Geohydrologic characterization and qualification of a high-level waste site in basalts

Geohydrologic studies are under way on the Hanford Site, Washington, to characterize the Columbia River basalt as a possible geologic medium for disposal of nuclear wastes. Findings indicate that deformation of the Pasco Basin and reference repository location has occurred slowly (averaging < 0.1 mm/yr) and is expected to continue at a slow rate along existing geologic structures. Most groundwater in the deep basalts moves laterally through portions of basalt flow tops under low ( about 10-4 m/m) hydraulic gradients. These flow tops are separated by basalt flow interiors having hydraulic conductivities in the range of 10-11 to 10-13 m/s. Using site-specific characterization data, performance assessment modeling indicates that groundwater traveltimes to the accessible environment are > 10,000 yr. Because of the limiting solubilities of major radionuclides and the sorptive capacity of basalt, most radionuclides are effectively immobilized in the basalt medium