## 5 Discussion
This implies that rainwater (and local leaks from municipal water pipelines) removed the residual NAPLs trapped in the aquifer pores, causing an increase in MTBE, ETBE, and the total hydrocarbon concentration in the groundwater where radon favorably dissolved with a reduction in the water radon contents. This process was already proposed in [23] for this site and [22] for a close fueling station in Roma. Factor 2 accounted for a further 20% of variance (Table 4). Since a direct correlation was found between the depth of the groundwater and the concentrations of ETBE and total hydrocarbons, with approximately zero scores for radon and MTBE levels, we can \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Variable** & **Factor 1** & **Factor 2** & **Factor 3** \\ \hline Radon & \(\mathbf{-0.635}\) & \(0.007\) & \(\mathbf{-0.740}\) \\ MTBE & \(\mathbf{0.782}\) & \(0.056\) & \(\mathbf{-0.431}\) \\ ETBE & \(\mathbf{0.840}\) & \(\mathbf{-0.231}\) & \(\mathbf{-0.153}\) \\ Total hydrocarbons & \(\mathbf{0.831}\) & \(\mathbf{-0.113}\) & \(\mathbf{-0.003}\) \\ Groundwater depth & \(\mathbf{-0.260}\) & \(\mathbf{-0.957}\) & \(0.007\) \\ Variance & \(2.4795\) & \(0.9859\) & \(0.7578\) \\ \% Variance & \(49.6\) & \(19.7\) & \(15.2\) \\ \hline \hline \end{tabular} Bold fonts highlight higher factor scores.  
\end{table} 
Table 4: Factor analysis results in site 1. Variables are \({}^{222}\)Rn activity concentration (Bq L\({}^{-1}\)), MTBE, ETBE, and total hydrocarbon concentrations (\(\mu\)g L\({}^{-1}\)) in groundwater and groundwater table depth (m). Figure 5: Site 1. Schematic map of NAPL saturation in groundwater and plume reconstruction (**a**). Conceptual model of the contaminated area at well PZ13 (**b**). B is Backfill; RP stands for Reworked Pozzolan; P is Pozzolanelle Unit; TL1 is Tufo Lionato Unit (lithoid facies) and TL2 Tufo Lionato Unit (granular facies). attribute this component to a degradation process of the first two substances in aerobic conditions. Precipitations (consistent with a reduction in the groundwater depth) introduced oxygen and nutrients into the aquifer promoting rapid biodegradation of ETBE and total hydrocarbons in the vadose zone, with reduced efficiency for MTBE [34; 35]. The additions of biosurfactants (natural extracts of marine algae and plants) improved the separation and dispersion of hydrocarbon blobs, allowing the development and growth of natural microorganism that further degraded the NAPLs. Factor 3 explained a supplementary 15% of variance (Table 4); it could be interpreted as volatilization of organic products (mainly MTBE) and radon degassing in the unsaturated part of the aquifer, also linked to the action of soil vapor extraction practices. MTBE is characterized by a higher vapor pressure and thus volatilizes more easily from the residual NAPL [36]. The low score of the groundwater table depth demonstrated no role of the small aquifer fluctuations. NAPL saturation of polluted wells in site 2 was estimated according to Equation (1), based on the radon deficit values. The average radon level (17.1 Bq L\({}^{-1}\)) of the groundwater extracted from PZ01, PZ09, SN01, SN03, and SN09 was used as a reference. The NAPL saturation in groundwater, represented on a schematic map (Figure 6a), ranged from about 8 to 54%, with the highest values in correspondence with the lower radon activity concentrations and stronger contamination. Such high saturations, compared with those calculated for site 1, are well explained by the recent age of this diesel spill. The presence of dissolved NAPLs was presumably associated with blobs of residual droplets of diesel and, to a lesser extent, gasoline in the soil pores that were mobilized by rainfall and groundwater table fluctuations. Their location was probably at a depth of 1-2 and 2-3 m below ground level, as suggested by traces of VOCs detected using a photoionization detector (PID) during the drilling of monitoring wells PZ02, PZ04, PZ06, and PZ07 in 2019, as notified by MARES.