Using 222-Radon as tracer for areal and vertical distribution of hydrocarbon contaminations

Non-aqueous Phase Liquids (NAPLs), including crude oil and its refined products, produce a long lasting impact on the enviroment due to unwilling releases. Storage sites of fuel and pump stations are often involved in unwilling NAPL spills in the subsoil and/or groundwater. The non-polar nature of these contaminants confers them a slight solubility in water, causing a reduced efficency of the most common remediation techniques. These limitations result in long-term remediation activities with high related costs. Unconventional characterization methods like the Radon (222Rn)-deficit technique are meant to supplement and optimize the conventional approach by providing dense spatial information in quasi-real time, and at a reasonable cost (De Miguel et al., 2020). Schubert (2015) presented a comprehensive review of the use of 222Rn as a natural tracer to assess subsurface NAPL contamination. In the present work Radon deficit technique was applied to determine the vertical distribution of Rn in groundwater inside a gasoline contaminated area. Twelve passive accumulators PDMS-AC (polydimethylsiloxane mixed with activated carbon) were inserted at different depth inside fenestrated piezometers and left for two weeks to maximize the Rn accumulation. Then they were measured by high resolution gamma spectometry. PDMS-AC are characterized by a high permeability to Rn and impermeability to water (Voltaggio & Spadoni, 2013). The obtained results suggest the possible application of Radon deficit to find not only the areal but also the vertical distribution of a NAPL contamination in the saturated level. This datum could offer interesting applications to estimate initial and residual volumes of the contaminant during remediation procedures.