Evaluating Noble Gas Transport through Fractured Rock at a Legacy Nuclear Test Site

Understanding noble gas migration in fractured rocks and estimating their transport and fate in the subsurface is important for detecting underground nuclear explosions (UNEs). As part of the Underground Nuclear Explosion Signatures Experiment (UNESE), tracer gases including radioactive xenon and argon gas were injected into the chimney of the Barnwell nuclear test site in 2016 and migration of those tracers to the surface was monitored within several boreholes. Incorporating data from this field campaign, simulations were performed to model the tracer behavior using STOMP-GT, a multiphase subsurface flow and transport simulator developed at Pacific Northwest National Laboratory. Additionally, a suite of simulations was performed of ³⁷Ar and ³⁹Ar transport from hypothetical underground cavities to the ground surface to explore the likelihood of these gases persisting in the ground for years or even decades after an UNE. A range of properties was evaluated representing geologic media with different fracture apertures and frequencies in dual-porosity/permeability models for the constitutive relative permeability-saturation-capillary pressure relations of fractured rock units. The arrival time and concentrations of ³⁷Ar and ³⁹Ar at the surface were evaluated, and the persistence of ³⁹Ar in the subsurface, due to its longer half-life, was demonstrated. The persistence of ³⁹Ar suggests that this species may be an ideal long-term indicator of an UNE.