Background concentrations of Argon-39 in shallow soil gas

While radioisotopes of noble gases are known to be indicators of underground nuclear explosions (UNE), McIntyre et al. (2017) was the first to report the presence of ³⁹Ar in shallow soil gas in association with a decades old UNE. While this finding hinted at the potential application of ³⁹Ar to be used as an indicator of a UNE, doing so would also require an understanding of the natural concentrations of ³⁹Ar present in soil gas. Without knowing the expected range and variability of naturally occurring concentrations of ³⁹Ar, it is difficult to determine what measured concentrations would be indicative of an elevated concentration. This paper presents results from 16 soil gas samples and three atmospheric air samples collected from various locations across the western United States. Shallow soil gas samples were collected into self-contained underwater breathing apparatus (SCUBA) tanks using a custom-built soil gas sampling system and then processed and analyzed for ³⁹Ar. The measured concentrations of ³⁹Ar varied from atmospheric air concentrations to about 3.5 times atmospheric air concentrations (58 mBq/m³). The results presented here represent the first measurements of natural background ³⁹Ar concentrations in shallow soil gas. This data will be necessary if ³⁹Ar is to be used as an indicator of UNE.