Elsevier

Applied Geochemistry

Volume 26, Issue 4, April 2011, Pages 432-443
Applied Geochemistry

Characterization of recharge processes in shallow and deeper aquifers using isotopic signatures and geochemical behavior of groundwater in an arsenic-enriched part of the Ganga Plain

https://doi.org/10.1016/j.apgeochem.2011.01.003Get rights and content

Abstract

Arsenic concentrations in groundwater extracted from shallow aquifers in some areas of the Ganga Plain in the states of Bihar and Uttar Pradesh, exceed 50 μg L−1 and locally reach levels in the 400 μg L−1 range. The study covered 535 km2 of active flood plain of the River Ganga, in Bihar where a two-tier aquifer system has been delineated in a multi-cyclic sequence of Quaternary sand, clay, sandy clay and silty clay all ⩽∼250 m below ground surface. The research used isotopic signatures (δ 18O, δ 2Η, 3H, 14C) and major chemical constituents HCO3-,SO42-,NO3-,Cl-,Ca2+,Mg2+,Na+,K+,Astotal of groundwater to understand the recharge processes and groundwater circulation in the aquifers. Values of δ 18O and δ 2Η combined with 3H data indicate that the recharge to the As-enriched top 40 m of the deposits is modern (<50 a), predominantly meteoric, with some evaporation during infiltration, and partly from tanks and other surface water bodies. The lower part of the upper aquifer is vulnerable to mobilization of As with increasing groundwater extraction. The low As lower aquifer (max. 5 μg L−1) is hydrologically isolated from the upper aquifer and is characterized by lower 14C concentration and lower (more negative) δ 18O values. Groundwater in the lower aquifer is ∼3 ka old, occurs under semi-confined to confined conditions, with hydrostatic head at 1.10 m above the head of the upper aquifer during the pre-monsoon. The recharge areas of the lower aquifer lies in Pleistocene deposits in basin margin areas with the exposed Vindhyan System, at about 55 km south of the area.

Research highlights

► Sub-regional scale aquifers delineated in arsenic-enriched belt in the Ganga Plain. Isotopic fingerprint of the groundwater, from arsenic-enriched and arsenic-safe aquifers established for the first time in the Ganga Plain. ► Recharge processes and the water provenances of vertically separated Quaternary aquifers have been established. ► Mean residence time of groundwater in the deeper aquifers has been worked out using C-14 isotope. ► Water from the deeper aquifer has been correlated with the paleoclimatic model of the Middle Ganga Plain (Mid-Ganga Basin) for 6-2 ka.

Introduction

Groundwater with elevated As affects large tracts of the Bengal basin and is a serious public health hazard (Mandal et al., 1996, Bhattacharya et al., 1997, Nickson et al., 1998). Over 70 million people are exposed to As poisoning in Bangladesh and West Bengal, India, as they depend on As-enriched (>50 μg L−1) shallow aquifers for drinking water (Burgess et al., 2010). Groundwater in the Ganga Plain, in the upstream of the Garo-Rajmahal Gap, was considered to be relatively As-free, until As-enrichment was reported from a few villages on the bank of the River Ganga in the Bhojpur district of Bihar (Chakraborty et al., 2003). Subsequent analyses of spot drinking water sources has revealed As concentrations exceeding 50 μg L−1 in large tracts of the Middle Ganga Plain in Bihar and Eastern Uttar Pradesh (Acharya, 2005, CGWB and PHED, 2005). As reported from Bengal Delta Plain (BDP), As concentrations in the Middle Ganga Plain (MGP) exhibit a wide spatial variation resulting in a heterogeneous distribution of enriched wells. The hot spots are distributed in 15 districts of Bihar, on both banks of the Ganga where ∼10 million people reside in the risk zone (Saha, 2009). The enriched wells are confined to the younger alluvial deposits affecting the shallow aquifers (<50 m below ground), which are considered as the lifeline for drinking supply tapped by low-cost hand pumps (20–45 m) and open dug wells (8–12 m) (Acharya, 2005, Saha et al., 2009b). In the Sone-Ganga interfluve of the MGP, a sharp decline in As concentration 40 m below ground level (bgl) has been reported (Saha et al., 2009b). The aquifers at depth (>80 m bgl), tapped mostly by heavy duty deep tube wells for irrigation and urban water supply, have been found to contain relatively low As concentrations (max. 5 μg L−1).

The As-enriched part of the MGP in Bihar is underlain by a thick (>300 m), unlithified, multi-cyclic, fining-upward sequence of finer clastics e.g., sand, silt and clay with occasional gravel beds, of Quaternary age (Saha et al., 2007). However, in the downstream segment, south of the Ganga, between Munger Ridge and Sahebganj Plateau, the thicknesses of the Quaternary deposits are confined within 150 m.

The research on As-enrichment in the Ganga Plain carried out so far, has focused on regional hydrostratigraphy, spatial distribution of As and the geochemistry of enriched groundwater (Acharya, 2005, Saha et al., 2009b, Shah, 2007). The hydraulic parameters of As-free aquifers at depth have been discussed by Saha et al. (2009). Systematic study establishing the hydraulic regime and recharge mechanisms for aquifer systems in the contaminated area are yet to be carried out.

Determining stable isotope ratios (δ 18O and δ 2Η and radioactive isotope (3H and 14C) compositions from different aquifer systems help in understanding the sub surface hydrology in a better way (Soulsby et al., 2000, Mukherjee et al., 2006). In BDP various stable and radioactive isotopic compositions in groundwater have been studied to understand its geochemical behavior and recharge processes, particularly with reference to As-enrichment (Shivanna et al., 1999, Agrawal et al., 2000, Basu et al., 2002, Harvey et al., 2002, Klump et al., 2006, Sikdar and Sahu, 2009, Sengupta et al., 2008). No such study has been carried out in the As-enriched parts of the Ganga Plain.

This paper uses stable isotopic and geochemical signatures of groundwater from different aquifer systems, delineated on the basis of litholog analyses. The area is severely As-enriched, located in the western part of Bihar and underlain by multi-layered fluvio-lacustrine deposits of Quaternary age. The objective of the research includes understanding the groundwater regimes in different aquifer systems at depth <300 m through analyses of isotopic compositions (δ18O, δ2H, 3H and 14C) and hydrochemistry as well as differences in water levels. Attempts have also been made to delineate sub-regional extensions of As-enriched and As-free aquifers, using litholog analyses, and to shed light on the recharge processes affecting them, to gain a better understanding on possible downward spread of As-enrichment.

Section snippets

Study area setting

The study area (535 km2) is located in Bhojpur district, 70 km west of Patna, the capital of Bihar (Fig. 1). The River Ganga demarcates the northern boundary, while the Sone River, largest of the right hand tributaries of the Ganga, joins at 15 km east of the area. The younger alluvial belt along the Ganga in Bhojpur district has been identified as one of the most severely As-affected parts of Bihar, both in terms of concentrations and population residing in the risk zone (Saha et al., 2009b).

Materials and methods

The sedimentological characteristics of the Quaternary deposits of the area were studied from lithologs of four boreholes drilled during the study (Fig. 1 Inset C) at Bharauli (depth 250 m), Bariswan (depth 236 m), Paharpur (depth 286 m) and Sinhagram (depth 102 m) villages. Sub-meter scale lithologs were prepared based on visual analyses of drill-cut samples. The different litho-units classified on the basis of their permeability inferred from the sedimentological characteristics have been grouped

Hydrogeological framework

A seismic refraction study in the MGP has revealed a northerly thickening wedge of Quaternary sediments, overlying the Precambrian bedrock (Bose et al., 1976). The bedrock is an extension of the Peninsular craton, forming highlands in the south (Fig. 1). The thickness of the unlithified sediments on the banks of the Ganga is ∼700 m (Saha et al., 2007).

Exploratory drilling up to 300 m bgl by CGWB has indicated fluvio-lacustrine deposits with no trace of marine incursion (Saha, 1999). The thickness

Geochemical quality of groundwater

Out of the 13 samples collected from hand pumps, As concentrations exceeding 50 μg L−1 have been detected in 12. The hand pumps were selected from the available list of enriched wells (CGWB and PHED, 2005). Mean As concentration is 134 μg L−1 (range 43–376 μg L−1), indicating significant enrichment. Hand pump samples are mildly acidic (mean pH. 6.86), and fresh (Carrol, 1962), as the EC remains <850 μs/cm (Table 2). The anionic chemistry is overwhelmingly dominated by HCO3-, contributing 82% of the

Isotopic composition in ground water

Based on the aquifer configurations and groundwater chemistry, the samples are divided into three groups for isotopic investigations; Group-1: represents the dug well zone. Four dug well samples (nos. 2, 3, 4 and 5) and the shallow pieziometer at Bariswan (no. 6) comprise this group. Group-2: all hand pump samples, representing the upper slice and samples 19 and 20, from the lower slice of the upper aquifer, are included in this group. Group-3: deep groundwater samples (nos. 21, 22, 23 and 24),

Discussion

This research covers the As-enriched newer alluvial belt in the Sone-Ganga interfluve in MGP. A two-tier aquifer system exists within ∼300 m bgl. The top ∼25 m of the upper aquifer is made up of sandy clay and silty clay, with fine sand intercalations. Similar 3H and δ 18O signatures between dug well and hand pump samples suggest that this zone does not behave as an aquitard but rather forms a part of the upper aquifer. The top 40 m of the upper aquifer is As-enriched (up to 376 μg L−1). In spite of

Conclusions

The top 35 m slice of the upper aquifer which is As-enriched (>50 μg L−1), receives significant component of modern recharge from rainfall infiltration, beside percolation from tanks and other surface water bodies. The As-free lower part of the upper aquifer remains vulnerable to the spread of As-enrichment if shallow groundwater extraction increases significantly.

Environmental isotopic signatures (δ 18O, δ 2Η, 14C and 3H) reveal limited connectivity between the upper and the lower aquifer

Acknowledgements

The authors are thankful to B.M. Jha, Dr S.C. Dhiman and S. Kunar of the Central Ground Water Board and Dr K. Sivanna and Dr G. Singh from Bhaba Atomic Research Centre, Mumbai for their help and encouragement. Special thanks are extended to Dr P.C. Chandra for useful suggestions. The authors also express their sincere thanks to the hydrogeologists and chemists from CGWB and BARC for their assistance. The advice and initial review of the manuscript by A. Sarkar of IIT, Kharagpur is acknowledged.

References (40)

  • Bose, R.N., Bose, P.K., Mukherjee, B.B., 1976. A review of the seismic refraction and magnetic curves in the Gangetic...
  • W.G. Burgess et al.

    Vulnerability of deep groundwater in the Bengal aquifer System to contamination by arsenic

    Nature Geosci.

    (2010)
  • Carrol, D., 1962. Rain water as a chemical agent of geologic processes – a review. US Geol. Surv. Water Supply Paper,...
  • CGWB, GWD, 2007. Dynamic Groundwater Resources of Bihar State as on 31st March 2004. Central Groundwater Board,...
  • CGWB, PHED, 2005. A Report on Status of Arsenic Contamination in Groundwater in the State of Bihar and Action Plan to...
  • D. Chakraborty et al.

    Arsenic groundwater contamination in Middle Ganga Plain, Bihar, India: a future danger?

    Environ. Health Persp.

    (2003)
  • I.D. Clark et al.

    Environmental Isotopes in Hydrogeology

    (1997)
  • H. Craig

    Isotopic variations in meteoric water

    Science

    (1961)
  • S.K. Gupta et al.

    Groundwater isotopic investigations in India, what has been learnt?

    Current Sci.

    (2005)
  • C.F. Harvey et al.

    Arsenic mobility and groundwater extraction in Bangladesh

    Science

    (2002)
  • Cited by (39)

    • Floodplains landforms, clay deposition and irrigation return flow govern arsenic occurrence, prevalence and mobilization: A geochemical and isotopic study of the mid-Gangetic floodplains

      2021, Environmental Research
      Citation Excerpt :

      Therefore, from the above geochemical data, it can be concluded that the dissolution/weathering of carbonate rock acts as a major contributor for Ca2+, Mg2+, and HCO3−, however, alumino-silicates minerals are the major contributor for Na, K, and SiO2 in the Bhojpur district. Similar observations have been made by other researchers regarding the geochemistry in the middle Ganga plain (Saha et al., 2011). The trace metals concentration data are presented in Table 1.

    • Isotope and hydrochemical systematics of groundwater from a multi-tiered aquifer in the central parts of Indo-Gangetic Plains, India – Implications for groundwater sustainability and security

      2021, Science of the Total Environment
      Citation Excerpt :

      After publication of reports on arsenic contamination of shallow aquifers impacting ~10 million people (Chakraborty et al., 2003; Saha et al., 2009), there was a sudden rise in the number of wells tapping deeper aquifers. Vertical variations were also studied and found that high arsenic (370 μg/L) is mainly confined to shallow zones (<18 m below ground level, bgl) followed by lower values, typically 100 μg/L in deep zones (~50 m bgl) and further down to <5 μg/L below 100 m bgl (Saha et al., 2011; Laura et al., 2020). Even with a large volume of literature available on groundwater in Indo-Gangetic Plains (Saha and Ranjan, 2019 and references therein; Dwivedi et al., 2014; Sahu et al., 2015; Lapworth et al., 2018, 2020), further clarity is required regarding various aspects, particularly on groundwater recharge zones, linkage between surface water and groundwater, and groundwater flow dynamics in this multi-tiered aquifer system for efficiently implementing groundwater extraction plans or aquifer recharge activities.

    • Hydrogeochemical evolution and groundwater recharge processes in arsenic enriched area in central Gangetic plain, India

      2021, Applied Geochemistry
      Citation Excerpt :

      It can thus be hypothesized that the distribution of arsenic depends on geomorphological features of the rivers and depositional pattern of sediments in the study area. Other researchers also reported the higher concentration of arsenic in flood plain of the river Ganga and Ghaghra in the central Gangetic plains, India (Chauhan et al., 2009; Saha et al., 2011). It can be observed from Fig. 6 that the depositional environment and geological age are important factors in controlling As mobilization.

    View all citing articles on Scopus
    View full text