Geochemical control on spatial variability of fluoride concentrations in groundwater from rural areas of Gujrat in Punjab, Pakistan

Spatial variability and geochemical factors controlling fluoride (F−) concentrations were evaluated in five villages of subdistrict Gujrat in Pakistan. Groundwater samples (n = 70) were collected along the River Chenab and examined for F− concentrations. The average F−concentrations in groundwater of village Chhani Nikoani (1.8 mg/L) and Dhool Khurd (2.2 mg/L) were crossing the World Health Organization (WHO) permissible limit of 1.5 mg/L, while village Kalra Khasa, Ghazi Chak and Kula Chor were below the WHO guidelines. About 64 % of the samples contain fluoride concentrations that exceed the 1.5 mg/L drinking water standard set by WHO. Spatially F− concentrations in groundwater increased as moving away from the River Chenab because more residence time for water–rock interaction was available. High F− concentrations were associated with high electric conductivity (EC), total dissolved solids (TDS), pH and Na+ values. The overall groundwater quality is controlled by silicate mineral weathering and less by evaporation. Fluoride originates from anion exchange (OH− for F−) on clays and weathered micas under high pH conditions. High fluoride levels are associated with NaCl- and NaHCO3-type water produced by calcite precipitation and/or base ion exchange. The groundwater with higher fluoride concentrations is accompanied by lower Ca+2levels. This study enhances our understanding on geochemical behavior of F and highlights the importance of spatial variability on F release in aquifers of Punjab in Pakistan, as well as in other countries of Asia region.