The growing global demand for groundwater faces two naturally occurring, or geogenic, constituents that may be present at concentrations injurious to human health. These are fluoride (F) and arsenic. For each element, more than 200 million people are estimated to be chronically exposed through drinking water (primarily groundwater) exceeding the respective drinking-water standards (Edmunds and Smedley, 2013). The global map developed by Amini and others (2008) estimated about 260 million people exposed to high-F drinking water, which is corroborated by Wang and others (2020).
Many high-F groundwater provinces have been recognized worldwide, typically in arid and semi-arid regions. Prevalent regions have been documented from Argentina, China, India, Iran, Mexico, Pakistan, Sri Lanka, western USA and numerous countries in Africa. Occurrences are typically associated with large sedimentary basins, granites, and volcanic and geothermal terrains.
Fluoride is not an essential trace element for human health, but it is long-established that the element has a beneficial effect in protecting against dental caries (tooth decay), a factor which has led to the widespread use of fluoride toothpastes and mouthwashes. By contrast, long-term exposure to high concentrations of fluoride in drinking water can lead to fluorosis, which can range in severity from mild dental mottling to a crippling skeletal form (Fawell et al., 2006). Fluorosis has been recognized as affecting ancient civilizations from 2000 years ago (Lukacs, 1985; Yoshimura et al., 2006). Fluorosis symptoms are related to dose, but young children, the elderly and people with inadequate diets and poor health are particularly vulnerable (Irigoyen-Camacho et al., 2016; Malde et al., 2004). Once developed, the effects of fluorosis are irreversible. Evidence supporting fluoride as a neurotoxin has also been presented by Grandjean (2019), the National Research Council (NRC) of the United States National Academies of Science (2006), and Ozsvath (2009). The evidence for reproductive, developmental, endocrine, digestive, genetic, and cancer injury has been summarized by NRC (2006).
In countries with high concentrations of F in drinking water, defluoridation is included among the mitigation responses. By contrast, in some developed countries or regions with low concentrations, decisions have been made to treat public water supplies by fluoridation. As of 2011, nearly 370 million people across 25 countries were estimated to have been supplied with artificially fluoridated water, with another 18 million receiving naturally fluoridated water at near optimum concentrations (O’Mallone et al., 2016). Target doses for fluoridated water are typically around 1 mg/L. On grounds of health concerns, water fluoridation remains a highly controversial topic (Connett, 2007).
Our primary purpose is to introduce current knowledge of the occurrence of F in groundwater and to describe the major processes controlling the solubility and mobility of F in groundwater systems. Groundwater concentrations of F depend on water-rock interactions and therefore the aquifer and bedrock mineralogy and composition are of prime importance. Groundwater residence time, temperature, and the solution composition are equally important. The lithologic unit of the aquifer and its mineralogy are the source material while the solution composition, temperature, and residence time also determine its solubility or degree of reactivity.