Controls of Floodplain Evolution on the Occurrence of High Arsenic in Shallow Aquifers of the Ganges-Brahmaputra Delta

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0229600GoodbredDiscovered in the mid-1990s, elevated arsenic (As) levels in the groundwater of Bangladesh and West Bengal, India has proven to be a somewhat enigmatic problem. Toxic levels of dissolved-phase As have been found to be widespread across more than half of the 100,000 km2 Ganges-Brahmaputra (G-B) delta plain, affecting 10s of millions of people. Yet, at a local scale (101-103 m) the distribution of groundwater As is extremely heterogeneous, varying orders of magnitude (5-2000 mg/L) over short distances. These heterogeneities suggest that a highly specialized, but not uncommon, set of conditions are needed for elevated As to develop. It is becoming apparent from recent field studies that near-surface floodplain geology is an overarching control on this heterogeneity. Based on emerging ideas, the proposed study will focus on two main objectives: (1) determining the precise geological conditions necessary for the development of elevated groundwater As and (2) the history and processes of floodplain evolution that lead to these conditions. Impetus for the study arose from the recognition of a strong correlation between shallow tubewell As values and the mean sediment texture of near-surface ( 6 m) floodplain deposits. This relationship was recognized during a joint field program (Jan 2001) with the Columbia University arsenic research team funded through NIEHS/Superfund Basic Research Program. The natural-science component of the Columbia-SBRP study is focused on the geochemistry and hydrology of a 202 km study area (Araihazar) in Bangladesh. The study proposed here is designed complement the Columbia-SBRP project and will be conducted in collaboration. The fundamental goal is to develop a multidisciplinary understanding of the As problem by integrating biomedical, geochemical, hydrological, and geological components of the two studies. Individually, this study will identify the geological underpinnings of complex hydrological regimes and their role in the elevated As levels of local groundwater.