Radon diffusion in Non-Aqueous Phase Liquids (NAPLs): experimental data and their impact on the modelization of Rn deficit technique

From the point of view of a green economy and social sustainability, the use of investigating and monitoring techniques based on natural tracers are always more studied in order to reduce environmental and economical impact. The Radon deficit technique is proved to be a good instrument to apply in case of site contaminated by Non-Aqueous Phase Liquids (NAPLs)(De Miguel et al., 2020). This model can assess NAPL contamination of soil, in both vadose and satured section, when it is known the Rn partition coefficient between present fluid phases (air, water, NAPL) at equilibrium conditions (Semprini et al., 2000). The equilibrium condition can be considered respected only in two limit cases: stagnant water and relatively high groundwater flow. In fact, in natural systems Rn mobility in groundwater is generally controlled by a combination of diffusion and water transport. Assuming an istantaneous Rn exchange at the NAPL /groundwater interface depending on RnNAPL/ groundwater partition coefficient, Rn contact rate from groundwater to the interface must be valued. Moreover, the same evaluations are required for residual NAPLs, in which Rn only moves by diffusion. Generally, Rn diffusion in water is well known in literature and the groundwater flux can be measured. Instead, Rn diffusion in NAPLs can be theoretically calculated by Stockes-Einstein relationship, where the viscosity plays a major role. Thus, Rn diffusion in NAPLs is still an interesting parameter to investigate by lab tests due to lack of data in literature. The possible outcomes of this study include experimental data on diffusion coefficients in NAPLs, a theoretical improvement of Radon deficit technique and an extension to its modeling equations in case of non-equilibrium conditions (Morita,1996), that cannot be studied by this method at the moment. De Miguel E., Barrio-Parra F., Izquierdo-Díaz M., Fernández J., García-González J.E. & Álvarez R. (2020) - Applicability and limitations of the radon-deficit technique for the preliminary assessment of sites contaminated with complex mixtures of organic chemicals: A blind field-test. Environ. International, 138. https://doi.org/10.1016/j.envint.2020.105591. Morita A. (1996) - Non-equilibrium Partition Constants. J.Phys. Chem., 100, 12131-12134. https://doi.org/10.1021/ jp9607879. Semprini L., Hopkins O.S. & Tasker B.R. (2000) - Laboratory, Field and Modeling Studies of Radon-222 as a Natural Tracer for Monitoring NAPL Contamination. Transport Porous Med., 38, 223-240. https://doi.org/10.1023/A:1006671519143.