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 CO and CH, 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 CO mole fraction (CO) and C isotopic composition (δC-CO) and CH mole fraction (CH). Graphical analysis of the results indicates mixing of CO characteristic of the root zone (δC −18 to −19‰), deep soil gas of the capillary fringe (−13 to −15‰), and CO produced by microbial respiration of organic matter disposed in the LLRW trenches (−22 to −25‰). Distribution of CH 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 CH reflecting lateral transport from the LLRW area is decreasing with time in the deep profiles, deep unsaturated zone CO has changed little in recent decades. The results imply that CH and δC-CO may serve as good tracers of anthropogenic effects in the unsaturated zone even when CO primarily reflects natural processes