The evolution of aquifers and arsenic in Asia: a study of the fluvio-deltaic processes leading to aquifer formation and arsenic cycling and heterogeneity in Bangladesh, Vietnam, and Nepal

Abstract

Elevated groundwater arsenic in Southeast Asia is a problem, as it is not unusual for wells spaced several meters apart from one another and drilled to the same depth to have dramatically different concentrations of arsenic (i.e., 7-321 ìg/L). This is a compendium of sedimentological, geochemical, and geochronological methods, investigating why Asian aquifers evolve such heterogeneous arsenic distributions. Making use of preliminary arsenic data from sites in Van Phuc, Vietnam; Parasi, Nepal; and Araihazar, Bangladesh, ~1km high-resolution needle- and OSL- sampling transects were conducted between known areas of high and low groundwater arsenic. Results show that arsenic varies predictably with aquifer deposits of differing sources and ages, with groundwater arsenic decreasing in all locations as the age of the aquifer increases. This is important because the results show the degree to which aquifer deposits vary and define the local (i.e., sediment facies and fluvial history) and regional (i.e., sediment source and fluvial history) geomorphic processes that govern arsenic distributions in the region, such as the rates in which these aquifers ingrow and then, possibly, attenuate arsenic.