Groundwater fluoride concentrations in the watershed sedimentary basin of Quetta Valley, Pakistan

Litho-geochemical characteristics of low and high fluoride (F^-) groundwater along with hydrological processes were investigated to delineate its genesis and enrichment mechanism in a watershed sedimentary basin. In this study, groundwater F^- concentration ranged from 0 to 20 mg/L with a mean and standard deviation of 2.8 and ± 3.7 mg/L, respectively. Out of N = 87, 63% of samples exceeded the World Health Organization (WHO) limit of 1.5 mg/L. The order of cationic and anionic dominance in groundwater samples with mean was found in decreasing order as Na⁺  > Mg²⁺  > Ca²⁺  > K⁺ and HCO₃^-  > SO₄^2-  > Cl^-  > PO₄^3-  > NO₃^- measured in milligrams per liter. Groundwater chemistry changed from Ca-HCO₃ to Na-HCO₃ type and low to high fluoride as we moved from mountain foot towards the synclinal basin. Low fluoride groundwater reflected weathering, recharge, and reverse ion exchange processes with Ca-HCO₃- and Ca-Mg-Cl-type water while high fluoride groundwater revealed base ion exchange, mixing, and desorption as dominant hydrological processes with Na-HCO₃ and Na-Cl types of water. Gibb's diagram showed rock weathering and mineral dissolution as the major geochemical processes controlling water chemistry with an insignificant role of evaporation in the semi-arid area. Fluoride was undersaturated with mineral fluorite, indicating fluoride in groundwater is released by secondary minerals. However, due to complex geological features, groundwater fluoride enrichment was affected by a broad-scale process across a wide area such as depth, residence time, and most important geomorphological units hosting the aquifer.