FLUORIDE IN GROUNDWATER: INSIGHTS FROM GEOCHEMICAL MODELING (Invited Presentation)

Fluoride commonly occurs in groundwater at concentrations inadvisable for drinking (>1 mg/L) and reflects a distinctive chemical evolution that can be interpreted with geochemical modeling. This evolution requires a source mineral (fluorite or fluorapatite), suppression of dissolved calcium by the precipitation of a less soluble mineral (e.g. calcite) and/or by Ca-Na cation exchange, and it is affected by ionic strength, P_CO2, and temperature. Geogenic contamination of an aquifer by fluoride is often the result of mixing of ambient low-fluoride groundwaters with low-enthalpy high-fluoride thermal waters. High-temperature geothermal fluids have some of the highest fluoride concentrations ever reported for a natural water. Geochemical model simulations with graphical output are consistent with field data from aquifers in sedimentary and igneous rocks. Field examples from the Black Creek sedimentary aquifer, South Carolina, USA, the North China Plain sedimentary aquifer, the Lincolnshire limestone aquifer, UK, and the Stripa granite aquifer, Sweden, consistently show calcite and fluorite (or francolite) at or above saturation. A new plot of F/Cl against HCO₃/Cl ratios for field data is more revealing of these processes than the typical bivariant plot. When fluorite is added to the ion exchange model of Appelo (1984) for the Aquia aquifer, Maryland, fluoride concentrations can reproduce the field data. Fluoride concentrations can be simulated for Yellowstone National Park waters by simultaneously applying fluorite and calcite equilibrium solubility as a function of temperature. By comparing the solubility product constants of calcite and fluorite from 0-200°C, the solubility of calcite becomes lower than that of fluorite at about 50°C and fluoride concentrations increase twice as much as calcium concentrations which decrease at higher temperatures because of retrograde calcite solubility. The conclusion from this study is that for a wide range of lithologies and climatic differences, the main factors driving high fluoride concentrations are source rock and mineralogy, calcite precipitation, ionic strength, P_CO2, and temperature.