Hydrogeochemical contrast between brown and grey sand aquifers in shallow depth of Bengal Basin: Consequences for sustainable drinking water supply

摘要

Delineation of safe aquifer(s) that can be targeted by cheap drilling technology for tubewell (TW) installation becomes highly imperative to ensure access to safe and sustainable drinking water sources for the arsenic (As) affected population in Bengal Basin. This study investigates the potentiality of brown sand aquifers (BSA) as a safe drinking water source by characterizing its hydrogeochemical contrast to grey sand aquifers (GSA) within shallow depth (<70m) over an area of 100 km~2 in Chakdaha Block of Nadia district, West Bengal, India. The results indicate that despite close similarity in major ion composition, the redox condition is markedly different in groundwater of the two studied aquifers. The redox condition in the BSA is delineated to be Mn oxy-hydroxide reducing, not sufficiently lowered for As mobilization into groundwater. In contrast, the enrichments of NH_4~+, PO_4~3, Fe and As along with lower Eh in groundwater of GSA reflect reductive dissolution of Fe oxy-hydroxide coupled to microbially mediated oxidation of organic matter as the prevailing redox process causing As mobilization into groundwater of this aquifer type. In some portions of GSA the redox status even has reached to the stage of SC_4~(2-) reduction, which to some extent might sequester dissolved As from groundwater by co-precipitation with authigenic pyrite. Despite having low concentration of As in groundwater of the BSA the concentration of Mn often exceeds the drinking water guidelines, which warrants rigorous assessment of attendant health risk for Mn prior to considering mass scale exploitation of the BSA for possible sustainable drinking water supply.%KTH-lntemational Groundwater Arsenic Research Group, Department of Land and Water Resources Engineering, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden,Department of Chemistry, University of Kalyani, Kalyani, 741235, West Bengal, India;School of Geosciences, The University of Sydney, Sydney, NSW 2006. Australia;KTH-lntemational Groundwater Arsenic Research Group, Department of Land and Water Resources Engineering, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden;KTH-lntemational Groundwater Arsenic Research Group, Department of Land and Water Resources Engineering, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden,Department of Chemistry, University of Kalyani, Kalyani, 741235, West Bengal, India;Department of Chemistry, University of Kalyani, Kalyani, 741235, West Bengal, India;Department of Chemistry, University of Kalyani, Kalyani, 741235, West Bengal, India;Department of Geology and Geophysics, Indian Institute of Technology-Kharagpur, Kharagpur, 721302, West Bengal, India;Department of Chemistry, University of Kalyani, Kalyani, 741235, West Bengal, India;Department of Geological Sciences, Stockholm University, SE-106 91 Stockholm, Sweden;KTH-lntemational Groundwater Arsenic Research Group, Department of Land and Water Resources Engineering, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden;