FLUVIO-DELTAIC PROCESSES AND GEOMORPHIC DEVELOPMENT AT THE SCALE OF 100 TO 1000M: THEIR IMPORTANCE IN GOVERNING THE HETEROGENEITY OF GROUNDWATER ARSENIC IN ARAIHAZAR, BANGLADESH

While much has been learned in the past few decades about the origin of arsenic and the extent to which it contaminates Bangladesh's groundwater, many unresolved questions remain about the factors governing its heterogeneity. This is a key concern in Bangladesh and other impoverished Asian nations, where it is estimated that more than 60 million people are at risk of ingesting water with arsenic concentrations >50 µg/L (WHO limit is 10 µg/L). In looking for a common governance on arsenic heterogeneity, one shared aspect among these regions is that they comprise thick Holocene sequences of fluvio-deltaic sediment deposited by high-energy rivers on broad, flat coastal plains. Here, we show that the rivers within one of these deltas interact with the landscape to leave behind a complex sedimentological framework capable of explaining much of the arsenic heterogeneity over very small scales (i.e., <10 to >300 µg/L within 100 m). In the 25 square km Old Brahmaputra floodplain of Araihazar, Bangladesh, 100-1000 m lateral heterogeneities in shallow groundwater arsenic correspond to the deposits of two different fluvial transport regimes: villages overlying coarse-grained levee and bar features of a higher energy braided fluvial system have lower concentrations of dissolved arsenic versus villages overlying finer deposits of a lower-energy meandering system. Over 200 0-15 m borings into the floodplain indicate that areas overlain with these finer deposits (~3-12 m) have higher accumulations of arsenic in the groundwater. In addition, optically stimulated luminescence (OSL) dating of the aquifer sands indicates successive phases of channel reactivation and abandonment (1000, 600, and 400ya) over the past several hundred years. Here, we discuss how understanding local geomorphic development and resulting stratigraphic architecture is an important factor influencing shallow aquifer chemistry. In addition, the combination of geochemical and chronological studies from Araihazar can help clarify the role that geomorphology plays in arsenic heterogeneity on local spatial scales.