## 3 Results and Discussion
### Occurrence of Chlorinated Hydrocarbons and Their Spatial Distribution in Groundwater
Spatial Distribution of VOC Concentrations in Soil Gas and Its Relation to Groundwater Contamination with CHICs  
The values of all chemical parameters in soil gas from each sampling site are listed in Table S5. The semi-quantitative analyses of soil gases using a portable PID device showed that compared to the background concentration of VOCs (411 ug/m\({}^{3}\)), the study area exhibited higher VOCs concentrations. Considering the area as a whole, the mean VOCs concentration was 3090 ug/m\({}^{3}\), while the highest mean VOCs concentration (4850 ug/m\({}^{3}\)) was in source area A (Figure 5 and Table S5). However, these VOCs concentrations in soil gas were lower than those determined in the 1990s [93, 94], which is in line with the decrease in the dissolved concentrations of PCE, TCE and DCE in groundwater compared to older concentration data [70] (see also Section 3.1 for details). The highest VOCs concentrations within the source area (from 4530 ug/m\({}^{3}\) up to the upper detection limit of the instrument, >100,000 ug/m\({}^{3}\)) were found at the processing and cleaning sites for refrigerator components, at the chemical warehouse and at the pumping station for chlorinated solvents (site S268). The mean VOCs concentration in agricultural land (area B) and residential area C was 1470 and 3750 ug/m\({}^{3}\), respectively. The elevated VOC concentrations in the soil gas of residential area C, located approximately 600 m from source area A, clearly indicated the persistence of groundwater contamination over a large area. On the other hand, higher VOC concentrations could be due to additional sources in the residential area, especially road traffic. This was clearly seen from the semi-quantitative soil gas analysis of sample S286 located at the roundabout, with a VOCs concentration of 7620 ug/m\({}^{3}\) and TP of 2032 mg/m\({}^{3}\). Other soil probes along the busy road (orange circles at the lower boundary of the study area, Figure 1) had also higher concentrations of VOCs (3205-7480 ug/m\({}^{3}\)) and TP (13-116 mg/m\({}^{3}\)), which might be an obstacle for the success of delineating the lateral extent of the CHICs contamination plume. Studies confirmed that traffic emissions and spills of motor fuels and oils were a significant source of VOCs to soils and air [95, 96, 97]. However, our semi-quantitative analyses, without knowledge of the occurrence of individual VOCs in soil gas, could not confirm contributions to VOCs from other sources. A more detailed view of the spatial distribution of VOCs concentrations in soil gas in relation to the concentrations of other gases (CH\({}_{4}\), CO\({}_{2}\) and O\({}_{2}\)) and CHICs in groundwater helped to identify sites that can be considered as a source of groundwater contamination and to better understand the natural processes responsible for the observed spatial distribution of CHICs concentrations in groundwater. Particularly high VOCs concentrations (between 8954 and >100,000 ug/m\({}^{3}\)), often associated with high CH\({}_{4}\) concentrations, were found at sampling sites S264, S99, S10, S292 and S268, all from source area A (Table S5). These sampling sites, considered as the contamination sources, corresponded to high CL-HCs concentrations in groundwater (Figure 5), as confirmed by chemical analyses from wells O6, O13, O14, O16 and N18 (a concentration range from 111 (PCE), 12 (TCE), 9.24 (_cis_-DCE) and 3.73 (1,1,2-TCA) ug/L up to the limit of aqueous solubility; Table S4). Anomalous VOCs and CH\({}_{4}\) concentrations in soil gas (>100,000 ug/m\({}^{3}\) and 1191 mg/m\({}^{3}\), respectively) were observed at well site N18 with concentrations of all CLHCs in the groundwater at the level of 100% of their respective solubility. At this site, there was a pumping station for organic solvents; therefore, it is clear that the leakage of chlorinated solvents led to their accumulation in the form of DNAPLs due to their higher density than water [98, 99].