UNDERSTANDING HETEROGENEITY IN SEDIMENT GEOCHEMISTRY IN CONTRASTING GROUNDWATER ARSENIC BEARING ENVIRONMENTS IN MURSHIDABAD, WEST BENGAL, INDIA

Groundwater and shallow aquifer sediments were collected from six locations in Murshidabad, West Bengal, India, to study their hydrochemical and geochemical properties and examine relationships between dissolved arsenic (As) concentrations and solid-phase As concentrations. Regionally As is widespread, but locally distribution is highly heterogeneous, varying in one area from 0 to >4600 ppb at the same depth within a lateral distance of <50m, and in three other areas from 0 to ~700 ppb over a depth range of >20m. Four areas with high dissolved [As] (>600 ppb) in Holocene floodplains east of river Bhagirathi and two low As (<10 ppb) areas in Pleistocene sediments west of river Bhagirathi were targeted in this study. Mineralogy of aquifer sediments in high and low As areas is similar, consisting mainly of quartz and weathered feldspars, with more carbonate and accessory mineral grains in sediments from high As areas. Sediments with high As contents are gray and contain considerable amounts of apatite and Mg-rich clays, along with Fe- and Mn-rich minerals, which are found in greater proportions than in orange-brown, low As sediments. Indicative of reducing conditions, solid-phase Fe(II):FeT ranges from 0.6-1 in a core from a high As area and inversely correlates with sediment PO43- contents, possibly resulting from reductive dissolution of Fe-oxyhydroxides. Dissolved As concentrations increase from west to east for AsT and As(III) [~75% of AsT is As(III)], further indicating strongly reducing conditions. Specific conductance of high As waters ranges from 724-868μS/cm and for low As waters from 447-682μS/cm. DO and ORP ranged from 2.3-4.7mg/l and 57-388mV for high As waters, respectively. High concentrations of Fe, PO43-, and NH4+ and low concentrations of Cl- and SO42- are characteristic of high As groundwater. Stable isotope values of O, H, and DIC have been measured for groundwaters to determine recharge mechanisms and source of organic matter that may be facilitating As release from these sediments. Sequential extractions are being performed on sediment cores from high and low As areas to determine fractionation of As in sediment, which will help further understand As release mechanisms.