Insights into the arsenic contamination status and hydrogeochemical characteristics of major river basins in Bihar, India, after two decades of investigations Open Access

The first evidence of arsenic contamination in drinking water was identified in Argentina in 1917. Since then, groundwater poisoning has emerged as a widespread and serious public health concern, affecting many regions across the globe. Over the decades, the presence of arsenic in groundwater has endangered the health and well-being of millions, highlighting the urgent need for ongoing monitoring, mitigation efforts, and sustainable solutions to address this persistent issue (Ayeza 1917; Chakraborti et al. 2017). As per various reports, the arsenic contamination has been detected in >105 countries worldwide, with over 300 million people exposed to concentrations exceeding the World Health Organization's recommended limit of 10 μg/L. Of these, approximately 180 million individuals across 32 Asian countries face heightened risks due to elevated arsenic levels in their drinking water (WHO 1996; Hassan 2018; Kumar et al. 2021a, b; Shaji et al. 2021). In India, arsenic contamination in groundwater was first reported in 1976 in Chandigarh, located in the northern region of the country (Datta 1976; Chakraborti et al. 2017). Recent reports reveal that arsenic contamination in groundwater has been identified in 20 states and 4 Union Territories across India, putting >100 million people at risk (Shaji et al. 2021; Marghade et al. 2023). In India, the Ganga–Brahmaputra Plain is considered the most affected physiographic region, with arsenic contamination first reported in West Bengal in 1983. This issue has since impacted nearly 50 million people (Chakraborti et al. 2017; Marghade et al. 2023). Geographically, Bihar is among the most severely affected states in the Ganga–Brahmaputra Plain, situated within the Middle Ganga Plain region (Saha & Sahu 2016). In Bihar, arsenic contamination was first reported by Chakraborti et al. (2003) in Ojha Patti village, located in the Bhojpur district. Subsequently, in 2005, the Central Ground Water Board (CGWB) of the Government of India and the Public Health Engineering Department (PHED) of the Government of Bihar conducted investigations into arsenic contamination. Their findings revealed arsenic levels exceeding 10 μg/L in 11 out of the 38 districts in Bihar, with the highest concentration of 1,654 μg/L recorded in Bhojpur district. In 2019, the number of arsenic-affected districts in Bihar increased to 18 out of 38 (Rahman et al. 2019). After two decades of investigation into arsenic contamination, recent studies (Thakur et al. 2021; UKIERI 2021; Roshan et al. 2024) have reported that 22 out of 38 districts are now affected, with arsenic levels exceeding 10 μg/L. This contamination is exposing nearly 10 million people, including 1,600 inhabitants from 87 blocks in Bihar (Thakur et al. 2021, 2022).

Geologically, Bihar is predominantly composed of approximately 89% alluvial formations, spanning several major river basin regions from the Quaternary to the Recent Age (Mondal et al. 2020). Among these, the Ganga stem basin is significantly affected by arsenic contamination, with a higher population density compared to other river basins (Saha & Sahu 2016). In Bihar, the spatial distribution of arsenic contamination in groundwater, along with its associated hydrogeochemical characteristics, exhibits distinct regional patterns based on the major river basin frameworks (Mondal et al. 2020). Over the past two decades (from 2003 to 2022), numerous investigations have been conducted to assess arsenic contamination levels and their distribution patterns, both temporally and spatially, as well as with respect to depth across different districts. Some studies have also focused on understanding the mechanisms driving the evolution of arsenic contamination in groundwater (Saha et al. 2010; Mukherjee et al. 2012; Saha & Shukla 2013; Kumar et al. 2014; Singh 2014).

Most of these studies have been conducted at the local level, providing limited insights into the entire river basin, except for the Ganga stem basin in Bihar. The expanding arsenic contamination across numerous districts underscores the critical need for a detailed, river basin-wise assessment of groundwater characteristics. Such an evaluation should consider the unique geological formations, hydrological processes, and anthropogenic activities within each basin to develop targeted and effective mitigation strategies.

This paper provides a first-time review assessment of arsenic concentrations across major river basins in Bihar and identifies the key mobilization mechanisms driving arsenic pollution, along with their associated hydrogeochemical characteristics, based on past studies. This paper also reviews the temporal and spatial variations in arsenic contamination levels across major river basins in Bihar, highlighting existing gaps and emphasizing the need for further investigations. This paper also examines the changes in groundwater depth-to-water level (DTWL) behaviours over the past two decades (2003–2022), the influence of various geological formations and their characteristics on groundwater chemistry in major river basins, as well as the geomorphological features of these basins and their impact on arsenic contamination. This review paper also explores the feasibility of arsenic-free aquifers within the geological formations of major river basins in Bihar, based on the findings of past research studies.

The northern region of Bihar generally slopes from the northwest to the southeast and is divided into several river basins, including the Ghaghara, Gandak, and Burhi Gandak groups, the Bagmati-Adhwara group, the Kamla-Balan, Kosi, and Mahananda basins. All of these rivers ultimately flow into the Ganga River (FMIS 2011; Figure 1). The southern region of Bihar is situated between the Ganga River and the hills or plateaus along the Jharkhand state border (FMIS 2011; MSME-DI 2017; Figure 1). This area is mainly drained by rain-fed rivers that originate from the Chhotanagpur, Rajmahal, and Vindhyachal hills. During the non-monsoon season, these rivers either dry up or flow with a low discharge rate. The major river basins in this region include the Karmanasa, Sone, Punpun, Harohar, Kiul, Badua, Chandan, and others, all of which eventually flow into the Ganga River (FMIS 2011; Figure 1). Figure 2 presents the surficial and cross-sectional geological features, along with the groundwater characteristics, from different regions of Bihar. It is based on various aquifer mapping reports and includes a total of 8 lithological data points, labelled A to H (CGWB 2013 2016 2017 2019 2022). The southeastern districts of Gaya, Nawada, Banka, and Jamui in Bihar, which border Jharkhand, feature small sections of the Chhotanagpur Granite Gneissic Complex (CGGC) rocks, dating back to the Proterozoic and Archean periods. Additionally, the districts of Gaya, Nawada, and Munger are primarily composed of mica-bearing pegmatites, which are part of the Bihar Mica Belt from the Proterozoic era (Figure 2; Table 1). In some areas of the Munger, Gaya, and Nawada districts, Precambrian meta-sedimentary rocks, including phyllite, schist, and quartzite, are also found (CGWB 2023; Figure 2; Table 1). In these formations, the saprolite and weathered mantle zones are regions where groundwater is found in an unconfined environment near the surface and in a confined environment above the basement rocks (Figure 2; Lithologs E and F). Groundwater also exists under semi-confined conditions within aquitards, created by secondary porosities such as joints, cracks, and fractures (CGWB 2023).

The rocks of the Vindhyan Supergroup are confined to parts of the Karmnasa and Sone basin areas in the Sasaram, Kaimur, and Aurangabad districts of Bihar (Figure 2). This supergroup primarily consists of meta-sedimentary rocks such as schist, quartzite, limestone, and sandstone (CGWB 2023; Figure 2; Table 1). The Vindhyan sandstones are characterized by their compact nature and low primary porosity. Groundwater is found in the secondary porosity beneath the weathered residuum of this group. In the weathered mantle of the Vindhyan sandstone, groundwater exists under unconfined conditions (CGWB 2023; Figure 2; Litholog A). The Upper Tertiary Siwalik formations are located in the upper reaches of the West Champaran district, in the northwestern part of Bihar, along the border with Nepal (Figure 2). The Siwalik formations consist of red clay, sandstone, conglomerate, and porous limestone (Table 1). Groundwater is found in a confined environment within the sandstone of these formations (CGWB 2023). The Quaternary alluvial sediments of Bihar, ranging from the Pleistocene to sub-recent age, cover approximately 89% of the state's total land area (Mondal et al. 2020). These alluvial sediments cover the entire northern part of Bihar and a significant portion of the area between the Ganga River stem and the Kaimur Plateau, Chhotanagpur Plateau, and Rajmahal Hills in the southern part of Bihar (CGWB 2023). Geologically, the alluvial plains are typically divided into two layers: the Pleistocene older alluvium and the Holocene younger or calcareous alluvium. Together, these form a ‘two-tier aquifer system,’ with thin clay surficial layers on top (Singh 2015). The layers are separated by an aquitard made up of clay, sandy clay, boulders, gravels, and Kankars, with a thickness ranging from 15 to 32 m. This structure gives rise to shallow aquifer systems composed of fine to medium sand (<90 m deep) and deeper aquifer systems made up of medium to coarse sand (100–300 m deep) (Singh 2015; Saha & Sahu 2016; Figure 2; Lithologs A to H). According to various reports, deeper aquifers in the alluvial zones of the river basins exhibit significantly low arsenic contamination levels (<10 μg/L). However, shallow aquifers have been found to be contaminated with arsenic levels exceeding the 10 μg/L limit set by the WHO (1996) (Kunar et al. 2009; Singh 2015; Saha & Sahu 2016). Groundwater is observed in an unconfined environment up to 70 m below the surface in the phreatic aquifer. At greater depths, groundwater is found in confined environments (Saha & Sahu 2016; Figure 2).

These approaches involved a comprehensive analysis of groundwater level behaviour, groundwater quality parameters, an assessment of arsenic distribution and contamination levels spatial as well as temporal, and an exploration of the geochemical and environmental factors influencing arsenic mobilization. This multifaceted review aimed to provide a detailed understanding of the region's hadrochemical dynamics and the underlying processes driving arsenic behaviour in the river basins.

The consistent decline in the groundwater levels across the major river basins in Bihar observed indicates a growing demand for groundwater, leading to increased extraction from the basin aquifers (Dangar et al. 2021; Figure 4). The annual groundwater recharge across the different basins in Bihar is estimated to be 33.14 bcm, while the annual net extractable groundwater is estimated at 30.04 bcm (CGWB 2022). According to the CGWB 2022 report, 8 blocks are classified as overexploited, 12 as critical, 46 as semi-critical, and 469 as safe. Recent studies indicate that the excessive extraction of subsurface water from shallow aquifers causes a significant decline in groundwater levels, pushing them well below sustainable limits and resulting in substantial fluctuations in the DTWLs. These fluctuations mobilize harmful geogenic contaminants such as fluoride and arsenic from the soil into groundwater, eventually entering the food chain and impacting biotic systems (Chatterjee & Chowdhury 2020). Groundwater extraction and fluctuations in DTWLs facilitate the movement of dissolved oxygen within the soil zone of the aquifers. This dissolved oxygen oxidizes immobile minerals in reducing environments, resulting in the release of toxicants such as arsenic into the groundwater (Liu et al. 2003; Neidhardt et al. 2013; Chatterjee & Chowdhury 2020). Based on these observations, the greater fluctuations in the average DTWLs between the pre-monsoon and post-monsoon periods in the Ganga stem and northern river basins of Bihar contribute to a higher mobilization of arsenic metalloids compared to the southern river basins. These fluctuations enhance the movement of dissolved oxygen into the soil and aquifer zones, facilitating the reactions of arsenic-bearing minerals in reducing environments. As a result, arsenic is released into the groundwater more extensively in the Ganga stem and northern river basins than in the southern river basins, where such fluctuations are less pronounced.

Arsenic is a trace element in the Earth's crust, with an average concentration of 1.8 mg/kg. Common arsenic-rich minerals include realgar (As₂S₂), orpiment (As₂S₃), arsenopyrite (FeSAs), and enargite (Cu₂AsS₄) (Zhao et al. 2021; Marghade et al. 2023). The reduced form of arsenic, arsenite {As(III)}, is more soluble and mobile in water, typically occurring in anaerobic (low-oxygen) environments (Liu et al. 2021; Marghade et al. 2023). On the other hand, the oxidized form of arsenic is arsenate {As(V)} (Yadav et al. 2021; Marghade et al. 2023). In groundwater systems of river basins, organic arsenic compounds are typically associated with arsenic (III) and arsenic (V) (Raju 2022; Marghade et al. 2023). Arsenic mobilization in the environment occurs due to two primary factors: (a) natural or geogenic processes, such as atmospheric emissions and the desorption or dissolution of naturally occurring arsenic-rich minerals, and (b) anthropogenic activities, including mining, metal extraction, burning fossil fuels, use of wood preservatives, and poor landfill and dumpsite management (Rajmohan & Prathapar 2014; Shrivastava et al. 2015; Marghade et al. 2023; Figure 5). Globally, arsenic contamination in groundwater is primarily attributed to geological or natural processes rather than human activities (Smedley & Kinniburgh 2002; Sailo & Mahanta 2014; Shrivastava et al. 2015).

Over the past few decades, numerous studies have been conducted to investigate the various mechanisms and processes that lead to arsenic mobilization and its accumulation in aquifer systems (Tabelin et al. 2010; Marghade et al. 2023). However, debates continue regarding the mechanisms of arsenic mobilization and the factors that influence its movement (Charlet et al. 2007; Postma et al. 2007; Halim et al. 2009; Sailo & Mahanta 2014; Shrivastava et al. 2015). The release of arsenic from its sources into the soil, water, and atmosphere is controlled by numerous variables and complex geochemical processes. Four primary mechanisms are commonly recognized to explain arsenic mobilization (Das et al. 1996; Mandal et al. 1996; Rahman et al. 2001; Roy & Saha 2002; Ravenscroft et al. 2009; Islam et al. 2010; Bhattacharya et al. 2011; Rajmohan & Prathapar 2014; Shrivastava et al. 2015; Saha & Sahu 2016; Marghade et al. 2023; Figure 5). These mechanisms are

• (I) Dissolution of arsenic-bearing minerals

Natural weathering processes and human activities, such as mining, expose naturally occurring minerals like arsenopyrite (FeAsS), realgar (As₄S₄), and orpiment (As₂S₃) to atmospheric conditions, leading to their oxidation. The oxidative degradation process begins when molecular oxygen (O₂) from the environment and water (H₂O) react with the arsenopyrite sample. During this process, the arsenic in the sulphide minerals oxidizes, forming the highly soluble arsenate in groundwater (Das et al. 1996; Rahman et al. 2001; Roy & Saha 2002; Marghade et al. 2023; Wang et al. 2023; equation (II) in Figure 5). According to some researchers, this mechanism is a significant factor in the mobilization of arsenic into the groundwater system of West Bengal (Das et al. 1996; Rahman et al. 2001; Roy & Saha 2002).

• (III) Fe-oxyhydroxide reduction process

According to several studies, arsenic mobilization in the alluvial formations of the Ganga–Brahmaputra delta region is primarily attributed to microbial reduction, oxyhydroxide reduction, and the dissolution of arsenic-bearing minerals (Appelo et al. 2002; Zheng et al. 2004; Shrivastava et al. 2015; Marghade et al. 2023).

The mobility of arsenic in the environment is influenced by its lithological properties and the hydrochemical composition of the groundwater system. The concentration and mobilization of arsenic in groundwater are governed by several factors, including pH, redox conditions, adsorption reactions, the presence of organic matter, microbial activities, the role of competitive ions, and soil characteristics (Tabelin et al. 2010; Marghade et al. 2023; Figure 5).

Groundwater pH values play a crucial role in arsenic mobilization. Arsenic is primarily found as arsenite {As(III)} when the pH of the groundwater system is acidic to neutral (pH < 7). Arsenite is more mobile and toxic compared to arsenate {As(V)}. In neutral to slightly alkaline conditions (pH > 7), arsenate becomes more prevalent. Arsenate has a stronger tendency to adsorb onto mineral surfaces and exhibits lower ion mobility (Manning & Goldberg 1997; Anawar et al. 2004; Marghade et al. 2023).

Redox conditions in reducing environments are established when organic matter breaks down through microbial activity in groundwater systems. This initiates a series of complex redox processes that degrade Fe and Mn oxides, leading to the release of arsenic into the groundwater (Equation (2); Marghade et al. 2023).

Arsenic desorption can occur through various mechanisms in an alkaline groundwater system. When large amounts of hydroxide ions (OH⁻) compete with arsenic for binding sites on soil and sediment particles, desorption takes place. This process facilitates the degradation of iron and manganese oxides in alkaline conditions, releasing the adsorbed arsenic into groundwater (Equations (1)–(4); Kumar et al. 2022; Marghade et al. 2023).

Dissolved organic matter in the environment provides a carbon source and promotes the reductive dissolution process, which is supported by microbial activity. This microbial reduction of organic particles creates ideal conditions for arsenic solubility in reductive iron oxyhydroxide minerals. Additionally, the role of buried peat deposits in creating favourable redox conditions for the release of arsenic ions from iron oxyhydroxide (FeOOH) minerals has been documented (Equation (2); McArthur et al. 2001; Rowland et al. 2006; Marghade et al. 2023).

The binding rate of arsenic (III) in the groundwater system is reduced due to the strong interaction between negative ions and arsenic (III) ions. As the concentration of anions in the solution increases, arsenic is released into the solution due to enhanced inhibitory effects (Singh et al. 2014a, b; Marghade et al. 2023). Under these conditions, sulphate and chloride ions play a significant role in regulating the attachment of arsenic (III) at various stages of the chemical reaction process (Marghade et al. 2023).

The characteristics and compositional quantities of hydrochemical components in the soil affect arsenic mobilization conditions. It has been observed that calcium oxides adsorb anionic arsenic compounds to a lesser extent in alkaline-rich soils compared to acidic soils. In contrast, inorganic arsenic compounds exhibit a stronger affinity for hydrous oxides of Fe, Mn, and Al in alkaline soils (Woolson 1977; Shrivastava et al. 2015). As a result, crops grown in alkaline soils are able to absorb more anionic arsenic compounds. The adsorption capacity of soils, influenced by their compositional characteristics, plays a crucial role in determining the mobility and bioavailability of arsenic in the soil (Turpeinen et al. 2002; Shrivastava et al. 2015).

In addition to the Ganga stem basin, other river basins, particularly those in the northern part of Bihar, have also reported average arsenic contamination levels exceeding 10 μg/L (Saha & Sahu 2016; Thakur et al. 2022; Tables 3 and 4; Figure 6(a)). Arsenic contamination in the groundwater of the Ghaghara River basin has been reported as high as 244 μg/L in the Siwan and Saran districts of Bihar (Singh 2014; Jawaid & Kumari 2022; Kumar et al. 2022; Tables 3 and 4). The Gandak and Burhi Gandak river basins, along with the Mahi River sub-basin, have reported arsenic contamination levels up to 135 μg/L (Tables 3 and 4). It has been noted that the distal parts of these basins – such as the Saran, Vaishali, Begusarai, and Khagaria districts – are more contaminated compared to the proximal areas, including Siwan, Gopalganj, and the West and East Champaran districts. Additionally, more studies have been conducted in the distal parts of these basins (Kumar et al. 2014; Jangle et al. 2016; Arya & Kumar 2024; Table 3). In the Kamla-Baghmati basins, arsenic contamination in the groundwater system has been detected at concentrations as high as 91 μg/L (Table 4). However, there is limited research available on the extent of contamination and its impact on the local population (Rajmohan & Prathapar 2014; Abhinav et al. 2016; Table 3; Figure 6(a)). The Kosi basin is also affected by arsenic contamination, with levels reaching up to 400 μg/L (Table 4). The proximal areas of the basin, including the Supaul and Araria districts of Bihar, have been reported to exhibit higher contamination levels compared to the distal areas, such as the Khagaria, Purnea, and Katihar districts (Mukherjee et al. 2012; Jha & Gupta 2017; Deo 2018; Tables 3 and 4; Figure 6(a)). A portion of the MHB located in Bihar has been found to have arsenic contamination in its groundwater, with concentrations reaching up to 85 μg/L (Mukherjee et al. 2012; Rajmohan & Prathapar 2014; Tables 3 and 4; Figure 6(a)). The alluvial region in the northern part of the Ganga stem basin covers a larger area compared to the southern part, suggesting a broader spatial distribution of arsenic contamination in the northern regions (Kumar et al. 2018).

In the southern parts of Bihar, the groundwater in the Karmnasha-Sone, Harohar, and Badua-Chandan River basins has not been found to have significant arsenic contamination (Tables 3 and 4).

This suggests that these areas have relatively lower levels of arsenic in their groundwater compared to other river basins in Bihar, particularly those in the northern and central parts of the state (Mukherjee et al. 2012; FAHTC 2022a; Kumar & Maurya 2023; Tables 3 and 4; Figure 6(a)). The absence of arsenic contamination in these regions may be attributed to differences in geological conditions, aquifer characteristics, and water extraction patterns (Figure 2). The report by Krishan et al. (2018) found that arsenic contamination in the Punpun River basin was below 10 μg/L, which is within the safe limit as per the WHO (1996) guidelines (Tables 3 and 4; Figure 6(a)). The northern part of the KUB, particularly in Lakhisarai district, has been found to have arsenic contamination levels as high as 241 μg/L, especially near the Ganga stem basin. This concentration is well above the World Health Organization's recommended limit of 10 μg/L for safe drinking water (Rajmohan & Prathapar 2014; Tables 3 and 4; Figure 6(a)).

In contrast, the southern regions of the river basins generally show either no arsenic contamination or levels below 10 μg/L (Figure 6(a)). All the observations mentioned above are summarized in Figure 6(a) and Table 4. These data illustrate the basin-wise arsenic concentration levels across Bihar, highlighting that the Ganga stem and northern river basins have significantly higher arsenic concentrations (>10 μg/L) compared to the southern river basins.

The visual representation in Figure 6(a) clearly shows the areas with elevated arsenic levels, which are concentrated in the northern regions, particularly in river basins such as the Ganga stem basin, Ghaghara, Gandak, Burhi Gandak, Kamala Baghmati, Kosi and Mahananda, where arsenic contamination exceeds safe limits. In contrast, the southern river basins, including those of the Karmnasha-Sone, Harohar, and Badua-Chandan River basins, generally report arsenic levels below the 10 μg/L threshold, indicating relatively safer groundwater conditions in these regions.

The Ganga stem basin and the northern river basins of Bihar are characterized by a notably diverse and extensive range of spatial hydrogeochemical attributes. This variation is considerably more pronounced when compared to the hydrogeochemical characteristics observed in the southern river basins of the region (Saha & Sahu 2016; Table 3). The western section of the Ganga stem basin, extending up to Patna district, is dominated by calcareous alluvium, whereas its central and eastern regions are primarily characterized by younger alluvium (Kumar et al. 2020; Figure 2). The southern regions of the Ganga stem basin are characterized by older alluvium deposits interspersed with formations of various massive rock types, contributing to the area's geological diversity. The distinct soil properties of these alluvium deposits and hard rock formations play a significant role in shaping the groundwater hydrochemistry (Saha & Sahu 2016). In the western part of the Ganga stem basin, including areas like Patna, Bhojpur, and Buxar, detailed hydrochemical analysis has revealed a dominance of calcium (Ca) and magnesium (Mg) cations, along with bicarbonate (HCO₃) anions, particularly in regions characterized by calcareous and younger alluvium (Saha et al. 2010; Sahu 2013; Table 3; Figure 6(b)). Agrawal et al. (2011) reported that the groundwater system in the Begusarai district, situated in the central region of the younger alluvium within the Ganga stem basin, is predominantly of the Ca–HCO₃ facies. In contrast, Saha & Shukla (2013) identified a dominance of the Ca–Na–HCO₃ facies in the eastern part of the Ganga stem basin (Table 3; Figure 6(b)).

In the northern part of the Ganga stem basin in Bihar, calcareous and younger alluvium dominate the soil composition, outnumbering the older alluvium and other rock formations (Kumar et al. 2020; Figure 2). In the northern regions of Bihar, most aquifers exhibit a bicarbonate-dominant facies, with the Ca–HCO₃-type facies being particularly prevalent in the Ghaghara, Gandak, Burhi Gandak, Baya, and Kosi basins (Kumari & Rani 2008; Mukherjee et al. 2012; Mumtazuddin et al. 2012a, b; Kumar et al. 2014; Arya & Kumar 2024; Figure 6(b); Table 3). This bicarbonate-dominant facies are primarily attributed to the interaction between groundwater and calcareous alluvial deposits, which are rich in calcium carbonate (CaCO₃) (Agrawal et al. 2011). In the Purnea district, located within the MHB, the groundwater system predominantly exhibits a Ca–Na–HCO₃-type facies (Mukherjee et al. 2012; Table 3). In the Kamla-Bagmati River basins of Darbhanga district, Bihar, the groundwater chemistry predominantly shows a Ca–Mg–HCO₃-type facies, similar to the western part of the Ganga stem basin (Kumar 2019; Table 3).

The hydrochemical characteristics in the southern parts of Bihar differ from those observed in the Ganga stem and northern river basins due to distinct geological settings (Sahu 2014). Unlike the northern and Ganga stem basin parts, where calcareous and younger alluvium dominate, the southern region is primarily characterized by older alluvial deposits and hard rock formations, such as granite and gneiss (Kumar et al. 2020). These geological differences significantly influence groundwater chemistry, as older alluvium and hard rocks release different ions into the groundwater, including higher concentrations of sodium (Na) and silica (Ray et al. 2000). The groundwater in the Badua-Chandan basin, located in the southern part of Bhagalpur district and the entire Banka district of Bihar, has been characterized by the dominant Na–Ca–HCO₃–Cl type facies (Mukherjee et al. 2012; Table 3). In the Manpur Block of Gaya district, located in the Punpun basin of Bihar, the groundwater has been reported to predominantly exhibit a Ca–Cl type facies. This type of facies is characterized by the dominance of calcium (Ca) and chloride (Cl⁻) ions in the groundwater, reflecting distinct geological and hydrogeochemical processes (Krishan et al. 2018; Table 3). The groundwater system in the Mohinia block of Kaimur district, representing the Karmnasa basin of Bihar, has been reported to predominantly exhibit a Ca-SO₄ type facies. This hydrochemical composition is characterized by the dominance of calcium (Ca) and sulphate ions in the groundwater (Kumari 2017; Table 3). Kumar & Maurya (2023) reported the dominance of the Na–HCO₃-type facies in the groundwater system of Nawada district, located in the southern part of Bihar, specifically within the Kiul and Harohar basins. This hydrochemical facies is characterized by a high concentration of sodium (Na) and bicarbonate (HCO₃) ions in the groundwater, reflecting distinct geological and geochemical processes at play in the region (Kumar & Maurya 2023; Table 3; Figure 6(b)). In Nawada district, the presence of sodium ions is often linked to the dissolution of sodium-bearing minerals such as sodium feldspar or other sodium salts, which are prevalent in the local geological formations (Saha & Shukla 2013).

Figure 6(b) illustrates the dominance of Ca–Mg–HCO₃ and Ca–HCO₃-type hydrochemical facies in the Ganga stem and northern river basins of Bihar, with the exception of the Mahananda river basin. These facies are characterized by the presence of calcium (Ca), magnesium (Mg), and bicarbonate (HCO₃) ions in significant concentrations, reflecting groundwater's interaction with various geological formations in the region (Saha & Shukla 2013).

The mobilization of arsenic metalloids in aquifers is driven by a complex interplay of various anthropogenic and natural factors, which work in synergy to influence their behaviour. This process involves multiple interconnected mechanisms, each contributing to the release, transport, and distribution of arsenic within groundwater systems (Equations (1)–(5); Shrivastava et al. 2015; Saha & Sahu 2016; Marghade et al. 2023; Neog et al. 2024). Several studies have identified key factors influencing the hydrogeochemistry of arsenic-contaminated aquifers in river basins across Bihar. These factors include the pH levels of the groundwater, redox conditions prevalent in reducing environments, and the presence of organic matter within the alluvial soils. Microbial activities play a crucial role by mediating chemical reaction chains, while the concentration of hydrochemical ions contributes to the development of suitable reducing conditions. Additionally, geographical location and geological processes, such as erosion, transportation, and infiltration, significantly impact the mobilization and distribution of arsenic within these aquifers (Saha & Sahu 2016; Marghade et al. 2023; Neog et al. 2024). The mechanisms and factors driving the release of arsenic metalloids in aquifers have been comprehensively investigated and reported for the Ganga stem basin. In contrast, other river basins in Bihar, despite experiencing similar contamination challenges, have received relatively limited scientific attention (Saha & Sahu 2016). Silicate weathering, carbonate dissolution, and ion exchange are recognized as the dominant geochemical processes influencing the hydrochemistry of groundwater systems in the river basins of Bihar (Saha et al. 2010; Mukherjee et al. 2012; Saha & Sahu 2016). According to Saha et al. (2010), the dissolution of calcite and dolomite, derived from the Vindhyan Supergroup, plays a significant role in shaping the hydrochemistry of the western part of the Ganga stem basin region. In contrast, carbonate dissolution, silicate weathering, and ion exchange processes, originating from the Himalayan region, predominantly influence the hydrochemistry of the middle and eastern parts of the Ganga stem basin as well as the MHB (Agrawal et al. 2011; Mukherjee et al. 2012).

In the context of other northern river basins, carbonate dissolution emerges as a critical geochemical process influencing the hydrochemistry of the Ghaghara, Gandak, Burhi Gandak, and Kosi River basins (Kumari & Rani 2008; Kumar et al. 2014; Kumar 2019). This process involves the interaction of groundwater with carbonate minerals such as calcite, leading to the release of bicarbonate ions, which significantly influence the alkalinity and ionic composition of groundwater in these regions (Agrawal et al. 2011; Mukherjee et al. 2012). For the Kamla and Bagmati river basins, both carbonate dissolution and dolomite dissolution are the primary processes shaping groundwater hydrochemistry (Kumari & Rani 2008; Kumar et al. 2014; Kumar 2019). Dolomite dissolution, involving the breakdown of dolomite {CaMg(CO₃)₂}, releases calcium, magnesium, and bicarbonate ions into the groundwater, further altering its chemical profile (Saha et al. 2010; Agrawal et al. 2011).

The hydrochemistry of the river basins in Bihar, including the Punpun, Harohar, Kiul, Chandan, and Badua rivers, which are situated on Precambrian formations, is predominantly influenced by silicate weathering and ion exchange processes. These Precambrian formations are primarily composed of crystalline rocks such as granite, gneiss, and schist, which play a crucial role in determining the chemical composition of groundwater (Mukherjee et al. 2012; Rajmohan & Prathapar 2014; Krishan et al. 2018). The dominance of sulphate ions in the aquifers of the Karmnasa basin points to the dissolution of carbonate minerals and gypsum as the primary processes shaping the hydrochemistry of this region. These processes are sourced from the Vindhyan Supergroup, which consists of sedimentary rocks that are rich in carbonate minerals such as calcite and gypsum (CaSO₄·2H₂O) (Kumari 2017).

The arsenic-contaminated aquifers of the Ganga stem basin and other northern river basins of Bihar are characterized by a dominant presence of calcium (Ca²⁺) and bicarbonate ions, alongside moderately alkaline pH values and moderate electrical conductivity (EC) levels (Saha & Sahu 2016; Marghade et al. 2023; Table 3). This hydrochemical signature provides important insights into the geochemical processes occurring in these aquifers and their role in the mobilization of arsenic. The high concentrations of calcium and bicarbonate ions in these aquifers are primarily attributed to the dissolution of carbonate minerals, such as calcite, in the groundwater. Carbonate dissolution releases calcium ions into the water, which contribute to its hardness, while bicarbonate ions help buffer the pH, typically maintaining a moderately alkaline range (pH 7.5 to 8.5). This carbonate dissolution is a key process in the hydrochemistry of the aquifers (Agrawal et al. 2011). The moderately alkaline pH levels of the groundwater in these aquifers are conducive to the mobilization of arsenic. In reducing environments, which are common in these regions, arsenic exists primarily in the more soluble arsenite form (As³⁺), as opposed to arsenate (As⁵⁺), which is less mobile. The moderately alkaline conditions help maintain this reduction–oxidation state, promoting the dissolution of arsenic into the groundwater. Additionally, the presence of organic matter and reducing agents such as ferrous iron and sulphides in the sediments can enhance this reduction process, leading to increased arsenic mobility (Mukherjee et al. 2012). EC values in these aquifers are typically moderate, reflecting the presence of dissolved ions like calcium, bicarbonate, sodium, and magnesium. Moderate EC values indicate a balanced concentration of dissolved solids in the groundwater, which plays a role in the overall ionic strength of the water. This balance also influences the chemical behaviour of arsenic, as high ion concentrations can facilitate its mobility through ion exchange and redox processes (Saha et al. 2010; Agrawal et al. 2011; Equation (5)).

In the arsenic-contaminated aquifers of the Ganga stem basin and other northern river basins of Bihar, low concentrations of nitrate (NO₃) and sulphate (SO₄) ions have been reported, highlighting specific geochemical characteristics of these aquifers. The low concentrations of NO₃ and SO₄ suggest limited influence from anthropogenic activities like agricultural runoff or industrial pollution, which are typically significant sources of these ions (Saha & Sahu 2016; Marghade et al. 2023).

The aquifers in these basins are also characterized by a higher infiltration of organic matter, particularly in areas with sandy soils and freshwater recharge zones. These zones, which are often associated with flood-prone areas, provide a source of organic material that can influence both the chemical and biological conditions within the aquifers. During flooding events, organic matter from the surface is carried into the groundwater system, where it can be decomposed by microbial activity. This influx of organic material, along with the microbial processes it supports, can alter the redox conditions in the aquifers, promoting the reduction of arsenate (As⁵⁺) to the more soluble arsenite (As³⁺) form, thereby enhancing the mobilization of arsenic (Saha & Sahu 2016).

Apart from the reductive dissolution mechanism, arsenic mobilization in Bihar's river basins also occurs through the dissolution of arsenic-bearing minerals, particularly in the Ghaghara River basin, where arsenic is released from minerals like arsenopyrite and realgar (Singh 2014; Equation (1)). Additionally, the competitive ion exchange process in the Ganga stem and other northern river basins contributes to arsenic release, as ions like sulphate and phosphate displace arsenate from mineral surfaces, enhancing arsenic mobilization (Mukherjee et al. 2012; Saha & Sahu 2016; Equations (5) and (6)). These processes, along with reductive dissolution, are key to arsenic contamination in the region's groundwater.

Over the past two decades, numerous investigations into the status of arsenic contamination and its mobilization characteristics have been conducted by both government and private agencies in major river basins of Bihar. These studies have been critical in assessing the extent of arsenic contamination in the region's groundwater and understanding the factors that contribute to its release and spread. The first detection of arsenic contamination in Ojha Patti village, situated in the Bhojpur district of the western part of the Ganga stem basin in Bihar, led to widespread investigations throughout the Ganga stem basin, as well as other northern and southern river basins in Bihar. According to several reports, the Ganga stem basin is the most affected by arsenic contamination (>10 μg/L), particularly in the shallow aquifers (up to 90–100 m in depth) that are composed of younger, calcareous alluvium made up of fine to medium sand. The highest level of arsenic contamination reported in the western part of the Ganga stem basin is 1,861 μg/L, found in Bhojpur district. The groundwater system in this area exhibits a Ca–Mg–HCO₃-type hydrochemical facies. Arsenic contamination exceeding 10 μg/L has been widely documented in the groundwater systems of the Ganga stem basin, particularly in the middle regions, where the groundwater is characterized by a Ca–HCO₃-type hydrochemical facies. In addition to the Ganga stem basin, northern river basins in Bihar, including the Ghaghara, Gandak, Burhi Gandak, and Kosi, also show similar levels of arsenic contamination exceeding 10 μg/L. These basins are also characterized by shallow aquifers composed of younger and calcareous alluvium. The groundwater in these basins predominantly exhibits a Ca–HCO₃-type hydrochemical facies. In contrast, the Kamla-Bagmati River basins, located in the northern part of Bihar, exhibit a slightly different hydrochemical composition. In these basins, the groundwater primarily shows a Ca–Mg–HCO₃-type facies, indicating a higher presence of magnesium (Mg) ions alongside calcium and bicarbonate. Ca–Na–HCO₃-type hydrochemical facies have been predominantly reported in the Mahananda river basin, where arsenic contamination exceeds 10 μg/L. The presence of Ca–Na–HCO₃-type facies suggests that the groundwater in this basin is influenced by the dissolution of minerals such as calcite and soda-rich minerals, which contribute calcium and sodium ions to the water.

In some southern river basins, such as the Kuil River basin near the boundary of the Ganga stem basin, particularly in Lakhisarai district, arsenic contamination levels have been reported as high as 240 μg/L. These areas are characterized by a Na–HCO₃-type hydrochemical facies within a two-tier sandy aquifer system. In the Punpun river basin, the groundwater is predominantly characterized by a Ca–Cl type hydrochemical facies, with arsenic contamination levels reported to be below 10 μg/L in Gaya district, which is considered safe according to WHO (1996) guidelines. The Karmnasha-Sone (southern part), Harohar-Kiul (southern part), and Chandan-Badua River basins have reported arsenic contamination either below the detection level or <1 μg/L. These basins are characterized by Na–HCO₃-type hydrochemical facies in Karmnasha-Sone and Harohar-Kiul, and HCO₃–Cl type hydrochemical facies in the Chandan-Badua basin.

Reports indicate that the hydrochemistry of arsenic-contaminated aquifers in most of Bihar's river basins has been shaped by processes such as silicate weathering, carbonate and dolomite dissolution, and ion exchange. The evolution and mobilization of arsenic in shallow aquifers in most of Bihar's river basins are primarily of geogenic origin, with the Fe-oxyhydroxide reduction process acting as the dominant mechanism. This process is mediated by microbial respiration activities that occur in reducing environments, where organic matter plays a key role. The release of arsenic in the groundwater systems of river basins is facilitated by various factors, including freshwater infiltration, prolonged flooding and waterlogging, the geographical characteristics of the river basins, seasonal fluctuations in water table depth, and the hydrogeochemical properties of the aquifers. These conditions collectively contribute to the creation of a reducing environment, which supports arsenic mobilization. However, some studies have also reported the dissolution of arsenic-bearing minerals and competitive ion exchange processes as mechanisms contributing to arsenic mobilization in the Ganga stem and Ghaghara River basins of Bihar.

In the last two decades, arsenic contamination in Bihar's river basins has shown an upward trend in spatial distribution, temporal prevalence, and concentration levels. Initially concentrated in the Ganga stem basin, contamination has now spread to other basins like Ghaghara, Gandak, Burhi Gandak, Kosi, Mahananda and parts of southern basins, including Punpun and Kiul. Arsenic mobilization in Bihar's major river basins is driven by both geogenic factors, such as reductive dissolution of iron oxyhydroxides, silicate weathering, and mineral dissolution, and anthropogenic factors like groundwater overextraction, agricultural runoff, and urban wastewater discharge. These processes alter the natural geochemical balance of aquifers, intensifying arsenic release into shallow and, potentially, deeper groundwater systems. To tackle this issue, an integrated investigation strategy is needed. This should include systematic monitoring of both shallow and deep aquifers, detailed studies of arsenic release mechanisms, and a comparison of affected and non-affected regions. A multidisciplinary approach involving advanced geospatial tools, geochemical analysis, and community engagement is vital to understand contamination trends and develop effective mitigation strategies. Such studies will provide a comprehensive understanding of arsenic contamination from various perspectives, aiding in the identification of alternative viable aquifers and the development of effective management plans. These efforts will contribute to improving the livelihood standards of communities across all major river basins of Bihar.

The first author (V.K.), working under the guidance of the second author (S.S.), is the author of the study described in this article as part of his Ph.D. work. V.K. expresses gratitude to The Council of Scientific and Industrial Research (CSIR) for providing funding for his Ph.D. study through a Senior Research Fellowship (SRF).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.