Sediment Age and Arsenic Availability: Leaching Experiments on Aquifer Sands and Implications for Arsenic Mobilization

While arsenic heterogeneity is well documented in many of Asian's shallow aquifers, the cause for this heterogeneity still remains poorly understood. This is in part due to the fact that the aquifers are made of sediments that hydrostratigraphically vary over small distances (~10-100 meters) due to the dynamic depositional conditions created by this highly fluvial region. Sediment variability is evident in sediment properties such as grain size and the amount of combustible organics, as well as the time in which the sediments were deposited—as shown by luminescence dating of aquifer deposits. With one of the key features of our work showing trends in arsenic correlating to sediment depositional ages in study sites in Bangladesh, Vietnam, and Nepal, a question was raised as to whether older sediment, overall, leaches less arsenic than more recent deposits. Results from Na, As, and REE concentrations monitored in the effluent of flow-through columns packed with aquifer sands from heterogeneously affected sites in Bangladesh and Nepal show similar Na and different As and REE leaching patterns over four months of periodic sampling. The common effluent concentrations of Na (~100 &mug/L) combined with variable As (2 to <0.5 &mug/L) and REE (La/Lu ~1 in Northwest Bangladesh and ~10 in Parasi, Nepal) indicate that aquifer deposits, sourced from different regions and deposited at different times (3-20 ka), can weather similarly, and can still leach different concentrations of arsenic. This indicates that differences in arsenic are not solely explained by sediment age alone, and also correspond to grain size and the amount of combustible organics contained in a deposit.