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.


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