Geologic sources of radium to municipal wells in Wisconsin

Radium commonly occurs in groundwater obtained from the Midwestern, Cambrian-Ordovician aquifer system at activities (a measure of concentration) close to or exceeding the U.S. Environmental Protection Agency’s Maximum Contaminant Level (MCL) of 5 pCi/L for combined 226Ra and 228Ra. Many communities in Wisconsin and the north-central United States rely on these regionally extensive, dolomitic and sandstone aquifers for their principal source of drinking water. A recent U.S. Geological Survey study of radium occurrence and geochemistry found this aquifer in the Midcontinent and Ozark Plateau region has the highest frequency of Ra occurrence among the 15 major aquifer systems in the country. The 95 public water systems in Wisconsin with Ra levels exceeding the MCL have adopted various strategies to bring their systems into compliance. These strategies include well reconstruction, water treatment by ion exchange or blending, and/or abandonment of groundwater systems in favor of surface water sources. However, the source of Ra contamination to the groundwater has not been identified: therefore it is difficult to plan municipal well construction to avoid Ra contaminated water. The proposed research project is designed to address this gap in knowledge by developing a quantitative relationship between sediment and aqueous geochemistry and the concentration of Ra in groundwater.

Project Objectives
The goal of this project was to develop a geochemical model describing the relationship of Ra to specific aquifer solids, in order to provide increased scientific understanding for strategies to minimize Ra in groundwater used as a drinking water source. The specific objectives and the work to achieve them is discussed below:
1. Investigate the isotopic signature of ²²⁶Ra and ²²⁸Ra in groundwater from the Midwestern Cambrian-Ordovician aquifer system to determine potential sources.
2. Quantify potential solid-phase sources of Ra, and parent nuclides U and Th. This includes studying nuclide speciation, dissolution, and/or sorption potential in these same solids
3. Provide a geochemical basis for management decisions regarding amelioration of high Ra levels in municipal wells