# A Case Study of the Radon Hazard at the Boundary of a Coal Minefield
## 4 Discussion  
### VAR in Soil Air  
High average soil air VAR for the studied coal field implies the presence of two main factors acting separately or together: (1) a high content of uranium, thorium, and radium in the upper soil horizon; (2) the presence of a permeable zone for radon release from deeper horizons. Insufficient data are available to support the first factor. No anomalies were detected during previous exploration works at this site, and measurements of the gamma field with the radiometer (SRP-68-01) did not detect any anomalies either. The permeable zone, even with low concentrations of radon parent elements, will still transport a large amount of radon to the surface through diffusion-convection transport. Part of the examined area belongs to the mining area, another area was affected by disjunctive disturbance, and some measurements were made in the area outside these conditions. However, we did not notice any significant differences in soil radon levels between these locations. The absence of significant correlations between soil radon and the distance to the disturbance as well as to the mine indicates that our study area is probably entirely within the zone of influence of these factors, given the significant increase in soil VAR for such depths. However, this statement needs to be verified by studying a control area far away  
Figure 2: Spatial features of RFD (**A**) and soil VAR (**B**). from the mines. It is additionally affirmed by the lack of statistically significant variations between the mean values of soil radon inside and outside the mine area. These findings have been attributed to certain hypotheses. As previously mentioned, tectonic disturbances lead to zones of rock crushing, which allows for radon emanation [33; 34]. The primary factors responsible for the manifestation of gas anomalies of this nature have been recognized as fault dynamics (i.e., fault activity) and types (i.e., dynamic types, such as thrusting, faulting, and fault rupture, among others), as well as other factors [35; 36; 37]. Additionally, the unusual concentration of soil radon within the zone of disturbance and its crushing area is more extensive [16; 38]. Furthermore, it is plausible that the augmented release of radon through faults stems from the concentration of radioactive substances within the fault [39; 40]. Another factor to consider is underground coal mining. The mining process causes the collapse of the above-mine space. Even minor alterations to the stress-strain state result in modifying the radon field in the geological surroundings [41; 42]. Radon anomalies can facilitate the detection, in some cases, of erratically situated subterranean activities, which are distinctive of the initial phases of coal deposit formation [43]. Such findings are rather prevalent throughout the study area. Linear anomalies of radon gas release were previously observed along the worked-out longwall face and can be assumed to occur throughout the entire minefield. The stretching zone, in this case, may extend beyond the boundaries of the working area and the minefield. Based on field observations of the mine space and natural stretching structures, such as grabens and dumps, the stretching zones exhibit good permeability for deep gases [18; 23]. Similar to our study, remarkably elevated levels of soil radon were recorded in the Upper Silesian Basin, albeit from slightly deeper boreholes [23]. In areas where coal mining has taken place, the migration of gases from the mine continues uncontrollably and chaotically even after the end of the mining process [44]. These gases include methane, carbon dioxide, and radon. The effect of mining on the growth of soil VAR may not be related to the geodynamics of the space above the mine and the creation of zones of increased permeability. Radon can be transported directly from the mine through the natural fault zone to the surface, thus significantly enriching the soil with radon compared to other soil gases.