## 3 Results and Discussion
### Occurrence of Chlorinated Hydrocarbons and Their Spatial Distribution in Groundwater
Vinyl chloride also showed lower concentrations in the groundwater sampled in 2020, but the statistical comparison was meaningless due to the limited number of groundwater samples  
Figure 4: The concentrations of chlorinated hydrocarbons determined in this study (year 2020) compared to older concentration data (years 2005 and 2015) from the study area, taken from the final report of [70]. Different lowercase letters above concentration points indicate a statistically significant difference in concentrations among the sampling campaigns at a significance level of <0.05 (Kruskal–Wallis test with [PERSON]’s post hoc test). above the detection limit. This decrease over the last 15 years averaged 72% for PCE, 80% for TCE and 85% for the sum of _cis_- and _trans_-DCE. Two reasons could be behind the gradual decrease in CLHCs concentrations: (i) slow natural attenuation through microbial, physical and chemical processes [86] and (ii) small-scale remedial intervention using the pump-and-treat method targeting identified dense non-aqueous-phase liquids (DNAPLs) carried out in the study area [71; 72]. On the other hand, high CLHCs concentrations in source area A (Figure 5) and PCE, TCE and _cis_-DCE concentrations in several wells above the 1:1 concentration line (that is, a straight line expressing the same concentration in an individual well between the two evaluated sampling years), i.e., higher in the last groundwater sample than in previous years (Figure S2), were observed. This is evidence of the persistent contamination of the aquifer. These compounds accumulate in the deepest aquifer parts as DNAPLs. The fate of DNAPLs is complicated; they can be divided into smaller units that are trapped in low-permeability zones, releasing contaminants for a long time [87; 88]. It has been estimated that DNAPLs can persist for hundreds of years [89]. Also, the seepage of chlorinated solvents from still-operating smaller manufacturers in source area A and slow diffusional dissolution from the sorbed state into the pore spaces of the aquifer, a process that may take years [90], cannot be ruled out. The concentrations of CLHCs in the groundwater varied by up to five orders of magnitude with high coefficients of variation (379%, 683%, 430%, 237% and 336% for PCE, TCE, _cis_-DCE, _trans_-DCE and TCA, respectively) (Figure 2), indicating a heterogeneous spatial distribution of their concentrations, common in industrial and urban areas with this type of chemical contamination [2; 6; 7]. These concentrations represented -10\({}^{-6}\) up to 100% of the respective solubility of individual CLHCs. Dot distribution maps of CLHCs concentrations in groundwater showed that groundwater contamination with these compounds decreased with the increasing distance from source area A, i.e., in the direction of the groundwater flow (Figure 5 and Figure S3). The decrease was clearly visible for TCE, _cis_-DCE and TCA, with the highest number of wells where the concentration exceeded 10 \(\mu\)g/L in source area A. Although groundwater from this area is not used for drinking purposes, a comparison to drinking water standards could be taken as an indicator of the level of groundwater contamination with CLHCs. The concentration data were compared to the drinking water standards of the EU, USEPA and WHO (Table S3). The comparative results confirmed that PCE and TCE were of primary interest when more than 60% of sampling sites exceeded the EU limit for the sum of PCE and TCE (10 \(\mu\)g/L). Both WHO and USEPA list drinking water standards separately for PCE and TCE. In any case, 50% of all Figure 5: Dot distribution maps: (**a**) PCE, (**b**) TCE and (**c**) _cis_-DCE concentrations in groundwater from the study area on a background showing the spatial distribution of VOCs concentrations in soil gas. Maps for VC and TCA are shown in Figure S3 in Supplementary Materials. Specifically mentioned soil gas (symbol S) and groundwater (symbol N and O) sampling sites in the text are numbered. The direction of groundwater flow is from northeast to southwest.