Biological Intrusion of Low-Level Waste Trench Covers
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BIOLOGICAL INTRUSION OF LOW-LEVEL WASTE TRENCH COVERS THOMAS E. HAKONSON AND ERNEST S. GLADNEY Environmental Science Group, Los Alamos National Laboratory, Los Alamos New Mexico, USA
ABSTRACT The long-term integrity of low-level waste shallow land burial sites is dependent on the interaction of physical, chemical, and biological factors that modify the waste containment system. Past research on low-level waste shallow land burial methods has emphasized physical (i.e., water infiltration, soil erosion) and chemical (radionuclide leaching) processes that can cause waste site failure and subsequent radionuclide transport. The purpose of this paper is to demonstrate the need to consider biological processes as being potentially important in reducing the integrity of waste burial site cover treatments. Plants and animals not only can transport radionuclides to the ground surface via root systems and soil excavated from the cover profile by animal burrowing activities, but they modify physical and chemical processes within the cover profile by changing the water infiltration rates, soil erosion rates and chemical composition of the soil. One approach to limiting biological intrusion through the waste cover is to apply a barrier within the profile to limit root and animal penetration with depth. Experiments in the Los Alamos Experimental Engineered Test Facility were initiated to develop and evaluate biological barriers that are effective in minimizing intrusion into waste trenches. The experiments that are described employ four different candidate barrier materials of geologic origin. Experimental variables that will be evaluated, in addition to barrier type, are barrier depth and soil overburden depth. The rate of biological intrusion through the various barrier materials is being evaluated through the use of activatable stable tracers.
INTRODUCTION Low-activity and wastes suspected of being contaminated are generally buried in shallow trenches (1.5 to 45 m wide, 2 to 11 m deep, 6 to 300 m long) that are covered with from less than 1.0 to 2.5 m of material when the trenches are full (Duguid, 1977). Most waste burial facilities attempt to revegetate the trench covers to minimize soil loss and to increase aesthetic appearance of the site. Although it has been recognized (Lutton et al., 1979; Anonymous, 1979) that biological intrusion of low-level waste trenches can lead to transport of radionuclides from a burial site, little has been done to quantify the magnitude of the problem and to develop measures, when needed, to prevent the intrusion. The stability of low-level waste trench covers is a function of physical, chemical, biological, and climatological factors that interact in both obvious and subtle ways. The importance of biological factors in altering the integrity of trench covers is often overlooked despite evidence that plants and animals can influence trench cover stability and, as well, can mobilize radionuclides buried in the trench (Lutton et al., 1979; Anonymous, 1979). Biological interactions w
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