Unravelling the role of sediment disposition influenced redox-dominated geochemical processes in distribution of geogenic contaminants in groundwater of a major fluvial aquifer: Implications for safe drinking water sources.

Groundwater-sourced drinking water in large sedimentary basins across the world are highly vulnerable to geogenic contamination. The Brahmaputra River Basin (BRB) in the northeastern part of India is found to have widespread groundwater contamination from redox-sensitive elements, arsenic (As) and nickel (Ni), besides having elevated levels of iron (Fe), manganese (Mn) and fluoride (F-), deteriorating the drinking water quality. Here, we show that concentrations of As, Ni, Fe, and Mn exceeding the World Health Organization (WHO) drinking water permissible limits observed in the BRB groundwater is primarily controlled by the redox-dependent geochemical processes as a consequence of water-sediment/rock interactions. In situ measurements of pH, oxidation reduction potential (ORP), dissolved oxygen (DO), and concentrations of redox-sensitive parameters such as As, Ni, Fe, Mn, NO3-, and SO42- indicate that the groundwater redox state ranges from mildly oxidizing to strongly reducing. Chemical speciation of As (III) vs As (V) and Fe (II) vs Fe (III) from the groundwater samples demonstrate the dominance of reduced species of As (III) and Fe (II). Groundwater is found to be under mildly oxidizing conditions in the absence of clay layers, while it is reducing wherever groundwater is confined by clay layers leading us to hypothesize that the development of the reducing condition in the groundwater may be due to oxygen limitation by the clay layers. Under these reducing conditions, redox-sensitive elements such as As, Ni, Fe and Mn are mobilized into the groundwater, deteriorating the drinking water quality. This study underscores the vital influence of aquifer sediment heterogeneity in the fate and mobilization of geogenic contaminants in groundwater. This study emphasizes the need for detailed aquifer mapping to ensure safe and sustainable drinking water availability.