Natural organic matter in sedimentary basins and its relation to arsenic in anoxic ground water: the example of West Bengal and its worldwide implications |
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| Affiliation: | 1. Department of Earth Sciences, UCL, Gower St, London WC1E 6BT, UK;2. Department of Geology, University of Delhi, Chattra Marg, Delhi 110007, India;3. School of Earth Science, Birkbeck, University of London, Malet St., London WC1E 7HX, UK;4. Arcadis Geraghty & Miller International Inc., 2 Craven Court, Newmarket CB8 7FA, UK;5. Robens Centre for Public and Environmental Health, University of Surrey, Guildford GU2 7XH, UK;6. Central Ground Water Authority, Bidhan Nagar, Salt Lake City, Kolkata 700091, India;7. Department of Geology, Royal Holloway University of London, Egham, Surrey TW20 0EX, UK;8. Central Ground Water Authority, 18/11 Jamnagar House, Mansingh Road, Delhi 110011, India;1. Department of Environmental & Earth System Science, Stanford University, Stanford, CA 94305, United States;2. Parsons Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139, United States;3. Department of Geosciences, Boise State University, Boise, ID 83725, United States;1. Department of Geology, Union College, Schenectady, NY, USA;2. Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA;3. Department of Civil and Environmental Engineering, Bangladesh University of Engineering and Technology, Dhaka, Bangladesh;1. Earth System Science Dept., Stanford University, Stanford, CA, USA;2. State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan, Hubei, PR China;3. Department of Geosciences, Boise State University, Boise, ID, USA;4. Civil and Environmental Engineering, Pennsylvania State University, University Park, PA, USA;1. Department of Geology and Mining, University of Rajshahi, Rajshahi 6205, Bangladesh;2. Institute of National Analytical Research and Service (INARS), Bangladesh Council of Scientific and Industrial Research (BCSIR), Dhanmondi, Dhaka 1205, Bangladesh;3. Institute of Nuclear Science and Technology, Bangladesh Atomic Energy Commission (BAEC), Savar, Dhaka, 1349, Bangladesh;4. Department of Soil Science, Patuakhali Science and Technology University, Patuakhali 8602, Bangladesh;1. Karlsruhe Institute of Technology (KIT), Institute of Applied Geosciences, Karlsruhe, Germany;2. Swiss Federal Institute of Aquatic Science and Technology, Dübendorf, Switzerland;3. Center for Applied Geosciences, Geomicrobiology, University of Tübingen, Germany;4. Center for Applied Geosciences, Hydrogeology, University of Tübingen, Germany;5. Karlsruhe Institute of Technology (KIT), Institute for Photon Science and Synchrotron Radiation (IPS), Eggenstein-Leopoldshafen, Germany;6. School of Earth Sciences, University of Bristol, Bristol, UK;7. KLATEFOS, Vietnam National University, VietNam;8. Technische Universität Berlin, Institute of Applied Geosciences, Berlin, Germany |
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| Abstract: | In order to investigate the mechanism of As release to anoxic ground water in alluvial aquifers, the authors sampled ground waters from 3 piezometer nests, 79 shallow (<45 m) wells, and 6 deep (>80 m) wells, in an area 750 m by 450 m, just north of Barasat, near Kolkata (Calcutta), in southern West Bengal. High concentrations of As (200–1180 μg L−1) are accompanied by high concentrations of Fe (3–13.7 mg L−1) and PO4 (1–6.5 mg L−1). Ground water that is rich in Mn (1–5.3 mg L−1) contains <50 μg L−1 of As. The composition of shallow ground water varies at the 100-m scale laterally and the metre-scale vertically, with vertical gradients in As concentration reaching 200 μg L−1 m−1. The As is supplied by reductive dissolution of FeOOH and release of the sorbed As to solution. The process is driven by natural organic matter in peaty strata both within the aquifer sands and in the overlying confining unit. In well waters, thermo-tolerant coliforms, a proxy for faecal contamination, are not present in high numbers (<10 cfu/100 ml in 85% of wells) showing that faecally-derived organic matter does not enter the aquifer, does not drive reduction of FeOOH, and so does not release As to ground water.Arsenic concentrations are high (≫50 μg L−1) where reduction of FeOOH is complete and its entire load of sorbed As is released to solution, at which point the aquifer sediments become grey in colour as FeOOH vanishes. Where reduction is incomplete, the sediments are brown in colour and resorption of As to residual FeOOH keeps As concentrations below 10 μg L−1 in the presence of dissolved Fe. Sorbed As released by reduction of Mn oxides does not increase As in ground water because the As resorbs to FeOOH. High concentrations of As are common in alluvial aquifers of the Bengal Basin arise because Himalayan erosion supplies immature sediments, with low surface-loadings of FeOOH on mineral grains, to a depositional environment that is rich in organic mater so that complete reduction of FeOOH is common. |
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