Properties and Variability of Soil and Trench Fill at an Arid Waste-Burial Site

Arid sites commonly are assumed to be ideal for long-term isolation of wastes. Information on properties and variability of desert soils is limited, however, and little is known about how the natural site environment is altered by installation of a waste facility. During fall construction of two test trenches next to the waste facility on the Amargosa Desert near Beatty, NY, samples were collected to: (i) characterize physical and hydraulic properties of native soil (upper 5 m) and trench fill, (ii) determine effects of trench construction on selected properties and vertical variability of these properties, and (iii) develop conceptual models of vertical variation within the soil profile and trench fill. Water retention was measured to air dryness (y = 2 x 10 cm water suction). The 15 300-cm pressure-plate data were omitted from the analysis because water-activity measurements showed the actual suction values were significantly less than the expected 15 300-cm value (avg. difference = 8550 ± 2460 cm water). Trench construction significantly altered properties and variability of the natural site environment. For example, water content ranged from 0.029 to (K041 m mfor fill vs. 0.030 to 0.095 m m" for soil; saturated hydraulic conductivity was =10" cm s~' for fill vs. 10~ to =10~ cm s~' for soil. Statistical analyses showed that the native soil may be represented by three major horizontal components and the fill by a single component. Under initial conditions, calculated liquid conductivity (Ki) plus isothermal vapor conductivity (K,) for the upper two soil layers and the trench fill was = 10 ~ cm s", and KI was < Ay. For the deeper (2-5 m) soil, total conductivity was *> 10 ~ cm s~', and K\ was >K,. This study quantitatively describes hydraulic characteristics of a site using data measured across a water-content range that is representative of arid conditions, but is seldom studied. I VOLUMES of radioactive and other hazardous wastes produced each year are of major concern in that new sites will be required for disposal of these wastes. The primary objective for waste-burial facilities is to limit exposure of the public to hazardous wastes for 100 to lOOOOyr. Sites in arid regions commonly are assumed to be ideal for long-term isolation of wastes (Reith and Thomson, 1992) because the natural environment has features that can reduce the potential for waste migration (e.g., low precipitation, high evapotranspiration, thick unsaturated zone). The complex processes and interacting factors affecting water flow at arid sites, however, are not well understood (Gee and Hillel, 1988; Nativ, 1991; Andraski, 1992). Depending on specific but commonly transient conditions, water movement through either liquid or vapor flow may occur in response to matric suction and temperature gradients. Under dry conditions, movement in the vapor phase is likely to be an important transport mechanism for certain contaminants (e.g., tritium, C, radon). Evaluation of flow processes at arid sites requires the use of numerical models (Payer and Gee, 1992; Scanlon and Milly, 1994) because of these complexities. Adequate and complete data needed for U.S. Geological Survey, 333 W. Nye Lane, Carson City, NV 89706. Received 1 Mar. 1995. ""Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 60:54-66 (1996). numerical simulation of flow processes generally are lacking, however, because of difficulties in characterizing the physical and hydraulic properties of the commonly dry, stony desert soils (Mehuys et al., 1975; Russo, 1983; Andraski, 1991). Few attempts have been made to measure hydraulic properties across the extreme range of water content observed at arid sites. Although recent work at active or proposed arid waste-burial sites has increased our knowledge of soil properties and flow processes in undisturbed arid settings (Fischer, 1992; Istok et al., 1994; Scanlon and Milly, 1994), litde is known about how, or to what degree, the natural site environment is altered by the installation of a waste-burial facility. This work was prompted by the need to better define and quantify the factors and processes affecting waste isolation at a representative arid waste-burial site. The specific objectives of this study were to: (i) characterize the physical and hydraulic properties of native soil (upper 5 m) and trench fill, (ii) determine the effects of burialtrench construction on selected physical and hydraulic properties and vertical variability of these properties, and (iii) quantitatively develop preliminary conceptual models of vertical variation within the native soil profile and the trench fill for use in numerical simulations of unsaturated flow at the site. MATERIALS AND METHODS The study site is adjacent to the commercial waste-burial facility on the Amargosa Desert, about 17 km southeast of Beatty, NV, and 20 km east of Death Valley National Park. On the basis of disposal-facility records (R. Marchand, US Ecology, Inc., 1991, personal communication) and data collected at the study site (Wood and Andraski, 1995), annual precipitation during 1981 to 1992 averaged 108 mm. Depth to the water table ranges from 85 to 115 m below land surface (Fischer, 1992). Sediments in the area are largely alluvial and fluvial deposits (Nichols, 1987). Surface soils are mapped as the Yermo-Arizo association (W.E. Dollarhide, USDA-SCS, 1987, personal communication). The Yermo soils are loamyskeletal, mixed (calcareous), thermic Typic Torriorthents, and the Arizo soils are sandy-skeletal, mixed, thermic Typic Torriorthents. Vegetation in the area is sparse; creosote bush [Larrea tridentata (DC.) Cov.] is the dominant species. The waste facility has been used for the burial of low-level radioactive waste (1962-1992) and hazardous chemical waste (1970-present). The low-level radioactive waste facility was the first such commercially operated site in the USA. Steps used in construction of the low-level radioactive waste burial trenches include excavation and stockpiling of native soil, emplacement of waste, and backfilling with previously stockpiled, uncompacted soil. Dimensions of the trenches have changed since the facility opened in 1962. Until the mid-1970s, trenches were about 1 to 3 m wide, 2 to 6 m deep, and 100 to 200 m long. Trenches constructed later are 30 to 90 m wide, 15 m deep, and 250 m long. Two test trenches were constructed during September 1987 using methods that simulate those used during active disposal at the low-level radioactive waste-burial facility. The test trenches were excavated by backhoe to about 4 m in all three dimensions 54 Published January, 1996