RADIUM SOURCES AND MOBILITY IN DISCRETE HYDROSTRATIGRAPHIC UNITS WITHIN THE MIDWESTERN CAMBRIAN-ORDOVICIAN AQUIFER

Most municipal supply wells in eastern Wisconsin are completed in regionally confined portions of the Cambrian-Ordovician aquifer, but many of these wells produce water with combined radium (²²⁶Ra + ²²⁸Ra) that exceeds the 5 picoCuries/L (pCi/L) federal drinking water standard. Elevated Ra is also common locally to the west of the Maquoketa shale aquitard, including Madison, Wisconsin, where upper and lower sandstone aquifers are separated by fine-grained siltstone and shale facies within the Eau Claire Formation. The study included sampling groundwater from twenty-two short-screened monitoring wells completed in discrete hydrostratigraphic units. Aqueous analyses included major ion chemistry, Ra²²⁶, Ra²²⁸, uranium and thorium, pH, conductivity, and dissolved oxygen (DO). Analysis of aquifer solids included X-ray fluorescence and X-ray absorption spectroscopy. The groundwater samples ranged from less-than-detection to 5.2 pCi/L, with an average concentration of 1.31 pCi/L Ra. There was little intra-well variability over the two sampling periods in 2016 and 2017. In general, Ra is higher in hydrostratigraphic units with low DO or monitoring wells with elevated specific conductance. These findings suggest that possible factors contributing to elevated Ra concentrations may be loss of Ra(II)-retaining sorption sites due to anoxic conditions, or increased ion competition for sorption sites due to elevated total dissolved solids. Knowledge of the variation in water chemistry between hydrostratigraphic units can potentially inform well design and construction to reduce Ra in drinking water.