Borehole geophysics as applied to the management of radioactive waste: site selection and monitoring

The U.S. Geological Survey is conducting research on borehole geophysics, including the application of well logging to the investigation of radioactive waste disposal sites. During the past 14 years, eight such sites have been studied. Geophysical well logging provides in situ measuring techniques which can be used to supplement conventional coring and water sampling techniques in both site-selection studies and in the monitoring of operating disposal sites. Some of the advantages of well logging over conventional techniques include the relatively large volume of rock investigated and the ability to measure temporal changes. Logging also reduces the amount of expensive coring required. Logs provide a continuous record of data that can be correlated from hole to hole. Data from geophysical logs can be interpreted in terms of lithology, elastic moduli, bulk density, porosity, moisture content, water quality, and the location and orientation of fractures. After a site is in use, borehole geophysics provides a means to monitor changes in moisture content, water level, and radionuclide concentration in either cased or open holes. Water samples, usually taken by conventional methods through wells, provide data only on the permeable intervals below the water table. Gamma spectral logging techniques provide data on the vertical and lateral distribution of contaminants. Radionuclides that have migrated have been identified at several sites. Detection limits on the order of 0.2 picocuries per gram have been attained for 60Co and 137Cs. A number of other gamma-emitting radioisotopes can be identified using in-hole gamma spectrometry