Global groundwater contamination by geogenic fluoride.

Fluoride (F¯) is a global geogenic contaminant in groundwater, occurring at elevated concentrations across almost every continent. While optimal fluoride intake benefits dental health, levels exceeding the WHO safe limit (1.5 mg L-1) can cause dental fluorosis and even severe skeletal fluorosis, with drinking water being the primary exposure pathway. Understanding the global occurrence and geochemistry of fluoride in groundwater is essential for minimizing associated health risks. Therefore, this study reviews the incidence, distribution, mobilization mechanisms of fluoride in groundwater, and identifies the key knowledge gaps related to fluoride contamination worldwide. The study presents a large-scale synthesis of contaminated aquifers, cost-effective defluoridation methods, socio-economic challenges, and strategies for safe drinking water supply. This was done by adopting the PRISMA 2020 systematic review framework with review period between 1953 and 2025. Findings confirm that fluoride contamination is prevalent in Asia, Africa, the Middle East, Europe, and the Americas, driven mainly by fluorine-bearing minerals in aquifers. Key factors intensifying the contamination includes hydrogeochemical conditions such as high pH, Na-HCO3 water types, and low calcium (Ca2+) concentrations. Identified hotspot are the East African Rift Valley, Indo-Gangetic plains, northern China, and volcanic or geothermal regions in Mexico and Türkiye, with localized anomalies observed in Estonia, Poland, and Australia. Anthropogenic inputs contribute in some localized areas, although their impact is minimal. The fluoride levels are particularly high in arid and semi-arid regions, where evaporation and prolonged water-rock interaction accelerate fluoride mobilization. Despite decades of research, monitoring gaps persist, and mitigation remains challenging, especially for rural communities who are entirely dependent on untreated groundwater. This study recommends prioritizing efforts on scalable, cost-effective solutions to provide fluoride-safe water, particularly in areas where populations are largely vulnerable.