Arsenic in groundwater of the Bengal Delta Plain:: geochemical evidences for small scale redox zonation in the aquifer

The study focuses on characterisation of the hydrogeochemical environment of high As concentrations in groundwater and on the availability and mineralogical speciation of As in the aquifer sediments of an area of about 35 km(2) (Malda District, West Bengal, India), where hot-spots with As enriched groundwater occur in close vicinity of low As zones. Evidence has been found for the occurrence of distinct redox zones with high As concentrations, in which conditions for selective reduction of Mn, Fe or SO4 Occur. This implies that under specific local conditions Mn- and Fe-oxihydroxides represent important As-bearing phases in the aquifer. The comparison of the results of two field campaigns carried out in 2002 and 12 month later indicates a shift towards slightly stronger reducing conditions, along with local increases of arsenic concentrations in groundwater. High As concentrations in the shallow groundwater apparently are connected to abandoned river channels. In such oxbow channels fine grained sediments rich in C-org are typically deposited, which may accelerate the development of a reducing environment due to the microbial mineralization of organic matter. The percolation of sewage into the aquifer as an additional anthropogenic source of nutrients may be suggested, but its importance for the development of reducing conditions in the groundwater is hard to estimate, considering the regional scale of the As calamity. Statistical evaluation of mu-XRFA data suggests Fe bearing silicates (such as chlorite and biotite) as the main As carrier mineral phases in the aquifer sediments. Nevertheless, the redox-sensitive mobilization of As from disperse distributed Fe- and Mn-oxihydroxides is likely to be considered as the main driver for the enrichment of As in groundwater.