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Plant Cover and Water Balance in Gravel Admixtures at an Arid Waste-Burial Site

Environ. Sciences Lab., U.S. Dep. of Energy Grand Junction Projects Office, P.O. Box 14000, Grand Junction, CO 81503;

M. E. Thiede, D. J. Bates, L. L. Cadwell and G. W. Gee

Pacific Northwest Lab., P.O. Box 999, Richland, WA 99352;

C. J. Kemp

Westinghouse Hanford Co., P.O. Box 1970, Richland, WA 99352.

^* Corresponding author (jwaugh{at}gjpomail.doegipo.com).

ABSTRACT

Isolation of radioactive waste buried in unsaturated zones will require long-term control of recharge and erosion. Soil covers control recharge at arid 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 layer may control erosion over the long-term better than surface gravel layers. We postulated that gravel admixtures also would not influence plant establishment or soil water balance in waste-site covers. We measured the interactive effects of gravel admixture concentration, vegetation, and precipitation on soil water content and plant cover at the U.S. Department of Energy's Hanford Site in Washington state. Our results support the use of a combination of vegetation and gravel admixtures for erosion control. Vegetation seasonally depleted rootzone 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 in response to 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 (32 cm/yr), but persisted after the irrigation ceased. Cheatgrass (Bromus tectorum L.) and Russian thistle (Salsola kali L.) colonized areas that received 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.

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