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

In eastern India along the Ganges-Brahmaputra-Meghna (GBM) delta, arsenic concentrations greater than the WHO and U.S. EPA recommended maximum contaminant level (10μg/L) are commonly observed in groundwaters that supply millions of inhabitants in rural areas of West Bengal, India. The purpose of this research is to test whether river sediments can be regional hotspots that concentrate certain minerals which can associate with arsenic and later release that arsenic into the redox-sensitive aquifer sediments after their burial, resulting in arsenic mobility through the reduced aquifers. Once in the aquifer, arsenic can mobilize along current and paleo river systems, contaminating shallow tubewells tapped in to the aquifer.

Recent work done in West Bengal shows elevated As (>0.05mg/L) in tubewell water (35 samples from five locations) observed within a depth range of 10-30m in the sediments of the Ganges-Bhagirathi river system of the Murshidabad district. Through analysis of core samples taken along a transect from west of the river Bhagirathi (very low As) to the east of Bhagirathi (very high As), the possibility of Mg-bearing clay minerals and reworked silicates concentrating As on their surfaces is being explored. At each coring location, samples from shallow (<4m) and deeper subsurface (30-40m) sediments were collected to target understanding vertical extensibility of arsenic in these zones.

Geochemistry and mineralogy of cored sediments is being studied by heavy mineral separations, Scanning Electron Microscopy and X-Ray Fluorescence methods to determine elemental concentrations within the sediments. Additionally, the tubewell water samples are being analyzed with an Inductively-Coupled Plasma Mass Spectrometer to determine total As and As(III) speciation values and levels of major cations and anions present that correlate with high or low arsenic concentrations.

Based on concentration levels of arsenic in the sediments, determinations will be made whether arsenic is leached from shallow depositional areas into deeper aquifers. The concentrations of arsenic in the surface water and groundwater can be compared to that of the corresponding near-surface sediments to determine if surface water may be a recharge mechanism for the aquifer.