USGS Activities in Nevada -- Amargosa Desert Research Site

Significant Findings
Factors Affecting Water, Gas, and Contaminant Transport

Initial water-balance modeling of the Beatty waste site suggested that, under present-day climatic and bare soil conditions, the potential for infrequent percolation of precipitation below a depth of 2 m does exist, in spite of high annual evaporative demands (Nichols, 1987).

Chloride-mass balance estimates of long-term deep percolation rates through the unsaturated zone beneath an undisturbed, vegetated area at the ADRS suggested that percolation of precipitation below a depth of 10 m has been minimal or nonexistent for at least 6,000 (Fouty, 1989) to 16,000 years (Prudic, 1994). The estimated age of 16,000 years for pore water at a depth of 10 m approximates the time when climate in the area was wetter and cooler.

Initial field measurements to characterize the present-day soil-water regime in the upper 13 m of the unsaturated zone beneath an undisturbed, vegetated area documented the dryness of the native soils and sediments and provided data to develop a conceptual understanding of moisture movement (Fischer, 1992).

Laboratory and multiple-year field investigations determined how, and to what degree, features of the natural system can be altered by the installation of a waste disposal facility.

Test-trench construction significantly altered properties and variability of the natural site environment (Andraski, 1996). Results also demonstrated that the lower limit to which hydraulic characterization data normally are measured (i.e., -1.5 MPa, referred to as the permanent wilting point for crops) is not adequate for nonirrigated, desert soils and plants.

Native plants in an undisturbed setting effectively removed available moisture such that the potential for deep percolation of precipitation was limited. Episodic, deep drying during periods of below-average precipitation further limited the potential for deep percolation under natural, vegetated conditions (Gee and others, 1994; Andraski, 1997). In contrast, under nonvegetated waste-site conditions, continued accumulation and downward penetration of infiltrated water was observed during a 5-year test period. Results suggested that the lack of native plants can increase the potential for infiltrated water to contact and enhance the release of buried contaminants.

Field measurements of subsidence and erosion provided information on processes that potentially influence the accumulation and movement of water at a waste-burial site.

Subsidence of test trenches was affected by method of drum placement (stacked versus random) (Andraski, 1990; 1997). Rates of trench-cover subsidence showed a positive relation with cumulative precipitation, but subsidence did not have a measurable influence on the water balance during a 5-year test period. Subsidence continued after more than 5 years.

Erosion was decreased by rock-fragments armoring the near-surface backfill (Andraski and Prudic, 1997). Most of the observed soil loss was attributed to deflation (wind erosion) that occurred during the first 2 years following trench construction.

A new soil-water retention model (Rossi-Nimmo) was found to improve prediction of water potentials and temperatures (Andraski and Jacobson, 2000). Such improvements are important for calculations of liquid and vapor flow in near-surface soils and in deep unsaturated zones of arid and semiarid regions.

Unsaturated-zone gas samples collected from test hole UZB-2, about 100 m away from the waste facility, showed elevated tritium concentrations in water vapor to a depth of 108.8 m and elevated carbon-14 concentrations in carbon dioxide gas to a depth of 34 m (Prudic and Striegl, 1995). Tritium concentration of ground water collected from UZB-2 at the time of drilling was below detection. The carbon-14 dioxide distribution could be partly explained by gaseous diffusion or advective gas transport away from the waste facility. Hypothetical mechanisms that could explain the distribution of tritiated water vapor relied on lateral liquid flow along preferential flow paths (Striegl and others, 1996).

Analysis of pore water extracted from archived core samples and analyses of gas samples showed general increases in tritiated water vapor in the unsaturated zone (1992-96) and a bimodal distribution with depth, in which the greatest concentrations were in a gravelly layer at a depth of about 1-2 m (Prudic and others, 1997).

Isotopic composition of water in the deep unsaturated zone beneath the ADRS indicates that net gradients driving fluxes of water, gas, and heat are directed upwards for undisturbed conditions. Superimposed on the upward-directed flow field, tritium and carbon-14 are migrating away from waste in response to gradients in contaminant concentrations (Prudic and others, 1999; Stonestrom and others, 1999)

Tritium concentrations in shallow (about 1.5-m depth) soil gas samples measured on a grid adjacent to the waste burial area during 1997 and 1998 indicated a general trend of increasing concentrations toward the burial area and little difference in tritium concentrations between the two sampling periods (Striegl and others, 1998; Healy and others, 1999).