Overview of Plant-Based Remedies at U.S. Department of Energy Legacy Waste Sites

As long-term steward of Cold War legacy waste sites, the U.S Department of Energy (DOE) often turns to plant-based strategies, as highlighted by the following examples, with the goals of increasing sustainability of remedies and reducing long-term maintenance costs. The engineered cover for a uranium mill tailings disposal cell at the Monticello, Utah, Superfund site relies on evapotranspiration from planted sagebrush steppe vegetation and the water-storage capacity of a 160-cm layer of sandy clay loam soil overlying a 40-cm sand capillary barrier to limit percolation of rainwater. A 3.0-ha drainage lysimeter embedded in the cover during construction recorded an average percolation rate of 0.6 mm yr-1 over nine years, satisfying an EPA goal of <3.0 mm yr-1. Development of soil structure since construction changed soil hydraulic properties, apparently improved rooting habitat for shrubs and, hence, likely increased water extraction from deeper in the cover. In contrast, an increase in hydraulic conductivity attributable to soil development and root intrusion of a conventional, compacted soil cover at Lakeview, Oregon, a site with climate, soils, and vegetation similar to Monticello, has apparently been the cause of 10s of cm of annual percolation. A lysimeter study underway at the Grand Junction, Colorado disposal site is evaluating a method for accelerating the natural renovation or conversion of a conventional cover into an evapotranspiration cover. At a former uranium mill near Monument Valley, Arizona, phytoremediation using native desert phreatophytes has been shown to control percolation in the source area of a shallow nitrate plume, transpire enough water to slow plume dispersion, and apparently help enhance microbial denitrification in the plume and in source area soils. Finally, DOE is identifying and characterizing natural analogs of ecological change to help develop scenarios for evaluating the long-term performance of plant-based remedies.