Geochemical Variability of Arsenic in the Rural Aquifers of West Bengal, India

High exposure of arsenic (As) from drinking water is well recognized in SE Asia, notably in the Bengal Delta Plain (BDP). BDP aquifers are locally separated as As rich / As low and As free zones. The extent of arsenic contamination is heterogeneous with considerable spatial variability. The present study deals with groundwater quality and chemodynamics in As affected areas of BDP. Groundwater pH is circum-neutral to alkaline (pH 6.4-8.1). The major anions are dominated by HCO3- followed by Cl- and PO43-, and relatively low concentrations of NO3- and SO42-. Groundwaters (5-70 m) are often enriched with DOC (1.2-7.0 mg/L), whereas deep aquifers (100-304 m) contain low amount of DOC (0.4-1.9 mg/L). Concentration of dissolved As spans over four order of magnitude (<3-1059 µg/L) with high concentration of redox species (Fe and Mn ). The presence of DOC in shallow aquifers also indicates that organic matter is relatively young and more reactive. On the other hand, alkaline earth elements Ca2+ (23-277 mg/L), Mg2+ (14-69 mg/L), Sr2+(78-1015 mg/L) and Ba2+ (31-529 mg/L) also vary considerably in the shallow and the deep groundwater and controlled by carbonate mineralogy of the aquifers. High pCO2 values, relatively high redox sensitive elements, low Eh and the absence of DO are the principal chemodynamic patterns of the BDP shallow aquifers. On the contrary, deeper aquifers (upper delta plain) are As bearing with low concentration of chloride where as deeper aquifers (lower delta plain) are saline with or without dissolved As. The deep aquifer of the both regions also contains high bicarbonate, that indicates carbonate mineral dissolution play important role in As mobilization at least in deeper aquifers. The study also suggests that secondary minerals (mica/clay) are also important in As mobilization.