## 5 Discussion
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Variable** & **Factor 1** & **Factor 2** & **Factor 3** \\ \hline Radon & \(\mathbf{0.953}\) & \(-0.014\) & \(-0.049\) \\ MTBE & \(0.219\) & \(\mathbf{-0.512}\) & \(\mathbf{-0.790}\) \\ ETBE & \(\mathbf{0.872}\) & \(0.045\) & \(0.376\) \\ Total hydrocarbons & \(-0.128\) & \(\mathbf{0.726}\) & \(-0.215\) \\ Groundwater depth & \(0.302\) & \(\mathbf{0.590}\) & \(\mathbf{-0.450}\) \\ Variance & \(1.8230\) & \(1.1391\) & \(1.0165\) \\ \% Variance & \(36.5\) & \(22.8\) & \(20.3\) \\ \hline \hline \end{tabular} Bold fonts highlight higher factor scores.  
\end{table} 
Table 5: Factor analysis results in site 2. Variables are \({}^{222}\)Rn activity concentration (Bq L\({}^{-1}\)), MTBE, ETBE, and total hydrocarbons concentrations (ug L\({}^{-1}\)) in groundwater and groundwater table depth (m). Phenomena of natural attenuation are actively working in the site; biodegradation of ETBE and total hydrocarbons in aerobic conditions justifies about 20% of variance, while volatilization of organic products, coupled with radon degassing, explains a further 15%. The use of biosurfactants certainly promoted the activity of natural microorganism and biodegradation. Two areas of the residual NAPLs were recognized in site 2 around wells with lower radon and higher NAPL concentration (Figure 6). The presence of two single areas probably depends on the location and dynamics of the spills (gasoline first and diesel later) and on the change in the direction of the groundwater flow over the course of the year, as demonstrated in other areas [38]. In this site, the shallow depth of the groundwater table and its relevant fluctuations influence the mobilization of NAPLs more than in site 1, where rainfall mostly removed the residual contaminants. This hypothesis is supported by factorial analysis whose interpretation assigned 37% of the variance to rainfall (factor 1) and 23% (factor 2) to groundwater (and related capillary fringe). The radon deficit principle supported that and made it possible to calculate an average NAPL saturation of 8% to 54%, consistent with the younger age of the spills and the frequent presence of oil films in several wells. The residual NAPLs are probably located at a depth of 1-2 and 2-3 m, as confirmed by VOCs and it is plausible to expect MTBE blobs, and to a lesser extent residual ETBE, to be placed deeper than the total hydrocarbons due to the higher solubility of MTBE and the older age of the gasoline spill compared with the diesel release. A comparison table is provided to summarize the main findings from the factorial analysis applied to sites 1 and 2. The interpretation of factors is not always based on very high scores, but it is still supported by other descriptors (Table 6). In site 1, rainfall more than water table fluctuation removes NAPLs from the tick vadose zone, producing the radon deficit. This is supported by the greater depth (about 18 m) of the groundwater table in site 1 and the location of the residual NAPLs in the vadose zone (at \(-6/-10\) m and \(-11/-14\) m), quite distant from the area of fluctuation of the piezometric level (Table 6). At site 2, relevant fluctuations (up to 0.7 m) of a shallow water table (about 2 m below ground level) contribute to mobilize the residual NAPLs (mainly MTBE) located next to the water table surface (at \(-1/-2\) m and \(-2/-3\) m), promoting the radon deficit. This is demonstrated by the variance attributed to these driving factors.