# A Case Study of the Radon Hazard at the Boundary of a Coal Minefield
## 4 Discussion
### Rfd  
No correlation was found between soil VAR and RFD in any of the compared cases. In this study, we only discovered a correlation between radon concentration in soil and the frequency of observation points with high RFD (over 80 mBq\(\cdot\)m\({}^{-2}\).s\({}^{-1}\)) in the location outside the mine (r = 0.53, \(p\leq\) 0.05). This finding suggests that an increase in radon concentration in soil does not necessarily result in an increase in RFD, but it increases the chance of encountering high values. Similar results were previously obtained in loose rocks of a similar composition [68; 69]. It is known that the relationship between RFD and soil radon decreases as depth increases. Additionally, there are cases where RFD can decrease in clay soils, despite an increase in soil radon due to a reduction in its release rate into the atmosphere [70]. Therefore, when evaluating the risk of radon in a region, it is important to consider the presence of high radon levels in soil.  
A strong correlation between RFD and thermal-barometric conditions in the atmosphere was found. The occurrence of air temperature differences between the soil and the surface layer of the atmosphere leads to an increase in RFD due to the \"pump\" effect [24; 25]. Humidity is also known to be closely related to radon emission [71]. The lack of a relationship between humidity and radon concentrations in the atmosphere may be caused by a small temperature difference in a larger number of observations. This results in the humidity not reaching the dew point, thus preventing condensation on the soil surface. Consequently, this does not affect radon emanation from the soil during morning measurements.  
The obtained RFD values have a rather high level compared to previous studies [54]. However, the values obtained when estimating the RFD above the minefield were much higher, at 181.59 \(\pm\) 13.32 mBq\(\cdot\)m\({}^{-2}\).s\({}^{-1}\)[21]. In addition, the values in areas outside the mines (33.07 \(\pm\) 1.78 mBq\(\cdot\)m\({}^{-2}\).s\({}^{-1}\)) and the tectonic disturbance areas were significant. Areas with comparable RFD levels were found inside the minefield in both our previous work [21] and similar studies [23]. Thus, we can conclude that the minefield is not completely dangerous in terms of radon emanation, only if there are permeable zones and the composition of the surface layer is suitable. In general, the obtained results indicate that it is necessary to introduce anti-radon measures in all studied areas, provided a decision is made to develop this territory.  
The observed correlation between the distance from the minefield and the RFD may be evidence of the formation of zones with increased radon permeability in the vicinity of mines, especially old mines, and tectonic disturbance zones. Regarding the mine area, changes in radiological indicators can occur continuously and for quite a long time, even after the closure of the mine [27]. This suggests that decisions on the radon safety of geological environments may change over time and that mandatory monitoring is required.  
Figure 2 also shows the lack of a relationship between the RFD and VAR of soil air. Soil radon is characterized by spotty- or halo-shaped anomalies, although the values obtained are generally very high and similar in most measurements. RFD is also characterized by spotty-shaped anomalies, but it is offset relative to the soil VAR anomalies. The high clustering of the RFD data indicates similar conditions for radon emanations in different parts of the study area. For radon anomalies, the presented forms of anomalies are typical [72; 73; 74], and in our case, it is better to discuss the homogeneity of values for the whole study area, taking into account the measurement errors of the instrument. The most probable is that all the observation points are located within the areas of influence of the Zhurinsky Fault or the mining area.