Influence of advection on the soil gas radon deficit technique for the quantification of LNAPL

Alessandra Cecconi; Iason Verginelli; Fernando Barrio-Parra; Eduardo De Miguel; Renato Baciocchi

doi:10.1016/j.scitotenv.2023.162619

Influence of advection on the soil gas radon deficit technique for the quantification of LNAPL

Sci Total Environ. 2023 Jun 1:875:162619. doi: 10.1016/j.scitotenv.2023.162619. Epub 2023 Mar 4.

Authors

Alessandra Cecconi¹, Iason Verginelli², Fernando Barrio-Parra³, Eduardo De Miguel³, Renato Baciocchi¹

Affiliations

¹ Laboratory of Environmental Engineering, Department of Civil Engineering and Computer Science Engineering, University of Rome Tor Vergata, Via del Politecnico 1, 00133 Rome, Italy.
² Laboratory of Environmental Engineering, Department of Civil Engineering and Computer Science Engineering, University of Rome Tor Vergata, Via del Politecnico 1, 00133 Rome, Italy. Electronic address: verginelli@ing.uniroma2.it.
³ Prospecting & Environment Laboratory (PROMEDIAM), ETS de Ingenieros de Minas y Energía, Universidad Politécnica de Madrid, Alenza 4, 28003 Madrid, Spain.

PMID: 36878290
DOI: 10.1016/j.scitotenv.2023.162619

Abstract

The Radon (Rn) deficit technique is a rapid, low-cost, and non-invasive method to identify and quantify light non-aqueous phase liquids (LNAPL) in the soil. LNAPL saturation is typically estimated from Rn deficit using Rn partition coefficients, assuming equilibrium conditions. This work examines the applicability of this method in the presence of local advective fluxes that can be generated by groundwater fluctuations or biodegradation processes in the source zone. To this end, a one-dimensional analytical model was developed to simulate the steady-state diffusive-advective transport of soil gas Rn in the presence of LNAPL. The analytical solution was first validated against an existing numerical model adapted to include advection. Then a series of simulations to study the effect of advection on Rn profiles were carried out. It was found that in high-permeability soils (such as sandy soils), advective phenomena can significantly affect Rn deficit curves in the subsurface compared with those expected, assuming either equilibrium conditions or a diffusion-dominated transport. Namely, in the presence of pressure gradients generated by groundwater fluctuations, applying the traditional Rn deficit technique (assuming equilibrium conditions) can lead to an underestimation of LNAPL saturation. Furthermore, in the presence of methanogenesis processes (e.g., in the case of a fresh LNAPL of petroleum hydrocarbons), local advective fluxes can be expected above the source zone. In such cases, Rn concentrations above the source zone can be higher than those above background areas without advective phenomena, resulting in Rn deficits higher than 1 (i.e., Rn excess), and thus leading to a wrong interpretation regarding the presence of LNAPL in the subsurface if advection is not considered. Overall, the results obtained suggest that advection should be considered in the presence of pressure gradients in the subsurface to ensure an effective application of the soil gas Rn-deficit technique for quantitative estimation of LNAPL saturation.

Keywords: Advective transport; LNAPL delineation; Radon deficit technique; Soil gas transport modeling.