Combining Radon Deficit, NAPL Concentration, and Groundwater Table Dynamics to Assess Soil and Groundwater Contamination by NAPLs and Related Attenuation Processes

Abstract

Soil and groundwater contamination by NAPLs (Non-Aqueous Phase Liquids) is certainly a big issue for protecting the environment. In situ clean-up actions are routinely applied to mitigate the risk and are supplemented by monitoring surveys to assess the degree, extension, and evolution of the contamination. Radon gas is here used as a tracer of contamination because of its high solubility in non-polar solvents that produce a reduced concentration of the gas in polluted soil and groundwater with reference to radon levels in adjacent “clean” areas. This approach was employed in two sites where gasoline and diesel spillage occurred, causing soil and groundwater contamination. The two case studies were chosen because of their difference in terms of the hydrogeological features, age of the spillage, composition of residual NAPLs, and clean-up measures to test the advantages and limits of this approach in a variety of settings. Radon data, NAPL concentration in the groundwater (mainly total hydrocarbons, Methyl Tertiary-Butyl Ether and Ethyl Tertiary-Butyl Ether) and the depth of the groundwater table were periodically collected in surveys that spanned a period of two years. This dataset was statistically processed using principal component analysis to unravel which factors and attenuation processes are working in the sites and the response of the radon deficit approach to this complex series of phenomena concurrently occurring there.

Assessing Light Non-Aqueous Phase Liquids in the Subsurface Using the Soil Gas Rn Deficit Technique: A Literature Overview of Field Studies

“…The use of tracers to characterize and monitor LNAPL and to evaluate the effectiveness of in situ remediation has been investigated as one of the potential alternative approaches [17]. The potential use of 2 of 38 222 Radon (Rn), as a natural tracer characterized by a preferential partitioning into LNAPL, has been studied in this regard, as a sustainable alternative, since the late 1990s, e.g., [18][19][20][21][22][23][24][25][26][27][28][29][30].…”

“…This latter limitation was considered statistically significant only if no significant advective or co-advective transport of Rn in soil gas is assumed at the site. Mattia et al [27] measured Rn in soil gas by extracting it from two vapor extraction wells from a Soil Vapor Extraction (SVE) system, at different depths, and analyzing the samples in the laboratory using a semiconductor detector connected in a closed loop. Rn deficit observed in soil gas was used together with the more abundant Rn data obtained from groundwater sampling from the wells at the site, to identify the location of residual LNAPL.…”