Vertical Non-Aqueous Phase Liquid distribution by Rn prospecting for 3D plume modelling in polluted sites

Abstract

Abstract In the frame of a collaboration, between CNR-IGAG and Mares s.r.l., on the triennial project “CANDAC-Mares 2020” about the study of Non-Aqueous Phase Liquid (NAPL) contaminated sites, the Laboratory of Environmental Radioactivity, IGAG-CNR Montelibretti organized a study on the possibility to determine on-site radon vertical distribution as NAPL contamination tracer at different soil depths. Rn deficit technique, based on preferential solubility of natural radiogenic soil gas Rn into non-polar fluids, such as refine hydrocarbons, has been studied in various theoretical and applied researches so far. According to international scientific literature, Rn deficit can be used both in geochemical prospection of the areal irregular pollutant distribution and in monitoring of remediation activities successfully. Unfortunately the vertical location of the plume still represents a scientific challenge even though is well known this type of pollutant can be distributed along the vertical soil profile due to its density in comparison to water density, fluctuations of shallow aquifers, soil pore size distribution, aging of contamination and so on. In this article, a method to determine Rn vertical profile is tested and applied to assess Rn deficit technique applicability in vertical detection of pollutant presence for the first time in a fuel station. Two NAPL contaminated sites are selected for a pilot test. Results obtained seem to support the use of vertical Rn geochemical prospection for a 3D complete reconstruction of contaminant dispersion in different soil levels in order to monitoring plume changes during time due to natural attenuation and/or remediation activities.