## 4 Results
### Gas Chromatographic Analysis
Then, it gradually decreases till the end of the experiment, namely from 0.006 S/m to 0.004 S/m. The observed values of conductivity could be justified, according to the Archie model, by a decrease in water content from 0.2 to 0.175 (about 13%), but this explanation is not consistent with the dielectric permittivity measurement. The electrical conductivity variations are probably due to other phenomena as variations in salinity, for instance, which is one of the most important parameters that influence the electrical conductivity. As the evolution of electrical conductivity in soil is strictly connected to the modifications in the chemistry of the pore water, the initial decrease of this parameter could be due to the progressive depletion of nutrient salts present in MSMB solution used to stimulate bacterial activity. Anyway, the Archie model takes into account the electrolytic conductivity phenomena only; this cannot explain the overall variations of the observed changes in the conductivity. At a later stage of the experiment, the observed increase of the bulk conductivity is reported also by [PERSON] et al. [40] and [PERSON] et al. [17]. They linked this effect to an enhanced biological activity, which probably led to the production of acids as co-metabolites,or as products of the dissolution of CO\({}_{2}\) in water. Then, the consequent acidification of pH could dissolve some inorganic salts present in the soil, causing an increase of salinity in the pore water and therefore in the electrical conductivity. Studies at field-scale showed that bacterial mineralization of organic compounds augments CO\({}_{2}\) in the aquifer and produces organic acids that increase mineral weathering [41]. The last part of the plot shows a decrease in bulk conductivity; this was observed after about 45 days, and it was noticed also in other studies. The study by [PERSON] et al. [16] found a similar electrical conductivity trend, with a peak at about 30 days and then a decrease by doing measurements in the leachate produced in the process. [PERSON] et al. [17] worked at bigger scale (2 m\({}^{3}\)), monitoring separately the soil electrical resistivity and the fluid electrical resistivity for 100 days. The most interesting result was that, after 50 days, the microbial degradation activity induced an increase in fluid conductivity; meanwhile, the bulk electrical conductivity decreased, unexpectedly. The authors assumed that significant production of hydrophobic secondary metabolites occurred. To validate this assumption in our experiment, we performed a peak analysis of the gas chromatograms; however, we did not observe any peak that could be evidence of the secondary production of organic compounds. The TDR technology combines the measurement of the dielectric permittivity, related to the propagation of the electromagnetic field, the polarization of molecules, and the measurement of electrical conductivity, which refers to signal attenuation due to charge movement. The integration of electrical conductivity and dielectric permittivity is, therefore, suitable for the characterization of hydrocarbon-contaminated areas because it can detect ongoing biological processes. Therefore, it could be applied as a time-lapse monitoring tool during a bioremediation campaign. According to our knowledge, time-domain reflectometry measurements have been used in few studies to detect the presence of contaminants: a leakage of hydrocarbons that displaces the water fluid provokes a decrease in the parameters measured with TDR [11; 42]. However, we did not find any research about the connection of these measurements to biodegradation activity.