RADIOISOTOPES AND ORGANIC CONTAMINANTS IN A 110-M DEEP UNSATURATED ZONE, AMARGOSA DESERT RESEARCH SITE, NYE COUNTY, NEVADA

STONESTROM, D.A., USGS, MS-421, 345 Middlefield Rd., Menlo Park, CA 94025, dastones@usgs.gov; BAKER, R.J., USGS, 810 Bear Tavern Rd., West Trenton, NJ 08628; PRUDIC, D.E. and ANDRASKI, B.J., USGS, 333 W. Nye Ln., Carson City, NV 89706; EVANS, W.C. and MICHEL, R.L., USGS, Menlo Park, CA 94025

The U.S. Geological Survey is studying transport of mass and energy near disposal facilities ~20 km east of Death Valley National Park under auspices of the Toxic Substances Hydrology program. Low-level radioactive waste (LLRW) was buried in unlined trenches from 1962-92. Hazardous chemical waste was buried in unlined trenches at an adjacent facility from 1970-88. Elevated levels of tritium in water vapor and of carbon-14 in carbon dioxide (CO2) were initially detected in 1994 in soil-gas samples from a borehole 100 m from the LLRW facility. Subsequent samples included gas from tubes driven into gravelly sand ~1-2 m deep. In May 1999, tube and borehole samples were screened for 87 volatile organic compounds (VOCs). Of 19 VOCs detected at >1 ppb, ten were >10,000 times detection limits, including trichloroethene, chloroform, and carbon tetrachloride. The most abundant VOC was CFC113 (9.1 ppm at 12 m, ~100,000 times historic atmospheric levels). Vinyl chloride concentrations correlated weakly with tritium concentrations. In December 1999 a new borehole was completed ~32 m from the LLRW facility. The CO2 profile there exhibits an anomalous bulge above 50 m, with CO2 reaching ~2% at 24 m. The tritium profile is similar, reaching ~9,400 TU at 24 m. Carbon-13 in elevated CO2 is ~10 permil lighter than deeper in the profile and at comparable depths farther from the disposal facility. Ground water at the new borehole has a tritium level of 3.8 TU and VOCs near detection thresholds. This suggests that transport is primarily through unsaturated sediments. Preliminary models for diffusive movement of tritiated water through unsaturated sediments fail to match observed concentration histories. To resolve this discrepancy, studies are investigating physical and chemical coupling between organic compounds and radioisotopes, roles of thermal and barometric pumping, and roles of microorganisms and plants in contaminant transport and transformations.