The Hanford Site 1000-Year Cap Design Test

Surface barrier or capping technology is needed to isolate buried wastes. A successful cap must prevent the intrusion of plants, animals, and man into the underlying waste, minimize wind and water erosion, require minimal maintenance, and limit water intrusion to near-zero amounts. For some sites where wastes are long-lived, caps should potentially last a thousand years or more. At the U.S. Department of Energy (DOE) Hanford Site in Washington State, a surface cap with a 1000-year design life was constructed and then tested and monitored for performance under wetting conditions that are extreme for the region. The cap was built in 1994 over an existing waste site as a part of a Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) treatability test. The above-grade barrier or cap consists of a 2-m-thick silt-loam soil overlying layers (from top down) of sand, gravel, basalt rock (riprap), and a low-permeability asphalt. Two sideslope configurations, a clean-fill gravel on a 10:1 slope and a basalt riprap on a 2:1 slope were part of the overall design and testing. Design considerations included constructability; water-balance monitoring; wind and water erosion control and monitoring; surface revegetation, biointrusion control, subsidence, and sideslope stability; and durability of the asphalt more » layer. « less