GROUNDWATER ARSENIC IN SHALLOW AQUIFERS OF BENGAL DELTA PLAIN (WEST BENGAL): A MICROBIOLOGICAL APPROACH

The natural occurrence of groundwater As in shallow aquifers of the Bengal Delta Plain causes widespread human health problems in the region, including highly elevated rates of cancer and arsenicosis. The major issue is high concentrations of geogenic As in shallow drinking water aquifers. Arsenic concentrations in the aquifers are spatially heterogeneous and also vary throughout the year. We conducted a study in Chakdaha block, Nadia district, West Bengal, India, which has been identified as an arsenic “hot-spot”. A time scale monitoring program of several shallow tube wells was conducted to study variations in As concentration and other groundwater chemistry parameters over a time period of one year. Our results show that wells located closest (2-3 m) to impounded water systems (ponds) show the greatest variation (80-326 µg/L) in shallow groundwater As concentrations over the monitored time. Conservative ions including Cl exhibited a similar pattern of variation, as did HCO₃ and DOC, suggesting a signature of high local reducing condition. The pond water As concentrations increase following the monsoons and decrease in the dry season, along with HCO₃, DOC and Fe. We are currently conducting microcosm studies to characterize seasonal and spatial variation in the composition of As cycling microorganisms in the ponds compared the aquifers, as well as studies to characterize dissolve organic matter in pond and aquifer water. Our preliminary results suggest that dissolved, labile organic matter derived from the waste water ponds might stimulate geomicrobiological As cycling and reductive As mobilization in the aquifers, thereby controlling the release of As from aquifer sediments to groundwater. Localized and spatially variable reducing conditions within the aquifer may result from microbiological reduction of As- and Fe-oxyanions in response to the availability of labile surface-derived organic matter and nutrients in recharge water from the ponds.