Is organic matter a source or redox driver or both for arsenic release in groundwater?

Arsenic (As) contamination in groundwater is a global public health issue, because groundwater is the main source of drinking water worldwide. Arsenic contamination in groundwater is directly or indirectly associated with organic matter (OM). Therefore, this study investigated the role of OM in mobilization of As in the subterranean aquifers. The reduced concentrations of terminal electron acceptors (dissolved O2, NO 3 - , and SO 4 2 - ), enhanced concentrations of dissolved organic carbon (DOC), HCO 3 - and NH 4 + ions, and strong correlations between NH 4 + and DOC indicate the reducing geochemical environment of aquifer in Bangladesh, where DOC might have originated from microbial oxidation of sedimentary OM, humic substances, surface derived terrestrial OM and petroleum. Groundwater samples exhibit variations in the concentrations of DOC (0.20–5.09 mg/l), fluorescence intensity and As depending on the depth and locations of the sampling site suggesting that the aquifers have different characteristics and different sources of DOC. The DOC concentrations in the upper aquifer (up to 41 m) are higher than those in the middle and lower aquifers (>55 m) suggesting that the recent inflow of surface derived terrestrial OM with recharge water and petroleum in the shallow parts of the aquifer promoted the peak of high dissolved As, where older water mixes with recent recharge water containing organic carbon. Hydrogeochemical data and sediment geochemistry indicates that As mobilization from the sediment occurs through the microbial-mediated reductive dissolution of iron oxyhydroxide, where OM is the main redox driver facilitating As release into groundwater. Organic matter can strongly influence the solubility and mobility of As mainly through redox reactions, competitive adsorption, desorption and complexation reactions. Furthermore, based on the results of sediment and groundwater geochemistry, and As speciation in plant tissue, it can be concluded that OM is not only a redox driver, but also one of the sources of As in groundwater.