## 2 Materials and Methods
Note that the ERT line geometry determines the maximum depth of investigation, which can be estimated to reach one-quarter to one-fifth of the line length [40]. In the case considered here, it was possible to deploy only 72 electrodes with 1 m spacing along 3 parallel lines, and thus we could only reach a depth of about 15 m. This implies that the ERT investigation only covered the shallow aquifer. The electrode configuration employed in electrical prospecting was a dipole-dipole array, and more precisely a skip-4 dipole-dipole, i.e., the dipole size equals 5 times the electrode spacing, or 5 m in total. The acquisition was conducted using a Syscal Pro 72, produced by Iris Instruments. A full direct and reciprocal acquisition scheme was used, for a total of about 5000 measurements along each line, and an acquisition time in the range of 30 minutes. The direct-reciprocal acquisition (i.e., exchange of current and potential dipoles) was necessary to assess the measurement error [41], which in turn was needed in the inversion process. For the data inversion, and consequently the production of imaging results, we used the open software ProfileR, provided by [PERSON] [42]. The error threshold for resistance measurements and inversions was consistently fixed at 10%, which is a value compatible with the challenging electrical contact conditions posed by the partially paved soil surface and the need to drill holes for the electrodes through it. The evolution of the groundwater contamination status was reproduced based on a chemical analysis of water sampled in the monitoring network. A total of 180 monitoring campaigns were conducted in the period 2005-2019. Several thematic maps were produced by considering some key contaminants, which were correlated with the previously ascertained state of contamination. The concentration values were compared to the Italian threshold limits (CSC), the limits between each class correspond to multiples of the CSC value for each contaminant. We created thematic maps for the most relevant parameters, TCE and PCE concentration values, to visualize the evolution of groundwater pollution and identify intervention areas. To investigate the contamination dynamics, we grouped the data, taking into account the different phases of construction of the train station. For each parameter and each considered aquifer formation, we produced contour maps for the following phases:  
1. Preoperative (October 2004 to May 2006)
2. Preliminary excavation (May 2006 to October 2009)
3. Excavation (October 2009 to August 2011)
4. Post-excavation (August 2011 to January 2013)
5. Postoperative (January 2013 to December 2014)  
Following an accurate reconstruction of the qualitative status evolution of the 2 aquifers, the laboratory tests (microbiological and microcosm tests) were aimed at evaluating spontaneous dechlorination activity [29; 30; 31]. The purpose of the microbiological and microcosm chemical tests (not shown here) was to verify possible usage of an injectable colloidal activated carbon (PlumeStop(r), Regenesis) at the identified contamination spots both for removal by adsorption of contaminants from the dissolved phase and for possible acceleration of biological processes of reductive dechlorination [10; 11]. Based on the results obtained from the site characterization and microbiological [31] and microcosm [29; 30] studies, a pilot test was designed to optimize the operating conditions at the field scale. The complete multidisciplinary, multiscale, and multitemporal characterization identified the intervention areas and supported the choice of remediation technology to be deployed. The pilot test was intended to ascertain the feasibility of the proposed remediation solution. It was designed in one of the intervention areas to optimize the layout and calibrate the implementation of an optimized full-scale intervention in order to check its efficiency. Time-lapse geophysical surveys and soil and groundwater sampling during pilot testing were intended to prove effective product diffusion in the 2 aquifers.