The occurrence and geochemistry of fluorides in natural waters in Finland and East Africa with reference to their geomedical implications

Abstract Coarse-grained K-rich granites in Finland, particularly rapakivi granites, have a higher concentration of fluorine (0.2–0.4%) than other plutonic or metamorphic rocks. The main accessory minerals containing appreciable amounts of F are fluorite and fluorapatite, although OH-bearing minerals such as micas and amphiboles may also contain abundant F (0.2–1.1%). High F concentrations in rocks are reflected as regional F anomalies in soils, natural surface waters and shallow groundwaters. In streams and shallow groundwater draining areas composed of rapakivi granites. F concentrations are more than an order of magnitude higher (1–2 mg/l) than elsewhere (below 0.1 mg/l). This level of F concentrations in drinking water has proved to diminish the incidence of dental caries in Finland and elsewhere. In some parts of the world, granites (India, China) and particularly some recent or Neogene volcanics (East Africa) may release abundant F into waters to be further concentrated by strong evapotranspiration (up to 10–20 mg/l in potable water). As a consequence, F intake may reach harmful levels causing dental or skeletal fluorosis.