Sediment Geochemistry and Mineralogy of Arsenic Affected Areas of Northern SonarBangla Aquifer, Murshidabad, West Bengal, India

Several hypotheses have been proposed to explain the source of As in severely As-affected Bengal Basin and the mechanisms by which As is mobilized from the sediments into the ambient groundwater. The primary source of As is thought to be geogenic, possibly originating in As-rich lithologies in the headwater regions of the Ganges river system, as the river Ganges is the primary source of sediments to West Bengal. The river Ganges enters the Bengal Basin from the cratonic area to create its flood plains through the entire district of Murshidabad. Both surface and cored (7 cored sites:2m-20m)sediment samples (from 46 locations) from the western banks of the river Ganges and along the transect of the main distributary of Bhagirathi-Hooghly river have been mineralogically analysed mainly exploring the possibility of clay minerals concentrating Arsenic on their surfaces. Heavy mineral separation was also done on the river bank samples to identify any possible mineral phase that concentrates Arsenic in its solid state. The major minerals that dominate both the aquifer and river bed sediments are several phyllosilicates, apatite, prismatic Fe-Mg rich minerals (possibly hornblende or hypersthene), and magnetite, apart from expected quartz and orthoclase. Leaching (1.2N HCl and phosphate) of the separated minerals from the river bed sediments revealed an average As concentration range of 15-25 ppm, mostly on the phyllosilicates (median [As]:18ppm), apatite (median [As]: 25 ppm) and magnetite (median [As]: 15 ppm). Heavy mineral concentrations are higher in river bank and river bed sediments as compared to deeper aquifer sediments. Dissolved As concentrations of the pore water samples from our sediment sampling locations ranges from <1ppb to >180 ppb (median value of 46 ppb). The shallow groundwater is mostly CaHCO3 water with near neutral pH , is high in Fe [chiefly Fe(II)] and the Eh in the range of Fe reduction.