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The Lake St. Martin bolide has a big impact on groundwater fluoride concentrations

¹ GNS Science, P.O. Box 30-368, Lower Hutt, New Zealand
² Geological Survey of Canada, 601 Booth Street, Ottawa, ON K1A 0E8, Canada
³ Department of Earth and Environmental Sciences, Tulane University, New Orleans, Louisiana 70118, USA
⁴ Geological Survey of Canada, 601 Booth Street, Ottawa, ON K1A 0E8, Canada

The majority of residents of Manitoba (Canada) outside of the capital, Winnipeg, rely on groundwater for their drinking water. Between lakes Winnipeg and Winnipegosis, most aquifers occur in Paleozoic carbonate lithologies. Proximal to the town of Gypsumville, however, lithologies associated with the Lake St. Martin impact structure and younger basin-filling red bed and evaporite (gypsum/anhydrite) sedimentary rocks complicate the hydrology and hydrochemistry. Here, domestic wells have elevated salinities (up to 8000 mg/L total dissolved solids), elevated sulfate (up to 4000 mg/L), and elevated fluoride concentrations that are in excess of health limits (F^– up to 15.2 mg/L, with 20% over 1.5 mg/L). Groundwaters with elevated fluoride occur exclusively within the impact structure. The impact melt rocks and younger red beds consistently have the highest fluoride abundances, up to 2160 ppm. Groundwater pH values are alkaline, ranging up to 10.7, with highest groundwater pH from wells in the impact melt rocks. The spatial associations of impact melt rocks and red beds with elevated fluoride, strong positive correlations between fluoride and pH, sodium, chloride, sulfate, boron and lithium, greater Fe₂O₃ and Al₂O₃ concentrations of the host rocks, and cation exchange capacity (CEC) all indicate that fluoride concentrations in groundwaters are enhanced as a result of anion exchange wherein OH^– and CO₃^2– displace F^– adsorbed onto Fe- and Al-oxyhydroxide surfaces. Thus, the elevated fluoride contents of groundwaters at Gypsumville are a consequence of the composition of the impact melt rocks and enhanced permeability and grain-size reduction produced by bolide impact.

Key Words: fluoride • groundwater • ion exchange • impact melt