Hydrogeochemical controls on contrasting co-occurrence of geogenic Arsenic (As) and Fluoride (F) in complex aquifer system of Upper Indus Basin, (UIB) western Himalaya.

Arsenicosis and fluorosis have become severe health hazards associated with the drinking of Arsenic (As) and Fluoride (F) contaminated groundwater across south-east Asia. Although, significant As and F concentration is reported from major Himalayan river basins but, the hydrogeochemical processes and mechanisms controlling their contrasting co-occurrence in groundwater is still poorly explored and understood. In the present study, groundwater samples were collected from phreatic and confined aquifers of Upper Indus Basin (UIB), India to understand the hydrogeochemical processes controlling the distribution and co-occurrence of geogenic As and F in this complex aquifer system. Generally, the groundwater is circum-neutral to alkaline with Na-HCO, Ca-Na-HCO and Ca-Mg-HCO water facies signifying the dominance of silicate and carbonate dissolution. The poor correlation of As and F in groundwater depicted that these geogenic elements have discrete sources of origin with distinct mechanisms controlling their distribution. As enrichment in groundwater is associated with high pH, Fe, Mn and NH-N suggesting dominance of metal oxide/hydroxide reduction with organic matter degradation. However, F enrichment in groundwater is associated with high pH, HCO and Na, which is assisted by the incessant dissolution of fluorinated minerals. The study also revealed that high HCO facilitates the exchange of hydroxides (OH) with As and F on sediment surfaces that contribute to As and F enrichment in groundwater through desorption. 70% groundwater samples have As and F concentration above the permissible limit given by WHO. Therefore, continuous exposure to these contaminants may pose severe health hazard of arsenicosis and fluorosis to people living in the region and downstream. The study provides insights into geological sources, hydrogeochemical processes and mechanisms controlling distribution of As and F in groundwater that will help in developing the appropriate measures to mitigate the impact these contaminants on human health.