Geothermal waters are known for having the highest geogenic F concentrations of any natural water, e.g., 1,980 mg/kg in Rincón de la Vieja crater lake, Costa Rica (Kempter and Rowe, 2000) and 1,926 mg/kg in Kawah Ijen crater lake, Indonesia (Delmelle et al., 2000). Outside of acid crater lakes, hot springs and geysers more commonly have F concentrations of 5 to 50 mg/L (Deng et al., 2011). The subject of geothermal F is large enough to merit a separate paper which is being prepared. It should be noted, however, that the subject is of great importance to drinking water because when groundwater F is too high for drinking purposes, it is often caused by geothermal water leaking into an aquifer without an obvious increase in temperature (Armienta and Segovia, 2008; Carrillo-Rivera et al., 2002; Chae et al., 2007; Forrest et al., 2013; Murray, 1996b; Navarro et al., 2011; Parrone et al., 2020). A distinctive chemical feature of geothermal water is the associated elevation of Li, B, and As concentrations in addition to F (White, 1957). When ratioed to Cl, these elements differ little in water samples from deep drill holes compared to those in neutral-pH NaCl-type hot springs (Ellis and Mahon, 1964, 1977).