## 5 Discussion
It is worth noting a good correspondence between higher NAPL saturation and the presence of soil VOCs. The statistical analysis was also applied to the groundwater data of site 2, providing a different scenario (Table 5). The three main components accounted for 80% of the total variance. Factors 1, 2, and 3 explained about 37%, 23%, and 20% of variance, respectively. The first two components could be ascribed to the radon deficit process driven by rainfall (factor 1) and groundwater table fluctuations (factor 2) dynamics. Factor 3 could explain the biodegradation in anaerobic conditions. Figure 6: Site 2. Schematic map of NAPL saturation in groundwater and plume reconstruction (**a**). Conceptual model of the contaminated area (**b**). A is asphalt; FM stands for filling material; AD is alluvial deposit; C is clay. The interpretation of both factors 1 and 2 as a radon deficit process induced by the removal of the residual NAPLS was due to the shallow depth of the groundwater table (about 2 m) and its large annual fluctuations (up to 70 cm). In this case, it was possible to distinguish the contribution of rainfall that washed out the total hydrocarbons from the unsaturated zone and the effect of the groundwater table and the related capillary fringe on MTBE. Rainfall (factor 1) was effective with the total hydrocarbons related to the recent diesel spill and located at shallower depth where they were absorbed by backfill materials containing expanded clay, as demonstrated by the inverse correlation of the water table depth and radon concentration with the total hydrocarbons. The direct correlation of the groundwater depth with MTBE and ETBE contents suggests that these substances (linked to a previous gasoline spill) are located below the groundwater table and consequently are not removed by rainfall. In case of factor 2, the groundwater depth is inversely correlated with MTBE, consistent with the assumed location of residual MTBE at or just below the ground water surface. The direct correlation between the water table and total hydrocarbons strengthens the hypothesis that the total hydrocarbons are placed above and mobilized by rainfall. The absence of significant correlation with ETBE could be explained by its low degradation in anaerobic condition [37], No correlation with radon is evident, but this could be explained by the smoothing effect of the groundwater flow reversal on the transport and location of NAPLs and in turn on radon levels. Factor 3 accounted for the biodegradation of MTBE and total hydrocarbons in anaerobic condition (Table 5). The negative score of the groundwater depth indicated that rainfall supplied oxygen and nutrients promoting the activity of microorganism on these compounds [36]. The reverse correlation with ETBE could be justified by a low degradation of ETBE in anaerobic conditions [37]. Based on these results, it was possible to develop the conceptual models of sites 1 (Figure 5b) and 2 (Figure 6b). The residual NAPL source zone is located at well PZ13 (at a depth of 6 to 11 m) where the lowest radon and highest NAPL and VOC concentrations were detected (Figure 5), probably absorbed onto or trapped among mineral grains (for example zeolites [22; 23]. Rainfall and, to a lesser extent, groundwater table fluctuations mobilize NAPLs that dissolve in the aquifer and are transported downstream, creating temporary plumes with a dissipation half-life of about 3 weeks [22; 33]. The plume is perfectly confined in the fueling area by the mitigation system consisting of a series of pump-and-treat and vapor extraction wells. Radon deficit traces the NAPL location in the vadose and saturated parts of the aquifer due to its preferential solubility in organic phases; it accounts for about 50% of the variance in the groundwater data, as demonstrated by the principal component analysis. Average NAPL saturations, ranging from about 0.1 to 2.4%, are low, as expected by the age of the spill (about 20 years).