontracts. We classified every unique Central Arizona Project subcontract delivery between 2000–2019 by its final water use to derive an estimated split between agricultural and MCI uses. Central Arizona Project subcontract delivery data were obtained from the current and archived versions of the project’s website summaries in addition to being directly obtained from the agency through a public information request. Subcontract deliveries were classified based on the final end use, including long-term and temporary leases of project water. This accounting also includes the storage of water in groundwater basins for later MCI or agricultural use. Additionally, water allocated to Native American agricultural uses that was subsequently leased to cities was classified as an MCI use.

Data for the Gila River basin was obtained from two sources. The Arizona Department of Water Resources has published data for surface water use in five “Active Management Areas” (AMAs) located in the Gila River basin: Prescott AMA, Phoenix AMA, Pinal AMA, Tucson AMA, and Santa Cruz AMA 31 . The water-use data for these AMAs is compiled from annual reports submitted by each water user (contractor) and then reviewed by the Arizona Department of Water Resources. The AMA water-use data are categorized by purpose of use, facilitating our separation into MCI and agricultural uses. These data are additionally categorized by water source; only surface water sourced from the Gila River hydrologic system was counted (deep groundwater use was not). The AMA data were supplemented with data for the upper Gila River basin provided by the University of Arizona 32 . We have assumed that all water supplied by the Gila River Basin is fully consumed, as the river is almost always completely dry in its lower reaches (less than 1% flows out of the basin into the Colorado River, on average 33 ).

Data for Mexico were obtained from Hernandez-Cruz et al. 10 based on estimates for 2008–2015. Agricultural demands were estimated from annual reports of irrigated area and water use published by the Ministry of Agriculture and the evapotranspiration estimates of the principal crops published by the National Institute for Forestry, Animal Husbandry, and Agricultural Research of Mexico 10 . The average annual volume of Colorado River water consumption in Mexico estimated by these researchers is within 1% of the cross-border delivery volume estimated by the Bureau of Reclamation for 2000–2019 in its Colorado River Accounting and Water Use Reports 3 .

Exported water consumption

Annual average inter-basin transfer volumes for each of 46 canals and pipelines exporting water outside of the Upper Basin were obtained from Reclamation’s Consumptive Uses and Losses spreadsheet 34 . Data for the Colorado River Aqueduct in the Lower Basin were obtained from Siddik et al. 30 Data for exported water in Mexico was available from Hernandez-Cruz et al. 10 . We assigned any seepage or evaporation losses from inter-basin transfers to their proportional end uses. All uses of exported water are considered to be consumptive uses with respect to the Colorado River, because none of the water exported out of the basin is returned to the Colorado River Basin.

We relied on data from Siddik et al. (2023) to identify whether the water exported out of the Colorado River Basin was for only MCI or agricultural use. When more than one water use purpose was identified, as well as for all major inter-basin transfers, we used government and inter-basin transfer project websites or information obtained directly from the project operator or water manager to determine the volume of water transferred and the end uses. Major recipients of exported water include the Coachella Valley Water District (California); Metropolitan Water District of Southern California (particularly for San Diego County, California); Northern Colorado Water Conservancy District; City of Denver (Colorado); the Central Utah Project; City of Albuquerque (New Mexico); and the Middle Rio Grande Conservancy District (New Mexico). We did not pursue sectoral water-use information for 17 of the 46 Upper Basin inter-basin transfers due to their relatively low volumes of water transferred by each system (<247,000 cubic meters or 2000 acre-feet), and instead assigned the average MCI or agricultural percentage (72% MCI, 28% agricultural) from all other inter-basin transfers in the Upper Basin. The export volume of these 17 inter-basin transfers sums to 9.76 million cubic meters (7910 acre-feet) per year, equivalent to 1% of the total volume exported from the Upper Basin.

Reservoir evaporation

Evaporation estimates for the Upper Basin and Lower Basin are based upon Reclamation’s HydroData repository 35 . Reclamation’s evaporation estimates are based on the standardized Penman-Monteith equation as described in the “Lower Colorado River Annual Summaries of Evapotranspiration and Evaporation” reports 17 . The Penman-Monteith estimates are based on pan evaporation measurements. Evaporation estimates for the Salt River Project reservoirs in the Gila River basin were provided by the Salt River Project in Arizona (Charlie Ester, personal communication, 2023).

Another consideration with reservoirs is the volume of water that seeps into the banks or sediments surrounding the reservoir when reservoir levels are high, but then drains back into the reservoir as water levels decline 36 . This has the effect of either exacerbating reservoir losses (consumptive use) or offsetting evaporation when bank seepage flows back into a reservoir. The flow of water into and out of reservoir banks is non-trivial; during 1999–2008, an estimated 247 million cubic meters (200,000 acre-feet) of water drained from the canyon walls surrounding Lake Powell into the reservoir each year, providing additional water supply 36 . However, the annual rate of alternating gains or losses has not been sufficiently measured at any of the basin’s reservoirs and therefore is not included in Tables  1 and 2 .

Riparian and wetland vegetation evapotranspiration

We exported the total annual evapotranspiration depth at a 30 meter resolution from OpenET 37 using Google Earth Engine from 2016 to 2019 to align with OpenET’s data availability starting in 2016. Total annual precipitation depths, sourced from gridMET 38 , were resampled to align with the evapotranspiration raster resolution. Subsequently, a conservative estimate of the annual water depth utilized by riparian vegetation from the river was derived by subtracting the annual precipitation raster from the evapotranspiration raster for each year. Positive differentials, indicative of river-derived evapotranspiration, were then multiplied by the riparian vegetation area as identified in the CO-RIP 16 dataset to estimate the total annual volumetric water consumption by riparian vegetation across the Upper, Lower, and Gila River Basins. The annual volumetric water consumption calculated over four years were finally averaged to get riparian vegetation evapotranspiration in the three basins. Because the entire flow of the Colorado River is diverted into the Canal Alimentador Central near the international border, very little riparian evapotranspiration occurs along the river south of the international border in the Mexico basin.

In addition to water consumed by riparian evapotranspiration within the Lower Basin, the Salton Sea receives agricultural drain water from both the Imperial Irrigation District and the Coachella Valley Irrigation District, stormwater drainage from the Coachella Valley, and inflows from the New and Alamo Rivers 39 . Combined inflows to the Sea during 2015–2019 were added to our estimates of riparian/wetland evapotranspiration in the Lower Basin.

Similarly, Mexico receives drainage water from the Wellton–Mohawk bypass drain originating in southern Arizona that empties into the Cienega de Santa Clara (a wetland); this drainage water is included as riparian/wetland evapotranspiration in the Mexico basin.

Crop-specific water consumption

The volumes of total agricultural consumption reported for each sub-basin in Tables  1 and 2 were obtained from the same data sources described above for MCI consumption and exported water. The portion (%) of those agricultural consumption volumes going to each individual crop was then allocated according to percentage estimates of each crop’s water consumption in each accounting unit using methods described in Richter et al. 20 and detailed here.

Monthly crop water requirements during 1981–2019 for 13 individual crops, representing 68.8% of total irrigated area in the US in 2019, were estimated using the AquaCrop-OS model (Table SI- 3 ) 40 . For 17 additional crops representing about 25.4% of the total irrigated area, we used a simple crop growth model following Marston et al. 41 as crop parameters needed to run AquaCrop-OS were not available. A list of the crops included in this study is shown in Table SI- 3 . The crop water requirements used in Richter et al. 11 were based on a simplistic crop growth model, often using seasonal crop coefficients whereas we use AquaCrop-OS 40 , a robust crop growth model, to produce more realistic crop growth and crop water estimates for major crops. AquaCrop-OS is an open-source version of the AquaCrop model 42 , a crop growth model capable of simulating herbaceous crops. Additionally, we leverage detailed local data unique to the US, including planting dates and subcounty irrigated crop areas, to produce estimates at a finer spatial resolution than the previous study. We obtained crop-specific planting dates from USDA 43 progress data at the state level. For crops that did not have USDA crop progress data, we used data from FAO 44 and CUP+ model 45 for planting dates. We used climate data (precipitation, minimum and maximum air temperature, reference ET) from gridMET 38 , soil texture data from ISRIC 46 database and crop parameters from AquaCrop-OS to run the model. The modeled crop water requirement was partitioned into blue and green components following the framework from Hoekestra et al. 47 , assuming that blue and green water consumed on a given day is proportional to the amount of green and blue water soil moisture available on that day. When applying a simple crop growth model, daily gridded (2.5 arc minutes) crop-specific evapotranspiration (ETc) was computed by taking the product of reference evapotranspiration (ETo) and crop coefficient (Kc), where ETo was obtained from gridMET. Crop coefficients were calculated using planting dates and crop coefficient curves from FAO and CUP+ model. Kc was set to zero outside of the growing season. We partitioned the daily ETc into blue and green components by following the methods from ref. 41 It is assumed that the crop water demands are met by irrigation whenever it exceeds effective precipitation (the latter calculated using the USDA Soil Conservation Service method (USDA, 1968 48 ). We obtained county level harvested area from USDA 43 and disaggregated to sub-county level using Cropland Data Layer (CDL) 49 and Landsat-based National Irrigation Dataset (LANID) 50 . The CDL is an annual raster layer that provides crop-specific land cover data, while the LANID provides irrigation status information. The CDL and LANID raster were multiplied and aggregated to 2.5 arc minutes to match the AquaCrop-OS output. We produced a gridded crop area map by using this resulting product as weights to disaggregate county level area. CDL is unavailable before 2008. Therefore, we used land use data from ref. 51 in combination with average CDL map and county level harvested area to produce gridded crop harvested area. We computed volumetric water consumption by multiplying the crop water requirement depth by the corresponding crop harvested area.

Data availability

All data compiled and analyzed in this study are publicly available as cited and linked in our Methods section. Our compilation of these data is also available from Hydroshare at: http://www.hydroshare.org/resource/2098ae29ae704d9aacfd08e030690392 .

Code availability

All model code and software used in this study have been accessed from sources cited in our Methods section. We used AquaCrop-OS (v5.0a), an open source version of AquaCrop crop growth model, to run crop simulations. This model is publicly available at http://www.aquacropos.com/ . For estimating riparian evapotranspiration, we used ArcGIS Pro 3.1.3 on the Google Earth Engine. Riparian vegetation distribution maps were sourced from Dryad at https://doi.org/10.5061/dryad.3g55sv8 .

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Acknowledgements

This paper is dedicated to our colleague Jack Schmidt in recognition of his retirement and enormous contributions to the science and management of the Colorado River. The authors thank James Prairie of the US Bureau of Reclamation, Luke Shawcross of the Northern Colorado Water Conservancy District, Charlie Ester of the Salt River Project, and Brian Woodward of the University of California Cooperative Extension for their assistance in accessing data used in this study. The authors also thank Rhett Larson at the Sandra Day O’Connor School of Law at Arizona State University for their review of Arizona water budget data, and the Central Arizona Project for providing delivery data by each subcontract. G.L., L.M., and K.F.D. acknowledge support by the United States Department of Agriculture National Institute of Food and Agriculture grant 2022-67019-37180. L.T.M. acknowledges the support the National Science Foundation grant CBET-2144169 and the Foundation for Food and Agriculture Research Grant No. FF-NIA19-0000000084. R.R.R. acknowledges the support the National Science Foundation grant CBET-2115169.

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Contributions

B.D.R. designed the study, compiled and analyzed data, wrote the manuscript and supervised co-author contributions. G.L. compiled all crop data, estimated crop evapotranspiration, and prepared figures. S.D. compiled all riparian vegetation data and estimated riparian evapotranspiration. L.S.S. and R.R.R. accessed, compiled, and analyzed data from the Central Arizona Project. D.W. compiled data and prepared figures. A.H.-C. and S.S.-S. compiled and analyzed data for Mexico. J.C.S. compiled and analyzed reservoir evaporation data and edited the manuscript. L.M., B.L.R., and K.F.D. supervised data compilation and analysis and edited the manuscript.

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Richter, B.D., Lamsal, G., Marston, L. et al. New water accounting reveals why the Colorado River no longer reaches the sea. Commun Earth Environ 5 , 134 (2024). https://doi.org/10.1038/s43247-024-01291-0

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Groundwater quality assessment using water quality index (WQI) under GIS framework

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• Arjun Ram 1 ,
• S. K. Tiwari 2 ,
• H. K. Pandey 3 ,
• Abhishek Kumar Chaurasia 1 ,
• Supriya Singh 4 &
• Y. V. Singh 5  

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Groundwater is an important source for drinking water supply in hard rock terrain of Bundelkhand massif particularly in District Mahoba, Uttar Pradesh, India. An attempt has been made in this work to understand the suitability of groundwater for human consumption. The parameters like pH, electrical conductivity, total dissolved solids, alkalinity , total hardness, calcium, magnesium, sodium, potassium, bicarbonate, sulfate, chloride, fluoride, nitrate, copper, manganese, silver, zinc, iron and nickel were analysed to estimate the groundwater quality. The water quality index (WQI) has been applied to categorize the water quality viz: excellent, good, poor, etc. which is quite useful to infer the quality of water to the people and policy makers in the concerned area. The WQI in the study area ranges from 4.75 to 115.93. The overall WQI in the study area indicates that the groundwater is safe and potable except few localized pockets in Charkhari and Jaitpur Blocks. The Hill-Piper Trilinear diagram reveals that the groundwater of the study area falls under Na + -Cl − , mixed Ca 2+ -Mg 2+ -Cl − and Ca 2+ - \({\text{HCO}}_{3}^{ - }\) types. The granite-gneiss contains orthoclase feldspar and biotite minerals which after weathering yields bicarbonate and chloride rich groundwater. The correlation matrix has been created and analysed to observe their significant impetus on the assessment of groundwater quality. The current study suggests that the groundwater of the area under deteriorated water quality needs treatment before consumption and also to be protected from the perils of geogenic/anthropogenic contamination.

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Introduction

In India, there has been a tremendous increase in the demand for groundwater due to rapid growth of population, accelerated pace of industrialization and urbanization (Yisa and Jimoh 2010 ). The availability and quality of groundwater are badly affected at an alarming rate due to anthropogenic activities viz. overexploitation and improper waste disposal (industrial, domestic and agricultural) to groundwater reservoirs (Panda and Sinha 1991; Kavitha et al. 2019a , 2019b ). Consequently, human health is seriously threatened by the prevailing agricultural practices particularly in relation to excessive application of fertilizers; unsanitary conditions and disposal of sewage into groundwater (Panigrahi et al. 2012 ). The groundwater quality also varies with depth of water, seasonal changes, leached dissolved salts and sub-surface environment (Gebrehiwot et al. 2011 ). According to the World Health Organization (WHO 2017 ), about 80% of all the diseases in human beings are water-borne. Once the groundwater is contaminated, it is difficult to ensure its restoration and proper quality by preventing the pollutants from the source. It, therefore, becomes imperative to monitor the quality of groundwater regularly, and to device ways and means to protect it from contamination. The quality of groundwater is deciphered using various physical, chemical and biological characteristics of water (Diersing and Nancy 2009 ; Panneerselvam et al. 2020a ). It is a measure of health and hygiene of groundwater concerning the need and purpose of human consumption (Johnson et al. 1997 ; Panneerselvam et al. 2020b ).

In recent years, the assessment and monitoring of groundwater quality on a regular basis is being carried out using Geographic Information System (GIS) technique added with the IDW interpolation method and has proved itself as a powerful tool for evaluating and analysing spatial information of water resources (Aravindan et al. 2010 ; Shankar et al. 2010 , 2011a , b ; Venkateswaran et al. 2012 ; Selvam et al. 2013b; Magesh and Elango 2019 ; Balamurugan et al. 2020b ; Soujanya Kamble et al. 2020 ). It is an economically feasible and time-efficient technique for transforming huge data sets to generate various spatial distribution maps and projections revealing trends, associations and sources of contaminants/pollutants. In this work, GIS technique has been used for spatial evaluation of various groundwater quality parameters.

In this study, the physicochemical properties of forty-three groundwater samples collected from wells and hand pumps were determined and compared with international standards of WHO for drinking and domestic uses based on Water Quality Index (WQI). The WQI was first developed by Horton ( 1965 ) based on weighted arithmetical calculation. A number of researchers (Brown et al. 1972 ; GEMS UNEP 2007; Kavitha and Elangovan 2010 ; Alobaidy et al. 2010 ; Shankar and Kawo 2019 ; Bawoke and Anteneh 2020 developed various WQI models based on weighing and rating of different water quality parameters which is derived by the weighted arithmetic method. The WQI is a dimensionless number with values ranking between 0 and 100. The WQI is a unique digital rating expression that expresses overall water quality status viz. excellent, good, poor, etc. at a certain space and time based on various water quality parameters. Thus, the WQI is being used as an important tool to compare the quality of groundwater and their management (Jagadeeswari and Ramesh 2012 ) in a particular region; and is helpful for selecting appropriate economically feasible treatment process to cope up with the concerned quality issues. It depicts the composite impact of different water quality parameters and communicates water quality information to the public and legislative policy-makers to shape strong policy and implement the water quality programs (Kalavathy et al. 2011 ) by the government.

Mineral intractions strongly influence groundwater hydrochemistry in aquifers and disintegration of minerals from various source rocks (Cerar and Urbanc 2013 ; Modibo Sidibé et al. 2019 ). Hydrochemistry of the analysed samples indicates that the mean abundance of major cations is present in order of Na ++  > Ca 2+  > Mg 2+  > K + while major anions in order of \({\text{HCO}}_{3}^{ - }\)  >  \({\text{NO}}_{3}^{ - }\)  > Cl −  >  \({\text{SO}}_{4}^{2 - }\)  > F − . The study shows that the sodium is dominant alkali while calcium and magnesium are the dominant alkaline earth metal leached in the aquafer due to rock water interaction affecting the quality of groundwater. Sodium in aquafer is derived from the weathering of halite and silicate minerals such as feldspar (Khan et al. 2014 ; Mostafa et al. 2017 ). The critical evaluation of Hill-Piper Trilinear diagram reflects Na + -Cl − , mixed Ca 2+ -Mg 2+ -Cl − , Ca 2+ - \({\text{HCO}}_{3}^{ - }\) , mixed Ca 2+ -Na + - \({\text{HCO}}_{3}^{ - }\) , Na + - \({\text{HCO}}_{3}^{ - }\) and Ca 2+ -Cl − type hydro-chemical facies in decreasing order of dominance. The Hydro-chemical characterization of groundwater reveals that the nature of aquifer is controlled by type of water, source and level of contamination (Aghazadeh et al. 2017 ; Brhane 2018 ). Hence, in order to keep the health of any aquaculture system, particularly an aquifer system at an optimal level, certain water quality indicators or parameters must be regularly monitored and controlled. Therefore, the objective of the study is to calculate the WQI of groundwater in order to assess its suitability for human consumption using the GIS interpolation technique and statistical approach in the study area.

Mahoba district is the south-western district of Uttar Pradesh which is adjacent to the state of Madhya Pradesh in south and Hamirpur (UP) in the north. The study area falls under the survey of India (SOI) toposheets no. 54O and 63C lies between latitude N25°01′30″ to N25°39′40″ and longitude E79°15′00″ to E80°10′30″ and covers an area of approximately 2933.59 km 2 . River Dhasan separates the district Mahoba from Jhansi in the west. A certainpart of Jhansi and Banda district has been merged in newly constructed Mahoba district in 1995 (bifurcated from Hamirpur). Mahoba district consists of three tehsils Kulpahar, Charkhari, Mahoba and four blocks Panwari, Jaitpur, Charkhari, Kabrai (Fig.  1 a). Kabrai is the biggest block fromaerial coverage as well as population point of view. Jaitpur is the smallest block from aerial coverage and Charkhari from population point of view. The study area experiences a typical subtropical climate punctuated by long and intense summer, with distinct seasons. The area receives an average annual precipitation of 864 mm mainly from the south-west monsoon. The temperature of the coldest month (January) is 8.3°C while the temperature of the hottest month (May) shoots upto 47.5°C. The entire area under investigation is characterised by highly jointed/fractured Bundelkhand granite (Archean age) with thin soil cover. Physiographically , the area is characterised by Bundelkhand massif terrain and is marked by the occurrence of solitary or clustered hillocks and intervening low relief with undulating plains. Two major physiographic units are: (1) Southern part having high relief with hillocks- This is south of 20°25′ N latitude & maximum altitude is 340 mamsl, reserved forest. Granitoids and intervening pegmatitic veins and numbers of quartz veins are observed. (2) Northern part relatively low relief with lower hillocks- In between 25°25′N and 25°39′N latitude and maximum altitude is 310 mamsl. The area in and around Panwari is mainly covered with thick alluvium, and hard rock is encountered only below 35 mbgl, coverage with seasonal forest. Pedi plain, pediment inselberg and buried pediplains are present.

a Study area map depicting the sampling sites. b Geological map of study area

Geological and hydrogeological set-up

The granite, particularly leucogranite, older and younger alluvium consisting of clay, silt, sand and gravel mainly comprises the study area. The geological set-up of the study area indicates that the most dominant lithology is leucogranite covering mainly central and eastern part while recent alluvium covers the northern part (Fig.  1 b). At places, few patches of pink granite have also been recorded which appears enclosed in leucogranite or adjacent to its outcrop.The occurrence of groundwater is highly uncertain and unpredictable in this hilly and rugged terrain as it does not allow percolation and storages underground. The presence of porosity depends on the intensity of weathering and rock fracture which is responsible for groundwater occurrence, its quantity and flow mostly in permeable zones of weathered rock formations and under secondary porosity in the deep fractured zone. Groundwater recharge in the study area is triggered by the depth of overburden 7 m (Jaitpur-Kulpahar area) to 35 m (parts of Mahoba Tahsil and Charkhari block) as well as the intensity of weathering.

Materials and methods

The groundwater samples were collected during pre-monsoon (June 2016) period from the study area according to standard procedures of the American Public Health Association (APHA, 2017). The sampling locations were marked with the help of global positioning system (GPS) as shown in the Fig.  1 a. Samples were collected from the location through hand pump (depth: approx. 40 m) and dug wells (depth: 8–30 m ) as shown in Fig.  2 a–t. The collecting bottles (High-Density Polythene, HDPE) of one-litre capacity each were sterilized under the aseptic condition to avoid unpredictable contamination and subsequent changes in the characteristics of groundwater. Water samples were filtered using Whatman 42 filter paper (pore size 2.5 μm) prior to collection in the bottle. The sample was kept in the ice-box (portable) and brought to NABL accredited (ISO 17,025: 2017) laboratory of Central Ground Water Board (CGWB), Lucknow and Department of Soil Science & Agricultural Chemistry, Banaras Hindu University, Varanasi, UP, India. The samples were stored in a chemical laboratory at temperature 4–5 °C. The samples for metallic parameters were added 2 ml elemental grade nitric acid to obtain the pH 2–3 after acidification. The samples were pre-filtered in the laboratory to carry out the analysis. In the present study, a total of 20 groundwater quality parameters of forty-three samples were analysed as per test standard methods (APHA 2017) in the laboratory except for unstable parameters viz. hydrogen ion concentration (pH), electrical conductivity (EC) and total dissolved solids (TDS) which are determined by portable device (pH-meter, EC-meter and TDS-meter) in situ. Alkalinity (AK), Total hardness (TH), calcium (Ca 2+ ), magnesium (Mg 2+ ), bicarbonate ( \({\text{HCO}}_{3}^{ - }\) ) and chloride (Cl − ) were analysed using volumetric titrations; sodium (Na + ) and potassium (K + ) were analysed using systronics flame photometer model 129; nitrate ( \({\text{NO}}_{3}^{ - }\) ), fluoride (F − ), sulfate ( \({\text{SO}}_{4}^{2 - }\) ), were analysed using shimadzu 1800 spectrophotometer. Prior to analysis of the heavy metals viz. copper (Cu), manganese (Mn), silver (Ag), zinc (Zn), iron (Fe) and nickel (Ni); the groundwater samples were acidified with 1:1 nitric acid and concentrated ten times. The samples were subjected to analysis using Shimadzu 6701 Atomic Absorption Spectrophotometer (AAS) on flame mode with hollow cathode lamps of metal under analysis. The concentration of metal is displayed on the monitor. The standards of the metallic parameters were prepared from National Institute of Standards and Technology (NIST) certified (Certified Reference Materials) CRM as per NABL guidelines of 17,025:2017.

a – b Spatial distribution map of pH and EC. c – h Spatial distribution map of TDS, AK, TH, Ca 2+ , Mg 2+ and Na + . i – n : spatial distribution map of K + , \({\text{HCO}}_{3}^{ - }\) , \({\text{SO}}_{4}^{2 - }\) , Cl − , F − and \({\text{NO}}_{3}^{ - }\) . o – t Spatial distribution map of Cu, Mn, Ag, Zn, Fe and Ni

The quality assurance and quality control (QA/QC) procedure of the data has been considered during the study. Approximately half of the volume (500 ml) of samples were specially separated and checked in the laboratory to ensure QA/QC mechanisms. The accuracy of the chemical analysis has been validated by charge balance errors and samples < 5% error were considered.

The inverse distance weighted (IDW) interpolation technique used in this study is now-adays an effective tool for spatial interpolation of groundwater quality parameters leading to the generation of spatial distribution maps (Magesh et al. 2013 ; Kawo and Shankar 2018 ; Balamurugan et al. 2020b ; Sarfo and Shankar 2020 ). The weights were assigned to various parameters at each location based on distance and were calculated, taking into consideration the closest specified locations. The distribution of each groundwater quality parameter has been demarcated in different zones on spatial distribution map viz. acceptable/desirable and permissible limits according to BIS (2012, 2015) and WHO ( 2017 ) for drinking purpose. The statistical analysis and correlation matrix of the analysed groundwater quality parameters have been laid down as shown in Tables 1 and 2 , respectively.

The water quality index (WQI)

The WQI has been determined using the drinking water quality standard recommended by the World Health Organization (WHO 2017 ). The Water Quality Index has been calculated using the weighted arithmetic method, which was originally proposed by Horton ( 1965 ) and developed by Brown et al. ( 1972 ). The weighted arithmetic water quality index (WQI) is represented in the following way:

where n  = number of variables or parameters, W i  = unit weight for the i th parameter, Q i  = quality rating (sub-index) of the i th water quality parameter.

The unit weight ( W i ) of the various water quality parameters are inversely proportional to the recommended standards for the corresponding parameters.

where, W i  = unit weight for the i th parameter, S n  = standard value for i th parameters, K  = proportional constant,

The value of K has been considered ‘1′ here and is calculated using the mentioned equation below:

According to Brown et al. ( 1972 ), the value of quality rating or sub-index ( Q i ) is calculated using the equation as given below:

where V o = observed value of i th parameter at a given sampling site, V i = ideal value of i th parameter in pure water, S n = standard permissible value of i th parameter.

All the ideal values (V i ) are taken as zero for drinking water except pH and dissolved oxygen (Tripathy and Sahu 2005 ). In case of pH, the ideal value is 7.0 (for natural/pure water) while the permissible value is 8.5 (for polluted water). Similarly, for dissolved oxygen, the ideal value is 14.6 mg/L while the standard permissible value for drinking water is 5 mg/L. Therefore, the quality rating for pH and Dissolved Oxygen are calculated from the equations respectively as shown below:

where, V pH  = observed value of pH, V do  = observed value of dissolved oxygen.

If, Q i  = 0 implies complete absence of contaminants while 0 < Q i  < 100 implies that, the contaminants are within the prescribed standard. When Q i  > 100 implies that, the contaminants are above the standards.

The classification of water quality, based on its water quality index (WQI) after Brown et al. ( 1972 ); Chatterjee and Raziuddin ( 2002 ) and Shankar and Kawo ( 2019 ) have been considered here in this study for further reference which is mentioned in Table 3 .

Result and discussion

Groundwater quality parameters.

In this study based on the selected parameters as discussed above the groundwater quality maps have been prepared with the help of ArcGIS software 10.1 as shown in Fig.  2 a–t. In the following lines, the various parameters considered in the study are being discussed: The Bureau of Indian Standard (BIS 2012, 2015) and World Health Organization (WHO 2017 ) of drinking water standards have been considered as a reference in this study.

Hydrogen ion concentration (pH)

It is an important indicator for assessing the quality and pollution of any aquifer system as it is closely related to other chemical constituents of water. The presence of hydrogen ion concentration is measured in terms of pH range. Water, in its pure form shows a neutral pH which indicates hydrogen ion concentration. In the present study, the range of pH varies between 6.81 (minimum) to 8.32 (maximum) which is within the acceptable limit (6.5–8.5, avg: 7.81) indicating the alkaline nature of groundwater (ideal range of pH for human consumption: 6.5–8.5).

Electrical conductivity (EC)

In fact, it is a measure of the ability of any substance or solution to conduct electrical current through the water. EC is directly proportional to the dissolved material in a water sample. The desirable limit of EC for drinking purpose is 750 µS/cm. In this study, the electrical conductivity varies between 286 and 1162 µS/cm. High EC at some sites suggests the mixing of sewage in groundwater as these sites are near dense urbanization.

Total dissolved solids (TDS)

The weight of residue expresses it after a water sample is evaporated to dry state. It includes calcium, magnesium, sodium, potassium, carbonate, bicarbonate, chloride and sulfate. In the present study, it ranges between 280 to 879 mg/l (< 500 mg/l TDS for potable water as per BIS.). The agricultural practices, residential runoff, leaching of soil causing contamination and point source water pollution discharge from industrial or sewage treatment plants are the primary sources for TDS (Boyd 2000 ).

Alkalinity (AK)

It is a measure of the carbonate, bicarbonate and hydroxide ions present in water. The desirable limit of alkalinity in potable water is 200 mg/l, above which the taste of water becomes unpleasant. In the study area, the alkalinity ranges between 50 to 452 mg/l, which is within the permissible limit (600 mg/l).

Total hardness (TH)

It is the amount of dissolved calcium and magnesium in the water. Water moving through soil and rock dissolves naturally occurring minerals and carries them into the groundwater as it is a great solvent for calcium and magnesium. In this study, hardness ranges between 70 to 592 mg/l, which is within the permissible limits (600 mg/l). The high concentration of TH in groundwater may cause heart disease and kidney stone in human beings.

Calcium (Ca 2+ )

It enters into the aquifer system from the leaching of calcium bearing minerals. In the study area, the calcium concentration ranges from 12 to 112 mg/l and is within the permissible limit (200 mg/l). The lesser concentration of Ca 2+ in the groundwater satisfies the chemical weathering and dissolution of fluorite, consequently resulting in an increase of fluoride concentration.

Magnesium (Mg 2+ )

It is an important parameter responsible for the hardness of the water. In the study area, the concentration ranges between 2.4 to 120 mg/l and is present in little excess of the permissible limit (100 mg/l).

Sodium (Na + )

It is a highly reactive alkali metal. It is present in most of the groundwater. Many rocks and soils contain sodium compounds, which easily dissolves to liberate sodium in groundwater. In the study area, it ranges from 48.71 to 244.4 mg/l. The high concentration of Na + indicates weathering of rock-forming minerals i.e., silicate minerals (alkali feldspars) and/or dissolution of soil salts present therein due to evaporation (Stallard and Edmond 1983 ). In the aquifers, the high Na + concentration in groundwater may be related to the mechanism of cation exchange (Kangjoo Kim and Seong-Taekyun 2005).

Potassium (K + )

It is present in many minerals and most of the rocks. Many of these rocks are relatively soluble and releases potassium, the concentration of which increases with time in groundwater. In this study, it varies between 0.87 to 2.7 mg/l.

Bicarbonate ( \({\text{HCO}}_{3}^{ - }\) )

It is produced by the reaction of carbon dioxide with water on carbonate rocks viz. limestone and dolomite. The carbon-dioxide present in the soil reacts with the rock-forming minerals is responsible for the presence of bicarbonate, producing an alkaline environment in the groundwater. In the study area it varies between 36.61 to 536.95 mg/l and is within the permissible limit of 600 mg/l.

Sulfate ( \({\text{SO}}_{4}^{2 - }\) )

It is dissolved and leached from rocks containing gypsum, iron sulfides, and other sulfur bearing compounds. In the present study, it ranges between the 2.23 to 75.17 mg/l, which is well within the acceptable limit of 200 mg/l.

Chloride (Cl − )

In the present study the Cl − ranges between 70.92 to 276.59 mg/l which exceed the permissible limit (250 mg/l). The higher value of chlorine in groundwater makes it hazardous to human health (Pius et al. 2012 ; Sadat-Noori et al. 2014 ).

Fluoride (F − )

In groundwater fluoride is geogenic in nature. It is the lightest halogen, and one of the most reactive elements (Kaminsky et al. 1990 ). It usually occurs either in trace amounts or as a major ion with high concentration (Gaciri and Davies 1993 ; Apambire et al. 1997 ; Fantong et al. 2010 ). The groundwater contains fluorides released from various fluoride-bearing minerals mainly as a result of groundwater-host rock interaction. The study area comprising granite, granitic gneiss etc. is commonly found to contain fluorite (CaF 2 ) as an accessory mineral (Ozsvath 2006 ; Saxena and Ahmed 2003 ) which plays a significant role in controlling the geochemistry of fluoride (Deshmukh et al. 1995 ). In addition to fluorite it is also abundant in other rock-forming minerals like apatite, micas, amphiboles, and clay minerals (Karro and Uppin 2013 ; Narsimha and Sudarshan 2013 ; Naseem et al. 2010 ; Jha et al. 2010 ; Rafique et al. 2009 ; Carrillo-Rivera et al. 2002 ). In the present study, the fluoride concentration ranges from 0.11 to 3.91 mg/l. The concentration of fluoride exceeds the permissible limit (1.5 mg/l) in about 25% of the groundwater samples.

Nitrate ( \({\text{NO}}_{3}^{ - }\) )

Nitrate is naturally occurring ions and is a significant component in the nitrogen cycle. However, nitrate ion in groundwater is undesirable as it causes Methaemoglobinaemia in infants less than 6 months of age (Egereonu and Nwachukwu 2005 ). In general, its higher concentration causes health hazards if present beyond the permissible limit, 45 mg/l (Kumar et al. 2012 , 2014 ). In the study area, its concentration ranges from 86.95 to 210.4 mg/l. It is in excess of the permissible limits throughout the study area. The higher values of nitrate in potable water increases the chances of gastric ulcer/cancer, and other health hazards to infants and pregnant women (Rao 2006 ) also birth malformations and hypertension (Majumdar and Gupta 2000 ). The area under study is granite-gneiss terrain where the atmospheric nitrogen is fixed and added to the soil as ammonia through lightning storms, bacteria present in soil and root of plants. Further, animal wastes, plants and animals remain also undergo ammonification in the soil producing ammonia which undergoes nitrification/ammonia oxidation by Nitrosomonas and Nitrobacter bacteria to form nitrate (Rivett et al. 2008 ; Galloway et al. 2004). Granitic rocks contain nitrogen concentrations up to 250 mg Nkg −1 with ammonium partitioned into the orthoclase feldspar to a greater extent than muscovite or biotite (Boyd et al. 1993 ). Geologic nitrogen (nitrogen contained in bedrock) contribute to the ecosystem with nitrogen saturation (more nitrogen available than required by biota) leading to leaching of nitrogen and consequently elevating nitrate concentrations in groundwater (Dahlgren 1994 ; Holloway et al. 1998 ). Nitrogen released through weathering has a greater impact on soil and water quality. Also, denitrification is significant in modifying the level to which nitrogen released through weathering of bedrock influencing the supply of nitrate in groundwater (McCray et al. 2005 ).

Copper (Cu)

It is a naturally occurring metal in rock, soil, plants, animals, and groundwater in very less concentration. The concentration of Cu may get enriched into the groundwater through quarrying and mining activities, farming practices, manufacturing operations and municipal or industrial waste released. Cu gets into drinking water either by contaminating of well water or corrosion of copper pipes in case of water is acidic. In this study, it ranges between 0 and 0.0078 mg/l, which is within the permissible limit (0.05 mg/l).

Manganese (Mn)

It occurs naturally in groundwater, especially in an anaerobic environment. The concentrations of Mn in groundwater is dependent upon rainfall chemistry, aquifer lithology, geochemical environment, groundwater flow paths and residence time, etc. which may vary significantly in space and time. It may be released by the leaching of the overlying soils and minerals in underlying rocks as well as from the minerals of the aquifer itself in groundwater. In the present study, manganese ranges between 0.005 and 0.221 mg/l, which is within the permissible limit (0.3 mg/l).

It naturally occurs usually in the form of insoluble and immobile oxides, sulfides and some salts. It is rarely present in groundwater, surface water and drinking water at concentrations above 5 µg/litre (WHO 2017 ). In the present study, the silver ranges between 0.000 and 0.021 mg/l, which is within the permissible limit (0.1 mg/l).

Though it occurs in significant quantities in rocks, groundwater seldom contains zinc above 0.1 mg/l. In the present study, the groundwater shows the negligible concentration of Zn (0.0136 mg/l) which is well within the acceptable limit (5 mg/l).

The most common sources of iron in groundwater is weathering of iron-bearing minerals and rocks. The iron occurs naturally in the reduced Fe 2+ state in the aquifer, but its dissolution increases its concentration in groundwater. Iron in this state is soluble and generally does not create any health hazard. If Fe 2+ state is oxidised to Fe 3+ state in contact with atmospheric oxygen or by the action of iron-related bacteria which forms insoluble hydroxides in groundwater. So, the concentration of iron in groundwater is often higher than those measured in surface water. In the present study, the iron ranges between 0.0994 and 0.4018 mg/l, which is within of the permissible limit 1.0 mg/l (BIS 2015).

Nickel (Ni)

The primary source of nickel in groundwater is from the dissolution of nickel ore bearing rocks. The source of nickel in drinking water is leaching from metals in contact such as water supply pipes and fittings. Ni usually occurs in the divalent state, but oxidation states of  +  1,  + 3, or  + 4 may also exist in nature. In the study area, it ranges between 0 and 0.0408 mg/l, and it crosses the permissible limit (0.02 mg/l).

Statistical analysis, correlation matrix and relative weightage

The relative weightage, general statistical analysis and correlation matrix of groundwater quality parameters are tabulated in Tables 4 , 1 and 2 , respectively. The correlation matrix of various 20 groundwater quality parameters, including 6 heavy metals was created and has been analysed using MS Excel 2016 Table 2 . Out of these, eight parameters viz. TDS, EC, Na + , Alkalinity, TH, Ca 2+ , Mg 2+ , \({\text{HCO}}_{3}^{ - }\) are significantly correlated, reflecting more than 0.50 correlation value. Further, TDS vs EC, Na + vs Alkalinity, TH as CaCO 3 − vs Ca 2+ and Mg 2+ , \({\text{HCO}}_{3}^{ - }\) vs Alkalinity and Na + indicates most relevant correlation having a significant impetus on the overall assessment of the quality of groundwater than any other major radicals and physical parameters. However, the majority of quality parameters are positively correlated with each other. A critical analysis of the correlation matrix for the heavy metals indicates that Cu is positively correlated with EC, TDS, Na + , K + , Cl − and \({\text{NO}}_{3}^{ - }\) . Similarly, Mn is positively correlated with pH, EC, TDS and Cu. While, Ag is positively correlated with pH, Ca 2+ , Mg 2+ , K + , TH, Cl − , \({\text{NO}}_{3}^{ - }\) and Mn. Further, Fe is positively correlated with TDS, Mg 2+ , Na + , TH, \({\text{HCO}}_{3}^{ - }\) , \({\text{SO}}_{4}^{2 - }\) , \({\text{NO}}_{3}^{ - }\) , Cu and Ag. Similarly, Ni is positively correlated with pH, EC, TDS, Ca 2+ , K + , \({\text{NO}}_{3}^{ - }\) and Mn.

The higher concentration of Ni, Fe and Mn may trigger the presence of other heavy metals viz. Pb, Cd and Cr which are very sensitive and significant heavy metal and needs to be observed carefully in future for groundwater quality in the study area. The presence of Fe, \({\text{SO}}_{4}^{2 - }\) and \({\text{NO}}_{3}^{ - }\) may trigger the presence of Cd (Chaurasia et al. 2018 ).

Spatial distribution pattern

The spatial distribution pattern of the contour maps of the groundwater quality parameters have been generated as represented in Fig.  2 a–t. The spatial distribution pattern of the pH indicates that the central part along NW–SE across the district with some scattered small patches throughout indicating the presence of alkaline groundwater (Fig.  2 a). In acidic water, fluoride is adsorbed on a clay surface, while in alkaline water, fluoride is desorbed from solid phases; therefore, alkaline pH is more favourable for fluoride dissolution, (Keshavarzi et al. 2010 ; Rafique et al. 2009 ; Saxena and Ahmed 2003 ; Rao 2009 ; Ravindra and Garg 2007 ; Vikas et al. 2009 ). The southern portion of the district in Kabrai Block is having high TDS (> 750 mg/l) in groundwater (Fig.  2 c) due to poor fluxing and highly weathered rock formations. Similarly, EC is mainly highest (> 900 mg/l) in the southern part with small scattered patches in central and NE part of the district (Fig.  2 b). This is in consonance with the higher TDS (significant positive correlation with EC) as evidenced by the correlation matrix of the quality parameters (Table 2 ). The alkalinity map clearly and significantly indicates that it is highest in the central part surrounded by gradually decreasing alkalinity outwards (Fig.  2 d). The bicarbonates trigger the alkalinity in groundwater (Adams et al. 2001 ). The quality of groundwater in a major portion of the study area is alkaline in nature, indicating that the dissolved carbonates are predominantly in the form of bicarbonates. A positive correlation is observed between the alkalinity of groundwater and fluoride (Table 2 ), consequently releasing fluoride in the groundwater. The spatial distribution map of Ca 2+ suggests varying concentration within permissible limit throughout the study area (Fig.  2 f) due to the presence of alkali feldspar in granite. Similarly, Mg 2+ is also distributed unevenly but falls within permissible limit with an exception in NE part of the district (Fig.  2 g). The spatial distribution pattern of TH reflects that the study area is characterized by moderately hard groundwater.

Figure  2 e The Ca 2+ and Mg 2+ ions present in the groundwater are possibly derived from leaching of calcium and magnesium bearing rock-formations in the study area. The fluoride in groundwater shows a negative correlation with Ca 2+ , indicating the high value of fluoride in groundwater in association with low Ca 2+ content. The correlation matrix clearly marks a significant positive correlation among Na + , alkalinity and TDS, which is being reflected from their respective spatial distribution maps (Fig.  2 c, d and h). Na + is highest in the central part (with small patches in the eastern part and insignificantly in the western part) which is in conformity with the alkalinity and TDS spatial distribution patterns. Although, the presence of K + is insignificant and its lower concentration within the permissible limit is covering a major portion of the district due to poor weathering of orthoclase. Its distribution pattern indicates conformity more or less with the TDS and Na + (Fig.  2 c, h, and i). \({\text{HCO}}_{3}^{ - }\) is an important quality parameter showing significant positive correlation (> 0.50) with alkalinity and Na + (Table 2 ) which is also reflected in the spatial distribution pattern of these parameters (Fig.  2 d, h, j). Although sulphate ( \({\text{SO}}_{4}^{2 - }\) ) is an important quality parameter. It is present within the permissible limit in the study.

area (Fig.  2 k). Chloride is slightly in excess in a larger patch, particularly in SE-part of the study area which may cause a health hazard. It is revealed from the spatial distribution map of chloride (Fig.  2 l). This is due to poor fluxing and presence of halite mineral. Fluoride (F − ) is an important quality parameter, especially with respect to the study area where it is present noticeably in scattered patches throughout the district. It is observed that mainly in NE part, the central part and SE part of the district the concentration of fluoride is in excess (2.82 mg/l to 3.91 mg/l) of permissible limit 1.5 mg/l (Fig.  2 m). The higher concentration (> 3.0 mg/l) of fluoride may lead to skeletal fluorosis (Raju et al 2009 ). Several factors viz. temperature, pH, presence or absence of complexing or precipitating ions and colloids, the solubility of fluorine bearing minerals (biotite and apatite), anion exchange capacity of the aquifer (OH − with F − ), size and type of geological formations traversed by groundwater and the contact time during which water remains in contact with the formation are responsible for fluoride concentration in groundwater (Apambire et al. 1997 ). The lithology of fractured rock reveals that it contains more fluoride bearing minerals than massive rocks (Pandey et al. 2016 ). Nitrate (NO 3 − ) in groundwater is mainly anthropogenic in nature which could be due to leaching from waste disposal, sanitary landfills, over-application of inorganic nitrate fertilizer or improper manure management practice (Chapman 1996 ). In this study, it is observed that nitrate is in excess of the permissible limits with varying degree of concentration throughout the district, causing health hazard (Fig.  2 n). The area under study is granite-gneiss terrain where the atmospheric nitrogen is fixed and added to the soil as ammonia through lightning storms, bacteria present in soil and plants roots. Further, animal wastes, plants and animals remain also undergo ammonification in the soil producing ammonia which undergoes nitrification. The high values of nitrate in groundwater samples in the area may be due to unlined septic tanks and unplanned sewerage system that contaminates to the phreatic aquifer (Hei et al. 2020 ). Proper monitoring and concerned regulated effort are consistently required to get the assessment of nitrate impact on human health.

As far as heavy metals concentration in groundwater is concerned, Cu does not mark its noticeable presence (Fig.  2 o). Another, naturally occurring quality parameter is Mn which shows its presence within the permissible limit (Fig.  2 p). Silver and Zinc do not show any remarkable presence in the study area (Fig.  2 q and r). The study reveals a higher concentration of iron in groundwater in the Eastern part of the district due to secondary porosity and where ferrous (Fe 2+ ) ion usually occurs below the water table. The Fe 2+ after converting into Ferric (Fe 3+ ) state, becomes harmful and precipitated. This condition can be avoided naturally by raising the water table through groundwater recharging the affected area (Fig.  2 s). Nickel shows its remarkable presence in smaller patches in different areas (Fig.  2 t) due to the presence of heavy minerals like rutile and apatite.

Water quality index

The water quality index (WQI) map has been prepared using ArcGIS 10.1 on the basis of the selectively chosen quality parameters to decipher the various quality classes viz. excellent, good, poor, very poor and unsuitable at each hydro-station for drinking purpose (Tables 3 and 5 ; Fig.  3 ). The WQI Map of the study area indicates that major portion is having excellent (0–25) quality of groundwater while very poor (75–100) to unsuitable (> 100) quality is prevailing in small pockets in SW part (Fig.  3 ). The map clearly indicates that the quality of groundwater in Panwari Block belongs to excellent to good categories as for as potability for human consumption is concerned.

Water quality index map of the study area, District Mahoba

There is gradual variation in groundwater quality from very poor to excellent at the central part and outwards in the Charkhari Block. There is no noticeable change in the quality of groundwater except in the SW part of the Kabari Block. In the Jaitpur block, there is a significant.

variation in the quality class and the SW part (Nanwara, Ajnar and Khama) is characterized by poor, very poor and unsuitable categories (Fig.  3 ). Remaining part of the block falls under good to excellent groundwater quality. Overall, the quality of groundwater belongs to the excellent category in a major portion of the study area and is suitable for drinking as well as domestic uses.

Hydro-chemical facies

The major ions analysed are unevenly distributed and have been plotted on a Hill-Piper Trilinear diagram (Fig.  4 ). This diagram is comprised of two triangles at the base and one diamond shape at the top to represent the major significant cations and anions responsible for the nature of groundwater (Balamurugan et al. 2020a ). The piper diagram is used to categorize groundwater into various six types such as Ca 2+ - \({\text{HCO}}_{3}^{ - }\) type, Na + -Cl − type, mixed Ca 2+ -Mg 2+ -Cl − type, Ca 2+ -Na + - \({\text{HCO}}_{3}^{ - }\) type, Na + - \({\text{HCO}}_{3}^{ - }\) type and Ca 2+ -Cl − type. A critical evaluation of the diagram reflects that 32.56% of the samples fall under Na + -Cl − type, 30.23% of the samples under mixed Ca 2+ -Mg 2+ -Cl − type, 16.28% of the samples under Ca 2+ - \({\text{HCO}}_{3}^{ - }\) type, 13.95% of the samples under mixed Ca 2+ -Na + - \({\text{HCO}}_{3}^{ - }\) type, 4.65% of the samples under Na + - \({\text{HCO}}_{3}^{ - }\) type and 2.33% of the samples under Ca 2+ -Cl − type. Further, the observation reveals that the samples are distributed mainly into Na + -Cl − type, mixed Ca 2+ -Mg 2+ -Cl − type and Ca 2+ - \({\text{HCO}}_{3}^{ - }\) type reflecting higher concentration of sodium and calcium bearing salt/mineral. Hydrochemistry of the analysed samples indicate that the major cations are present in order Na +  > Ca 2+  > Mg 2+  > K + of mean abundance while anions are present in the mean abundance order of \({\text{HCO}}_{3}^{ - }\)  >  \({\text{NO}}_{3}^{ - }\)  > Cl −  >  \({\text{SO}}_{4}^{2 - }\)  > F − (Table 1 ). This reveals that sodium, chloride and bicarbonate dominate the ionic concentration in the groundwater due to action of weathering of minerals like halite and dolomite as well as ion exchange process.

Types of groundwater

The outcome of the present research in the hard rock area of the Bundelkhand region of India reveals that the groundwater has been deteriorated due to both geogenic and anthropogenic activities.

The study area is comprised mainly of granite and alkali granite, specifically in extreme southern which is responsible for leaching of fluoride in groundwater.

The thickness of overburden (loose soil and weathered rock) in the northern part of the study area is negligible. Therefore, there is a poor fluxing of groundwater which in turn triggers the concentration of TDS, fluoride and bicarbonate in groundwater.

Anthropogenic activities like unlined septic tanks and unplanned sewerage system have triggered the nitrate concentration in groundwater, particularly in the central and northern part of the study area. The rest of the area is safe and has potable groundwater. In addition, the area under study is granite-gneiss terrain where the atmospheric nitrogen is fixed and added to the soil as ammonia through natural lightning, bacteria present in soil and plants roots. Further, ammonification of animal wastes, plants and animal remains produces ammonia which undergoes nitrification.

Hydro-chemical facies reveal that the nature of groundwater is Na + -Cl − , mixed Ca 2+ -Mg 2+ -Cl − and Ca 2+ - \({\text{HCO}}_{3}^{ - }\) type in the study area.

The high value of WQI has been found, which is due to the higher values of chloride, fluoride, nitrate, manganese, iron, and nickel in the groundwater, which warrants immediate attention.

On the basis of WOI, it is concluded that the groundwater is safe and potable in the study area except for localized pockets in Jaitpur and Charkhari Blocks.

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Acknowledgements

Our thanks go to Central Groundwater Board (CGWB), Lucknow, NR, Region and Department of Soil Science & Agricultural Chemistry, Banaras Hindu University, Varanasi for their valuable support during the chemical analysis of groundwater samples.

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S. K. Tiwari

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H. K. Pandey

Central Groundwater Board, Lucknow, 226021, UP, India

Supriya Singh

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Ram, A., Tiwari, S.K., Pandey, H.K. et al. Groundwater quality assessment using water quality index (WQI) under GIS framework. Appl Water Sci 11 , 46 (2021). https://doi.org/10.1007/s13201-021-01376-7

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The Omnipresence of PFAS—and What We Can Do About Them

PFAS are raising a red flag among public health and environmental advocates.

Morgan Coulson

Per- and poly-fluoroalkyl substances (PFAS)—also known as “forever chemicals”—are everywhere.

Created in the 1940s, these synthetic compounds are an unseen ingredient in many items that we use in our daily lives, like cleaning products, food packaging, nonstick cookware, cosmetics, personal care items like dental floss,  water-repellent clothing, as well as stain-resistant carpets and upholstery. Since the 1970s, they have also been used in firefighting foams and by the military.  

“Food is another potential source,” says  Carsten Prasse , PhD, MSc, assistant professor in  Environmental Health and Engineering . “Unfortunately, PFAS are also present in biosolids which are used as agricultural fertilizer,” creating a pathway from contaminated soil to produce in the grocery store.  

Because of their longevity and resistance to disintegration—a characteristic born of their carbon-fluorine chemical bonds—PFAS can last thousands of years. These “attributes also make them very resistant to degradation in our treatment systems,” says Prasse.

The most common method of destroying PFAS is incineration, but some studies  indicate that this fails to eliminate all the chemicals, and instead releases the remaining pollution into the air.

In water treatment systems, the main methods for destroying PFAS are reverse osmosis, activated carbon, and ion-exchange resins—but these technologies are costly. Other methods include  supercritical water oxidation ,  plasma reactors , and most recently,  sodium hydroxide (lye) and dimethyl sulfoxide , chemicals used in soap and as a medication for bladder pain syndrome, respectively.

But when items containing PFAS inevitably reach landfills, the compounds leach into the environment. And every day, people flush PFA-laden products—like shampoo, cleaning liquids, even some toilet papers—down the drain.

“If they're not removed in our wastewater treatment plants, [PFAS] get into our rivers, streams, and groundwater, which are commonly used for drinking water production,” Prasse says. “Around 50% of our rivers and streams contain measurable PFAS concentrations.”  

According to a  2020 study published in Science by the Environmental Working Group,  an estimated 200 million Americans are served by water systems that contain PFAS. And it’s not just public systems— a  2023 study by the U.S. Geological Survey found that approximately 20% of private wells are contaminated. 

These compounds are now so ubiquitous, that  an estimated 98% of the U.S. population has detectable concentrations in their blood. That’s concerning, since studies have shown that exposure to some PFAS may be linked to harmful health effects, both in animals and humans.

“We know today that even very low concentrations can impact the reproductive system, [have] developmental effects, increase risk of certain cancers, reduce immune response, as well as increase cholesterol levels,” Prasse says. The Environmental Protection Agency  also links the compounds to thyroid disorders, obesity, and asthma.

Individuals who may have had high exposure to PFAS—in firefighting or chemical manufacturing industries, for example—should consider blood testing, Prasse says. “I think it is valuable …  because it allows them at least to talk to medical professionals, to think about follow-up examinations, to really monitor potential health effects.”

Prasse says we still know very little about the health impacts of PFAS, especially on a population level. While these compounds have been around for some time, there is insufficient research to answer many questions that have emerged over decades.

But some action is being taken. Last year, the EPA proposed  the first federal limits on forever chemicals in drinking water. And in February 2024, the agency proposed  that nine PFAS be categorized as hazardous to human health —a designation only applied to substances that are toxic or cause cancer, genetic mutation, or embryo malformation.

“The main reason for the step that the EPA is taking is that there's increasing evidence that there are toxic effects on a variety of levels,” Prasse says. “It will hopefully lead to more research to address the presence of these compounds in the environment, but also to more efforts to really elucidate the health impacts of these chemicals.”

  The proposal would classify the chemicals as "hazardous constituents" under the Resource Conservation and Recovery Act, making it easier for the agency to clean up contaminated sites—and to allocate funds to treat affected drinking water.  

But these nine compounds are only the tip of the iceberg. 

“We estimate there are more than 12,000 individual PFAS compounds, and unfortunately for most of them, we have basically no understanding about toxicity, and we don't really know a lot about their occurrence in the environment,” Prasse says. “I think the step by the EPA is really urgently needed to protect our drinking water and ultimately our health.”

  A  small study published in Environment International this month showed that cholestyramine—a cholesterol-lowering drug—could help scrub toxic forever chemicals from the blood of people who have been highly exposed. But the most efficient way to reduce contamination is preventatively, Prasse says, by regulating PFAS production and cleaning up the environment—especially waterways—and ensuring that our drinking water facilities are equipped to remove these compounds.

  “The issue at this point is really that we don't know what levels are concerning or lead to health effects, and which don't,” Prasse adds. “That's something that only the future will tell.”

Prasse and other experts recommend a variety of actions to minimize exposure to PFAS:  

• Avoid using nonstick cookware.
• Limit use of food packaging, such as grease-resistant takeout containers.
• Filter your water at the tap, with pitchers that are certified for PFAS.
• Avoid wearing water-resistant textiles.
• Seek out PFAS-free retailers’ products—including  menstrual products and large items like carpets or furniture.

Morgan Coulson is an editorial associate in the Office of External Affairs at the Johns Hopkins Bloomberg School of Public Health.

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Biden Administration Restores Wildlife Protections Weakened Under Trump

The rules give federal officials more leeway to protect species in a changing climate. Industry groups are expected to sue.

By Catrin Einhorn

After three years of planning and navigating the slow bureaucracy of federal rule-making, the Biden administration is restoring a series of protections for imperiled animals and plants that had been loosened under President Donald J. Trump.

The rules, proposed last year and now finalized, give federal officials more leeway to protect species in a changing climate; bring back protections for animals that are classified as “threatened” with extinction, which is one step short of “endangered”; and clarify that decisions about whether to list a species must be made without considering economic factors.

They come as countries around the world grapple with a biodiversity crisis that has taken hold as humans have transformed the planet .

“As species face new and daunting challenges, including climate change, degraded and fragmented habitat, invasive species, and wildlife disease, the Endangered Species Act is more important than ever to conserve and recover imperiled species now and for generations to come,” said Martha Williams, director of the U.S. Fish and Wildlife Service, which issued the finalized rules along with the National Oceanic and Atmospheric Administration’s fisheries service. “These revisions underscore our commitment to using all of the tools available to help halt declines and stabilize populations of the species most at-risk.”

Republicans and industry groups had assailed the initial proposal and are expected to do the same with the finalized version. Representative Bruce Westerman, an Arkansas Republican who leads the Natural Resources Committee, accused the Biden administration on Thursday of “undoing crucial reforms and issuing new regulations that will not benefit listed species.”

The rules are expected to set off a new round of lawsuits.

“The imposed Endangered Species Act restrictions are especially harmful to those, such as our farmer/rancher members, who depend on being able to produce their livelihoods through access to and use of natural resources,” the Nevada Farm Bureau Federation wrote in a comment to the proposed changes. Others that have spoken out against them include the oil and gas industry, foresters and states that want more control over managing wildlife.

Conor Bernstein, vice president of communications at the National Mining Association, said that while his group supports the conservation goals of the Endangered Species Act, the law imposes unnecessary restrictions on development and creates regulatory uncertainty.

Environmental groups, on the other hand, have been eagerly awaiting the reversal of the Trump-era rules, but many criticized the Biden administration for leaving some provisions in place.

“This administration is restoring some really important rules for endangered species,” said Mike Leahy, a senior director at the National Wildlife Federation. “But given all the threats they face, we would have liked to see them restore more protections, so their critical habitats can’t be picked apart piece by piece, or past harms to these species can’t be ignored.”

Mr. Leahy said rules protecting threatened and endangered species are especially important because Congress is not providing the funding that federal, state and tribal biologists need to recover them.

The Endangered Species Act, which turned 50 last year, is both lauded and loathed. Those who prioritize environmental health and the protection of America’s wildlife see it as a landmark law that has saved untold species from extinction. Others criticize it for curtailing economic activity and stomping on the rights of states and individuals.

During the Trump administration, officials weakened the law , undoing protections for animals categorized as threatened and allowing regulators to conduct economic assessments when deciding whether a species warrants protection. Environmental groups had argued those assessments had no place in such decisions.

The Biden administration had previously reversed a Trump-era change related to the definition of habitat for endangered animals.

During the public comment period for the new rules, officials received about 468,000 comments from a wide range of groups including those representing various industries, environmental advocates, states and tribes.

Some comments came from individuals, like Carol Ellis of Spokane, Wash., who wrote in support of strengthening the law. “We humans are creating the 6th extinction!” she wrote. “Get with the science.”

Lisa Friedman contributed reporting.

Catrin Einhorn covers biodiversity, climate and the environment for The Times. More about Catrin Einhorn

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In Indonesia, deforestation is intensifying disasters from severe weather, climate change

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Roads turned to murky brown rivers, homes were swept away by strong currents and bodies were pulled from mud during deadly flash floods and landslides after torrential rains hit West Sumatra in early March, marking one of the latest deadly natural disasters in Indonesia.

Government officials blamed the floods on heavy rainfall, but environmental groups have cited the disaster as the latest example of deforestation and environmental degradation intensifying the effects of severe weather across Indonesia.

“This disaster occurred not only because of extreme weather factors, but because of the ecological crisis,” Indonesian environmental rights group Indonesian Forum for the Environment wrote in a statement. “If the environment continues to be ignored, then we will continue to reap ecological disasters.”

A vast tropical archipelago stretching across the equator, Indonesia is home to the world’s third-largest rainforest, with a variety of endangered wildlife and plants, including orangutans, elephants, giant and blooming forest flowers. Some live nowhere else.

For generations the forests have also provided livelihoods, food and medicine while playing a central role in cultural practices for millions of Indigenous residents in Indonesia.

Since 1950, more than 285,700 square miles of Indonesian rainforest — an area twice the size of Germany — have been logged, burned or degraded for development of palm oil, paper and rubber plantations, mining and other commodities, according to Global Forest Watch.

Indonesia is the biggest producer of palm oil, one of the largest exporters of coal and a top producer of pulp for paper. It also exports oil and gas, rubber, tin and other resources. And it also has the world’s largest reserves of nickel — a crucial material for electric vehicles, solar panels and other goods needed for the green energy transition.

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Indonesia has consistently ranked as one of the largest global emitters of plant-warming greenhouse gases, with its emissions stemming from the burning of fossil fuels, deforestation and peatland fires, according to the Global Carbon Project.

It’s also highly vulnerable to climate change impacts, including extreme events such as floods and droughts, long-term changes from sea level rise, shifts in rainfall patterns and increasing temperatures, according to the World Bank. In recent decades the country has already seen the effects of climate change: more intense rains, landslides and floods during rainy season, and more fires during a longer dry season.

But forests can help play a vital role in reducing the impact of some extreme weather events, said Aida Greenbury, a sustainability expert focusing on Indonesia.

Flooding can be slowed by trees and vegetation soaking up rainwater and reducing erosion. In dry season, forests release moisture that helps mitigate the effects of droughts, including fires.

But when forests diminish, those benefits do as well.

A 2017 study reported that forest conversion and deforestation expose bare soil to rainfall, causing soil erosion. Frequent harvesting activities — such as done on palm oil plantations — and the removal of ground vegetation lead to further soil compaction, causing rain to run off the surface instead of entering groundwater reservoirs. Downstream erosion also increases sediment in rivers, making them shallower and increasing flood risks, according to the research.

After the deadly floods in Sumatra in early March, West Sumatra Gov. Mahyeldi Ansharullah said there were strong indications of illegal logging around locations affected by floods and landslides. That, combined with extreme rainfall, inadequate drainage systems and improper housing development, contributed to the disaster, he said.

Experts and environmental activists have pointed to deforestation worsening disasters in other regions of Indonesia as well: In 2021 environmental activists partially blamed deadly floods in Kalimantan on environmental degradation caused by large-scale mining and palm oil operations. In Papua, deforestation was partially blamed for floods and landslides that killed more than 100 people in 2019.

There have been some signs of progress: In 2018 Indonesian President Joko Widodo, known as Jokowi, put a three-year freeze on new permits for palm oil plantations. And the rate of deforestation slowed in 2021-22, according to government data.

But experts warn that it’s unlikely deforestation in Indonesia will stop anytime soon as the government continues to move forward with new mining and infrastructure projects such as new nickel smelters and cement factories.

“A lot of land use and land-based investment permits have already been given to businesses, and a lot of these areas are already prone to disasters,” said Arie Rompas, an Indonesia-based forestry expert at Greenpeace.

President-elect Prabowo Subianto , who is scheduled to take office in October , has promised to continue Jokowi’s policy of development, include large-scale food estates, mining and other infrastructure development that are all linked to deforestation.

Environmental watchdogs also warn that environmental protections in Indonesia are weakening, including the passing of the controversial Omnibus Law, which eliminated an article of the Forestry Law regarding the minimum area of forest that must be maintained at development projects.

“The removal of that article makes us very worried [about deforestation] for the years to come,” Rompas said.

While experts and activists recognize that development is essential for Indonesia’s economy to continue to go, they argue that it should be done in a way that considers the environment and incorporates better land planning.

“We can’t continue down the same path we’ve been on,” said Greenbury, the sustainability expert. “We need to make sure that the soil, the land in the forest doesn’t become extinct.”

Milko writes for the Associated Press.

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Sea levels are rising, but is South Florida also sinking? What the research shows

I n South Florida, sea levels have already risen several inches since the start of the century and could be around six feet higher by 2100. But another factor could be making those sunny day floods in South Florida worse: We’re sinking.

Well, only a little bit. And only in some places. That’s according to new and old research on the phenomena of sinking land — also known as subsidence — along the entire U.S. coast.

Earlier this month, new research published in the journal Nature showed the potential risk of a one-two combo of sinking land and rising seas to cities along the coast, and Miami topped the list as a location that could see quite a bit of flooded property by mid-century.

The paper, led by researchers at Virginia Tech, suggested that Miami could have around 80,000 properties flooding by mid-century, a multibillion-dollar risk, due to the combination of sinking land and rising seas.

Associate Professor Manoochehr Shirzaei at Virginia Tech’s Earth Observation and Innovation Lab, co-author of the study, said he believes cities aren’t taking the slipping elevation of their coastal land as seriously as they take sea level rise.

“The coastal hazard through 2050 is more likely driven by land subsidence than sea level rise, and this has to come across very clearly in every sea rise strategy,” he said. “I’m confident in this moment in most places, this is not the case.”

Subsidence is caused by several things, from glaciers moving or melting in the North Pole to overpumping oil, gas and groundwater, or, in South Florida’s case, compacting soil.

While the nationwide maps make clear that the real problem area for subsidence is in the Gulf Coast, specifically in places like Louisiana, Shirzaei said South Florida should still be paying attention to the future risk.

Specifically, he said, on the glitzy, expensive island communities that ring the coast of the Sunshine State.

“You do have some of the famous barrier islands. All of them are sinking,” he said.

How South Florida stacks up

Local research into subsidence in South Florida has found that yes, indeed, there are spots along the coast that are lower than they were decades ago. That’s especially true on barrier islands, said Shimon Wdowinski, a geophysics professor at Florida International University who has published several papers on subsidence.

But it’s only a minuscule amount.

His 2020 paper compared Miami Beach to Norfolk, Virginia, and found that Miami Beach experienced very little subsidence overall. It also happens in small patches, typically around new construction. In that paper, he noted that the Champlain Towers condominium building in Surfside sank at a rank of 2 millimeters a year from 1993 to 1997 — about the width of two credit cards stacked on top of each other.

His research, released before the tragic 2021 collapse, initially led some to believe that sinking earth could have played a role in the disaster, but the investigation has so far not pointed to land change as a culprit.

Wdowinski said that his research suggests that land subsidence is mostly localized to the space of a single building in South Florida, and it’s by design. Heavy buildings compact the soil, so good engineers design buildings with the knowledge that they will slowly settle over a long period of time.

And that’s not just because of South Florida’s dirt. It can be what’s underneath, too. When barrier islands like Miami Beach were first built or expanded, it was common practice to mow down trees and mangroves and use those cuttings to expand the island. Decades later, those trees have rotted, sinking the earth down a bit.

“In southeast Florida, we have mostly localized subsidence, which is associated with construction of new buildings,” Wdowinski said. “The weight of the building is actually pushing down, and if you have the proper foundation it should be fine.”

The problem comes when one side of the building sinks faster or further than another side, potentially straining the building’s foundation or the pipes underneath.

That is, unless the seas are rising.

South Florida is already a low-lying place, so millimeters matter here. Continued subsidence, paired with rising seas, rising groundwater levels and more intense rainstorms could leave South Florida at risk of more frequent and more intense floods.

That’s why Wdowinski is pursuing more grants and more research into the phenomenon all across the state. While he’s confident that sinking is a bigger problem on the Gulf Coast — including Florida’s west coast — he noted that all information is important when it comes to planning for climate change.

“It’s a factor. It’s not the most important one. But still, if we can get a better grip on that it would be helpful,” he said.

©2024 Miami Herald. Visit miamiherald.com. Distributed by Tribune Content Agency, LLC.

Lost homes, lost traditions, lost habitats: the cost of Indonesia’s brand new city

Residents of Balikpapan Bay in eastern Borneo dismiss claims that Nusantara will be a sustainable city that coexists with nature

I n eastern Borneo, beyond the thick jungle forests, an epic building project is under way. Giant trucks, cement mixers and diggers lumber along battered roads. Cranes tower overhead. Yellow dust clouds the air, caking everything in reach: the leaves of eucalyptus trees, the sides of passing vehicles and the homes of nearby residents.

This site – a 2,560 sq km area encompassing industrial plantations, mines, Indigenous communities and agricultural land – is to form Nusantara, Indonesia’s new administrative capital.

The decision to move the country’s capital to a new site was taken because Jakarta is rapidly sinking. In a single year, some areas of the capital subside by as muchas 11cm , a problem driven by excessive groundwater extraction and rapid urban development. On top of this, the climate crisis is making storm surges and extreme weather more likely, and causing sea levels to rise. By 2050, about 25% of the capital could be submerged if there is no effective action, according to a study by the government’s National Research and Innovation Agency.

Nusantara’s location, in the province of East Kalimantan, means the new capital will be at the centre of Indonesia’s archipelago of 17,000 islands, to help spread power and wealth more evenly across the country.

The development is welcomed by many in the wider province, who hope it will bring investment and better infrastructure. Officials promise the capital will be a modern, sustainable forest city that coexists with nature and is carbon neutral by 2045.

Others are less convinced that a new capital is an effective solution to Jakarta’s subsidence, or the best way to decentralise wealth – and it is seen by many as an attempt by the outgoing president, Joko Widodo, to create a grand legacy. Officials, however, promise the capital will be a modern, sustainable forest city that coexists with nature and is carbon neutral by 2045. The presidential palace – to be shaped like the country’s emblem, the mythological bird Garuda – is due to be inaugurated in August.

However, critics say the development is too ambitious and rushed. They also warn it could come with high costs, not only to the state – which will fund 20% of the $32bn bill – but also to the surrounding environment and local Indigenous communities.

Construction started in July 2022, and by 2045 the area is expected to be home to 1.9 million people – more than twice the current population of Balikpapan, the nearest city.

“Nusantara is changing the shape of everything,” says Pandi, a member of the Indigenous Balik community. His family has lived in the area, and depended on nature, for seven generations. He has already witnessed the damage brought by industrialisation over the decades, as areas have been deforested to make way for plantations.

“You can see how the plantation company changed the shape of the hill above us now – it made this area prone to flooding in the rainy season,” Pandi says, sitting in the front room of his house, which is built on stilts to avoid intruding waters. The impact of Nusantara, which is far greater in scale, will be worse, he says.

Already, development has affected the local environment and Balik traditions. A dam has been built nearby, Pandi says, which has altered the flow of water at the nearby river that the local population uses for transportation, as well as fishing and picking nipa leaves. A sacred stone, where his community leaves offerings, has been removed. Graves belonging to Indigenous people have been relocated in some areas.

Most people in Pandi’s community do not have the papers to prove land ownership, or the resources to fight a legal battle in court.

I n November last year, 33-year-old Yati Dalia returned home to find a notice plastered to the wall. It ordered her to vacate her home within two weeks. She has lost the house, as well as the small adjoining shop she ran. Her siblings lost their farmland. “It makes us feel so far away from the area and from our families,” says Yati, of members of the Balik Indigenous community who have been forced out.

She has been promised 150m rupiah (£7,500) in compensation, but this is yet to materialise, and it is unlikely to cover the cost of another home nearby, she says; land has become more expensive since the development began.

Myrna Asnawati Safitri, the Nusantara authority’s deputy for environment and natural resources, says regulation is being finalised that will recognise areas of historical significance to local communities. Issues such as land disputes are long-running and complex, she says, and until recently were the responsibility only of the East Kalimantan provincial government, which is a separate entity.

The scale of Nusantara – and its huge need for water, energy and infrastructure – means that its impact will be felt far beyond the core of the city, where government buildings and offices will eventually stand, through to outer rings of the development and beyond. On an island known as the “ lungs of the world ”, which is home to some of the most endangered species, this makes planning decisions especially sensitive.

L amale has spent more than two decades restoring stretches of mangrove trees that line the serene waters near his home in Mentawir. The trees were previously destroyed to make way for prawn and fish farms, and to build harbours.

His area has been selected as an eco-tourism location in the outer ring of the capital, and so is not at risk of demolition. But a section – about 15km by 2km – of mangrove has fallen victim to the construction of electricity lines, Lamale says, and there’s now a plan to build a toll road that will cut through the area.

It is still not clear how much would be removed. “We can imagine how the mangrove will be affected,” says Lamale. “I hope the development will be as minimal as possible.”

So far, in total, 1,700 hectares (42,000 acres) of mangrove have been cut down, says Mappaselle, a director with the local environment group Pokja Pesisir. He worries that the entire stretch of the estimated 12,000 hectares of mangrove that lines Balikpapan Bay is vulnerable.

“The more mangrove is cut down, the greater the catastrophe,” Mappaselle says. Destroying mangrove could increase sedimentation in the bay, which sticks to the gills of some fish species, smothers their eggs and damages the coral. It also clouds the water, preventing the seagrass from photosynthesising. When seagrass is gone, there’s nothing for the dugong – a marine mammal, sometimes known as a sea cow – to eat.

Such changes could also leave the local fishing communities with no choice but to leave. “The easiest way to push the fishermen out of the area is to damage their three essential parts of the sea: to destroy the mangrove, the seagrass and the coral. There will be no fish there that can be caught by the fishermen,” says Mappaselle.

Nusantara authorities say that mangrove within the city’s perimeters is protected. However, areas outside are not, and, regardless, enforcement is a challenge.

It is also unclear how the critically endangered local population of Irrawaddy dolphins will be affected in the long term by the project, which has seen an increase in ship traffic.

S ome fear that, in an effort to attract private investment – to fund 80% of the development – environmental standards could be weakened. Environmental groups have long warned of companies operating in the area with little oversight.

Sulfikar Amir, an associate professor at Nanyang Technological University in Singapore, was a spokesperson for the opposition presidential candidate Anies Baswedan in last month’s elections. He says it does not appear to be an attractive offer for investors, pointing to a similar project, Forest City in Jahor, Malaysia, which was backed by Chinese funding. “It has become a ghost city and it’s only 20 minutes from Singapore,” he says.

Foreign investment for the development has been slow to arrive. The president, Joko Widodo, widely known as Jokowi, said in November last year that the project had received a lot of interest from potential investors, but had yet to draw in foreign funding.

Back in Pandi’s stilted house, he expresses fears his village will be demolished to make way for a water management facility. He cannot comprehend leaving. “My parents’ graveyard is near this house,” he says. “If I must go, I must abandon my tradition, my ancestors’ legacy – and all of the memories here.”

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