FLUORIDE CONCENTRATIONS IN A SOLE SOURCE, CRYSTALLINE BEDROCK AQUIFER, MARATHON COUNTY, WISCONSIN

Residents of rural Marathon County, Wisconsin typically rely upon groundwater from fractured, crystalline bedrock for their drinking water supplies. A study involving 2,786 private water-supply wells from this county finds that dissolved fluoride concentrations in the crystalline bedrock aquifer range from <0.01 to 7.6 mg/L, with 0.6 percent of the values exceeding the EPA maximum contaminant level (MCL) of 4.0 mg/L, and 8.6 percent exceeding the EPA secondary maximum contaminant level (SMCL) of 2.0 mg/L. Roughly a quarter of these wells contain fluoride within the range that is considered optimal for human health (between 0.5 and 1.5 mg/L), whereas 63.2 percent fall below 0.5 mg/L. The Marathon County Health Department is using the results of this study to encourage residents with private wells to have their drinking water supplies tested for fluoride.

Consistent with studies conducted in other regions, the felsic rocks in Marathon County have significantly higher fluoride concentrations than do rocks from other lithochemical groups (i.e., mafic and metasedimentary rocks). Amongst the felsic rock types, syenites yield the most fluoriferous groundwater, but the highest median concentration occurs in a biotite, sodium-plagioclase granite. A relationship between plagioclase composition and fluoride concentrations suggests that dissolved fluoride levels are controlled by fluorite solubility and that higher fluoride concentrations are generally found in soft, sodium-rich groundwater. Maps depicting the spatial correlations between rock type and dissolved fluoride concentrations provide residents with a means to assess the likelihood of their water supplies containing too much or too little fluoride.