GROUNDWATER CHEMISTRY OF DEEP (>300 FEET) HIGH-AS AQUIFERS ACROSS THE INDIA-BANGLADESH BORDER

Arsenic (As) in drinking water in parts of West Bengal and Bangladesh is a major public health issue. It is estimated that over 50 million people in West Bengal and Bangladesh could be drinking water with As concentrations above WHO guideline of 10 µg/L. To avoid drinking water contaminated with As, numerous community wells >300 feet deep were installed across the Bengal basin. A number of recent surveys have indicated the presence of high As in these wells. We conducted a cross-border survey between Santipur (West Bengal, India) and Bagherpara (Jessore, Bangladesh) in January 2015 targeting wells >300 feet deep. We report here radiocarbon data for dissolved inorganic carbon (DIC) along with cations, trace metals, and anions for a total of 33 wells whose depth was verified to range between 308 and 820 feet with a down-hole camera. A total of 18 wells (out of 33 studied) had As concentrations >50 µg/L (max. 150 µg/L) along the 100-km long latitudinal (23.2^oN) transect. On the western side of the transect (in India), 18 wells were sampled between 308 and 615 feet and 9 wells with As >50 µg/L at depths of 308-581 feet. In Bangladesh, 15 wells were sampled between 403 and 820 feet and 9 wells with As >50 µg/L at depths of 485-820 feet. We found no systematic difference in other water quality parameters between wells containing As >50 µg/L and <50 µg/L. Electrical Conductivity ranged from 0.5 to 1.2 mS/cm whereas concentrations of the redox-sensitive elements Fe, Mn, and P varied between 0.3-7.8, 0.03-0.4, and 0.03-0.5 mg/L, respectively. With the exception of two wells, dissolved organic carbon (DOC) concentrations did not exceed 3 mg/L. A more striking feature of the data is that radiocarbon age in groundwater from 33 wells >300 feet deep (excluding wells containing bomb ¹⁴C, Fm >1.0) was found to average 1.7±0.7 kyr ¹⁴C years without corrections. The presence of young groundwater to considerably greater depth compared to elsewhere in Bengal basin (~10 kyr in the same depth range in Araihazar, Bangladesh, for instance) suggests a process that might be related to deep groundwater recharge carrying As or reactive DOC that release As to groundwater. It is presently unclear whether the deep penetration of radiocarbon and occurrence of As in this particular region is a reflection of natural processes or deep pumping.