[en] 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 the deformation of the Pasco asin and reference repository location has occurred slowly (averaging < 0.1 mm/y) and is expected to continue at a slow rate along existing geologic structues. Most groundwater in the deep basalts moves laterally through portions of basalt flow tops under low (approx. 10^-₄ m/m) hydraulic gradients. These flow tops are separated by basalt flow interiors having hydraulic conductives 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 y. Because of the limiting solubilities of major radionuclides and the sorptive capacity of basalt, most radionulcides are effectively immobilized in the basalt medium