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

High Ar-37 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 Ar-37 in the subsurface, mainly due to Ca activation by cosmic rays. A better understanding and improved capability to predict Ar-37 activity concentration in the subsurface and its spatial and temporal variability is thus required. A numerical model integrating Ar-37 production and transport in the subsurface is developed, including variable soil water content and water infiltration at the surface. A parameterized equation for Ar-37 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 Ar-37 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 Ar-37 activity concentrations. The influence of soil water content on Ar-37 production is shown to be negligible to first order, while Ar-37 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. (C) 2016 Elsevier Ltd. All rights reserved.