Transport of Tritium Through a 110-m Deep Vadose Zone, Amargosa Desert Research Site, Nye County, Nevada

D.E. Prudic, R.L. Michel, D.A. Stonestrom, and B.J. Andraski

Transport of tritium through a 110-m deep vadose zone at a site in the Amargosa Desert 18 km south of Beatty, Nevada is being studied as part of the U.S. Geological Survey's Toxic Substances Hydrology Program. The site is near a disposal facility that buried commercial low-level radioactive waste in unlined trenches from 1962-92. Elevated levels of tritium in water vapor initially were detected in 1994 in gas samples collected from 10 sampling ports between depths of 6 and 109 m in a borehole (UZB-2) 160 m south of the nearest trench. In December 1999, a new borehole (UZB-3) was drilled 103 m northeast of UZB-2, and 96 m south of the nearest trench. UZB-3 has 12 gas-sampling ports between depths of 5 and 104 m and a well screened across the water table from 109 to 111 m. In addition, a tube next to each borehole was driven to 1.5 m in a layer of gravelly sand. Tritium concentrations were highest at 1.5 m in April 2000. Concentrations were 12,840 tritium units (TU) at UZB-2 and 22,800 TU at UZB-3. Secondary peaks of 10,020 TU and 2,400 TU were observed at 48 and 109 m, respectively, in UZB-2. A secondary peak of 9,380 TU was observed at 24 m in UZB-3. Tritium at 48 m in UZB-2 has doubled each year since May 1997. In contrast, tritium at 1.5 m declined on average 890 TU each year during the same period. Tritium from ground water collected at UZB-2 in September 1993 was below detection. Tritium in ground water at UZB-3 in March 2000 was 3.8 TU. Thus, tritium appears to be moving through the vadose zone with preferential transport in three zones: (1) gravelly sand layer about 1-2 m below land surface, (2) above an interval of elevated water potential, and (3) just above the water table. Although tritium at 1.5 m may represent documented waste spills onto the land surface, tritium at depth probably originated in the trenches. Preliminary models for diffusive movement of tritiated water vapor from the trenches are unable to match observed concentration trends. To resolve this discrepancy, studies are investigating chemical and isotopic equilibrium assumptions, physical and chemical coupling between organic compounds and radioisotopes, roles of heat flow and barometric pumping, and roles of micro-organisms and plants in contaminant transport.

This abstract was published in Eos, American Geophysical Union Transactions, v. 81, no. 48, p. F387