## 1 Introduction
Soil and/or groundwater sampling with chemical analysis should be applied as the last step to confirm and interpret identified contamination and to quantify the contaminant levels to avoid the potential misinterpretation of the findings [31, 52]. The application of soil gas measurements to delineate subsurface organic contamination is well-known and has been used extensively since the 1980s [53, 54, 55, 56, 57, 58]. Soil gas measurements were used as a tool to assess VOCs-containing hazardous waste land-fills [59, 60] or to monitor the non-aqueous-phase liquids and hydrocarbon accumulation by measuring radon concentrations and other gaseous compounds [61, 62, 63]. Soil gas measurements (active and passive), defined as the collection, analysis and interpretation of soil gas data, are typically used in site investigation and remediation projects where VOCs are the primary constituents of concern [64]. Measurements of vapours in the soil pore space can help to determine the occurrence, composition, source and type of release and distribution of contaminants in the subsurface. In addition, they can be used to delineate VOCs vapour plumes as a screening tool to refine soil and groundwater sampling and to monitor soil remediation progress. Active soil gas measurement is conducted by analysing soil gas from specific depths, which is pumped from the ground through probe holes. Volatile organic compounds can be detected directly by soil gas sampling methods, while semi-volatile organic compounds (SVOCs) and non-volatile organic compounds can be detected indirectly by measuring gases (e.g., O\({}_{2}\), CO\({}_{2}\), H\({}_{2}\)S, and CH\({}_{4}\)) [65]. Soil gas measurement is a rapid, non-invasive screening method, but limited only to VOCs in the unsaturated zone with medium-to-high permeability, i.e., it is not feasible in a low permeable rock environment. The disadvantage is the fact that soil gas sampling captures a limited area, but this can be overcome by thickening the sampling grid [26, 38]. Soil gas measurement and analysis are a constantly evolving method. Considerable research is focused on the determination of greenhouse gases [66, 67] and the latest technological developments in soil gas screening, such as the development of modern spectrometry methods, the soil gas sampling technique, involving AI approaches, the application of mathematical statistics and regression analysis or the influence of design features on the accuracy of measurements over time, were attentively summarised in a recent extensive review [68]. More than 1800 environmental burdens of different industrial and agricultural origins were identified in Slovakia. It was estimated that around 1200 of these contaminated areas pose a serious risk to human health and the environment and more than 100 areas are at high risk, requiring immediate remediation [69]. One of these contaminated areas is located in the city of Zlate Moravce (western Slovakia)--the former refrigerator manufacturer, Calex company. The improper handling of chemicals during -50 years of refrigerator manufacturing led to extensive groundwater contamination with CLHCs, mostly with PCE, TCE, _cis-_ and _trans-_DCE and VC [70]. Compared to drinking water standards applicable in the European Union (EU) [15], the maximum concentrations of PCE, TCE and VC in groundwater for 2015 (6482, 85,200 and 5186 \(\upmu\)/L for PCE, TCE and VC, respectively) exceeded the limit values by more than 8500 and 10,000 times, respectively. Despite some minor remedial pump-and-treat interventions in the restricted area of the northern part of the industrial area [71; 72], severe groundwater contamination persists to this day. It is likely that not all sources of groundwater contamination were identified and remediated. To identify these potentially \"hidden\" and persistent contamination sources, an extensive active soil gas measurement was carried out.