ARSENIC IN GROUNDWATER OF YOUNG BENGAL DELTA SEDIMENT: ITS DISTRIBUTION AND GEOCHEMISTRY IN SHALLOW AQUIFER

Groundwater from shallow aquifers (<50 m) of Holocene deltaic sediments of West Bengal and Bangladesh are often enriched with elevated level of arsenic (As). The occurrence of this pollution is highly heterogeneous both spatially and vertically. It is urgently necessary to find out the reasons behind such type of heterogeneity, to formulate suitable mitigation strategies. A regular piezometric monitoring study in Sahispur village, Chakdaha block, Nadia, West Bengal was conducted to examine groundwater hydrochemistry and prevailing redox processes responsible for this high As groundwater at different depth of shallow aquifer. The concentration of major aqueous solutes is highest in the wells with lowest screening depth (well A and B) and decreases with increasing depth. Both As and Fe mostly exist in lower oxidation states (III and II, respectively) indicating persistent reducing environment within the aquifer. The calculation of SI for different mineral phases reveals that precipitation of Fe(III) phases is thermodynamically favorable in groundwater of all monitoring wells, whereas concentration of Fe(II) and Mn(II) in groundwater is limited by co-precipitation with CaCO₃. The release of As in the near-surface aquifer (well A and B) is primarily not due to redox processes. In well A carbonate mineral dissolution and related desorption of As from the surface of metal hydroxide is mainly responsible for the observed variation in As concentration. In well B competitive exchange of anions between PO₄^3- and AsO₄^3- may be appropriate to explain mobilization of As. In contrast, reductive dissolution of Fe and Mn oxyhydroxide is primary processes for mobilization of As in deeper screened piezometers (well C, D and E). The positive relationship of U and V with As, Fe and Mn suggest this redox processes in the deeper screened monitoring wells is possibly microbially controlled.