ISOTOPIC ANALYSIS OF RADIONUCLIDE PRESENCE AT DISCRETE INTERVALS IN THE MIDWESTERN CAMBRIAN-ORDOVICIAN AQUIFER SYSTEM

Naturally occurring contaminants like radium (Ra) pose a danger to groundwater as a drinking water source. The Midwestern Cambrian-Ordovician aquifer system (MCOAS), a source of drinking water for many municipalities in the Upper Midwest of the USA, is frequently observed to contain combined Ra (²²⁶Ra + ²²⁸Ra) above the maximum contaminant level of 5 pCi/L. Here, we investigate ²²⁶Ra associations with aquifer solids through a sequential extraction and digestion experiment, using sensitive multi-collector inductively coupled plasma mass spectrometry analysis to quantify the ultra trace Ra levels. We also compare parent ²³⁸U and daughter ²²⁶Ra isotope activities to determine Ra leachability. Equilibrium between parent and daughter isotope activities varies across stratigraphy, suggesting that Ra mobilization depends on minerals present in the bedrock layer in addition to the influence of local geochemical conditions releasing Ra to the aqueous system. The association between Ra and the aquifer solids varies across stratigraphy. Aquifer geochemical conditions will impact the release of Ra to the aqueous system differently in different stratigraphic units. Due to the heterogeneous nature of the aquifer system, the variation of stratigraphy presence across the extent of the aquifer suggests that Ra mobility varies due to local geochemical conditions and available mineral associations. This suggests that changes in groundwater flow from one part of the aquifer system to another may impact local geochemistry and mobilize naturally occurring contaminants.