Information Circular 36. Radium in the Mt. Simon-Hinckley Aquifer, East-Central and Southeastern Minnesota

Abstract

Studies conducted in the 1960s showed that radium was a fairly common constituent in ground water throughout much of the Midwest. Concentrations of 226Ra ranging from 1 to 80 pCi/L (picoCuries per liter) were found in northern illinois, Iowa, and eastern Wisconsin (Rowland and others, 1977). The high radium values were associated with water from deep sandstone aquifers of Cambro-Ordovician age, but not with aquifers above or below. Additional studies and reports of radium in the Cambro-Ordovician aquifers by Gilkeson and Cowart (1982, 1987), Hahn (1984), and Weaver and Bahr (1991) further delineated distribution patterns and have suggested sources for the observed concentrations. Radium is a naturally occurring radioactive element, formed by radioactive decay of uranium and thorium. The most abundant radium isotope, 226Ra, has a half-life of 1600 years and is part of the 238U decay series. Two other isotopes, 228Ra (half-life = 5.76 years) and 224Ra (half-life = 3.7 days), are part of the 232Th decay series. In 1976, the U.S. Environmental Protection Agency published regulations that established contaminant levels for radioactivity and radioactive nuclides in public water systems. The maximum contaminant level (MCL) for gross alpha activity was not to exceed 15 pCi/L and the combined 226Ra and 228Ra activity in the water was not to exceed 5 pCi/L (Federal Register, 1976). The U.S. Environmental Protection Agency is reviewing the contaminant levels for radioactive nuclides in drinking water. Testing of public water supplies by the Minnesota Department of Health showed that many communities in the southern half of Minnesota were not in compliance with the drinking water standards for radium. Sampling of some municipal wells indicated that high radium levels were chiefly associated with water from the Mt. Simon-Hinckley aquifer, a sandstone aquifer of Late Cambrian/Middle Proterozoic age. A few wells in the Jordan Sandstone of Late Cambrian age also showed elevated radium levels. Because these data were obtained from municipal wells in communities with known radium problems, it was difficult to detect a pattern or to isolate specific radium-producing horizons within the aquifer(s). Therefore, the Minnesota Geological Survey sampled water from sole-source Mt. Simon-Hinckley wells to identify the distribution of radium within the regional geologic framework. Base-level data were also acquired on other radionuclides, the age of the water, and the water chemistry. Sole-source Mt. Simon-Hinckley wells were selected for sampling based on existing water-well information, and included wells previously monitored by the Minnesota Department of Health. The study area extended north to south from the town of Hinckley to the Root River basin, and west to east from the Mankato area to the Mississippi River. Complete coverage could not be obtained because of a lack of sole-source Mt. Simon-Hinckley wells in the south-central part of the aquifer. Several Jordan aquifer wells were also selected for sampling. The data accumulated in this study provide a starting point for selecting mitigation strategies or for choosing appropriate sites for future wells.