Arsenic in Shallow Groundwater Adsorbed by Deeper Aquifer Sediments of Araihazar, Bangladesh

Natural arsenic contamination of shallow groundwater represents a serious health risk to millions of people in Bangladesh. Deeper (often Pleistocene) aquifers currently provide a safer alternative in many parts of the country. However, due to the increased pumping for drinking water supply, a downward hydraulic gradient has developed between the shallow and the deep aquifers in some areas. This might increase the probability of contamination of the deeper aquifers with high-As shallow groundwater through broken well casings or, on a broader scale, through leakage between clay lenses separating the aquifers. To ensure the sustainability of current water usage practices, response of the deep aquifer to potential shallow groundwater intrusion must therefore be evaluated. We performed three push/pull experiments in Araihazar upazila of Bangladesh that involved controlled injections of ~1000 L of groundwater into the deeper aquifer (60 m). Two experiments involved a minimal alteration of the deep groundwater by adding 250 µg/L of As(III) or As(V) and injecting it back into the same aquifer, thus changing only As levels in water chemistry. The third injection was carried out with shallow (38 m) groundwater containing 200 µg/L As to simulate a potential leakage. The injected water was allowed time to react for a period of ~10 days over which samples were regularly collected. Total volume extracted was about twice that injected. Arsenic was adsorbed by the deep aquifer in each experiment, however with notable differences in kinetics. As expected, As(V) was adsorbed more quickly than As(III), with e-folding times of ~2 hours and ~1 day, respectively. Compared to altered deep water, As was removed more slowly from shallow aquifer water, with an e-folding time of ~2 days. Deep aquifer sediment, therefore, can adsorb As from potential shallow groundwater intrusions, but further studies are necessary to evaluate its total adsorption capacity.