ASSESSING RADIUM ACTIVITIES WITH RESPECT TO RECHARGE IN THE MIDWESTERN CAMBRIAN-ORDOVICIAN AQUIFER SYSTEM

Radium (Ra) is a radioactive contaminant produced in groundwater systems by decay of parent nuclides (e.g., ²³⁸U, ²³²Th). Long-term ingestion of Ra in drinking water is linked to development of bone cancer and is therefore regulated by the EPA at a Maximum Contaminant Level of 5 picocuries/L (²²⁶Ra + ²²⁸Ra). The Midwestern Cambrian-Ordovician aquifer system (MCOAS) exceeds the Ra MCL at many locations, presenting challenges to communities using the MCOAS for drinking water. Radium activities are generally higher where the MCOAS is regionally confined and modern recharge is limited; however, elevated activities also occur in the regionally unconfined portion of the aquifer. Here, we use a suite of isotopes (δ¹⁸O, δD, δ³⁴S_SO4, ²³⁴U/²³⁸U) to examine Ra occurrence with respect to recharge along a groundwater flow path in Wisconsin, where the MCOAS straddles the transition from regionally unconfined to regionally confined. Thirty-two wells sampled along the flow path vary in depth from shallow bedrock to deep, Mt. Simon sandstone. Groundwater evolves from a mixed cation-bicarbonate type to calcium-sulfate and calcium-chloride types along the west-to-east flow path. Isotopic results reveal Pleistocene glacial recharge at the end of the flow path, where local conditions (e.g., local confinement) limit modern recharge. Here, Ra activities rise above the MCL as residence time increases and geochemical conditions become favorable for Ra mobility (e.g., elevated total dissolved solids). The highest Ra activities occur in samples classified as ‘mixed’ redox, suggesting that these municipal wells, with long open boreholes, produce groundwater from a variety of redox environments within the aquifer system. Multiple mechanisms, including competitive sorption and the dissolution or absence of iron and manganese (hydr)oxides, contribute to Ra activities. Results demonstrate the importance of local hydrogeologic conditions and aquifer recharge history on Ra activities in groundwater.