Carbon isotopes in unsaturated-zone gases and ground water near a radioactive-waste disposal area, Amargosa Desert Research Site, Nye County, Nevada

 

David A. Stonestrom¹, Robert L. Michel¹, William C. Evans¹, Tyson R. Smith^1,2, David E. Prudic³, Robert G. Striegl, Jr. ⁴, Herbert Haas⁵, Fred J. Brockman⁶, and Brian J. Andraski³

 

To test hypotheses about radionuclide distribution and transport, vertical profiles of ¹⁴C in carbon dioxide were determined on gas samples from a 110-m deep unsaturated zone 32, 100, and 3,000 m from a low-level radioactive-waste disposal area. A direct-scintillation-counting method for radiocarbon was developed that minimized sample handling and compared favorably with the more labor-intensive benzene-synthesis method. Values of d¹³C in pore gas were determined by isotope ratio mass spectrometry. Gross gas compositions were determined by chromatography. Gross gas compositions 100 m from the disposal area were similar to those at the 3,000-m site, i.e., relatively unperturbed, except that O₂ levels at the 100-m site were slightly depleted in the 34-48 m depth interval (~19.5% versus ~20.8% O₂ by volume). Radiocarbon levels at the 100-m site peaked at ~2,000 percent modern carbon (PMC) near land surface and decreased monotonically to <100 PMC at depths below 58 m. Gross gas composition 32 m from the disposal area displayed a well-defined CO₂ peak that reached 2.0-2.5% by volume at a depth of 24 m. ¹⁴C levels showed a roughly coincident peak that reached 6´10⁵ PMC at the 24 m depth. Elevated levels of tritium and volatile organic-carbon compounds accompanied the CO₂ and ¹⁴C peaks. Corresponding d¹³CO₂ values were shifted -7 to -10 permil from unperturbed values. This shift to lighter values is suggestive of fractionation during microbial production of ¹⁴CO₂ from disposed waste. Core samples from the affected depths were dry and virtually devoid of microbes, with <4´10³ colony forming units per gram of sediment, suggesting that the hypothesized microbial activity occurs closer to the emplacement trenches. Ground water at the 32-m site had a ¹⁴C level in dissolved inorganic carbon of 845 PMC in March 2000. Ground water from an adjacent well had a ¹⁴C level of 26 PMC in 1989 and 323 PMC in 1999. The low levels of ¹⁴C in ground water relative to those in unsaturated-zone gases suggest that radiocarbon transport is primarily by lateral gas transport through the unsaturated zone in this environment.

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¹ U.S. Geological Survey, Menlo Park, CA (dastones@usgs.gov, rlmichel@usgs.gov, wcevans@usgs.gov)

² Currently at Lewis and Clark College, Portland, OR (tsmith@lclark.edu)

³ U.S. Geological Survey, Carson City, NV (deprudic@usgs.gov, andraski@usgs.gov)

⁴ U.S. Geological Survey, Lakewood, CO (rstriegl@usgs.gov)

⁵ Retired Director, Radiocarbon Laboratory, Desert Research Institute, Las Vegas, NV (hhaas@concentric.net)

⁶ Environmental Microbiology Group, Pacific Northwest National Laboratory, Richland, WA (fred.brockman@pnl.gov)

 

This abstract was published in Eos, Transactions, American Geophysical Union, v. 82, no. 47, Fall. Meet. Suppl., p. F-516.