Unsaturated Zone CO2, CH4, and δ13C‐CO2 at an Arid Region Low‐Level Radioactive Waste Disposal Site. Issue 1 (14th June 2018)

• Record Type: Journal Article
• Authors: Conaway, C.H.; Walvoord, M.A.; Thomas, R.B.; Green, C.T.; Baker, R.J.; Thordsen, J.J.; Stonestrom, D.A.; Andraski, B.J.
• Is Part Of: Vadose zone journal. Volume 17:Issue 1(2018)
• Page Start: 1
• Page End: 11
• Publication Date: 2018-06-14
• Subjects: Soil science -- Periodicals; Zone of aeration -- Periodicals; Groundwater flow -- Periodicals; Groundwater flow; Zone of aeration; Periodicals; Electronic journals; 631.4
• DOI: 10.2136/vzj2017.11.0200
• Languages: English
• ISSNs: 1539-1663

Abstract

Abstract : Core Ideas: CO2 –C isotopic composition discriminates sources and shows mixing. The shallow root zone is key in shallow CO2 and CH4 production and cycling. Differences in CO2 and CH4 distributions are explained by interactions with pore water. 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 ( x CO2 ) and C isotopic composition (δ 13 C‐CO2 ) and CH4 mole fraction ( x CH4 ). Graphical analysis of the results indicates mixing of CO2 characteristic of the root zone (δ 13 C −18 to −19‰), deep soil gas of the capillary fringe (−13 to −15‰), and CO2 produced 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 x CH4 reflecting lateral transport from the LLRW area is decreasing with time in the deep profiles, deep unsaturated zone x CO2 has changed little in recent decades. The results imply that CH4 and δ 13 C‐CO2 may serve as good tracers of anthropogenic effects in the unsaturated zone even when CO2 primarily reflects natural processes.