High levels of fluoride contamination in groundwater of the semi-arid alluvial aquifers, Pakistan: evaluating the recharge sources and geochemical identification via stable isotopes and other major elemental data

Hydrogeochemical methods were integrated to delineate the geochemical factors controlling fluoride (F - ) contamination in groundwater at four sites in the districts of Lahore ( Samada ) and Kasur ( Sari Chimba , Kot Maiga , and Chah Fatehwala ) in Panjab province of Pakistan. Hydrochemical data and stoichiometric ratios indicate Na–Cl and Na–HCO 3 as the dominant water types with silicate weathering influencing overall hydrogeochemistry of the study area. The groundwater F - concentrations ranged between 0.54 mg/L and 17.5 mg/L, with more than 70% samples having F - concentrations above the World Health Organization (WHO) provisional drinking water guideline (1.5 mg/L). Saturation indices determined that 100% samples were saturated with respect to calcite and 96% samples were undersaturated with respect to fluorite, indicating the influence of calcite precipitation on fluoride enrichment. A positive correlation was observed between fluoride with pH, Na + , and HCO 3 − , confirming that high fluoride concentrations were the result of weathering of silicate minerals and the exchange of OH - on clay surface under the alkaline pH conditions. The isotopic values of δ 18 O and δ 2 H in groundwater ranged from 9.14 to − 5.51‰ and 56.57 to − 39.5‰, respectively. The stable isotope data indicated the meteoric origin of groundwater with some evaporative effect, which is partly influencing groundwater quality such as high pH and salinity, as a result facilitating anion exchange (OH - for F - ) on clays surface. The research indicates that the groundwater quality of the study area is not recommendable for drinking due to its high total dissolved solids (TDS) and elevated fluoride concentrations.