Mobility of arsenic in the sub-surface environment: An integrated hydrogeochemical study and sorption model of the sandy aquifer materials
Introduction
Occurrence of As in groundwater has long been detected in tube well and drinking water supplies in SE Asia (e.g., Thailand, Vietnam, Loa PDR, Cambodia and Myanmar), India, Bangladesh, China, Mongolia, Nepal and Pakistan (Smedley and Kinniburgh, 2002, Bhattacharyya et al., 2003a, Sun et al., 2004, Polya et al., 2005, Stanger et al., 2005, Kohnhorst, 2005, Nickson et al., 2005, Tetsuro et al., 2006, Nath et al., 2008a, Berg et al., 2008). It was estimated that more than 100 million people are at risk due to consumption of harmful toxic groundwater in these countries (e.g., Nordstrom, 2002, Sun et al., 2004, SOES, 2008). Currently, the situation in Bengal delta plain (BDP, West Bengal and Bangladesh) is a subject of concern, since a large portion of the inhabitants (>40 million), mostly in rural villages depend on As-enriched groundwater for drinking purpose (Nordstrom, 2002, SOES, 2008). This may results in the exposure to As, which is a significant cause of skin pigmentation, hyperkeratosis, carcinogenicity, cardiovascular disease and may affect mental development of children, among other possible adverse effects (e.g., Guha Majumder et al., 1988, Smith et al., 2000, Bhattacharyya et al., 2003b, Kapaj et al., 2006).
As-enriched groundwaters are typically observed in the young quaternary deltaic environments where groundwater is mostly under reducing condition. Numerous studies on groundwater As have been conducted to understand the release mechanism and the cause for its heterogeneous distribution. However, there are a lot of ambiguities concerning release and distribution of As in the sub-surface environment. The most common and widely accepted mechanism is attributed to Fe(III)-reduction model (Bhattacharya et al., 1997, Nickson et al., 1998, McArthur et al., 2001), validated by many others (e.g., BGS and DPHE, 2001, Bhattacharyya et al., 2003a, Nath et al., 2008a). McArthur et al. (2004) observed that high groundwater As (>0.66 μM) was associated with extensive reduction of Fe-oxyhydroxide (i.e., grey aquifer sediments); whereas low groundwater As (<0.66 μM) was characterized by incomplete reduction (i.e., brown aquifer sediments). The extensive groundwater pumping may have triggered Fe(III)-oxyhydroxide reduction, by inducing and enhancing groundwater flow, associated organics flow and thus release sorbed As (e.g., Harvey et al., 2002, Acharyya, 2002). However, Nath et al. (2008b) found no relation between high groundwater As and high groundwater pumping. Beside Fe-oxyhydroxides in the sediments, Fe-bearing minerals such as biotite, chlorite and carbonates were found to be the main carrier phases of As, and they may act as potential As pollutants in groundwater (Pal et al., 2002, Nath et al., 2008c). In particular siderite, carbonate green rust and calcite have been shown to be excellent As scavengers (Jönsson and Sherman, 2008, Román-Ross et al., 2006). Recently, Polizzotto et al., 2006, Polizzotto et al., 2008 suggested that As may be released at the oxic–anoxic boundaries, i.e., closed to the surface, and be drawn down from the near-surface through the aquifer to well-depths. Contrary, Sengupta et al. (2008) observed that surface recharge (through pond) does not pose any threat to As contamination. However, none of these studies convincingly addressed why neighbouring wells may have very contrasting aqueous geochemical behavior, as illustrated in Bangladesh by van Geen et al. (2003).
The purpose of this study is to characterize an aquifer close to the Hooghly river, the largest tributary of the Ganges in West Bengal, India, in terms of groundwater chemistry, redox peculiarities in aqueous phase, sediment geochemistry and sorption behaviour of the aquifer materials. The study site is located 65 km North of Kolkata, beside Hooghly river. The hydrochemical study was made to highlight the behaviour and distribution of redox species (e.g., As, Fe and Mn) in groundwater, while geochemical characterization of the sediments to evaluate the role of lithology (e.g., grain size) and mineral species (e.g., career phases: sources and sink) on the distribution of As in the sub-surface environments. The sorption model of the sandy aquifer material enhanced our understanding to decipher natural geochemical processes that have an important control on groundwater geochemistry.
Section snippets
Geology and physiographic setting
The study area (∼16 km2, Chakdaha Municipality, West Bengal, Fig. 1) is an integral part of the BDP, the world largest delta, by its size, density of population and low elevation leading to frequent flooding. The BDP has a characteristic geomorphic and geologic feature. Various geomorphological units were mapped (Morgan and McIntire, 1959, Umitsu, 1987, Brammer, 1996), which include piedmont plains, fluvial flood plains (e.g., the moribund delta), the Chandina plain (Bangladesh) and upland
Chemical characteristics of the groundwater
The results from preliminary survey reveals that the study area can be categorized into two major domains, high (mean: 1.8 μM) and low groundwater As areas (mean: 0.28 μM). In high groundwater As areas, the As concentrations generally varies 3–4 orders of magnitude and remain above National standard (0.66 μM or 0.05 mg L−1) and WHO (0.13 μM or 0.01 mg L−1) guidelines, the later being itself one order of magnitude lower than our “high”/“low” As content limit. The groundwaters have low Eh (range: −153 to
Conclusions
The occurrence of simultaneous high groundwater As and Fe concentrations and high pCO2 shows that As mobilization (through reductive dissolution of Fe-oxyhydroxides via organic matter decomposition) is active where Fe concentration is controlled by siderite, or possibly green rust (observed by Jönsson and Sherman, 2008). High groundwater As areas are found where Fe-oxyhydroxide and mica are rare, which suggests a release of As from these sorbent solid phases. The sorption experiment further
Acknowledgements
The authors gratefully acknowledge RGNDWM, Govt. of India, IFCPAR and CNRS (EC2CO Programme) for giving financial support. One of the authors (BN) is thankful to the Deutscher Akademischer Austausch Dienst (DAAD) for a fellowship and the Institute für Mineralogie und Geochemie, Universität Karlsruhe, Germany for laboratory support. Thanks for analytical assistance from Dr. U. Kramar for ED-XRF analysis and Dr. Zsolt Berner for his support during this research.
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