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Findings Suggest VOCs May Help Explain Radionuclide Transport

USGS scientists collecting gas samples from the unsaturated zone.
USGS scientists collecting gas samples from the unsaturated zone. Subsurface gases are drawn through a small glass tube (in foreground hand) filled with adsorbing resins that trap volatile organic compounds for later analysis. Photo credit: Brian Andraski, USGS.
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Volatile organic compounds (VOCs) from low-level radioactive wastes disposed of in landfills can migrate long distances in the subsurface and may help explain puzzling observations of tritium transport at a Nevada landfill. A team of scientists from the U.S. Geological Survey (USGS) and Portland State University working at the Amargosa Desert Research Site (ADRS) in Nevada found elevated levels of VOCs throughout a 110-meter-thick unsaturated zone and as much as 300 meters laterally from a closed low-level radioactive waste landfill. The subsurface distribution of VOCs around the landfill was similar to the distribution of tritium (a radioactive form of hydrogen) at the site. The tritium plume has long been taken as evidence of tritiated water-vapor transport through the unsaturated zone, but attempts to model tritium transport have not fully explained the observed distribution. Results from the present study suggest that organic compounds could play an important role in the long-distance transport and fate of tritium in the environment.

In this first-of-its-kind study, chlorofluorocarbons (CFCs) were determined to be the predominant VOCs associated with low-level radioactive wastes. The CFC concentrations are much greater than those reported for conventional landfills and may be a unique characteristic of VOCs associated with these wastes, although common gasoline components such as xylene, benzene, and toluene, together with common industrial solvents such as trichloroethylene (TCE) and tetrachloroethylene (PCE), were also detected frequently in subsurface gas samples.

Additional measurements showed that VOCs laden with CFCs and other solvents are escaping to the atmosphere from the unsaturated zone surrounding the landfill. CFCs, which are associated with the depletion of ozone in the atmosphere, are roughly 10,000 times more potent than carbon dioxide (CO2) as a greenhouse gas. CFC from these types of landfills may represent a substantial source of CFC to the atmosphere more than two decades after the international treaty banning CFC production, the Montreal Protocol on Substances that Deplete the Ozone Layer, took effect.

The results of this study will help waste management professionals anticipate the potential environmental impact of low-level radioactive wastes that have been disposed of in landfills. The results also have implications beyond the management and detection of low-level radioactive waste because many of the VOCs (TCE, PCE, benzene, toluene, methylene chloride, and vinyl chloride) are known or suspected carcinogens and are commonly present in other types of landfill waste.

The USGS Toxic Substances Hydrology, Hydrologic Research and Development, and Groundwater Resources Programs provided funding for the study.

Reference

Baker, R.J, Andraski, B.J., Stonestrom, D.A., and Luo, Wentai, 2012, Volatile organic compounds in the unsaturated zone from radioactive wastes: Journal of Environmental Quality, v. 41, no. 4, p. 1324-1336, doi:10.2134/jeq2011.0480.

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Created on Febrary 5, 2013