Fluoride contamination of groundwater in different geological settings of Punjab Province, Pakistan: Levels, possible mechanisms and health risks.

Groundwater is a primary drinking water source, particularly in developing countries, and its contamination with different anions and/or metal ions is a major concern on a global scale (Lapworth et al., 2022, Younas et al., 2019). Different natural (geogenic) processes such as mineral weathering and dissolution, volcanic activities and anthropogenic processes such as agricultural and mining activities, surface runoff etc. leads to the disturbance of the natural aquifer system, and may result in varying amounts of chemicals/contaminants into groundwater Qin et al., 2013; Rehman et al., 2022). Fluorine is an abundant trace element present in earth’s crust/soil which ranges from about 500–700 mg/kg and may leach into groundwater through different hydrological processes (Nordstrom and Smedley, 2022). Previous studies reported that natural groundwater fluoride enrichment is controlled by various hydrogeological factors including groundwater levels, the local geology and geochemical conditions (pH, HCO₃^?, Na⁺/Ca⁺, Cl). Other important factors that may affect the solubility of fluoride in groundwater includes residence time, cation and anion exchange processes and arid to semiarid climatic conditions (Farooqi et al., 2007a; De et al., 2023).

Fluoride is often a natural constituent of groundwater and is considered hazardous at high concentrations Viz; beyond the World Health Organization (WHO) guideline of 1.5 mg/L and may lead to dental and/or skeletal fluorosis (Wang et al., 2024). On the other hand, lower concentrations (e.g. <0.5 mg/L) into drinking water may cause dental caries. According to the WHO, about 65 % of waterborne human diseases are associated with F contaminated water sources (De et al., 2023). Therefore, this issue is a global problem that needs detailed insight into the fluoride dynamics of natural aquifer systems for water resource management purposes. Globally, 180 million people are potentially exposed to fluoride-contaminated groundwater and at risk of fluorosis due to natural/geogenic elevated groundwater fluoride concentrations, especially in Africa and Asia. In Asia, more than 90 million people are at risk of fluorosis with the largest affected populations living in India (49 million), China (22 million), and Pakistan (7.6 million) (Podgorski and Berg, 2022; Araya et al., 2022; Podgorski et al., 2018). Given that excessive chronic intake of fluoride poses serious health threats, it is considered an emerging environmental and public health issue in various countries, including Pakistan (Rahman et al., 2020; Bhattacharya et al., 2020).

Pakistan is among the countries with widespread groundwater fluoride contamination due to its natural sedimentary setting and a semi-arid/arid climate (Ling et al., 2022). To date, several studies have reported fluoride contamination on a small spatial scale (Rasool et al., 2015; Rasheed et al., 2022; Rehman et al., 2022). Furthermore, a recent study utilizing over 5000 fluoride data points highlighted extensive fluoride contamination at different hotspot areas, suggesting detailed insights of underlying mechanisms at these locations, including the Thal desert and Indus floodplain (Ling et al., 2022). This study focuses on three areas of the Punjab province: Lower Indus Areas (LIA), Upper Indus Areas (UIA), and Thal Desert Areas (TDA). By comparing groundwater chemistry in these spatially distinct areas within the same basin, more attention can be devoted to the influence of hydrogeochemical conditions. The specific objectives of this study are I). To assess the spatial distribution of fluoride and other water quality parameters, II). To understand the key hydrogeochemical controlling factors involved in fluoride enrichment. III). To assess the human health risk due to fluoride contaminated drinking water at different areas of Punjab, Pakistan.