Riddle of arsenic in groundwater of Bengal Delta Plain—role of non-inland source and redox traps |
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| Authors: | D Chatterjee R K Roy B B Basu |
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| Institution: | (1) Department of Chemistry, Kalyani University, West Bengal, India;(2) Arsenic Research Cell, School of Fundamental Research, Kolkata, 700026, India;(3) Former of Geological Survey of India, Government of India, , India;(4) Arsenic Technology Park, Baruipur, India;(5) School of Fundamental Research, 29, Pratapaditya Road, Kolkata, 700 026, India |
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| Abstract: | In the Bengal Delta Plain (BDP) the primary arsenic sourcing appears to be different from the global scenario. Here, the Terminal Pleistocene–Holocene depositional platform, the interactive early Holocene depositional morphology with fluvio-estuarine and marine incursions played a crucial role for arsenic sourcing and enrichment. The lenticular silt-fine sand layer between anoxic clay beds favoured entrapment of dissolved organic carbon with decayed phyto-planktons debris. The Terminal Pleistocene–Holocene transgression and regression processes may have acted as major events in the BDP. Interestingly, at the end of the last glacial maxima, the Pleistocene delta had undergone block movements, wherein some parts of the platform were raised above the level of Holocene deposition. Those blocks were found to be free from arsenic in the groundwater. The sea, during re-emerging inundation (10–7 ka BP), has witnessed a monsoon-induced environment in the BDP with the resultant oscillation of sea level leading to higher upsurge towards the north. This might have resulted in the marine incursion and inundation in pre-existing land depressions. Meanwhile arsenic entrapments through marine incursion as well as enrichment in the presence of organic carbon/DOC and/or Fe/Mn/Al catalytic agents could have developed into localised redox traps. It may be of relevance that due to the repetitive transgressive–regressive phases in Holocene, resulting in periodic exposure and weathering of iron-bearing minerals and consequent iron enrichment in the aquifer system. The iron, thus present, had free charge to host arsenic as a sink. It appears that arsenic, wherever found, would likely be of atypical localised exhaustible phenomenon, both in horizontal and vertical context. It also rationalises the cause of the absence of arsenic in the other nearby Pleistocene platform, which has not come across Holocene interaction and marine incursion, as to the likely limiting condition for the search for arsenic in the BDP or beyond. |
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| Keywords: | Arsenic Bengal Delta Plain (BDP) Terminal Pleistocene– Holocene Marine incursion DOC |
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