Groundwater quality in the alluvial aquifer system of northwest India: New evidence of the extent of anthropogenic and geogenic contamination.

Groundwater depletion has been widely studied in northwest India, but water quality concerns are still poorly constrained. In this study, we explore the hydrochemistry of the top 160m of the aquifer system, through detailed field studies in the Bist-Doab region, considering both anthropogenic and geogenic controls. A detailed comparison is made between sites dominated by urban and agricultural landuse. Salinity, nitrate, chloride and lead concentrations are significantly higher in the shallow (0-50m) groundwater system due to surface anthropogenic contaminant loading from agricultural and urban sources. The widespread occurrence of oxic groundwater within the aquifer system means that denitrification potential is limited and also enhances the mobility of selenium and uranium in groundwater. Geogenic trace elements (e.g. As, Se, F), are generally found at concentrations below WHO guideline drinking water values, however elevated U concentrations (50-70μg/L) are found within the deeper part of the aquifer and shallow urban aquifers associated with higher bicarbonate waters. Higher concentration of Se (10-40μg/L) are found exclusively in the shallow groundwater system where Se is mobilised from soils and transported to depth in the shallow aquifer due to the prevailing oxidising aquifer conditions. New evidence from a range of environmental tracers shows elevated concentrations of anthropogenic contaminants in the deeper part of the aquifer (50-160m deep) and demonstrates vulnerability to vertical migration of contaminants. Continued intensive groundwater abstraction from >100m deep means that water quality risks to the deep aquifer system need to be considered together with water quantity constraints.