Groundwater fluoride contamination: A reappraisal

Dissolution of fluorite (CaF2) and/or fluorapatite (FAP) Ca5(PO4)3F, pulled by calcite precipitation, is thought to be the dominant mechanism responsible for groundwater fluoride (F−) contamination. Here, one dimensional reactive–transport models are developed to test this mechanism using the published dissolution and precipitation rate kinetics for the mineral pair FAP and calcite. Simulation results correctly show positive correlation between the aqueous concentrations of F− and and negative correlation between F− and Ca2+. Results also show that precipitation of calcite, contrary to the present understanding, slows down the FAP dissolution by 106 orders of magnitude compared to the FAP dissolution by hydrolysis. For appreciable amount of fluoride contamination rock–water interaction time must be long and of order 106 years.