Abstract
In this research, the radioactive noble gas radon was used as a tracer for Non-Aqueous Phase Liquids (NAPLs) contamination, since it is much more soluble in these substances than in air or water. Soil radon remains trapped within the NAPLs, resulting in a local reduction in the radon concentration within close proximity to the contaminated area. This technique was applied to a contaminated site in Roma (Italy). The main residual NAPLs are total hydrocarbons and methyl-tertiary-butyl ether (MTBE), a water-soluble additive. The monitoring activities included two sampling campaigns of groundwater from 18 wells in February and May 2020. Concentration maps were produced using radon data. The results show that the radon deficit traces the location of NAPLs in the fuelling station very well, with a residual source zone extending in a NNW-SSE direction. A good correspondence between a low amount of radon and a higher concentration of NAPLs was found. A reduction in the average amount of radon in the May 2020 survey indicated a stronger remobilization of NAPLs compared to that of the February 2020 monitoring campaign. The peaks of Volatile Organic Compounds (VOCs) detected between 8–9 and 11–12 m depths indicate the presence of residual blobs of NAPLs in the vadose zone of the aquifer.
Highlights
The monitoring and detection of organic contaminants—e.g., Non-Aqueous Phase Liquids (NAPLs)—in the subsoil using geochemical and non-invasive methods are crucial due to the direct and strong impact that these substances have on groundwater resources
The chemical analyses of the groundwater show that the main residual NAPLs still present in the site are total hydrocarbons, expressed as n-hexane and methyl-tertiary-butyl ether (MTBE), an additive introduced in gasoline in place of lead
This is consistent because when light non-aqueous phase liquids (LNAPLs), such as total hydrocarbons or MTBE, infiltrate the soil, being lighter than water they tend to float on top of the water table and follow its flow direction
Summary
The monitoring and detection of organic contaminants—e.g., Non-Aqueous Phase Liquids (NAPLs)—in the subsoil using geochemical and non-invasive methods are crucial due to the direct and strong impact that these substances have on groundwater resources. Effective remediation planning requires the identification and assessment of residual NAPLs in the subsoil or contaminants dissolved in groundwater to form a NAPL plume. Geochemical tracers such as radon gas are often used to study the spatial distribution of residual contaminants in aquifers and to deduce information on subsoil contamination [1,2]. Due to its ubiquitous presence in nature, its chemical and physical properties, and its straightforward detectability, radon fulfils all the requirements for use as an environmental tracer [2,3]. The equilibrium radon concentration in the pore space of an aquifer or soil depends on the aquifer emanation coefficient, the aquifer porosity, the radium activity concentration, and the bulk density of the mineral matrix [4]
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