Evaluation of a capillary barrier at the Rocky Mountain Arsenal

A monitoring program is currently being implemented to assess the performance of a capillary barrier constructed at the Rocky Mountain Arsenal, a high-profile hazardous waste site located northeast of Denver, Colorado, USA. Monitoring includes regulatory-required pan lysimeters to measure basal percolation as well as water content reflectometer probes to measure soil moisture profiles. The moisture content profiles allowed continued evaluation of the cover performance in response to natural precipitation and irrigation. The results to date indicate that the comparatively low density of the cover soils, selected to aid in vegetation establishment, appears to have been detrimental to the overall cover performance. The RMA site, which is regulated under the US Environmental Protection Agency’s (USEPA) Superfund program, was once considered to contain the “most contaminated square mile on earth” (Frumkin 2005). A primary remedy component at this site involves in-situ consolidation of contaminated soils to be covered with a total of six unsaturated soil covers. These six covers, which span over 160 ha of RMA, are deemed ‘alternative’ covers, as they were required to be ‘equivalent’ to a prescriptive RCRA-Subtitle C cover (FWENC 1996). Based on the site-specific conditions and studies available at the time (e.g. Melchior 1997), a quantitative threshold percolation of 1.3 mm/year was adopted in 1998 for the RMA alternative covers (RVO 1998). The first RMA alternative cover consisting of a capillary barrier was constructed in 2007 over an 8.5 ha area in central RMA. Known as the Shell Cover, it includes a 1.22 m-thick soil layer underlain by a capillary break composed of a nonwoven geotextile placed over coarse gravel (chokestone layer). A comparatively low relative compaction (ranging from 75% to 85% of the maximum dry density from Standard Proctor tests) was specified for the cover soils in order to promote vegetation growth.