# A Case Study of the Radon Hazard at the Boundary of a Coal Minefield
## 4 Discussion
### VAR in Soil Air
Coal mines can accumulate significantly high concentrations of radon in the air, especially in dead-end and collapsed areas after mining [45; 46]. This mechanism has been proposed in studies by [PERSON] et al. [44; 47; 48]. This phenomenon can occur when coal is mined up to the fracture zone of fault rocks (most often, large faults are the natural boundaries of the mine and the coal deposits). In our study, we examined the area surrounding the Zhurinsky Fault, which acts as the boundary for coal mining. Radon can be transported from the mine through this fault, as well as the crushing zones formed as a consequence of mining activities. In previous research, we acquired elevated volumetric radon activity measurements within residential buildings near the East Kamyshan Fault, situated within the mine area. Specifically, in one of the buildings, in close proximity to this radon source, the radon concentration in the basement reached 20,000 Bq/m\({}^{3}\) during the summer, when the \"pump\" effect of soil radon into the room was reduced. Furthermore, the RFD was significantly greater than in other monitored areas [21]. To confirm the presence of radon removal from the tectonic disturbance zone rather than the mine, it is necessary to analyze the gas composition of the soil above the disturbance zone. If free hydrogen (H2) is present, it can be concluded that removal is occurring from deep horizons in the Earth's crust along the fault zone, rather than from the mine. The simultaneous presence of high levels of methane, hydrogen, and radon may indicate that radon has migrated from deeper horizons, which is possible in the presence of a permeable medium that is characteristic of faults. Conversely, low levels of methane and hydrogen in combination with high levels of radon suggest radon formation through the decay of radium within near-surface geological conditions. The stimulation of radon emission to the surface can occur due to coal-bearing strata gases, such as methane, carbon dioxide, etc., under both mine and fault influences [49; 50; 51]. Any of the described possibilities will make it possible to predict hazards in the surrounding areas of mines. The obtained soil radon levels are significantly high, which may result in the migration of similar levels of radon into residential buildings. As is widely acknowledged, there is a strong correlation between the concentration of radon in soil air and radon levels found in residential buildings [52; 53]. Previously, we detected significant levels of radon in residential structures situated on the periphery of a coal mine. The highest recorded VAR values reached 1705 Bq/m\({}^{3}\), and approximately 21% of residential buildings had values above 200 Bq/m\({}^{3}\). High VAR levels were also detected in dwellings located within 1 km of the Zhurinsky Fault, with a mean value of 331.9 Bq/m\({}^{3}\)[21; 22; 54]. Coal and coal-bearing rocks can contain high concentrations of uranium and radium, which also affect the radon content in mine workings and faults crossing coal layers [55; 56]. According to the Swedish Radon Criteria [57], if soil radon content is below 10,000 Bq/m\({}^{3}\), no further measures are needed during residential building construction. For values between 10,000 Bq/m\({}^{3}\) and 50,000 Bq/m\({}^{3}\), additional measures are required to decrease indoor radon levels. For levels above 50,000 Bq/m\({}^{3}\), increased protective action is necessary during construction. About 48% of the studied surveyed areas are within these criteria. However, it should be noted that the level of risk is both positively correlated with the permeability of the topsoil layer and influenced by local conditions [58]. This figure may fluctuate in either direction. The moderate correlation between meteorological conditions (air temperature and humidity) and soil radon confirms the connection between the upper lithospheric air and the atmosphere moving via convection, diffusion, and other forces [59; 60; 61].