MONSOONAL INFLUENCE ON STABLE ISOTOPE SIGNATURE AND ARSENIC DISTRIBUTION IN GROUNDWATERS: A CASE STUDY FROM WEST BENGAL, INDIA

In the Bengal delta, the groundwater concentrations of As have been found to exceed 10 µg L^-1. It has recently been suggested that extensive ground water withdrawal for agriculture in these areas may accelerate the release of As to groundwater. The area investigated is located in the Chakdaha block of Nadia District, West Bengal. Groundwater has been collected from 49 wells in and around the study area for (hydro)chemical and stable isotope (δ¹⁸O and δ²H) analysis during the pre-monsoon season (Apr-May, 2009) and the sampling was re-executed during the post-monsoon season (Oct-Nov, 2009). In the present study, 85% of pre-monsoon samples and 82% of post monsoon are found to have total As concentration higher than 10 µg L^-1. However, the mean concentration of total As increased (19%) after the monsoon from being 51.8 to 61.4 µg L^-1. The mean concentration of As(III) significantly increased (114%) passing from pre-monsoon to the post-monsoon season from being 30.9 to 66 µg L^-1. The groundwater samples have stable isotopic composition which falls sub-parallel to the GMWL. This probably indicates that evaporated surface water could be mixed with groundwater, the surface water sources being possibly surface ponds or re-infiltrating irrigation water. However, the δ¹⁸O profile during the pre-monsoon are heavier (with more positive values for) than the post-monsoon samples. This can be explained by a greater mixing of isotopically enhanced regional humid sources (e.g., from evaporation of surface water bodies) with the pre-monsoon groundwater in the dry season. Extensive groundwater withdrawal for agricultural purposes in the area may contribute to the drawdown of these evaporated surface waters. Precipitation has seemingly the main control on the distribution of isotopes in this region. The highest degree of evaporation as measured by the deviation from GMWL is shown by the dry season precipitation. This process could trigger As mobilization through reductive dissolution of Fe-oxyhydroxide minerals and wide-spread withdrawal of groundwater accelerates the rate of As release. The spatial variability of As concentrations in the top 20 m of shallow aquifers can to a large extent be attributed to groundwater age (determination underway) controlled by the (hydro)geological heterogeneity in the local groundwater flow system.