Possible Reasons for Elevated Fluorine Concentrations in Groundwaters of Carbonate Rocks

The concentration of dissolved F did not change in long-term (four months) experiments on the interaction of crushed limestone and marl with water (at R/W = 1) at room temperature. The comparison of the activity products and equilibrium constants of the mineral dissolution reactions indicates that the experimental solutions were undersaturated with respect to fluorite and oversaturated with respect to F-apatite. The long-term existence of such unequilibrated solutions is explained by the steady state of the system at which the primary F-bearing mineral (fluorite, mica, or palygorskite) is transformed into a secondary one (F-apatite). The dissolved F concentration is controlled under these conditions by the ratios of the dissolution reaction rates for the primary mineral and the precipitation rate of the secondary one. In the experiments with the association fluorite + calcite + dolomite (duration up to seven months), the solution was saturated with respect to these minerals. Their dissolution constants were utilized to derive dependences of the activity of the dissolved F ion on the activities of the Ca, Mg, and carbonate ions in the solutions. The experimental data are consistent with these dependences. If the solution is saturated with respect to gypsum, the activity of the dissolved F ion should also depend on one more parameter: the activity of the sulfate ion. When groundwaters filtrate for a long time or are stagnant, the association F-apatite + calcite + dolomite (+ gypsum) is stable; dependences of the activity of the F ion on the activities of aqueous species in the solution were derived for this association. These dependences are much stronger than those for the association fluorite + calcite + dolomite (+ gypsum).