Unsaturated Zone CO2, CH4, and δ13C-CO2 at an Arid Region Low-Level Radioactive Waste Disposal Site

Elevated tritium, radiocarbon, mercury, 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. Here we present an examination of sources, mixing, and biogeochemistry of CO₂ and CH_4, two additional important species in the unsaturated zone at ADRS. In spring 2015 and 2016, shallow unsaturated zone gas samples were collected from 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 carbon isotopic composition (δ¹³C-CO₂), and CH₄ mole fraction (xCH₄). Graphical analysis of results indicates mixing of CO₂ characteristic of the root zone, deep soil gas of the capillary fringe, and CO₂ produced by microbial respiration of organic matter disposed in the LLRW trenches. Higher xCO₂, lower δ¹³C-CO₂, and lower xCH₄ in shallow gas samples in 2016 compared to 2015 are consistent with increased soil moisture conditions in 2016. 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. The proportion of CO₂ to CH₄ near the LLRW area suggests that a relatively small portion of the waste is decomposed anaerobically. Although xCH₄ reflecting lateral transport from the LLRW area is decreasing with time in the deep profiles, deep unsaturated zone xCO₂ has changed little over 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.