RADIUM RELEASE FROM BEDROCK TO GROUNDWATER DUE TO GEOCHEMICAL CONDITIONS ASSOCIATED WITH THE DEGRADATION OF AN ORGANIC CONTAMINANT PLUME

Secondary water quality impacts associated with hydrocarbon spills that reach the subsurface can include the release of naturally occurring contaminants from bedrock to groundwater under changing geochemical conditions within the organic plume. The biodegradation of organic carbon can be coupled with reduction of electron acceptors including iron (Fe(III)) and manganese (Mn(IV), where reductive dissolution of Fe and Mn (hydr)oxides can release sorbed contaminants, such as arsenic, to groundwater. Radium (Ra), a carcinogen regulated in U.S. drinking water at a total ²²⁶Ra and ²²⁸Ra activity of 185 millibecquerels per liter (mBq/L), is another geogenic contaminant with a strong sorption affinity for Fe and Mn (hydr)oxides with potential to be released under the geochemical conditions in hydrocarbon-contaminated aquifers.

Radium activities were investigated in a sandstone aquifer near Madison, Wisconsin, contaminated with a mixture of chlorinated solvents, ketones, and aromatics occurring as a dense non-aqueous phase liquid in the source zone. Groundwater samples were collected at Westbay® multi-level systems (MLS) from ports within the Tunnel City Group and Readstown Member of the Cambrian-Ordovician aquifer system. MLS MP-16, located outside the dissolved phase plume, was used for background ²²⁶Ra activities. MLS MP-24S and MP-19S are located 60 and 600 m east of the source zone, respectively, in the direction of contaminant plume migration. Samples were analyzed for ²²⁶Ra and ²²⁸Ra using multi-collector inductively-coupled plasma mass spectrometry. ²²⁶Ra activities are up to ten times higher than background 60 m downgradient from the source zone, where pH is lower, total dissolved solids concentration is higher, and conditions are methanogenic. Correlations indicate Fe and Mn (hydr)oxide reduction and sorption site competition are likely responsible for the elevated Ra activities. ²²⁶Ra activities are attenuated to near background within the Fe(III)/SO₄^2--reducing zone 600 m downgradient from the source via sorption to secondary minerals (e.g., clays). Although the maximum total Ra activity of 100 mBq/L is well below the EPA standard, elevated activities compared to background emphasize the importance of examining cycling of Ra and other trace elements in hydrocarbon-contaminated aquifers.