Geogenic sources and chemical controls on fluoride release to groundwater in the Independence Basin, Mexico. (December 2020) - LaFayette, Gabrielle N.
Abstract: High fluoride (F − ) concentrations in groundwater are a recognized environmental health concern, and over 200 million people around the world routinely drink well water with F − concentrations that exceed safety guidelines. Groundwater in the Independence Basin of Central Mexico reaches F − concentrations of 15.5 mg L −1, which are over an order of magnitude greater than the World Health Organization limit of 1.5 mg L −1, but knowledge of sources and geochemical conditions responsible for releasing F − into solution is preliminary. The objectives of this work were to define F − hosts within a range of Independence Basin aquifer rock types and quantify chemical controls on F − release into solution. Solid-phase concentrations of F − in well-drill cuttings display no apparent depth dependence but are greatest within andesite, near-surface uncemented alluvium, and well-cemented Paleocene conglomerate samples, and they are least within volcanic glass, rhyolite, and ash matrix samples. Chemical extractions showed that large amounts of F − are bound with metal oxides/oxyhydroxides in all samples. Batch experiments examining F − release from aquifer materials under evolving groundwater conditions (i.e. increasing pH and specific conductance) revealed that F − may be released into solution with elevated pH but only slightly mobilized with increasing specific conductance. Therefore, declining aquifer water levels tapping into older, more mineralized groundwater (with high conductivity) in the Independence Basin may not have as great of an effect on F − release to solution than does evolving and increasing groundwater pH. Accordingly, the degree of rock weathering and pH of solution play the greatest roles in regulating the release of F − to groundwater, and results from this study may be used to better predict where F − groundwater contamination is most probable in aquifers around the world. Highlights: Fluoride is a widespread groundwater contaminant in the Independence Basin, Mexico. Fluoride naturally occurs in all aquifer rock types in the present study. Metal oxides/oxyhydroxides are important hosts of fluoride in aquifer materials. Fluoride in solution increases with rising pH. Fluoride in solution slightly increases with specific conductance.