THE ROLE OF DISSOLVED TIDAL MIXING AND DISOLVED ORGANIC CARBON ON FE-OXIDE PERMEABLE NATURAL REACTIVE BARRIER FORMATION ALONG THE MEGHNA RIVER, BANGLADESH

Geogenic groundwater arsenic (As) contamination in shallow alluvial aquifers across the Ganges-Brahmaputra-Meghna Delta is an ongoing health crisis. This groundwater is anoxic, As, and Fe(II) rich, and bidirectionally mixes with oxic riverwater through tidally mediated flow reversals Meghna Riverbanks. These riverbanks commonly contain high solid-phase As (>500 mg/kg) associated with Fe(III)-oxides. Tidal mixing during the dry season may cause Fe(II) oxidation and precipitate Fe(III) oxides, which scavenge As—a permeable natural reactive barrier (PNRB). However, microbially mediated reductive dissolution could break down Fe(III)-oxides during the wet season. Microbial reduction rates are likely influenced by organic carbon (OC) liability within the riverbank mixing zone. The relative influence of OC lability vs the extent of mixing is difficult to discern from the field and models.

Our objective is to mimic the riverbank mixing zone with reversing flow 1-D column experiments and to quantify any associated Fe and As losses from sediments sourced from the Meghna River, Bangladesh. We hypothesize that PNRBs fail to form when highly labile OC is present, irrespective of sand permeability.

Two reversing flow 1-D column experiments simulated a gaining riverbank with 12 hr tidal reversals using different DOC analogues over two weeks. These were L-lactate and acetate which mimic easily and poorly metabolized DOC compounds. Refrigerated sediment collected from the Meghna riverbank was packed into 20 cm columns. Anoxic artificial groundwater (AGW) containing dissolved Fe (50 mg/L) and As (100 µg/L) entered one end of the column during low tide, while oxic artificial surface water (ASW) entered the opposing end during high tide. Effluent water was analyzed for dissolved Fe with the ferrozine method and As with the modified molybdenum blue method. The profile of solid-phase concentrations of Fe and As in Fe-oxides was measured with 1M HCl extractions before and after the experiments. The results will be analyzed using a numerical reactive flow and transport model to constrain oxidation and reduction kinetics. This physical and numerical modeling of the initial conditions needed for PNRB formation is expected to give new insights into Fe-As deposit stability within the tidally fluctuating riverbanks of Meghna River.