Modeling geochemical factors controlling fluoride concentration in groundwater

Abstract

The fluoride in groundwater can affect human health. The concentration of fluoride ions in groundwater is influenced by geogenic processes and is limited by the solubility of fluoride-containing minerals, especially fluorite. In this work, we summarize and assess fluorite solubility and the Pitzer interaction model parameters of related species, by which a quantitative model for calculation of the phases and chemical speciation in the Na–Ca–H–F–Cl–SO4–OH–HCO3–CO3–CO2–H2O aqueous system for temperatures 0–100 °C, and a pressure of 1 bar was established. Based on this model, we evaluate factors that govern the levels of fluoride, e.g. temperature, solution composition, pH, CO2 partial pressure, and concentration of carbonate minerals in the aquifer. The fluorite solubility increases with temperature and varies in different aqueous solutions. The fluoride concentration is highest in NaHCO3 type aqueous solutions. In acidic solution, HF is the dominant species, and free fluoride becomes dominant with increasing pH. Dissolved CO2 in solution slightly increases the concentration of fluoride in non-bicarbonate aqueous solution and non-calcite-bearing aquifer systems. In contrast, it has a significant negative effect in calcite and bicarbonate-bearing aquifer systems, where it greatly decreases the fluoride concentration. In calcite-bearing aquifer systems, the fluoride concentration is lower than in non-calcite mineral aquifer systems.