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

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 CO2 produced by microbial res...