Distribution of Radium in Minnesota Drinking Water Aquifers

Radium is a naturally-occurring radioactive metal produced by decay of geologically abundant uranium or thorium. Public water systems monitor radium-226 (226Ra) and radium-228 (228Ra) because ingesting combined radium (226Ra + 228Ra) above the Maximum Contaminant Level (MCL) of 5.0 picocuries per liter (pCi/L) may present increased cancer risk to humans (bone and blood cancers). The presence of radium in source water above the MCL adds expense to public water systems because of the need for treatment or blending. The health and economic difficulties that radium causes can be reduced by understanding factors that control its occurrence. Studies of radium in groundwater across many parts of North America indicate that southern Minnesota is within a zone where radium is present at high levels (Focazio, et al., 2001). Lively et al., (1992) defined the distribution of radium within the Mt. Simon Aquifer in Minnesota. From 20072010, the Minnesota Department of Health (MDH) investigated patterns of radium occurrence in two Cambrian sandstone drinking water aquifers (Mt. Simon and Jordan) beneath southern Minnesota to guide future well-drilling efforts away from radium-producing aquifers and to identify areas where treatment may be required to meet federal drinking water standards. This memorandum summarizes three steps taken to fulfill those objectives: 1) capturing all available radium data (all aquifers) from existing water quality databases; 2) generating new data in selected locations through the sampling of wells completed in the Mt. Simon Aquifer or the St. Peter-Prairie du Chien-Jordan Aquifer system, and 3) mapping the resulting radium distributions. Steps 1 and 2 were carried out in close collaboration with David Vinson (Ph.D. candidate, hydrogeochemistry, Duke University) while he was employed by MDH.