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

Abstract

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 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 xCH4 reflecting lateral transport from the LLRW area is decreasing with time in the deep profiles, deep unsaturated zone xCO2 has changed little in 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.