# Radon Dynamics in Granite and Calcareous Soils: Long-Term Experiments in a Semi-Arid Context
## 4 Discussion
### Radon Potential  
The detailed analysis of long-term time series has demonstrated that radon dynamics are largely determined by weather parameters and soil properties, and their interactions. Soil water content (SWC) was the parameter likely to exert the most significant influence on radon in soil air. However, its response to rainfall events and variability within the soil depth depends on the type of soil. Barotmetric pressure, temperature, and wind also affect radon soil activity, although their impact was less significant. Figure 8: WTC and XWT of the calcareous soil between \({}^{222}\)Rn and SWC (**a,b**), Soil T (**c,d**), ET (**e,f**), P (**g,h**), and Wind (**i,j**); spectral strength and coherence range from blue (weak) to yellow (strong) colours. Arrows indicate the relative phase relationship (in phase pointing right, antiphase pointing left, one signal leading the other by 90\({}^{\circ}\) pointing up/down). Curved lines on scalograms indicate the cone of influence where the edge effects are important. Thus, processes that alter radon activity and its mobility in the soil have an impact on the radon hazard assessment. As previously introduced, the geogenic radon potential can be considered an indicator of radon activity hazard, since radon gas exhalating from the soil is the main contributor to indoor environmental accumulation. It conceptualises 'what Earth delivers in terms of radon' from geogenic sources--e.g., radionuclides in soils, rocks, and fractures--to the atmosphere [9; 77]. The widely used Neznal equation for the radon potential of a building site (Equation (1) [7]) includes soil gas permeability and radon activity, and it leads to a radon index class. Therefore, soils with high radium and uranium content and high permeability present high RP values, implying a higher probability of indoor radon infiltration due to geological factors. RP can be classified as low (RP \(<\) 10), medium (10 \(\leq\) RP \(<\) 35), and high (RP \(\geq\) 35), according to the [PERSON] et al. [7] classification. The soils studied are classified as low risk, due to the relatively low concentrations of radium and uranium (Table 1). Initially, we chose granite soil, because it belongs to a radon-prone area, to compare it with the local calcareous soil. However, the radon activity recorded in the long-term series, the gas permeability, and the warm semi-arid climate define this soil as low risk. Consequently, the RP assessment could not be performed properly. Nevertheless, our findings on radon dynamics and their response to changes in the environment and soil properties can be used to parametrise and model RP variations. Soil water content and soil texture are quantitatively related to radon activity and gas permeability, and therefore to radon potential. Radon activity can be estimated from radium activity, since \({}^{222}\)Rn is the \({}^{226}\)Ra decay product. Therefore, the theoretical radon activity can be obtained by the following consolidated expression (Equation (4))--e.g., [PERSON] [78], [PERSON] et al. [79], and [PERSON] et al. [80]:  
\[C_{Rn}=C_{Ra}\cdot\varepsilon\cdot\rho_{b}, \tag{4}\]  
where \(C_{Ra}\) (Bq kg\({}^{-1}\)) is \({}^{226}\)Ra concentration, \(\varepsilon\) is the emanation coefficient (-), and \(\rho_{b}\) (g cm\({}^{-3}\)) is the bulk density. The radon emanation coefficient in soil is a function of soil texture, water content, and temperature, and it is obtained from Equation (5) [81; 82]:  
\[\varepsilon=\varepsilon_{0}\cdot[1+a(1-\exp(-b\cdot S_{W}))]\cdot[(1+c)\cdot T], \tag{5}\]  
where \(\varepsilon_{0}\) is the radon emanation coefficient at 25 \({}^{\circ}\)C for dry soil; \(S_{w}\) is the water saturation (-); \(T\) (\({}^{\circ}\)C) is the soil temperature; and \(a\), \(b\), and \(c\) are three parameters fitted that depend on the soil properties (Table 5). If, as a first approximation, the residual (volumetric) water content is considered zero and the saturated (volumetric) water content is equal to the porosity, \(\phi\), then \(S_{w}=\text{SWC}/\phi\).