STABLE ISOTOPIC EVALUATION OF ARSENIC HOTSPOTS AND LOW AS AREAS IN MURSHIDABAD, WEST BENGAL, INDIA

The source and distribution of high levels of dissolved arsenic in the shallow aquifers of the Bengal Delta remains controversial. Microbial mediated oxidation of organic matter, possibly from the localized ponds, is understood to be a driving mechanism in the geochemical transformations by which As is released from the shallow anoxic sediments into the shallow groundwaters. The objective of the current study is to delineate the sources of groundwater recharge (in both high As and low As areas) by identifying regional trends in δ¹⁸O and δD compositions of the groundwater, surface waters, and precipitation. To understand the kinetics in this recharge process of shallow aquifers, a study has been conducted over 2 field seasons. Regionally in Murshidabad, high dissolved As concentrations are widespread, but local distribution is highly heterogeneous, varying in one area from 0 to >4600 ppb at the same depth within a lateral distance of <50 m, and in three other areas from 0 to ~700 ppb over a depth range of >20 m. This study incorporated four high As hot spot areas and subsequently compared the geochemical parameters to low As areas both in terms of sediment and water stable isotope chemistry. Dissolved As concentrations increase from west to east in the study region (crossing the river Bhagirathi) for both As_T and As(III) [~75% of As_T is As(III)], further indicating strongly reducing conditions. A total of 35 water samples were analyzed for stable oxygen and hydrogen isotopes. Chemically distinct groundwaters (Ca-Na-HCO₃-Cl) have DIC values that indicate CO₂ outgassing, a process which enriches DIC in these waters in heavier oxygen isotopes. Heavier δ¹⁸O values indicate an exchange with atmospheric CO₂, whereas the lighter values indicate carbonate and/or organic rich components in the matrices. On δ¹⁸O vs δD plots, the majority of the pond waters lie below both the GMWL and LMWL, whereas the major rivers of this region (Ganges and Bhagirathi) plot above the GMWL with δ¹⁸O values of -8.0‰ to 8.3‰. The surface pond waters and the groundwaters in high As areas show contrasting slopes when crossplots of the intercept and slope values are plotted. Hence, the groups are statistically distinct. The present data suggest that dry season recharge is a chief source of groundwater in the arsenic hot spots of Murshidabad, West Bengal.