GEOCHEMISTRY OF ARSENIC HOTSPOTS IN MURSHIDABAD AND SURROUNDING AREAS IN WEST BENGAL, INDIA

Groundwater and shallow aquifer sediments (2m-40m) have been collected (June 2009) from four areas (thanas) of the Murshidabad district in West Bengal, India, to study their hydrogeochemical and mineralogical properties in order to correlate groundwater arsenic concentrations and solid-phase arsenic concentrations. Samples collected from the older river terraces west of the Bhagirathi River show low levels of arsenic (<1 ppb), with dissolved oxygen and conductivity ranging from 8.40 - 8.74 (mg/L) and 454 - 1153 (μS/cm), respectively, suggesting no direct correlation to arsenic. In the four areas sampled east of the Bhagirathi in the younger terraces, analysis shows arsenic in a range of 2 ppb - 4035 ppb. Most of the arsenic in water is As(III), and Fe and As do not necessarily correlate at each depth. Dissolved oxygen [4.64 - 9.02 (mg/L)], conductivity [608 - 1457 (μS/cm)] and pH [6.98 - 7.45] for these areas show no correlation with arsenic concentrations. Here we present preliminary results on the geochemistry and mineralogy of water and sediment core samples from contrasting (age, geographical setting) areas in West Bengal. Anion and cation analysis, along with total Fe and As analysis, are being done. The major minerals that dominate aquifer sediments are phyllosilicates, apatite, prismatic Fe-Mg rich minerals, magnetite, and phosphates such as vivianite, along with siderite, apart from the expected quartz and feldspars. Particle size analysis and characterization, Scanning Electron Microscopy, X-ray Diffraction, and sequential extractions studies are being done to better understand the association of arsenic in the sediments and the leaching mechanism(s) of arsenic from these reworked sediment grains under changing redox conditions to account for observed elevated and varying concentrations of arsenic.