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

High 37Ar 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 37Ar in the subsurface, mainly due to Ca activation by cosmic rays. A better understanding and improved capability to predict 37Ar activity concentration in the subsurface and its spatial and temporal variability is thus required. A numerical model integrating 37Ar production and transport in the subsurface is developed, including variable soil water content and water infiltration at the surface. A parameterized equation for 37Ar 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 37Ar 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 37Ar activity concentrations. The influence of soil water content on 37Ar production is shown to be negligible to first order, while 37Ar 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.