ASSESSING RADIUM LEACHING POTENTIAL FROM STRATIGRAPHIC UNITS WITHIN THE MIDWESTERN CAMBRIAN-ORDOVICIAN AQUIFER SYSTEM

Radium (Ra) is a naturally occurring contaminant produced by the decay of parent isotope decay chains (e.g., thorium (²³²Th) and uranium (²³⁸U)). Radium exceeds the U.S. EPA Maximum Contaminant Level at some locations within the Midwestern Cambrian-Ordovician aquifer system (MCOAS), an important source of drinking water in the Midwest. In the MCOAS, Ra solid-phase associations vary across stratigraphic units, suggesting that geochemical conditions will have different impacts on Ra release from the solid to aqueous phase within each unit. Here, we evaluate Ra leaching potential from stratigraphic units within the MCOAS to determine the effects of mineralogy and increasing total dissolved solids on Ra release to groundwater.

Batch experiments were performed with MCOAS core samples with differing mineralogy. For example, the Decorah dolostone contains numerous green shale partings, while the dolomitic contact between Platteville dolostone and St Peter sandstone is characterized by high sand content, and pyrite-filled vugs and pyrite-healed vertical fractures. Subsamples were taken from each stratigraphic unit, pulverized and kept in an anoxic environment throughout the experiment. Then, 200 mL of an anoxic 10 mM NaCl solution was added to 20 g of each core sample and stirred continuously. Aqueous samples were then collected after 6, 12, 24, 48, 72, and 168 hours. Upon collection, samples were centrifuged, filtered, and analyzed for sulfate and metals (Ca, Mg, K, Ba, Fe, Mn, Sr, U, Th). A subset of samples were analyzed for ²²⁶Ra using multi-collector inductively-coupled plasma mass spectrometry. Concentrations of dissolved sulfate and metals stabilize by 72 hours. Additional experiments will utilize stirred-flow reactors and higher concentrations of NaCl solution (e.g., 100 mM, 1000 mM) to examine the effect of competitive sorption on Ra leaching. Results will enhance understanding of Ra release from stratigraphic units to the aquifer system, provide data for models examining sources of Ra to municipal wells, and help inform strategies (e.g., well reconstruction) to minimize Ra in drinking water.