Collaborative Research: Vertical control of groundwater arsenic concentrations in shallow Bangladesh aquifers

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0345777GoodbredElevated levels of arsenic (As) in groundwater of the Ganges-Brahmaputra-Meghna Delta (GBMD) consumed by tens of millions of people has caused what may be the largest case of human poisoning in recorded history. Although the source of As is natural, the spatial distribution of As in groundwater is highly variable. Field investigations conducted by the team of PIs in Araihazar upazila, Bangladesh, over the past 3 years suggest that in the case of the shallowest aquifers, this spatial variability across 101-102 m is driven vertically by the surface geology: (a) groundwater As concentrations tend to remain low to ~20 m depth wherever sandy deposits extend to the surface, (b) As concentrations are typically elevated in shallow aquifers that are capped by a surface layer of fine silt or clay. Demonstrating the expression of this relationship in a complex fluvio-deltaic environment required the collection of spatially dense data using a novel combination of geophysical and geochemical tools. In this proposal, we propose to apply the new approach developed in Araihazar, which includes on-site measurement of labile sediment properties, to investigate the spatial controls of As in shallow aquifers of several other areas of Bangladesh. NSF Criterion 1 - Science: Fluvio-deltaic deposits are intrinsically complex systems because their architecture reflects changes in the position and order of stream channels over time. Somewhat serendipitously, this team of PIs has discovered what appears to be a very consistent relationship between the nature of surfacial deposit and the redox state of the underlying aquifers on spatial scales of 101-102 m. If this relationship holds on such scales elsewhere in Bangladesh, it could also explain regional differences in the mean and variability of groundwater As concentrations on spatial scales of 104-105 m throughout the GBM, as wells as similar environments where elevated As in groundwater As has been reported, such as Vietnam and Argentina. The proposed investigation will also set the stage for parallel studies of the impact of surface geology on the microbiology and hydrology of shallow aquifers. NSF Criterion 2 - Impact: The vast majority of the ~10 million tube wells in Bangladesh are shallow ( 20 m). As more of these wells are tested, the usage of high-As wells for drinking and cooking (as opposed to washing) will hopefully drop. If the hypothesis that redox conditions in shallow aquifers are anchored to the local surface geology can be confirmed in different fluvio-deltaic settings of the GBMD, this knowledge could be used for targeting safe aquifers more effectively. A critical question facing policy makers, however, is whether tube well As concentrations could change over time as irrigation, which draws water primarily from the shallow aquifers, continues. An implication of the hypothesized vertical control by surface geology would be that the spatial distribution of groundwater As concentrations in the shallow aquifers is relatively insensitive to changes in hydrology caused by large-scale groundwater pumping for irrigation. This is clearly important to know since it would reduce the need for repeated testing of tube wells over time and the need for switching irrigation from groundwater to surface water. Finally, our observation might lead to field-scale manipulation of surface deposits to produce low-As groundwater in areas with very few other options.