H23B-1570
Unsaturated zone carbon dioxide flux, mixing, and isotopic composition at the USGS Amargosa Desert Research Site
Tuesday, 15 December 2015
Poster Hall (Moscone South)
Christopher H Conaway1, James Joseph Thordsen1, Burt Thomas1,2, Karl Haase3, Michael T Moreo4, Michelle A Walvoord5, Brian J Andraski6 and David A Stonestrom5, (1)US Geological Survey, National Research Program, Menlo Park, CA, United States, (2)Willamette University, Salem, OR, United States, (3)US Geological Survey, National Research Program, Reston, VA, United States, (4)USGS Nevada Water Science Center, Henderson, NV, United States, (5)US Geological Survey, National Research Program, Denver, CO, United States, (6)USGS Nevada Water Science Center, Carson City, NV, United States
Abstract:
Elevated concentrations of tritium, radiocarbon, and volatile organic compounds at the USGS Amargosa Desert Research Site, adjacent to a low-level radioactive waste disposal facility, have stimulated research on factors affecting transport of these contaminants. This research includes an examination of unsaturated zone carbon dioxide (CO2) fluxes, mixing, and isotopic composition, which can help in understanding these factors. In late April 2015 we collected 76 soil-gas samples in multi-layer foil bags from existing 1.5-m deep tubes, both inside and outside the low-level waste area, as well as from two 110-m-deep multilevel gas-sampling boreholes and a distant background site. These samples were analyzed for carbon dioxide concentration and isotopic composition by direct injection into a cavity ring-down spectrometer. Graphical analysis of results indicates mixing of CO2 characteristic of the root zone (δ13C -18 ‰ VPDB), deep soil gas of the capillary fringe (-20‰), and CO2 produced by microbial respiration of organic matter disposed in the waste area trenches (-28‰). Land-surface boundary conditions are being constrained by the application of a novel non-dispersive infrared sensor and traditional concentration and flux measurements, including discrete CO2 flux data using a gas chamber method to complement continuous data from surface- and tower-based CO2 sensors. These results shed light on radionuclide and VOC mobilization and transport mechanisms from this and similar waste disposal facilities.
