## 3 Results and Discussion
### Occurrence of Chlorinated Hydrocarbons and Their Spatial Distribution in Groundwater
The positive correlation between VOCs and CH\({}_{4}\) concentrations (\(r_{\rm S}=0.28\): \(p<0.001\)) is consistent with this statement. A positive correlation was also found between VOC and CO\({}_{2}\) concentrations (\(r_{\rm S}=0.32\): \(p<0.05\)) because CO\({}_{2}\) is produced during the aerobic degradation of organic compounds [112; 113; 114]. Moreover, CO\({}_{2}\) can also be formed by the oxidation of CH\({}_{4}\) in the unsaturated zone [115; 116]. During the aerobic degradation of VOCs, the concentration of O\({}_{2}\) decreases [117]. As expected, lower O\({}_{2}\) concentrations were measured at sites with higher VOCs concentrations (\(r_{\rm S}=-0.33\): \(p<0.05\)). Furthermore, a strong negative correlation between O\({}_{2}\) and CO\({}_{2}\) was statistically demonstrated (\(r_{\rm S}=-0.84\): \(p<0.001\)). An almost constant inverse correlation between CO\({}_{2}\), O\({}_{2}\) and hydrocarbon vapour anomalies was observed by [118]. Organic compounds volatilising from groundwater undergo further degradation in the unsaturated zone with the production of CH\({}_{4}\) and CO\({}_{2}\), as described above. Groundwater sources can, therefore, be traced by analysing gases from progressive degradation, such as CH\({}_{4}\) and CO\({}_{2}\), whose correlations to subsurface organic contamination were confirmed in several studies [26; 62; 118]. Spatial distribution maps of CH\({}_{4}\) and VOCs concentrations (Figure 7) showed their similarity, but were partially disturbed by higher CH\({}_{4}\) concentrations in the agricultural land (area B), where the decomposition of farmyard manure could be a source of CH\({}_{4}\)[119; 120; 121]. These observations were further elaborated using PCA and the bi-plot of the first two principal components is shown in Figure 6b. Three principal components explained most of the variability of the dataset (up to 98%). Chlorinated hydrocarbons in groundwater and VOCs in soil gas corresponded to PC1 with the complete overlap of the component scores, which indicated their close inter-relationship. This supports the use of soil gas sampling as a useful screening tool for identifying contamination in the shallow saturated zone. Both O\({}_{2}\) and CO\({}_{2}\) were correlated to PC2, with the antagonistic position of these two variables pointing to O\({}_{2}\) consumption and CO\({}_{2}\) production during the degradation of organic compounds in groundwater. The third principal component was loaded by TP and partly by CH\({}_{4}\), which also had a close relationship with PC1 and thus with CHICs in groundwater. Figure 6: (a) Bi-plot of first two principal components for observations (soil gas measurement sites) and (b) bi-plot of first two principal components for variables (CLHCs concentrations in groundwater and soil gas chemical indicators). Samples shown in red in (a) are outliers. Red ellipse in (a) represents the area bounded by three times the standard deviation (\(\pm\)3 \(\times\) SD) of the principal components PC1 and PC2. It can be concluded, based on the results of this study and previous findings [26; 34; 42; 51; 122], that groundwater contamination with VOCs is strongly related to VOCs concentrations in soil gas under certain conditions. Despite several limitations (source concentration, impermeable horizons above the water table and the presence of pipelines and utility networks) [101; 123; 124]  
Figure 7: Comparison of spatial distribution maps of VOCs and CH\({}_{4}\) concentrations in soil gas from 300 soil probes. The similarity of the spatial concentration distributions of VOCs and CH\({}_{4}\) is evident, with the exception of agricultural land (area B) due to crop plant and animal production. Red frame indicates the study area. clearly delineated areas of the highest concentrations of CLHCs in groundwater and was even able to detect DNAPLs.