RESULTS FROM ONGOING HYDROGEOLOGICAL AND HYDROGEOCHEMICAL FIELD INVESTIGATIONS OF GEOGENIC ARSENIC CONTAMINATION IN GOTRA, NADIA DISTRICT, WEST BENGAL, INDIA

The geological surveys of India and Canada, and the University of British Columbia are collaborating to investigate the occurrence of geogenic arsenic in an aquifer supplying potable water to the village of Gotra, 60 km northeast of Kolkata. This paper presents selected results from instrumented monitoring wells and from a survey of groundwater chemistry in 60 domestic wells. The main aquifer forms a fairly continuous layer between 22 and 30 m below the surface consisting of fine to coarse, grey, micaceous sands with a geometric mean hydraulic conductivity of 1.37 x 10-3 m/s. Throughout the village, which is located on a former channel levee, the aquifer is confined by up to 20m of calcareous clayey silt. In lower lying land north of the village, the confining layer is absent. Hydrographs show that heads in the aquifer vary by up to 5 m annually, with a peak in late September 2006 associated with an intense rainfall event. Hydrographs from different monitoring wells are usually within 0.1 m and respond almost simultaneously to aquifer stresses, consistent with very low horizontal gradients and high conductivities. Chemographs of electrical conductivity and pH are generally stable throughout the year although EC reflects a noticeable freshening of groundwater associated with the hydrograph peak, suggesting aquifer recharge. The possibility of recharge from surface ponds is supported by limited isotopic data. Groundwater chemistry results show that waters of the main aquifer belong to the Ca-HCO3 facies, with alkalinity and TDS values reaching 682 mg/L (as CaCO3) and 546 mg/L, respectively. Waters from the fewer, deeper (>40m) domestic wells are of a Ca-Na-HCO3 type, and are fresher. In the main aquifer, SO4 is virtually absent whereas it is often present in minor amounts in the deeper wells. The spatial distribution of As in the main aquifer shows a clear transition from high (>50 ppb) to low values when moving north, across a line that follows the trend of the channel levee. Although As values as high as 200 ppb are found in deeper wells, they reach 516 ppb in the main aquifer. Active degassing of CH4 and CO2 is observed in the most affected wells. Concentrations of As are highly correlated with those of alkalinity, dissolved Fe, PO4 and NH3. On the other hand, the presence of SO4 appears to preclude significant concentrations of As.