Plant cover and water balance in gravel admixtures at an arid waste-burial site

Isolation of radioactive waste buried in unsaturated zones will require long-term control of recharge and erosion. Soil covers control recharge at and sites by storing rainwater close enough to the surface to be removed by evapotranspiration. Surface layers of rock or gravel control erosion at sites with sparse vegetation, but can also alter plant habitat and cause recharge through interred waste. As an alternative, gravel mixed into the uppermost soil law may control erosion ever the king-term better than surface gravel layers. Gravel admixtures may also not influence plant establishment or sod water balance in waste-site covers. The interactive effects of gravel admixture concentration, vegetation, and precipitation on soil water content and plant cover were measured at the US Department of Energy's Hanford Site. Results support use of a combination of vegetation and gravel admixtures for erosion control. Vegetation seasonally depleted root zone water storage to about 6.5 volume % regardless of precipitation amount or the presence of gravel admixture amendments. In contrast, yearly increases in soil water storage as deep as 225 cm in plots without vegetation may be a leading indicator of recharge. The composition and abundance of vegetation changed over time and with precipitation amount, but was not influenced by gravel amendments. Seeded wheatgrasses [Agropyron sibericum Wilde and Agropyron dasystachyum (Hook.) Scribn.] established only when irrigated with twice average precipitation, but persisted after the irrigation ceased. Cheatgrass (Bromus tectorum L.) and Russian thistle (Salsola kali L.) colonized areas receiving both irrigation and ambient precipitation. Stands with wheatgrasses extracted water more rapidly and depleted soil water to lower levels than cheatgrass-dominated stands. Increases in gravel cover and near-surface gravel concentrations after 5 yr were evidence of the formation of a protective gravel veneer. 44 refs., 8 figs., 2 tabs