IMPACT OF FLUCTUATING RIVER LEVELS ON SEDIMENTARY ARSENIC MOBILIZATION ALONG THE DYNAMIC SURFACE WATER-GROUNDWATER INTERFACE OF THE MEGHNA RIVER, BANGLADESH

Arsenic (As) contamination in the shallow aquifers of Bangladesh jeopardize the health of millions of people who rely on this groundwater for drinking purposes. Currently, the paradigm for As mobilization is the microbially mediated reduction of As-bearing iron (Fe) (oxy)hydroxide-rich sediments fueled by labile organic matter (OM). Along the Meghna River in Bangladesh, tidal fluctuations drive oxic river water into the aquifer causing oscillating redox conditions which may either mobilize As following the dissolution of Fe(III)-oxides, or sequester As on the surface of freshly precipitated Fe(III)-oxides. To constrain the As-mobilizing processes in this dynamic zone, the bulk geochemistry of three sediment cores along the surface water-groundwater mixing front was analyzed, including two subaqueous cores (2 and 6 m into river) and one core 5 m from river edge. Our previous studies show that these sediments contain low molecular weight, labile OM. For this study, total elemental concentrations were determined by microwave digestion to identify zones of enriched Fe, Mn, and As. The Fe(II) and Fe_t concentrations were determined by ferrozine method following 1.2 N HCl leach, whereas a sequential extraction procedure targeting As was employed to elucidate which mineral phases As was associated with. Results show average As concentrations of 3.5±1.6 mg/kg with relative enrichment found near the surface 5 m from river. Similarly, Fe and Mn concentrations (averaging 18,900±8,500 mg/kg and 346±226 mg/kg, respectively) were enriched in the sediment 5 m from the river. The proportion of Fe(III)/Fe_t decreases from 80% to 30% in the subaqueous cores from 0 to 40 cm depth, whereas the Fe(III)/Fe_t in the core 5 m from the river increases from 15% to 60% along this depth. On average, ~80% of sedimentary As was associated with four phases, amorphous Fe-oxides (~40%), crystalline Fe-oxides (~20%), residual minerals (~15%), and OM (~5%). A pronounced redox gradient in the subaqueous cores, marked by a decrease in Fe(III)/Fe_t with depth, facilitates the precipitation of amorphous Fe(III) oxides. Here, the co-occurrence of As-bearing amorphous Fe-oxides and labile OM suggest that a large portion of sedimentary As can be easily mobilized when reducing conditions promote the microbially mediated reduction of amorphous Fe-oxides.