Geochemical characteristics of sediments from a potential natural reactive barrier and arsenic contaminated aquifer in Bangladesh.

The presence of geogenic arsenic (As) in the major deltas of Southeast Asia and its severe health effects have been well known for decades. Recently, the roles of tidally and seasonally fluctuating major rivers that interact with iron (Fe) rich-reducing groundwater have been highlighted as these interactions may form a natural reactive barrier (NRB) within the permeable riverbank. These NRBs, rich in reactive Fe-oxides, may act as sink or source of As and Fe. We studied geochemical characteristics of sediments from a potential NRB site along the Meghna River in Bangladesh. Pristine sediment cores were collected from 0-3m depth along 83m transect orthogonal to the river, and aquifer sediments were collected from drill cuttings of three monitoring wells up to 36m in depth and at 35, 83 and 133m from the river. Grainsize distribution of the sediment was obtained using CAMSIZER and Mastersizer, and elemental composition was determined by X-ray fluorescence. Loss on ignition of dried sediments was measured as an estimation of sedimentary organic matter (SOM). Sediments were incubated with deionized water under oxic conditions to simulate the sediment-river water interactions, the extracts were analyzed for concentrations of water-extractable cations, anions, organic carbon and nitrogen, as well as the absorbance and fluorescence signatures of water-extractable organic matter. Results revealed that riverbank sediment (fine-medium sand) was separated from aquifer sediment (fine-coarse sand) by a silt layer (5m depth), with a clay layer at 37m. Concentrations of As (7 mg/kg) and Fe (41 g/kg) were found to be similar in both riverbank and aquifer sediments. Fe enrichment was observed at 40m from the river, the approximate extent of tidal influence on the groundwater table. Other redox sensitive elements (Co, Ni, Cu, As, V and U) accumulated within 40m from the river. Riverbank sands, silt, aquifer sands and clay contained 0-2, 4-6, 0-2, and 7-8 mass% of SOM, respectively. Although containing similar mass, 10-12% of SOM in aquifer sand was water-extractable compared to only 0-2% SOM in riverbank sand. Spectroscopic analyses indicated that the SOM in riverbank sand was freshly derived and labile, while that in aquifer sand was older and recalcitrant. These results highlight unique geochemical characteristics of NRB sediments.