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Volatile and fluid transport in deep, arid soils, FY97 LDRD Final Report

The legacy of nearly five of rapid industrialization throughout the Southwest includes sites where volatile contaminants have been accidentally or intentionally released at or immediately below the surface. Understanding the mechanism and rate of volatile transport trough the vadose zone is important to assessing the potential impact on groundwater resources. This is particularly significant in and environments where the inseminated (vadose) zone above the water table may be more than 300 m thick. While numerical models have been developed to predict the movement of volatiles trough the unsaturated zone, there are only limited opportunities to verify predictions against field data. Field measurements of vadose zone transport are important in terms of constraining model parameters and can be applied to a variety of contaminant issues. This includes the ability to monitor and detect deep underground explosions in violation of nuclear test ban treaties. We have investigated the movement of vadose zone gases in a deep alluvial basin at the Nevada Test Site. The opportunity to study the migration of soil gases in this setting is unique due to the access afforded by the Joint Test Organization`s U-la tunnel complex, mined at a depth of approximately 300 m below ground surface in the alluvium of Yucca Flat (Allen, 1995; Allen, 1996). The tunnel complex is more than 180 m above the standing water level (Figure 1). In this portion of Yucca Flat the alluvium is poorly sorted and consists of channel cut and overbank deposits that contain a mixture of Tertiary volcanic and pre-Tertiary sedimentary detritus locally derived from nearby volcanic and sedimentary sources. The porosity of the alluvium ranges from 31 to 35%. Although high angle faults dissect other portions of Yucca Flat, there are no surface expressions of faults in the immediate vicinity of the U-la tunnel complex.