INVESTIGATION OF SOLUTE SOURCES TO THE MIDWESTERN CAMBRIAN-ORDOVICIAN AQUIFER SYSTEM OF EASTERN WISCONSIN

The Midwestern Cambrian-Ordovician aquifer system (MCOAS), an important source of drinking water, contains levels of radium (Ra) greater than the EPA Maximum Contaminant Level at many locations in eastern Wisconsin (WI). Elevated levels of Ra often correspond with anoxic aquifer conditions and elevated total dissolved solids. However, natural sources of solutes contributing to total dissolved solids may vary throughout the aquifer. In this study, we use a suite of isotopes (δ¹⁸O/δD, δ³⁴S – SO₄^2-, ⁸⁷Sr/⁸⁶Sr, ²³⁴U/²³⁸U) to provide new constraints on the sources of solutes to the MCOAS in Fond du Lac, WI. Groundwater samples were collected from 19 deep wells open to multiple units of the MCOAS and 13 shallower wells open to the uppermost units of the MCOAS. Groundwater from deep wells ranges from Mg-HCO₃ to Ca-Cl and Ca-SO₄-types, while groundwater from shallow wells is predominantly Mg-HCO₃-type. The δ¹⁸O-δD of samples indicates a meteoric origin with a temperature signal that suggests deep groundwater was recharged during cooler conditions, likely during the Pleistocene. Additionally, the δ³⁴S of SO₄^2- in most deep groundwater samples indicates a source of sulfate derived from the dissolution and transport of Silurian evaporites in the Michigan Basin. The ⁸⁷Sr/⁸⁶Sr of samples exhibit little variability, ranging from 0.7092-0.7112. The ²³⁴U/²³⁸U activity ratios range from 2-18 and reflect the differences in relative water-rock interaction. Results demonstrate the influence of multiple geochemical factors on combined Ra (²²⁶Ra + ²²⁸Ra) levels, including elevated total dissolved solids in the deep portion of the MCOAS affected by the transport of solutes from the Michigan Basin during the Pleistocene.