Radon is a radioactive gas naturally occurring in soils and groundwater. It was used as a natural tracer for Non -Aqueous Phase Liquids (NAPLs) contamination, since it is much more soluble in a wide range of these substances than in air or water, resulting in a concentration-deficit compared to background values in nearby unpolluted areas. The mapping of this process, known as the “radon deficit technique” (Semprini et al., 2000) allows identifying the contamination by NAPL which affects both the vadose zone and the phreatic zone of an aquifer. This technique was applied to a contaminated site in Roma (Italy). In the site there is a gas station, where more than twenty years ago the first oil spill occurred with other episodes over time. The remediation of the area began in 2016 when a network of sampling wells was set up to monitor the extent and evolution of the contamination. The remediation system consists of pumping wells of the “pump & treat” system and injection wells for the reintroduction of treated water. A soil vapour extraction system completes the remediation plant. The main residual NAPLs in the site are total hydrocarbons expressed as n-hexane and Methyl-TertiaryButyl Ether (MTBE), a water-soluble additive. The monitoring activities included five sampling campaigns of water from the piezometers from February 2020 to May 2021. Radon measurements were collected with the radonometer Rad7 (Durridge Company Inc.) equipped with the Big Bottle RAD H2O accessory. Concentration maps, produced using radon data from water analyses, allow observing the gas concentration distribution in the waters of the sampled piezometers. The results show that the radon deficit accurately traces the location of NAPLs in the gas station, with a residual source zone extending in the NNW-SSE direction, following the flow direction of the aquifer. A good correlation between a low radon concentration and a higher presence of NAPLs was found. Finally, the presence of underground cavities around the study site, which could affect the circulation of fluids in the subsoil, is known by previous works (Nisio et al., 2017). Therefore, in order to deepen the study of the area surrounding the contaminated site, it was decided to carry out measurements of total gamma radiation, with a gamma detector.
Mattia, M., Tuccimei, P., Soligo, M., Portaro, M., Carusi, C., Rainaldi, E., et al. (2021). Using Radon as a natural tracer for NAPL (MTBE and total hydrocarbons) contamination: a case study in Roma (Italy). In 90° Congresso della Società Geologica Italiana. Abstract book [10.3301/ABSGI.2021.03].
Using Radon as a natural tracer for NAPL (MTBE and total hydrocarbons) contamination: a case study in Roma (Italy)
Mattia M.;Tuccimei P.;Soligo M.;Portaro M.;
2021-01-01
Abstract
Radon is a radioactive gas naturally occurring in soils and groundwater. It was used as a natural tracer for Non -Aqueous Phase Liquids (NAPLs) contamination, since it is much more soluble in a wide range of these substances than in air or water, resulting in a concentration-deficit compared to background values in nearby unpolluted areas. The mapping of this process, known as the “radon deficit technique” (Semprini et al., 2000) allows identifying the contamination by NAPL which affects both the vadose zone and the phreatic zone of an aquifer. This technique was applied to a contaminated site in Roma (Italy). In the site there is a gas station, where more than twenty years ago the first oil spill occurred with other episodes over time. The remediation of the area began in 2016 when a network of sampling wells was set up to monitor the extent and evolution of the contamination. The remediation system consists of pumping wells of the “pump & treat” system and injection wells for the reintroduction of treated water. A soil vapour extraction system completes the remediation plant. The main residual NAPLs in the site are total hydrocarbons expressed as n-hexane and Methyl-TertiaryButyl Ether (MTBE), a water-soluble additive. The monitoring activities included five sampling campaigns of water from the piezometers from February 2020 to May 2021. Radon measurements were collected with the radonometer Rad7 (Durridge Company Inc.) equipped with the Big Bottle RAD H2O accessory. Concentration maps, produced using radon data from water analyses, allow observing the gas concentration distribution in the waters of the sampled piezometers. The results show that the radon deficit accurately traces the location of NAPLs in the gas station, with a residual source zone extending in the NNW-SSE direction, following the flow direction of the aquifer. A good correlation between a low radon concentration and a higher presence of NAPLs was found. Finally, the presence of underground cavities around the study site, which could affect the circulation of fluids in the subsoil, is known by previous works (Nisio et al., 2017). Therefore, in order to deepen the study of the area surrounding the contaminated site, it was decided to carry out measurements of total gamma radiation, with a gamma detector.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
