THE ORIGIN OF ELEVATED MOLYBDENUM IN SOUTHEAST WISCONSIN GROUNDWATER

Molybdenum (Mo) is an essential trace nutrient but has negative health effects at high concentrations. Groundwater has typically low Mo (<2 µg/L), and elevated levels have been associated with anthropogenic input, although geogenic sources have also been reported. Here we investigated the occurrence and distribution of Mo in an attempt to evaluate its source in drinking water wells from shallow glacial sand and gravel and Silurain dolomite aquifers of southeast Wisconsin, USA. The legacy of massive coal ash disposal throughout the region has been suggested as the source of Mo in the regional aquifers. Groundwater well samples were collected in areas near and away from known coal ash disposal sites and analyzed for Mo along with diagnostic geochemical and isotopic tracers, and tritium-helium indicators for groundwater residence time. Molybdenum concentrations ranged from <1 to 150 µg/L with concentrations exceeding the U.S. Environmental Protection Agency health advisory of 40 µg/L in alkaline (pH >7.5), anoxic groundwater dominated by sodium and sulfate ions. High Mo was found in older groundwater (mean residence time >300 y) and primarily in confined areas. The boron (δ¹¹B= 22.9±3.5‰) and strontium (⁸⁷Sr/⁸⁶Sr=0.70923±0.00024) isotope ratios were not consistent with the expected isotope fingerprints of coal ash but rather mimic the compositions of the aquifer rocks. Given the older residence time, the location in confined areas of the aquifer and the geochemical composition, we hypothesize that geogenic Mo is released from oxidation of sulfide minerals (i.e. pyrite) in the carbonate aquifer and/or from upward migration of Mo-rich groundwater from the Mauqoketa shale that underlies the regional aquifer.