DETAILED VERTICAL PROFILES OF SEDIMENT AND GROUNDWATER CHEMISTRY: COMPARISON BETWEEN HIGH AND LOW ARSENIC ZONES IN GOTRA, NADIA DISTRICT, WEST BENGAL, INDIA

BECKIE, Roger D.¹, SENGUPTA, Subhasish², PAL, Taraknath², MUKHERJEE, Pradip², HALL, Gwendy E.M.³, RENCZ, Andrew³, DESBARATS, Alexandre J.³, and KOENIG, Cassandra E.M.¹, (1) Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, BC V6T 1Z4, Canada, rbeckie@eos.ubc.ca, (2) Geological Survey of India, 27, Jawaharlal Nehru Road, Kolkata, 700 016, India, (3) Northern Canada Division, Geological Survey of Canada, 601 Booth St, Ottawa, ON K1A 0E8, Canada

Since 2004, The Geological Survey of India , The Geological Survey of Canada and the University of British Columbia have been jointly investigating naturally occurring arsenic in groundwater at a field site in the village of Gotra, West Bengal, India. Within the village high (> 50 ppb) and low arsenic groundwater zones are separated by a transition zone of less than 30 m. In 2004, groundwater samples were collected from 48 domestic and irrigation tube wells in Gotra and surrounding countryside. A push-in tool was also used to collect water samples in two vertical profiles, one each in the high and low arsenic zones. pH, EC, alkalinity and ammonia were measured in the field. Water samples were analyzed by ICP-MS, ICP-OES and ion chromatography. In addition, in each zone a continuous 6 cm diameter core was collected to 40 m depth below ground surface. A total of 133 distinct sub-samples of sediment were taken from both cores at intervals of 0.5 m for a near-total digestion analysis. The groundwater geochemistry showed highly variable arsenic values between sample locations, although there was no clear distinction in major - element chemistry between high-dissolved-arsenic and low-dissolved-arsenic zones. The sediment cores showed arsenic values between 1 ppm As in sandy zones up to 20 ppm As in finer-grained material. Sediment arsenic peaks did not coincide with dissolved-arsenic peaks although the highest dissolved and sediment arsenic was observed near the surface. The sediment geochemistry showed that solid-phase arsenic was strongly correlated with solid-phase iron, manganese and the metals copper, nickel, chromium and to a slightly lesser extent zinc. The correlation was stronger in the low-dissolved-arsenic zone than in the high-dissolved-arsenic zone. There was a poor correlation between solid-phase arsenic and sulfur, although any correlation could have been masked by the high solid-phase sulfur detection limits.