Determining the source of anthropogenic or naturally occurring contaminants in public-supply wells in Missouri is complicated by the long open intervals of these wells. While samples from the wellhead confirm the presence of a contaminant, the mixed geochemical signature in such samples provides little information about the fate or vertical distribution of the contaminant in the aquifer. In 1999, the U.S. Geological Survey (USGS) began using borehole geophysical methods in combination with discrete sampling and borehole packer tests to determine the source of tetrachloroene (PCE) and trichloroethene (TCE) in public-supply, domestic, and monitoring wells at several sites in Missouri. Investigations at the Riverfront and Oak Grove Village Superfund sites in Franklin County indicated complex flow patterns within the wells and vertical stratification of contaminants. For example, at Oak Grove Village, the primary TCE source in one public-supply well that had wellhead TCE concentrations of about 20 micrograms per liter (ug/L) was a narrow permeable zone about 20 feet (ft) thick within the Potosi Dolomite that contained TCE concentrations of 100 ug/L. Similar investigations have determined the source of salinity in other public-supply wells and been used to successfully reconstruct domestic wells to minimize contamination.

During 2007, the USGS, in cooperation with the Missouri Department of Natural Resources (MDNR), Division of Geologic and Land Survey (DGLS), and the Public Drinking Water Branch (PDWB), began using these same techniques in an investigation of naturally occurring radionuclides in public-supply wells in several areas of the State. The objective of this investigation is to determine the vertical distribution of radionuclides in the ground-water system penetrated by these wells and use this information in combination with geohydrologic and geochemical data to assist the MDNR and water suppliers in developing cost effective long-term solutions to bring these systems into compliance.