Baker, R.J., Andraski, B.J., Walvoord, M.A., Stonestrom, D.A., Prudic, D.E., Luo, Wentai, Eos. Trans. AGU, 84(46), Fall Meet. Suppl., Abstract H32A-0509, 2003.

Copyright 2003 American Geophysical Union

Volatile Organic Compounds (VOCs) and Elevated Concentrations of Carbon Dioxide (CO2) in Unsaturated-Zone Vapors Near a Chemical and Low-Level Radioactivity Waste-Disposal Facility, Amargosa Desert Research Site, Nye County, Nevada

Abstract

As part of its Toxic Substances Hydrology Program, the U.S. Geological Survey is studying contaminant-transport processes in an arid environment at the Amargosa Desert Research Site (http://nevada.usgs.gov/adrs/). The site is near waste-disposal facilities 20 kilometers east of Death Valley National Park. Low-level radioactive waste was buried in unlined trenches of varying depth during 1962-92. Hazardous chemical waste was buried in unlined trenches at an adjacent facility during 1970-88. Mean annual precipitation at the site from 1981 to 2000 was 108 millimeters. The unsaturated zone is aerobic down to the water table, which is about 110 m (meters) deep. Sampling infrastructure south and west of the facility includes a grid of vapor probes 1.5 m deep, a 23.8-m-deep background borehole (JFDB), and two approximately 100-m-deep boreholes (UZB-2 and UZB-3), which are 160 m and 100 m from the nearest trench, respectively, and are instrumented for multi-level sampling. Analytes detected in unsaturated-zone-vapor samples include elevated concentrations of tritium and carbon-14; three chlorofluorocarbon (CFC) compounds, eight chlorinated solvent compounds, and toluene, all at concentrations exceeding 1,000 parts per billion (ppb) in UZB-3, and at lower concentrations in UZB-2 and in the shallow-vapor-probe grid; and CO2 in concentrations up to 2% in UZB-3, whereas maximum CO2 concentrations in JFDB are less than 0.2%. With the notable exception of toluene, VOCs that are known to be highly biodegradable are generally absent or occur at low concentrations (<100 ppb). The trends in the CO2 concentration profiles approximately parallel those of CFCs and radionuclides. The following preliminary conclusions have been drawn from the radionuclide, VOC, and CO2 data: 1. Biodegradation of organic substances is a reasonable explanation for the presence of CO2 in UZB-3 at concentrations greater those in JFDB (background), which are attributed to near-surface natural biological activity and abiotic geochemical processes near the water table. 2. Elevated CO2 concentrations and low concentrations (<1,000 ppb) of most biodegradable organic compounds in UZB-3 indicate that most of the degradation occurs near the waste trenches. 3. The VOCs that occur at high concentrations (>1,000 ppb) in UZB-3 are relatively recalcitrant to biodegradation in this environment. 4. The similarity among their vertical and radial concentration profiles indicates that the same vapor-transport phenomena are responsible for the observed elevated levels of CO2, hydrocarbons, and radionuclides. The CO2- and VOC-concentration data will be used to enhance ongoing efforts to develop process-based models of contaminant transport at the research site.