Subsurface geology of the Cold Creek syncline

Bedrock beneath the Hanford Site is being evaluated by the Basalt Waste Isolation Project (BWIP) for possible use by the US Department of Energy as a geologic repository for nuclear waste storage. Initial BWIP geologic and hydrologic studies served to determine that the central Hanford Site contains basalt flows with thick, dense interiors that have low porosities and permeabilities. Furthermore, within the Cold Creek syncline, these flows appear to be nearly flat lying across areas in excess of tens of square kilometers. Such flows have been identified as potential repository host rock candidates. The Umtanum flow, which lies from 900 to 1150 m beneath the surface, is currently considered the leading host rock candidate. Within the west-central Cold Creek syncline, a 47-km/sup 2/ area designated as the reference repository location (RRL) is currently considered the leading candidate site. The specific purpose of this report is to present current knowledge of stratigraphic, lithologic, and structural factors that directly relate to the suitability of the Umtanum flow within the Cold Creek syncline for use as a nuclear waste repository host rock. The BWIP geologic studies have concentrated on factors that might influence groundwater transport of radionuclides from this flow. These factors include:more » (1) intraflow structures within the interiors of individual lava flows, (2) interflow zones and flow fronts between adjacent lava flows, and (3) bedrock structures. Data have been obtained primarily through coring and geophysical logging of deep boreholes, petrographic, paleomagnetic, and chemical analysis, seismic-reflection, gravity, and magnetic (ground and multilevel airborne) surveys, and surface mapping. Results included in this document comprise baseline data which will be utilized to prepare a Site Characterization Report as specified by the US Nuclear Regulatory Commission.« less