Radon deficit technique applied to the study of the ageing of a spilled LNAPL in a shallow aquifer.

A recent diesel spill (dated January 2019 ± 1 month) in a refilling station is investigated by the Radon deficit technique. The primary focus was on quantifying the LNAPL pore saturation as a function of duration of ageing, and on proposing a predictive model for on-site natural attenuation. A biennial monitoring of the local fluctuating shallow aquifer has involved the saturated zone nine times, and the vadose zone only once. Rn background generally measured in external and upstream wells is elaborated further due to the site characteristics, using drilling logs and phreatic oscillations. Notably, this study marks the first application of the Rn deficit method to produce a detailed Rn background mapping throughout the soil depth. Simultaneously, tests are performed on LNAPL surnatant samples to study diesel ageing. In particular, they are focused on temporal variations of LNAPL viscosity (from an initial 3.90 cP to 8.99 cP, measured at 25 °C, after 34 months), and Rn partition coefficient between the pollutant and water (from 47.7 to 80.2, measured at 25 °C, after 14 months). Rn diffusion is also measured in different fluids (0.092 cm2 s-1, 1.14 × 10-5 cm2 s-1, and 2.53 × 10-6 cm2 s-1 at 25 °C for air, water and LNAPL, respectively) directly. All parameters and equations utilized during this study are introduced, discussing their influence on Radon deficit technique from a theoretical point of view. Experimental findings are used to mitigate the effect of LNAPL ageing and of phreatic oscillations on determination of LNAPL saturation index (S.I.LNAPL). Finally, S.I.LNAPL dataset is discussed and elaborated to show the pollutant attenuation across subsurface over time, induced by natural processes primarily. The proposed predictive model for on-site natural attenuation suggests a half-removal time of one year and six months. The significance of such models lies in their capability to assess site-specific reactions to pollutants, thereby enhancing the effectiveness of remediation efforts over time. These experimental findings may offer a novel approach to application of Rn deficit technique and to environmental remediation of persistent organic compounds.

Vertical Light Non-Aqueous Phase Liquid (LNAPL) distribution by Rn prospecting in monitoring wells.

In the frame of a collaboration between the Italian National Research Council (CNR) and Mares s.r.l., a study, about the possibility of determining radon vertical distribution at different soil depths in order to trace light non-aqueous phase liquid (LNAPL) contaminations, was developed. The radon deficit technique, based on the preferential solubility of soil gas radon into non-polar fluids, such as refined hydrocarbons, has been investigated by various theoretical and applied research so far. According to international scientific literature, radon deficit can be used both for geochemical prospection of the spatial irregular NAPL dispersion and for monitoring of remediation activities. Even though it is well known that this type of pollutants can be distributed along the vertical soil profile-firstly due to their density in comparison to water density, and secondly due to fluctuations of shallow aquifers, soil pore size, aging of contamination, and so on-the vertical localization of the plume still represents a scientific challenge. In this article, a method to determine the radon vertical profile is tested and applied to assess the potential use of the radon deficit technique in the vertical detection of pollutant presence for the first time in a fuelling station. Two LNAPL-contaminated sites were selected for a pilot test. Experimental findings seem to support the use of vertical radon geochemical prospection to delimit the depth range of a LNAPL pollution directly. Systematic data collection and modeling may lead to a 3D reconstruction of the dispersion of contaminant in different soil levels.