## 3 Results and Discussion  
### Occurrence of Chlorinated Hydrocarbons and Their Spatial Distribution in Groundwater  
The concentrations of studied CLHCs in groundwater from original and newly designed wells are listed in Table S4 and illustrated in the form of Tukey box plots in Figure 2. Many of the CLHCs, e.g., VC, most of the chlorinated ethanes, chloroform, BTEX, etc., were below the detection limit of GC-MS in most wells; therefore, the discussion was limited to the major CLHCs like PCE, TCE, _cis_- and _trans_-DCE and TCA. Comparing the median values of CLHCs concentrations in groundwater, PCE showed the highest median concentration, followed by TCE, _cis_-DCE, TCA and _trans_-DCE; however, the concentration difference was statistically significant only for PCE vs. _trans_-DCE, PCE vs. TCA, TCE vs. _trans_-DCE and _cis_-DCE vs. _trans_-DCE. This order reflects the fact that TCE was the primary chlorinated solvent used in production, while PCE was used as a substitute for TCE. In addition, part of TCE is an intermediate product of the microbial degradation of PCE, which is further degraded to form _cis_- and rarely _trans_-DCE [82; 83]. This fact was also indicated by highly positive correlations between PCE and TCE (\(r_{\text{S}}=0.41\): \(p<0.01\): Spearman rank-order correlation), PCE and the sum of _cis_- + _trans_-DCE (\(r_{\text{S}}=0.74\): \(p<0.0001\)), PCE and TCA (\(r_{\text{S}}=0.64\): \(p<0.0001\)) and TCE and _cis_- + _trans_-DCE (\(r_{\text{S}}=0.60\): \(p<0.0001\)). However, the broad absence of VC, being present only in three wells from source area A, suggests the incomplete dechlorination of PCE/TCE in this area [4], possibly due to a low population of suitable bacteria or the rapid conversion of VC through oxidation reactions [84]. A large part of the area has oxygen-saturated groundwater (Figure 3), in which VC is unstable and rapidly oxidises aerobically [10,84]. This could be the reason for the low frequency of its occurrence in groundwater. It was found that _cis_-DCE is also degraded by aerobic oxidation but to a lesser extent [85], which finally coincides with the higher frequency of occurrence of _cis_-DCE compared to that of VC (Figures 3 and 4). From the point of view of groundwater contamination and its impact on the environment and human health, this is a favourable situation, since VC is more toxic compared to _cis_-DCE, PCE and TCE. Figure 3: Evolution of PCE mole fraction relative to other less chlorinated ethenes (calculated as the ratio of PCE mole concentrations to the sum of mole concentrations of PCE, TCE, _cis_-DCE and VC) in groundwater with increasing distance from the source area A and its relation to the spatial distribution of dissolved oxygen concentrations (mg/L). Generally, PCE mole fraction in groundwater increased towards higher dissolved oxygen. Note that groundwater flows from northeast to southwest. The red frame indicates the studied area. Figure 2: Statistical summary of concentrations of the most frequently detected chlorinated hydrocarbons in groundwater in the form of Tukey box-plot. Vinyl chloride (VC) is not shown due to the small number of samples above the relevant detection limit (<1.0 ug/L). White circles represent outliers. The percentage numbers below the lower whisker are the coefficient of variation (CV). The first systematic data on the concentrations of PCE, TCE, _cis-, trans-_DCE and VC in the groundwater of this industrial area come from 2005 and 2015. One of the possibilities of demonstrating the natural attenuation of CLHCs is to compare existing historical concentration data with the current data obtained in this study. The median PCE concentration from the sampling campaign in 2020 was significantly lower than that of 2005, while the median concentrations of TCE and the sum of DCEs from the most current sampling campaign were lower than those of 2015 and 2005 (Figure 4).