GEOCHEMICAL CONTROLS ON ARSENIC RELEASE INTO GROUNDWATER FROM SEDIMENTS: THE EFFECTS OF A NATURAL REACTIVE BARRIER (NRB) ALONG THE MEGHNA RIVER, BANGLADESH

Arsenic is naturally sorbed onto iron (Fe) oxyhydroxides in aquifer sediments and then subsequently released when abiotic and microbally mediated reductive dissolution processes dominate in aqueous environments. The goal of this study is to characterize the hydro-geochemical interactions between the Meghna River and shallow aquifers along the river banks, i.e., in the hyporheic zone (HZ). HZ exchange significantly promotes release of As under alternating reducing and oxidizing conditions, thereby acting as a Natural Reactive Barrier (NRB). Groundwater (GW) samples from a range of depths were collected from a transect spanning up to 1 km east and west of the river banks. Samples were tested in the field for geochemical parameters including DO, pH, conductivity, and dissolved As and Fe. Groundwater δ¹⁸O and δ²H analyses point to the source of aquifer recharge to be local precipitation infiltrating terrestrially derived sediments. Dissolved organic matter (DOM) in the GW samples was characterized using absorbance and 3-D fluorescence spectroscopy. The results indicate DOM is terrestrially derived and OM characteristics change with distance from the riverbank. On the east riverbank, as distance from the river increases, the dissolved organic carbon (DOC) concentration, extent of humification and aromaticity increase. Higher freshness index (β:α) near the river indicate recently-derived, while lower β:α away from the river indicate more decomposed DOM in the GW. In addition to the GW, fifteen 1.5-m deep sediment cores were obtained along the east and west riverbank. Sediment cores were analyzed at 60-cm intervals, using hand held XRF for relative elemental abundance, and X-Ray Diffraction for detecting mineralogical changes along a depth profile. Elemental abundance reveals an enrichment of solid phase As and Fe in the upper 0-1.5 m of riverbank sediment column. Preliminary hydrochemical results indicate that >65% of GW samples contain As over the WHO MCL (0.01mg/L). The well samples extend a distance beyond the HZ during the dry season, but frequent flooding during the wet season can allow fresh DO and DOC to impact shallow depths of these aquifers. The intermittent influence of O₂ changes redox conditions in the NRB and can influence the mobilization of As sorbed to Fe-minerals.