Geogenic groundwater arsenic in high altitude bedrock aquifers of upper Indus river basin (UIRB), Ladakh

Many sedimentary aquifers across the globe are contaminated with As, a known toxicant and carcinogen, thereby making millions of people vulnerable to a health hazard. Although the source of As enrichment is being linked to the provenance of the sediments (i.e. hard rocks), there are limited studies of groundwater As enrichment in hard rock areas. In this study, we collected the groundwater samples from complex hard rock aquifer system across the Himalayan collision zone in Upper Indus River Basin (UIRB) to understand the source and to evaluate the hydrogeochemical processes responsible for groundwater As mobilization. The study suggests that groundwater As enrichment comes from volcanic rocks and ophiolitic melange. Such, As contaminated aquifers are widely used for groundwater exploitation for drinking and other purposes in Ladakh (UIRB). The results indicated that As enrichment occurs with an increase in temperature and depth. The results also indicated that the groundwater in Ladakh is described by multiple hydrochemical facies with pH circum-neutral to alkaline in nature. Higher pH, Fe2+ and Mn and lower ORP, NO3-, and SO(4)(2- )in groundwater are found to be associated with high As ground- water indicating metal oxides/hydroxides under reducing conditions are predominantly controlling the groundwater As mobilization.