RADIONUCLIDE ASSOCIATION WITH AQUIFER SOLIDS IN THE MIDWESTERN CAMBRIAN-ORDOVICIAN AQUIFER SYSTEM

Radium (Ra), a naturally-occurring contaminant in aquifer systems, is associated with elevated risk of cancer if ingested. The Midwestern Cambrian-Ordovician aquifer system is known to have frequent occurrence of Ra activity above the maximum contaminant level (MCL) of 5 pCi/L for the combination of the two major isotopes, ²²⁶Ra + ²²⁸Ra. Additionally, this aquifer system is used as a drinking water source for many municipalities. Radium is emitted from parent nuclides uranium and thorium (e.g., ²³⁸U and ²³²Th) via alpha decay. While these radionuclides are present in aquifer solids across the system, little is known about their direct relationship to Ra production in the groundwater. In this study, we examine Ra, U, and Th levels in bedrock, in order to measure the aqueous association between the radionuclides and the aquifer solids. To achieve this, we conducted sequential extraction experiments on selected sections of bedrock core from the regionally-confined portion of the Midwestern Cambrian-Ordovician aquifer system. The experiment examined metals associated with the water-extractable fraction, the weakly sorbed fraction, strongly sorbed fraction, and a complete digestion of the rock. Analysis of Ra isotopes was conducted by multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS). Due to the small concentrations, isolation of Ra was accomplished through a series of columns to separate interfering ions prior to MC-ICPMS analysis. Analysis of ²³⁸U and ²³²Th was accomplished by high resolution ICPMS. The association of these radionuclides with the aquifer solids will contribute further understanding about aquifer solids as a source of radionuclides to groundwater.