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

Abstract

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.

Keywords: Diesel fuel; LNAPL ageing; LNAPL contamination; Natural attenuation; Radon deficit technique; Shallow aquifer.