COLUMBIA RIVER BASALTS: DO THEY LEAK? A RETROSPECTIVE OF BWIP PROGRAM DATA

A key issue in deep repository performance is the degree of hydraulic communication or leakage of the repository horizon to overlying aquifers or other pervious hydrogeologic units. This was particularly relevant for the Basalt Waste Isolation Project (BWIP), which evaluated the suitability of basalts, a fractured crystalline rock-type, for high-level radioactive waste containment. The question of basalt intercommunication continues to be an issue for state and federal agencies concerned with groundwater-resource management and private companies interested in the viability of basalts for underground natural gas storage or as potential carbon sequestration reservoirs.

The Columbia River Basalts constitute a large flood-basalt province in the Pacific Northwest, covering 163,000 km². At its deepest location, the basalt has an aggregate, composite thickness of >4,000 meters. Thick interior sections within basalt flows at the Hanford Site in Washington State were identified as part of site characterization investigations in the mid-1980’s as possible candidate horizons for a high-level nuclear repository being considered by the United States. The Hanford Site, together with a salt site in Deaf Smith, Texas, was abandoned by U.S. Congressional mandate in 1987 in favor of a tuff site currently being investigated at Yucca Mountain, Nevada.

Inferential site characterization data collected as part of the BWIP program (hydrochemical/isotopic data, hydraulic properties, and opportunistic head responses induced by high-stress drilling operations) suggest that within the Hanford Site only low pervasive intercommunication exists, with significant vertical communication occurring along major structural features. Hydrochemical/isotopic data also reflect the distinct separation of aquifers except in areas where major faulting and deformation has occurred. A large-scale hydrologic stress test (pumping test) was planned during the BWIP program to assess intercommunication both laterally and vertically across selected basalt flows. Although the tests were not completed, the data gathered during the site characterization can be used to address questions about basalt intercommunication.