[en] Within the Basalt Waste Isolation Project, geohydrologic studies are focusing on repository site characterization, particularly those factors that might influence groundwater transport of radionuclides to the accessible environment. Beneath the Hanford Site, the Columbia River basalt consists of at least 70 basalt flows extending to a depth of >3 km. Within the 180-km² central Cold Creek syncline, these flows appear to be nearly flat lying; at least four are thick (>40 m) and laterally continuous and are presently considered potential candidates for repository development. These flows lie 850 to 1150 m below ground level within a 47-km² area currently considered to be the prime candidate site. Tectonic studies appear to indicate that the site area was deforming at a low average rate of strain in the Miocene and that this rate has continued into the late Cenozoic. Available head and hydrologic data appear to indicate that groundwater within the deeper basalts generally moves laterally through zones of higher hydraulic conductivity (flow tops) and vertically through the fractured flow interiors. Hydraulic head gradients appear to be low (approx. 10^-4 m/m). Horizontal hydraulic conductivity values of 10^-11 to 10^-14 m/s have been measured within flow interiors; tests needed to obtain vertical hydraulic conductivity values are planned. An iterative approach involving numerical modeling and data collection and interpretation is being used to develop a preliminary geohydrologic conceptual model of the potential site and surrounding area