## 2 Materials and Methods
### Soil Gas and Groundwater Sampling and Analysis  
For the purposes of this study, i.e., identification of groundwater contamination sources and gas plume delineation, active soil gas screening, including semi-quantitative measurements of VOCs and other parameters (CH\({}_{4}\), TP, CO\({}_{2}\) and O\({}_{2}\)) in soil gas, was carried out with a portable instrument equipped with photo-ionisation detector (PID) within temporary soil probes and uniform sampling and chemical analyses of soil gases. Temporary soil probes with a diameter of 8 cm were deepened with a hand auger and slam bar to a depth of 0.8 m below the ground in the entire studied area. After removing the auger or slam bar, the sampling device (hollow steel probe) was inserted into the opening and the atmospheric gases were purged. The space between the soil probe and the device was sealed with a sealing cone at the ground surface. The soil gas quality was analysed with a portable photo-ionisation detector (PID) device (Ecorprobe 5 Infrared, RS Dynamics, Zug, Switzerland). This device enables real-time chemical monitoring (contaminant detection) by measuring total VOCs concentration with PID calibrated using isobutylene and separate measurements of CH\({}_{4}\), TP and CO\({}_{2}\) with an infrared (IR) analyser calibrated using methane. In addition, temperature, pressure and O\({}_{2}\) are also measured. The device uses a small pump to draw gases through access tubes into the ionisation chamber. Simultaneous with determining the optimal depth of soil gas measurements and sampling (0.8 m), the pump flow rate (1.0 L/min), soil gas volume (0.4 L) and measurement duration (25 s) were adjusted to the level of soil contamination and soil air permeability tests in vertical profiles at selected sites prior to soil gas screening. The depth of 0.8 m below the surface for soil gas measurement and sampling was selected to minimise the effect of changes in barometric pressure, temperature, break-through of ambient air from the surface, high saturation, clay, organic matter and presence of anthropogenic sediments, which could limit soil gas transfer at the near-surface horizon. Carrying out soil gas measurements in this depth should avoid water-table depth variations, depending on the season and the rainfalls, that may also have an impact on the soil gas transfer. The PID detector was calibrated daily with 100 ppm isobutylene standard to provide fast in situ correction of instrument's response that might be changed due to internal dusting or depositing of various particles, etc. Before each measurement, the soil gas installation was purged by extracting three system volumes and measurement and sampling started only after the stabilisation of the physicochemical parameters monitored by the PID. In total, soil gas measurements were performed at 300 sampling sites across the study area (Figure 1). Soilgas measurements were also carried out at two sites outside the study area (marked as SG background 1 (above the industrial area) and SG background 2 (left bank of the local stream of Zitava) in Figure 1), i.e., at sites that are not in contact with source area A. Concentrations of chemical indicators in soil gas at these sites were considered as local background because groundwater contamination with CLHCs was not confirmed after repeated sampling [70]. As already known [13; 38; 49; 64; 65; 73; 74; 75; 76; 77; 78], there are limitations of soil gas measurement and sampling methods, such as meteorological conditions (rainfall, low barometric pressure, wind), depth of groundwater, lithology and properties of the overlaying soils (very-low-permeable or saturated soils, high soil moisture, preferential pathways), microbial activity and biodegradation (resistance of contaminants to biodegradation), etc.