Moisture can accumulate under asphalt covers for waste-burial areas, potentially mobilizing components of the waste. This case study describes an asphalt cover emplaced in 1961 to contain plutonium, uranium, beryllium, and lead wastes in an area at the Los Alamos National Laboratory used for underground nonnuclear detonation tests. In this semiarid environment, moisture, apparently from meteoric sources, has accumulated in the soil and tuff below the cover. Saturated conditions are observed to a depth of ~6 m, while adjacent uncovered areas remain dry. No migration of plutonium or other contaminants has been observed. Remediation of the saturated volume may consist of removal of the asphalt cover and its replacement with a multilayer engineered cover, possibly with venting of the subsurface. The potential for water accumulation should always be considered when asphalt has been used or is proposed as a cover for a waste site.
