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

Abstract

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 2 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 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 2 mole fraction (xCO 2 ) and C isotopic composition (d 13 C-CO 2 ) and CH 4 mole fraction (xCH 4 ). Graphical analysis of the results indicates mixing of CO 2 characteristic of the root zone (d 13 C −18 to −19‰), deep soil gas of the capillary fringe (−13 to −15‰), and CO 2 produced by microbial respiration of organic matter disposed in the LLRW trenches (−22 to −25‰). Distribution of CH 4 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 xCH 4 reflecting lateral transport from the LLRW area is decreasing with time in the deep profiles, deep unsaturated zone xCO 2 has changed little in recent decades. The results imply that CH 4 and d 13 C-CO 2 may serve as good tracers of anthropogenic effects in the unsaturated zone even when CO 2 primarily reflects natural processes.Abbreviations: ADRS, Amargosa Desert Research Site; LLRW, low-level radioactive waste; VOC, volatile organic compound; xCH 4 , methane mole fraction; xCO 2 , carbon dioxide mole fraction.

Spatial Fingerprinting of Biogenic and Anthropogenic Volatile Organic Compounds in an Arid Unsaturated Zone

“…Concentrations in that area are one to two orders of magnitude greater than other concentrations near and in the LLRW area. These results confirm previous postulation of multiple landfill source locations (with CCl 4 having a source to the east) based on VOC samples from outside the LLRW facility (Baker et al, 2012) and CO 2 and CH 4 concentration profiles (Conaway et al, 2018). The contours of m , p ‐xylene show a dispersed, patchy pattern with peaks separated from any known anthropogenic sources.…”

“…Loadings are similar for different dates and years at JFDB, indicating relative uniformity of VOCs with time and depth at that undisturbed, background location. For PC1, all loadings are positive, with the lowest values (least influence) at UZB3_23.8 and X488Y480_1.5 locations, which are associated with emissions from the LLRW area (Conaway et al, 2018). In other words, high scores for PC1 indicate elevated concentrations in all samples, especially at distal and intermediate locations beyond the direct influence of the LLRW sources.…”

“…3D). Lower bVOC concentrations at these locations may reflect the relative scarcity of plants on the LLRW landfill cover, greater microbial respiration of bVOCs near these locations as indicated by elevated CO 2 and relatively low δ 13 C‐CO 2 (Conaway et al, 2018), or displacement of bVOCs by increased gas advection in the LLRW facility. The VOCs plotting along the trend extending up and to the right consist of CFCs, chlorinated ethenes and ethanes, and cyclohexane.…”

“…Peaks at depth may relate to variations of geology, production rates, or reactivity. For example, the majority of ambient VOCs exhibit peaks at approximately 100 m in both UZB2 and UZB3, which may relate to georespiration, consistent with elevated CO 2 near the water table (Walvoord et al, 2005;Conaway et al, 2018;Keller and Bacon, 1998), and potential secondary effects of equilibrium gravitational settling (Seltzer et al, 2017). Greater concentrations at UZB2 (160 m from the nearest waste trench) than at UZB3 (100 m from the nearest waste trench) indicate a negative association of ambient VOC sources with the waste facility, consistent with results of the PCA analysis and the distributed shallow-sample observations for m,p-xylene (Fig.…”

“…Biological sources may emit VOCs in the subsurface directly, or aboveground emissions may enter the subsurface by diffusion and by barometric pumping (Auer et al, 1996; Choi et al, 2002; Stauffer et al, 2019). Elevated CO 2 near the water table at the ADRS indicated potential georespiration (Walvoord et al, 2005; Conaway et al, 2018), in which microbes respire relict carbon. Global emissions have led to accumulation in the atmosphere of refractory VOCs such as the CFCs, which has led to their use as tracers of atmospheric exchange with the unsaturated zone (Weeks et al, 1982) and as tracers of the time of recharge of groundwater (Busenberg and Plummer, 1992).…”