INFLUENCE OF SEDIMENTARY ORGANIC MATTER ON ARSENIC MOBILITY ALONG THE RIVER-AQUIFER INTERFACE: INSIGHTS FROM THE MEGHNA RIVER, BANGLADESH AND THE HOOGHLY RIVER, INDIA

Sedimentary organic matter (SOM) likely plays a crucial role in mobilizing arsenic (As) in the shallow aquifers of the Bengal basin. The chemical properties of SOM determine its lability and therefore its ability to influence redox conditions which regulate the mobility of As by driving the microbially-mediated reduction of As-bearing iron (Fe) oxides. In the hyporheic zone (HZ), surface water-groundwater mixing drives the precipitation of Fe-oxides, which can sequester As in the solid phase. However, if conditions in the HZ become reducing, the Fe-oxides may dissolve and mobilize As. The influence of SOM on the redox state of actively mixing HZs is unknown. This study aims to constrain the role of SOM on mobilizing As along the river water-groundwater interface using spectroscopic techniques to characterize the SOM from the HZ along the Meghna River in Bangladesh and the Hooghly River in India. The HZ at the sandy Meghna River site was found to actively release As from the reductive dissolution of Fe-oxides, whereas As was immobilized by Fe-oxide precipitation within the sandy Hooghly Riverbank HZ. Briefly, the Meghna River site consists of 4 lithologies; riverbank sands (0-3 m bgl), a shallow silt layer (3-7 m bgl), aquifer sands (7-32 m bgl) and a clay aquitard (32-37 m bgl). The riverbank sands and shallow silt layer (12 g/kg and 28 g/kg organic carbon, respectively) contain high proportions of fresh SOM rich in labile amide and polysaccharide functional groups, which may contribute to the microbially-mediated reduction of As-bearing Fe-oxides to prevent the accumulation of As within the HZ. Conversely, the SOM in the aquifer sands and clay aquitard (10 and 45 g/kg organic carbon, respectively) contain humified aromatic-carboxylate functional groups which readily form aqueous As-Fe-OM complexes. Along the Hooghly River, two sites were sampled to evaluate the SOM properties within the HZ: a silt-capped riverbank and a sand riverbank. Preliminary data from the Hooghly River shows that the silt SOM is rich in aromatic compounds, whereas the SOM from the sand riverbank contains negligible proportions of aromatic SOM. These findings show that riverbank HZs may contain SOM with varying chemical reactivities, however, higher As mobility is associated with more labile SOM, which promotes reducing conditions within the HZ.