CONTRASTING BIOGEOCHEMICAL CONTROLS ON ARSENIC MOBILIZATION IN REDUCING AQUIFER AND A POTENTIAL NATURAL REACTIVE BARRIER (NRB)

Elevated arsenic (As) in groundwater adversely affects the health of millions globally. The mobilization of As has been attributed to the microbially mediated reductive dissolution of iron (Fe)-oxy(hydroxides) fueled by labile organic carbon. Recent studies hypothesize that the mixing of oxic river water with Fe- and As-rich reducing groundwater in the hyporheic zone of fluctuating rivers cause Fe-oxide minerals to precipitate and accumulate As on their surfaces, forming a natural reactive barrier (NRB). Here we discuss sediment data obtained from the Meghna riverbank (1-3 m depth, 0-83 m from river) and from the adjacent aquifer (2-36 m depth, 35-130 m from river) in Bangladesh. The grain sizes of the sediment samples and the bulk elemental concentrations were analyzed to identify zones of enriched Fe, Mn, As and other redox sensitive elements. Sediments were incubated with oxygen-rich deionized water to simulate the river water-sediment interaction. Sediment-water extracts were then analyzed for major ion composition, dissolved organic carbon (DOC), and the absorbance and fluorescence spectroscopic properties of the dissolved organic matter (DOM). Results show that a silt layer at 5 m depth separated riverbank sands from aquifer sands below. There was no significant difference in the bulk concentrations of Fe (41 g/kg), Mn (1 g/kg), and As (7 mg/kg) between the aquifer and riverbank sediments. In the riverbank, Fe enrichment was found at 40 m from the river, while As and other redox sensitive elements (U, V, Co, Zn, Ni) accumulated at distances <40 m from the river, where the water table is influenced by river fluctuations, whereas Mn accumulated at depths <1 m within the vadose zone of the dry season riverbank at distances >40 m from the river. Aquifer extracts contained high total dissolved solids (TDS, 90 mg/L), DOC (1,294 mg/kg), and strong humic-like fluorescence. In contrast, riverbank sediments produced much less TDS (5 mg/L), DOC (116 mg/kg), and exhibited strong protein-like fluorescence. The findings suggest that As and Fe mobility within the riverbank sediment is regulated by fresh and labile organic matter from the infiltrating oxic river water. However, within the aquifer sediment, the mobilization of As and Fe is controlled by recalcitrant and humic-like organic matter under reducing conditions.