Alteration of natural 37Ar activity concentration in the subsurface by gas transport and water infiltration

Abstract

High (37)Ar activity concentration in soil gas is proposed as a key evidence for the detection of underground nuclear explosion by the Comprehensive Nuclear Test-Ban Treaty. However, such a detection is challenged by the natural background of (37)Ar in the subsurface, mainly due to Ca activation by cosmic rays. A better understanding and improved capability to predict (37)Ar activity concentration in the subsurface and its spatial and temporal variability is thus required. A numerical model integrating (37)Ar production and transport in the subsurface is developed, including variable soil water content and water infiltration at the surface. A parameterized equation for (37)Ar production in the first 15 m below the surface is studied, taking into account the major production reactions and the moderation effect of soil water content. Using sensitivity analysis and uncertainty quantification, a realistic and comprehensive probability distribution of natural (37)Ar activity concentrations in soil gas is proposed, including the effects of water infiltration. Site location and soil composition are identified as the parameters allowing for a most effective reduction of the possible range of (37)Ar activity concentrations. The influence of soil water content on (37)Ar production is shown to be negligible to first order, while (37)Ar activity concentration in soil gas and its temporal variability appear to be strongly influenced by transient water infiltration events. These results will be used as a basis for practical CTBTO concepts of operation during an OSI.

Advancing Physically‐Based Flow Simulations of Alluvial Systems Through Atmospheric Noble Gases and the Novel 37Ar Tracer Method

“…Both pathways lead to an exponentially decreasing production rate of 37 Ar with depth, which is characterized by the attenuation length ( L ). depends mainly on the bulk density of the sediments (including water and soil gas; Guillon et al, ). In case of predominantly horizontal GW flow and full saturation of the porous media, advective and diffusive vertical transport is negligible and the exponential production profile is maintained in the 37 Ar activity concentration of GW.…”

Explicit simulation of environmental gas tracers with integrated surface and subsurface hydrological models

“…Other production reactions, e.g., induced by muons or by neutrons from (α, n) reactions can be neglected in the depth range considered here (Spannagel and Fireman, 1972;Heisinger et al, 2002;Sramek et al, 2017). The depth dependent production rate 37 P (z)r in (atoms•yr −1 • cm −3 ) has been parameterized through a production function equation (Guillon et al, 2016) and can be described as follows:…”

“…HYDRUS, NUFT, and Getflows (Nitao, 1998;Tosaka et al, 2000;Simunek et al, 2016) are examples of multiphase flow and transport models. However, these mechanistic models are computationally demanding and are therefore usually preferred for controlled experiments and to model mass-transport in unsaturated soils (Guillon et al, 2016).…”

Two-Phase Flow, Heat and Mass Transfer and Tracer Transport to the Atmosphere from Underground Nuclear Cavities Through Fractured Porous Media

“…However, most papers use the dual permeability approach of Barenblatt et al (1960) with various degrees of sophistication and various softwares depending on their institutes. NUFT is used for single phase flows by Sun and Carrigan (2014) and for two-phase flows the thermal effects are neglected and only long-term by Carrigan (2014, 2016), Sun et al (2015) and Guillon et al (2016). FEHM is used by Jordan et al (2014Jordan et al ( , 2015, Harp et al (2018), and Bourret et al (2020) for instance.…”