Applicability of radon emanometry in lithologically discontinuous sites contaminated by organic chemicals

Abstract

The applicability of radon (Rn) measurements to delineate non-aqueous phase liquids (NAPL) contamination in subsoil is discussed at a site with lithological discontinuities through a blind test. Three alpha spectroscopy monitors were used to measure radon in soil air in a 25,000-m area, following a regular sampling design with a 20-m grid. Repeatability and reproducibility of the results were assessed by means of duplicate measurements in six sampling positions. Furthermore, three points not affected by oil spills were sampled to estimate radon background concentration in soil air. Data histograms, Q-Q plots, variograms, and cluster analysis allowed to recognize two data populations, associated with the possible path of a fault and a lithological discontinuity. Even though the concentration of radon in soil air was dominated by this discontinuity, the characterization of the background emanation in each lithological unit allowed to distinguish areas potentially affected by NAPL, thus justifying the application of radon emanometry as a screening technique for the delineation of NAPL plumes in sites with lithological discontinuities.

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

“…Theoretical and practical aspects of the Rn deficit technique were studied through laboratory experiments [20,53,55,57,60,110], modeling approaches [18,21,59,60,109], and field experiences [19,25,26,29,30,54,56,59,[114][115][116]. This section presents an overview of the technique's utilization in soil gas, especially from a field perspective.…”

“…Regarding the field applications, different configurations were developed and successfully applied to assess the occurrence of residual LNAPL also in the unsaturated portion of the aquifer (vadose and smear zones) [23,25,26,29,56,58,115,117]. Radon activity can be monitored in soil gas with different approaches, depending on the equipment used, the sampling method selected, and the characteristics of the site under investigation.…”

“…These probes are constructed from stainlesssteel hollow bars with free, pointed bottom ends ("lost" tips). The probes are then installed to a depth of 0.7-1 m, either manually or using tools such as electric drills [19,23,25,29,115]. This depth range is typical when using temporary probes and is mainly defined by the necessity to sample soil gas at a depth that will be minimally affected by atmospheric variables, such as temperature, atmospheric pressure, and wind velocity.…”

“…A direct correlation was found between the Rn deficit at shallow depths and the highest electrical resistivity at greater depths, which increases the reliability of the results of the methods. De Miguel et al [115] used temporary soil gas probes to measure soil gas Rn through hollow rods buried 75-100 cm in the subsoil, at a site with lithological discontinuities through a blind test. Rn was measured using alpha spectroscopy monitors, following a regular sampling design with a 20 m 2 grid and adding some external points to assess the background Rn activity concentration in soil gas at the site.…”

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

“…The efficiency of both approaches relies on environmental monitoring [2] and on a suitable geochemical tracer of ongoing transport, dilution, degradation, and volatilization phenomena [3,4]. Radon gas can be used to identify NAPLs in soil and groundwater because of its preferential partitioning in organic phases [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. The resulting radon deficit (regarding the site-specific radon background concentration in soil or groundwater) can be utilized as an NAPL indicator [17].…”