## 1 Introduction  
Groundwater in urbanised and industrial areas is highly vulnerable to contamination with a wide range of volatile organic compounds (VOCs). An important position among VOCs is occupied by chlorinated ethenes, which are used in industrial processes as dry-cleaning and degreasing agents [1; 2]. Tetrachloroethylene (PCE) and trichloroethylene (TCE) belong to the best-known and historically most studied chlorinated solvents. The long-term interest in the study of these compounds results from their high toxicity, mobility, persistence and the fact that they often accumulate in aquifers to a state of saturation as dense non-aqueous-phase liquids (DNAPLS), representing a challenge for the development of effective groundwater remediation methods [3; 4; 5]. The highest concentrations of PCE, TCE and their main anaerobic degradation products, _cis-_1,2-dichloroethylene (_cis-_DCE) and vinyl chloride (VC), in groundwater are usually associated with point sources. Groundwater in the close vicinity of a point source, e.g., the landfill of chemical waste and various industrial facilities, contained up to thousands of \(\mu\)g/L PCE and TCE with significant concentrations of _cis-_DCE and VC [6; 7; 8]. However, CLHCs concentrations of environmental concern may reach further from the source as a result of their transport under favourable conditions, limiting natural attenuation [9; 10]. Tetrachloroethylene is classified as probably carcinogenic to humans (Group 2A). still active industrial facilities, with buried installations or hazards such as cables. This entails an enhanced risk of overlooking single contamination sources or even high-risk areas, which, in consequence, can dramatically hinder the risk assessment and effective remediation [29, 30, 31]. Therefore, characterisation with sufficient spatial resolution is one of the main concerns and is still an open area of research. Today, there are multiple cost-effective, reliable and fast screening methods, e.g., phytoscreening [32, 33, 34], geophysical methods [35, 36, 31], direct-push with a membrane interface probe (MIP) and laser-induced fluorescence (LIF) sensors [37, 38, 39], as well as soil gas measurements and sampling [40, 41, 42, 43]. These, if applied correctly, minimise the risk of missing single contamination sources or high-risk areas without increasing the financial burden by directing and focusing the subsequent, more precise, but also more expensive methods [44, 45, 46, 47]. Volatile organic compounds, besides having a high vapour pressure, tend to be slightly soluble in water, in the order of tens to thousands mg/L. The high vapour pressures of most VOCs result in their strong partitioning from water into the gas phase [48]. Thus, almost all VOCs in soils and groundwater are also present in soil gas [49]. Over time, VOCs migrate from groundwater contaminant plumes into the overlying soil gas, which in turn may become a source of vapour intrusion [50]. Concentrations of VOCs in soil gas depend on their concentrations in groundwater, solubility in water, vapour pressure and soil/subsurface properties [40]. Particularly, chlorinated ethenes have low solubility in water, a high vapour pressure and diffusion coefficients and are relatively resistant to degradation in most soils. These properties make them suitable for detection using soil gas sampling [51]. In order to minimise the risk of missing a single groundwater contamination source and avoid the inaccurate reconstruction of the spatial contamination extent, a more thorough pre-screening of the site at a reduced cost with rapid analytical results was carried out using soil gas measurements and sampling, whose effective use, also in combination with phytoscreening or geophysical methods, has already been confirmed [26, 33, 43]. Moreover, results of soil gas measurements can be much more sensitive in some cases than those of soil and water sampling outputs [42]. On the other hand, screening techniques cannot replace conventional in situ sampling.