RADIONUCLIDES IN PUBLIC-SUPPLY WELLS IN MISSOURI

More than 20 Public Water Supply Systems in the state of Missouri have exceeded the maximum contaminant levels (MCLs) for gross alpha particle activity (greater than 15 picocuries per liter [pCi/L]) and combined radium-226 and 228 (²²⁶Ra and ²²⁸Ra) of 5 pCi/L. It has been known for decades that most systems with elevated radionuclides withdraw water from the Ozark aquifer and tend to be located where the Cambrian and Ordovician rocks become confined by younger rocks. Concentrations of gross alpha activity and ²²⁶Ra are the predominate radionuclides exceeding standards; however, ²²⁸Ra and potentially radium-224 (²²⁴Ra) are important gross alpha sources in some areas within and near the St. Francois Mountains. Determining the origin of the radionuclides is complicated by the long open intervals of public-supply wells. Wellhead samples confirm the presence of radionuclides in wells serving these systems, but mixed geochemical signatures in such samples provides little information about the origin and vertical distribution of the contaminant in the aquifer.

The USGS, in cooperation with the Missouri Department of Natural Resources (MDNR) Public Drinking Water Branch and Missouri Geological Survey, has been using borehole geophysical logging combined with discrete sampling and packer testing to investigate the origin of radionuclides in public-supply systems as wells become available for study such as when pumps are pulled for servicing. To date, five systems distributed around the state have been examined. Radionuclide profiles in four of the five systems have strong vertical gradients and are associated with increased specific conductivity and chloride concentrations typically in the lower intervals of the boreholes. Several wells were successfully reconstructed to seal off high-chloride and radionuclide intervals. Other wells, near the St. Francois Mountains and open to the Lamotte Sandstone, have radionuclides throughout the water-yielding intervals of the open borehole and tended to have increased ²²⁸Ra and potentially large ²²⁴Ra (estimated from the difference in 72-hour and 30-day gross alpha counts). In some cases, 72-hour gross alpha counts exceeded 300 pCi/L in samples from the Lamotte Sandstone, indicating a potential risk to domestic supply where this formation may be reached by domestic wells.