## 2 Materials and Methods
### Soil Gas and Groundwater Sampling and Analysis
To overcome these limitations, prior to soil gas measurements, a detailed historical study of the geology, hydrogeology and operational activity of the industrial area, as well as continuously cored borings in individual parts of the studied area, was carried out to collect information about lithology and depth of groundwater. Subsequently, all soil gas measurements were carried out following the same procedure, sampling sites spacing and frequency was thickened and, where possible, the measurements were carried out in a regular grid, decontamination procedures were practiced to prevent contaminant gain or loss that results from adsorption onto sampling equipment, blank samples were tested regularly and pressure, temperature, wind speed, depth of groundwater and rainfall were monitored during the whole soil gas measurement campaign. To limit the effect of soil moisture and seasonal variations on the results of the soil gas measurements to the greatest possible extent, soil gas measurements were realised during longer dry periods without precipitation and in the shortest possible time, while season, moisture content and water-table depth variations among others also affect the biodegradation of VOCs in soil gas available for measurements and sampling [65]. For qualitative/quantitative chemical analyses, ten soil gas samples were taken from probes evenly distributed throughout the study area (Figure 1). A volume of soil gas equal to three system volumes was vented from each soil probe before the soil gas samples were collected in a glass tube with sorbent (activated carbon). The sorbent tube was connected to a pump inserted into the soil probe. Soil gas samples were taken at a pump flow rate of 200 mL/min and a sampling duration of 10 min. After the end of the sampling, the test tubes were stored in the cold (<4 \({}^{\circ}\)C) and dark environment until the analysis. Soil gas samples were analysed for BTEX, CLHCs and TP using gas chromatography-mass spectrometry (GC-MS). Soil gas sampling was carried out according to the European standard ISO 18400-204:2017 Soil quality-Sampling-Part 204: Guidance on sampling of soil gas [79]. Before the screening, groundwater samples were taken from 25 existing monitoring wells (marked as O1-O25 in Figure 1). Based on the results of the soil gas screening, at sites with anomalously high concentrations of VOCs or other gases (CH\({}_{4}\), CO\({}_{2}\) and O\({}_{2}\)) in contrast to their mean concentrations in the study area as a whole, further conventional works were optimised, specifically where new monitoring wells for groundwater sampling were situated. A total of additional 40 monitoring wells (diameter = 110 mm) were drilled to a depth of 6 to 9 m b.g.s. (marked as N1-N40 in Figure 1), depending on the depth of the aquitard, at sites of potential sources of groundwater contamination identified by coefficient was used for the primary inspection of the relationships among the determined parameters in the soil gas. The effect of VOCs and TP, which are indicators of soil gas contamination, on parameters indicating possible VOCs degradation, and thus tracking groundwater contamination, was investigated by Spearman correlation and principal component analysis (PCA). Principal component analysis was used to simplify the data set, to identify outliers and to reveal hidden relationships among the variables and samples. [PERSON]'s post hoc tests were used to determine significant differences in the values of the variables among the designated zones within the study area. Results were considered statistically significant at \(p<0.05\) or lower. Since the raw data were measured in different units, standardisation of the original data was necessary. Data transformation was carried out according to the following equation:  
\[Y=(X-b)/a, \tag{1}\]  
where \(Y\) is the standardised value, \(X\) is the original value, \(b\) is the sample mean and \(a\) is the sample standard deviation.