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Unsaturated zone CO2, CH4, and δ13C-CO2 at an arid region low-level radioactive waste disposal site

November 1, 2018

Elevated tritium, radiocarbon, Hg, and volatile organic compounds associated with low-level radioactive waste (LLRW) at the USGS Amargosa Desert Research Site (ADRS) have stimulated research on factors and processes that affect contaminant gas distribution and transport. Consequently, we examined the sources, mixing, and biogeochemistry of CO2 and CH4, two additional important species in the unsaturated zone at ADRS. In spring 2015 and 2016, shallow unsaturated zone gas samples were collected from the 1.5-m depth both inside and outside the LLRW disposal area. Samples also were collected from two 110-m-deep multilevel gas-sampling boreholes and a distant background site. These samples were analyzed for CO2 mole fraction (xCO2) and C isotopic composition (δ13C-CO2) and CH4 mole fraction (xCH4). Graphical analysis of the results indicates mixing of CO2 characteristic of the root zone (δ13C −18 to −19‰), deep soil gas of the capillary fringe (−13 to −15‰), and CO2produced by microbial respiration of organic matter disposed in the LLRW trenches (−22 to −25‰). Distribution of CH4 overall reflects atmospheric sources and production in anaerobic microzones in the LLRW area and methanotrophy in the undisturbed shallow subsurface outside the LLRW area. Although xCH4 reflecting lateral transport from the LLRW area is decreasing with time in the deep profiles, deep unsaturated zone xCO2 has changed little in recent decades. The results imply that CH4 and δ13C-CO2 may serve as good tracers of anthropogenic effects in the unsaturated zone even when CO2 primarily reflects natural processes.

Citation Information

Publication Year 2018
Title Unsaturated zone CO2, CH4, and δ13C-CO2 at an arid region low-level radioactive waste disposal site
DOI 10.2136/vzj2017.11.0200
Authors Christopher H. Conaway, Michelle Ann Walvoord, Randall B. Thomas, Christopher Green, R.J. Baker, James J. Thordsen, David A. Stonestrom, Brian J. Andraski
Publication Type Article
Publication Subtype Journal Article
Series Title Vadose Zone Journal
Series Number
Index ID 70196863
Record Source USGS Publications Warehouse
USGS Organization National Research Program - Western Branch