## 4 Results  
### Site 1  
Table 1 shows the radon activity concentrations of the groundwater extracted from the 18 wells of site 1 in the period 2020-2022. Tables 1 and 2 report correspondent groundwater levels and contents of NAPLs (total hydrocarbons, MTBE, and ETBE), respectively. Radon levels ranged from 17 Bq L\({}^{-1}\) (well PZ04 in November 2020) to 185 Bq L\({}^{-1}\) (well PZ03 in May 2021). Lower values were generally recorded in some wells within the study area (wells PZ02, PZ04, PZ08, PZ09, PZ10, PZ13, and PZ19, see Figure 2 for locations), adjacent to or downgradient of the removed leaking underground tank. Higher radon concentrations, found in wells PZ03, PZ07, PZ14, PZ25, and PZ26 (see Figure 2 for locations) located away from the contamination, were used to calculate the average radon in the clean wells (119 \(\pm\) 11 Bq L\({}^{-1}\)); this value was then employed as a reference to estimate the radon deficit and the NAPL saturation of \"dirty\" wells. Radon activity concentrations in soil gas extracted at different depths from vapor extraction wells 1 (VEP01) in December 2021 and vapor extraction well 4 (VEP04) in May and October 2021 are reported in Table 2. The location of VEP01 and VEP04 can be found in Figure 2. Table 2 also reports VOC and CH\({}_{4}\) concentration detected in situ during soil gas sampling. Low radon values (<1100 Bq m\({}^{-3}\)) and/or high methane (up to 60,000 ppm) and VOC (up to 615 ppm) contents were detected at depths of 6-10 and 11-14 m, while no methane, negligible VOCs (never higher than 26 ppm), and relevant radon concentrations (up to 175,000 Bq m\({}^{-3}\)) were found at depths of 0-5 and 15-18 m. This inverse correlation between radon and residual organic compounds agrees with the radon deficit rationale according to which radon remains trapped within the pure residual NAPLs producing a concentration deficit compared to nearby unpolluted soil areas. Residual VOCs were detected in November 2020 on the cores of SN12 drilling (Figure 4). Highest levels were found between 6 and 13 m in agreement with methane and VOC data from VEP01 vapor extraction well in December 2021 (Table 2). \({}^{226}\)Ra specific activities of soil samples from SN12 drilling (Figure 4), measured by gamma spectrometry, are reported in Table 3. They range from 37 to 157 Bq kg \({}^{-1}\). \begin{table}
\begin{tabular}{l c c c c c c c c} \hline \hline \multirow{2}{*}{**Wells**} & \({}^{\textbf{222}}_{\textbf{Rn}}\) (Bq & \({}^{\textbf{222}}_{\textbf{Rn}}\) (Bq & \({}^{\textbf{222}}_{\textbf{Rn}}\) (Bq & \({}^{\textbf{222}}_{\textbf{Rn}}\) (Bq & \({}^{\textbf{222}}_{\textbf{Rn}}\) (Bq & \({}^{\textbf{222}}_{\textbf{Rn}}\) (Bq & \({}^{\textbf{222}}_{\textbf{Rn}}\) (Bq & \({}^{\textbf{222}}_{\textbf{Rn}}\) (Bq & \({}^{\textbf{222}}_{\textbf{Rn}}\) (Bq \\ L\({}^{-1}\))) & L\({}^{-1}\)) & L\({}^{-1}\)) & L\({}^{-1}\)) & L\({}^{-1}\)) & L\({}^{-1}\)) \\ \hline  & **February** & **November** & **February** & **July 2021** & **July 2021** & **November** & **April 2022** \\  & **2020** & **May 2020** & **2020** & **2021** & **May 2021** & **July 2021** & **November** & **April 2022** \\ \hline PZ25 & \(102\pm 7  
Total hydrocarbons were found in wells PZ02, PZ04, PZ09, PZ10, and PZ13 (Table 2); levels range from 50 \(\upmu\)g L\({}^{-1}\) (PZ09 in November 2020 and July 2021) to 137,280 \(\upmu\)g L\({}^{-1}\) (PZ13 in November 2020). The same wells were contaminated by MTBE, with concentration from 6 \(\upmu\)g L\({}^{-1}\) (PZ09 in November 2020) to 3295 \(\upmu\)g L\({}^{-1}\) (PZ02 in February 2020). ETBE was detected only in PZ02, PZ04, and PZ13; its levels ranged between 4 \(\upmu\)g L\({}^{-1}\) (PZ 02 in May 2021) and 1135 \(\upmu\)g L\({}^{-1}\) (PZ13 in February 2020). Other wells did not contain these substances in any sampling campaign. During the study period, the average depth of the groundwater table was 17.73 m with a fluctuation of 0.15-0.20 m (Table 1).