Do Ponds Cause Arsenic-Pollution of Groundwater in the Bengal Basin? An Answer from West Bengal S. SENGUPTA, † J. M. MCARTHUR,* ,‡ A. SARKAR, † M. J. LENG, § P. RAVENSCROFT, | ,# R. J. HOWARTH, ‡ AND D. M. BANERJEE ⊥ Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur, West Bengal 721 302, India, Department of Earth Sciences, University College London, Gower Street, London WC1E 6BT, U.K., NERC Isotope Geoscience Laboratory, British Geological Survey, Keyworth, Nottingham NG12 5GG, U.K. and School of Geography, University of Nottingham, NG7 2RD, U.K., Department of Geography, University of Cambridge, Downing Place, Cambridge CB2 3EN, U.K., and Department of Geology, University of Delhi, Delhi 110 007, India Received November 30, 2007. Revised manuscript received April 1, 2008. Accepted April 7, 2008. We report time-series data collected over two years for δ 18 O, δ 2 H, and Ca, Mg, K, and Cl, concentrations for 10 ponds in, and upflow of, an As-polluted region of southern West Bengal. We compare the compositions of As-polluted groundwaters from wells with the compositions of waters in ponds upflow, and within the range of influence, of the wells. Conservative tracers ( δ 18 O, δ 2 H, K), and other tracers (Ca, Mg) that are likely conservative in the waters, show that pondwater and groundwater are distinct and do not overlap in composition. These data show that water from ponds cannot be identified in As-polluted groundwater, so putative DOC in pondwater cannot be mixing into the As-polluted groundwater we have sampled. Separate estimates of the degree of recharge from ponds to groundwater, using calculations based on temporal variations in salt content and isotopic composition in ponds, and salt- balance, show that insignificant amounts of As-polluted groundwater are derived via pond recharge. It follows that pondwater in the study area does not contribute significant mass to arsenic-polluted groundwater and so does not provide organic matter to aquifers in amounts sufficient to drive reduction of iron oxyhydroxides and hence arsenic pollution. Introduction Serious pollution of groundwater in alluvial aquifers by dissolved arsenic occurs worldwide and is a cause for global concern. In the Bengal Basin, many millions of people have been exposed to water containing >10 μg/L As (the WHO guideline value) drawn from alluvial aquifers (1–5). In most cases of As-pollution of groundwater worldwide, the arsenic derives from microbial reduction of iron oxyhydroxides (FeOOH) which releases to groundwater the As sorbed to the dissolving host oxide (6–8, et seq.), although in a few localities other mechanisms might operate; e.g., in Argentina (9) and the southwest U.S. (10, 11). Reduction of iron oxyhydroxides is driven by microbial oxidation of carbon in organic matter (12, 13) and such microbial reduction is evident in aquifers of the Bengal Basin, both because they contain dissolved Fe(II), and because microbial activity has been observed in situ in the aquifer (14) and in laboratory culture using sediments from the aquifer (15–17). Identifying the source(s) of the organic matter (OM) that drives FeOOH reduction in alluvial aquifers is an important goal because the availability of OM is one of the factors that governs the degree and duration of the pollution process. Mobilization of arsenic into groundwater will continue as long as arsenic-bearing FeOOH is available for reduction, and organic matter is available to drive reduction. If the source of OM is anthropogenic, e.g., from ponds, unsewered sanitation, or surface soils, concentrations of OM will never decline, so any decline in pollution will depend entirely on exhausting the sedimentary store of arsenic in FeOOH. If the source of OM is in subsurface sediments, concentrations of OM will decline over time, so driving downward the degree of pollution (but at what rate is unclear). It follows that any link that can be established, or broken, between arsenic pollution and any source of OM will be valuable to aquifer development and remediation. Here we test the hypothesis that pond-derived OM drives subsurface redox reactions, and so arsenic pollution (14, 18). In doing so, we deliberately have not measured dissolved organic carbon (DOC) in pondwater or groundwater because DOC is not conservative in solution. For example, water leaking from ponds to underlying aquifers might either derive DOC from organic-rich bottom muds, or lose it through bacterial oxidation during passage through such muds. In addition, DOC may be sorbed to sediment particles during flow through the aquifer, or new DOC may be added by diffusion from aquitards, intercalated organic-rich lenses of sediment with the aquifer, or the aquifer itself. Finally, DOC may be present but not labile, so measurements of DOC concentration may mislead. To overcome such difficulty, we trace the degree to which pondwater mixes with groundwater using conservative tracers. Our approach is based on the fact that a putative OM-rich water that derives from a pond, and evolves into As-polluted groundwater, must carry with it the conservative tracers present in the original pondwater. A groundwater from which pondwater tracers are absent will not contain a component of pondwater and so will not contain a com- ponent of pond-derived DOC. The conservative tracers of pondwater we use are those in the water molecule itself (δ 18 O, δ 2 H), a conservative dissolved tracer (K), and two other tracers that are seemingly conservative in our waters (Ca and Mg). With these tracers, we examine whether As-polluted ground- water derives partly or wholly from ponds, and so test the role of pond-derived OM in driving reduction of FeOOH and arsenic pollution. We confirm our findings using mass- balances that provide estimates of leakage from ponds to underlying aquifers. The Field Area. The study region comprises the three contiguous villages of Joypur, Ardivok and Moyna, 40 km NE of Kolkata (JAM hereinafter; Figure 1 and Supporting Information Figure S1). The area, and the distribution of arsenic within it, have been described elsewhere (19), so only a brief description is given here, and only of the arsenic- * Corresponding author e-mail: j.mcarthur@ucl.ac.uk. † Indian Institute of Technology. ‡ University College London. § British Geological Survey. | University of Cambridge. # Current address: Entec UK Ltd, Trinity House, Cambridge Business Park, Cowley Road, Cambridge, CB4 0WZ. ⊥ University of Delhi. Environ. Sci. Technol. 2008, 42, 5156–5164 5156 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 14, 2008 10.1021/es702988m CCC: $40.75  2008 American Chemical Society Published on Web 06/07/2008