title: Alteration of natural (37)Ar 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.
authors.affiliation: GEOTOP, Département des sciences de la Terre et de l'atmosphère, Université du Québec à Montréal, CP8888 Succ. Centre-Ville, Montreal, QC, Canada. Electronic address: guillon@sca.uqam.ca.
authors.forename: Sophie
authors.initials: S
authors.lastname: Guillon
authors.affiliation: Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA, 94550, United States. Electronic address: sun4@llnl.gov.
authors.forename: Yunwei
authors.initials: Y
authors.lastname: Sun
authors.affiliation: Climate-Environmental Physics, Physics Institute, University of Bern, Switzerland, Sidlerstrasse 5, CH-3012, Berne, Switzerland. Electronic address: purtschert@climate.unibe.ch.
authors.forename: Roland
authors.initials: R
authors.lastname: Purtschert
authors.affiliation: Climate-Environmental Physics, Physics Institute, University of Bern, Switzerland, Sidlerstrasse 5, CH-3012, Berne, Switzerland. Electronic address: raghoo@climate.unibe.ch.
authors.forename: Lauren
authors.initials: L
authors.lastname: Raghoo
authors.affiliation: CEA, DAM, DIF, F-91297, Arpajon, France. Electronic address: eric.pili@cea.fr.
authors.forename: Eric
authors.initials: E
authors.lastname: Pili
authors.affiliation: Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA, 94550, United States. Electronic address: carrigan1@llnl.gov.
authors.forename: Charles R
authors.initials: CR
authors.lastname: Carrigan
mesh_terms: D001128:Argon / Q000032:analysis*; D005107:Explosions; D008962:Models, Theoretical; D054043:Nuclear Weapons; D011834:Radiation Monitoring; D011868:Radioisotopes / Q000032:analysis*; D012990:Soil Pollutants, Radioactive / Q000032:analysis*; D014867:Water / Q000032:analysis
chemical_list: D011868:Radioisotopes; D012990:Soil Pollutants, Radioactive; D014867:Water; D001128:Argon
keywords: (37)Ar; CTBTO; Gas transport; Numerical model; Uncertainty; Water saturation