Testing the hypothesis of fluoride and uranium co-mobilization into groundwater by competitive ion exchange in alluvial aquifers of Southern Punjab, India

Co-mobilizing fluoride (F^-) and uranium (U) into groundwater poses a drinking water quality problem globally. Competitive ion exchange with aquifer sediments has been hypothesized to cause their co-mobilization. However, this hypothesis has been postulated merely based on correlations of F^- and U with other groundwater parameters without characterizing that F^- and U were present as exchangeable in the aquifer sediments. The present study, therefore, tested this hypothesis by determining the abundance and association of F^- and U in the aquifer sediments and correlating these data with the groundwater composition in the alluvial aquifers of southern Punjab, India, where the groundwater contamination by F^- and U is severe. Our results support the hypothesis that competitive ion exchange can co-mobilize F^- and U into groundwater. However, the specific ion exchange reaction involved in the F^- and U mobilization can differ. In the study area, the U mobilization into groundwater was linked to increased ionic strength due to the increase in concentration of any ionic species. However, the mobilization of F^- was explicitly linked to the changes in OH^- and HCO₃^- concentrations rather than the overall ionic strength. Bicarbonate was the most critical ionic species that could cause F^- and U co-mobilization.