Effect of geochemical conditions on radium mobility in discrete intervals within the Midwestern Cambrian-Ordovician aquifer system

Abstract

Radium (Ra) commonly occurs in groundwater obtained from the Midwestern Cambrian-Ordovician aquifer system (C-O-AS) at activities approaching and exceeding the United States Environmental Protection Agency's Maximum Contaminant Level (MCL) of 5 pCi/L for combined 226Ra and 228Ra. The occurrence of Ra(II) in groundwater is dependent on a number of factors, including the prevalence of parent radionuclides within hydrogeological strata, as well as aquifer geochemical conditions. Interbedded aquifer and aquitard sequences within the Midwestern C-O-AS are stratified with respect to Ra(II) activity and geochemical conditions, and thus the formations that serve as Ra(II) sources to groundwater remain poorly constrained. This study analyzes aqueous samples collected from short-screened wells at various depths within the Midwestern C-O-AS near Madison, WI, USA, to determine geochemical parameters including dissolved oxygen; pH; major and minor ions; and metals, including 226Ra, 228Ra, and parent isotopes 238U and 232Th. Additionally, the elemental composition of aquifer solids is determined as a function of depth. Within solid phases, 238U and 232Th occur in both fine-grained facies and as coatings on sandstone minerals. Most groundwater samples contain dissolved combined Ra(II) lower than 2.5 pCi/L; however, one well completed in the unconfined and one well completed in the confined portion of the groundwater system exceed 3.5 pCi/L. In the confined system, anoxic conditions are associated with elevated Ra(II) concentrations, while in the upper, oxic aquifer, elevated total dissolved solids are positively related to Ra(II). These results demonstrate that multiple factors impact Ra(II) sources and mobility in the regionally unconfined portion of the Midwestern C-O-AS.