9 Natural Geochemical Processes Causing High-Fluoride Groundwater
High-F groundwaters originate overwhelmingly from natural geogenic processes having specific physical and chemical conditions. Assuming an F-bearing mineral occurs in the aquifer, F concentrations may increase by:
- development of a Na-HCO3 type groundwater;
- interaction with a F-rich aquifer such as felsic (silicic) and alkaline igneous rocks, or phosphoritic sediments;
- development of a saline type groundwater or low-Ca brine (ionic strength effect);
- calcite precipitation or precipitation of other relatively insoluble Ca minerals;
- dissolved inorganic carbon (DIC) increase and/or PCO2 decrease;
- increasing temperature (associated with precipitation of a Ca-bearing mineral and DIC); and,
- extremes of pH (high or low).
Many of these features are prevalent in arid and semi-arid environments, and high-F groundwater is commonly found in aquifers under such conditions. Other processes may well play a role, but the listed processes are likely the most important for aquifers not contaminated by industrial waste sources. As shown by the geochemical modeling examples, development of Na-HCO3 type waters, calcite precipitation, and PCO2 are all interrelated. From the examples that follow, a pattern emerges that aquifers with elevated F are saturated to supersaturated with respect to calcite and fluorite.