## 3 Results and Discussion
### Occurrence of Chlorinated Hydrocarbons and Their Spatial Distribution in Groundwater
Dense non-aqueous-phase liquids were found to persist in groundwater for several decades and serve as a persistent source of groundwater contamination [100]. In addition, CLHCs in the form of DNAPLs can spread further along the impermeable geological layer by the groundwater flow and settle in aquitard depressions, thus forming long-term secondary contamination sources [4]. It should be emphasised that high CLHCs concentrations in groundwater were not always reflected by increased VOCs levels in soil gas. An example of this situation was the significant groundwater contamination from well O3 of source area A (PCE = 48.3 ug/L, TCE = 97.2 ug/L, _cis_-DCE = 6020 ug/L and VC = 406 ug/L), but the relatively low VOC concentration from the vicinity of this well (a mean of 3810 ug/m\({}^{3}\) calculated from S246-248 and S255 soil probes), which was closer to the VOCs concentrations determined in soil probes outside the source area with lower median concentrations of CLHCs in groundwater. This could be due to the extensive development of this part of source area A with a high density of underground networks and pipelines, which can significantly affect the transport of soil gas vapours [101]. In addition, the topsoil of this part was removed during the reconstruction works, which in turn may constrain the transport of soil gas vapours [42; 102; 103; 104]. The opposite, i.e., lower or no groundwater contamination versus a high level of VOCs or TP, was demonstrated in wells N27 (only TCE presence at a concentration of 41.7 \(\mu\)g/L) and N40 (all CLHCs below the detection limit) in areas B and C, respectively, with VOCs concentrations of 14,705 and 7620 \(\mu\)g/m\({}^{3}\) and TP concentrations of 432 and 2032 mg/m\({}^{3}\) (S264-265 and S286), respectively. The explanation for this fact is the high density of road and rail traffic in the immediate vicinity of these sites. However, the general trend of decreasing VOCs concentrations with the increasing distance from the contamination source remained unchanged and copied the decreasing concentrations of CLHCs detected in the groundwater from source area A towards residential area C (Figures 5 and S3). The above-mentioned soil probes, characterised by an anomalous concentration of at least one parameter, were also evidently distinguishable from other soil probes in the bi-plot of the first two main components of observations (Figure 6a). Most of the observations formed one main cluster, situated in the middle of the bi-plot. Soil probe S268 showed VOCs concentrations above the upper detection limit of the instrument, while soil probes S264-265 and S286 were characterised by high TP levels or increased VOCs concentrations. Other soil probes, correlating negatively with PC1 and representing outliers, had a high CO\({}_{2}\) concentration (>50,000 mg/m\({}^{3}\)) compared to the rest. Soil gases from ten sampling sites were further analysed for selected chemical compounds, including PCE, TCE and _cis_-DCE (Table S6). The concentration of most organic compounds in the soil gas was below the detection limit of GC-MS and the most frequently detected were TCE and TP. Tetrachloroethylene and toluene were additionally detected only in sample PVZ3 from source area A. The concentration of TP in soil gas decreased with the increasing distance from industrial source area A, but the TCE concentrations were approximately constant and did not correspond to the decrease in TCE concentrations in the groundwater with distance from source area A (Figure 4). This discrepancy is probably related to the different rate of the sorption and degradation of TCE vapours in the unsaturated zone [105]. The more than fifty-year production of refrigerators, until its termination in 2002, was a source of groundwater contamination. The observed decrease in CLHCs concentrations over time (Section 3.1) suggests that their degradation in groundwater is still occurring [106; 107; 108]. The anaerobic degradation of organic compounds is accompanied by CH\({}_{4}\) production [109; 110; 111].