## 3 Results  
### Air Monitoring  
The SVE system was constantly monitored to perform with the best efficiency. The VOC IN-concentration trend allows us to evaluate the efficiency of the extraction system, where a reduction of VOCs pumped over time could presumably indicate a decrease of hydrocarbons in the vadose zone. The SVE system removed an estimated 540 kg of VOC from the subsurface during about 1300 days of continuous (24 h/day) operation, as shown in Figure 5. The contaminant removal was characterized by a high initial rate (1.5 kg/day) to a value approximately ten times lower (0.15 kg/day) after one year of operation. A general trend when using an SVE treatment consisted of initially high VOC removal rates (Figure 5a) due to flushing and evaporation, then a drop of the VOC removal rate occurred, reaching a steady state in which the mass transfer of VOC contamination was controlled only by diffusion [24]. For this reason, it was chosen to analyze and present the data about the GAC facility efficiency in two periods: (i) one related to the first working year, where concentrationsof treated pollutants were high (range 5000-50,000 \(\mathrm{\SIUnitSymbolMicro\mathrm{g}/\mathrm{N}m^{3}}\)) (Figure 5b), and (ii) one for the last three working years, where concentrations dropped and reached values lower than 5000 \(\mathrm{\SIUnitSymbolMicro\mathrm{g}/\mathrm{N}m^{3}}\) (Figure 5c). During the first year of remediation with the SVE system, the GAC facility performance initially followed linearly the trend of 100% efficiency (Figure 5), but in the later years, the general trend fell between 50% and 100% efficiency lines. It could be explained by the fact that the easily removable VOCs (already in the soil gas, readily transferrable from soil pore or readily transferred from organic matter) were already removed in the first year, and GAC efficiency decreased as less volatile components were treated. In fact, GAC adsorption capacity is based on VOC type, vapor temperature, and relative humidity. VOCs OUT concentrations recorded over the treatment period were below the permitted limit and followed an almost-linear distribution for all the four years of remediation, showing the efficacy of the GAC system. Another reason for monitoring the system is related to the GAC treatment unit, which needs to be replaced as soon as the total saturation occurs. In fact, the IN-OUT difference evaluates the  
Figure 5: **(a)** Linear trend estimation of distribution for volatile organic compounds (VOC) IN, VOC OUT, and VOC mass extracted, (b) correlation trend and VOC removal efficiency (%), (c) overview of 5b in the range [0–5000] \(\mathrm{\SIUnitSymbolMicro\mathrm{g}/\mathrm{N}m^{3}}\). GAC vessel efficiency, showing that a GAC unit is saturated and needs to be replaced immediately when IN and OUT have about the same value. However, the best time to change out the GAC unit is when a breakthrough is first noticed and verified. In addition, two GAC beds are aligned in series to prevent contaminant emissions from exceeding local regulatory requirements if saturation in the primary treatment vessel occurs. In correspondence with a facility downtime due to a strong rainfall event, occurring in November 2015, no IN-OUT VOC concentrations were recorded. After that event, an increase of VOCs was observed. The groundwater level (i.e., soil moisture), which varies with the rainfall [31], can strictly affect the volatilization of hydrocarbons. The rainfall distribution over time demonstrates that a significant soil moisture produces a drop in the extracted VOC amount that increases again as soon as the precipitation event ends. As stated by [PERSON] and [PERSON] [25], since oxygen diffusion through the air is around \(10^{4}\) times faster than that through water, an increased soil moisture corresponds to a reduced porosity for available gases.