Geochemical factors controlling the occurrence of high fluoride groundwater in the Nagar Parkar area, Sindh, Pakistan by Tahir Rafique, Shahid Naseemb , Tanzil Haider Usmani , Erum Bashir, Farooque Ahmed Khana , Muhammad Iqbal Bhanger

Fluoride concentrations in groundwater near Nagar Parkar in the Thar Desert of southeastern Pakistanrange from 1.13 to 7.85 mg/l, and roughly 78% of the samples contain fluoride in concentrations thatexceed the drinking water standard of 1.5 mg/l set by WHO. The groundwater is alkaline (pH 7.1–8.4),brackish (TDS 449–15,933 mg/l), and classified as Na–Cl type water. This prevailing chemical characterreflects the influence of saltwater intrusion, high evaporation rates, and ion exchange. Groundwater is alsosupersaturated with respect to calcite, which promotes the removal of Ca2+ and HCO3− fromsolution. As aresult, groundwater is generally undersaturated with respect to fluorite, the mineral that typically controlsthe upper limit of fluoride concentrations. This study examines a number of geochemical parameters in aneffort to discover the controls on fluoride concentrations in groundwater. High fluoride concentrations areassociated with high TDS, high pH, high Na concentrations, and high sodium absorption ratios (SAR). Thissuggests that elevated fluoride levels are the result of enhanced fluorite solubility due to Ca depletion andhigh ionic strength and the release of fluoride from colloid surfaces under high pH conditions. Contraryto what has been found in other studies, sample depth and water temperature do not appear to have asignificant role in the distribution of fluoride within the groundwater.