# Radon Dynamics in Granite and Calcareous Soils: Long-Term Experiments in a Semi-Arid Context
## 3 Results
### Influence of Weather and Soil Parameters on Radon Dynamics  
The impact of weather factors on radon was obtained from the analysis of continuous time series (Figures 2 and 3). In granite soil, the series began during the early spring rains of 2021, which resulted in elevated radon activity (above the mean 4215 Bq m\({}^{-3}\)) concurrent with the maximum CO\({}_{2}\) (Figure 2 (a)). Fluctuations in surface soil water content (SWC), mostly below the average (0.074 m\({}^{3}\) m\({}^{-3}\)), showed discrete peaks at the end of May 2021 (Figure 2 (b)), aligning with a significant rainfall episode. Thereafter, radon activity decreased in late summer, coinciding with a reduction in precipitation and an increase in temperature (Figure 2). In autumn, radon peaks increased during rainy periods. Upper-layer SWC values rose with autumn rainfall, along with the highest radon peaks (Figure 2 (c)). Wind gusts from late autumn 2021 until early winter 2022, without rain episodes, are related to radon fluctuations (Figure 2 (d)). Radon stabilised above the mean after January 2022. March 2022 rains led to a substantial increase in upper-layer SWC, while values in the lower layer remained unchanged (Figure 2 (e)). Radon and SWC increased, reaching their highest levels in April and May 2022. The radon peak in May 2022 coincided with the second-highest rainfall event, leading to a notable increase in CO\({}_{2}\) (Figure 2 (e)). At the beginning of June 2022, radon decreased again, in conjunction with a decrease in CO\({}_{2}\) and soil moisture, while temperatures rose. As observed in the granite soil, from July 2022 to the beginning of January 2023, temperature and SWC in the calcareous soil gradually decreased in the seasonal trend while the radon increased (Figure 3). SWC was affected by the most important rainfall event of the series (Figure 3 (a)). Rain events during mid- and late-autumn 2022 caused a notable impact on radon activity, particularly following two significant rainfall episodes in October and November 2022 (Figure 3 (a,b)). Then, the wind gusts coincided with a decline in radon. Additionally, the late autumn period (Figure 3 (c), concluding the December 2022 rain events) showed a low-pressure trend, aligning with a slight increase in radon activity. In winter 2022, strong wind gusts were recorded along with a decrease in radon activity (Figure 3 (c)). The two continuous soil series clearly show how the weather variables influence radon dynamics, especially in granite soil. Both SWC and temperature were identified as conditioning variables, and SWC was the parameter likely to exert the most significant influence on radon activity [43; 60]. The water content is also analysed in Section 3.3. High temperatures in summer (i.e., also higher ET) are seasonally associated with low SWC values and low radon activity [61; 62]. In fact, the response of the radon variations to the SWC depends on the initial SWC in each seasonal period. Therefore, for a semi-arid climate such as Alicante, radon activity generally increases more with longer periods of rainfall events (Figure 2 (e)). Thus, maximum values of radon peaks occurred during periods of high soil moisture linked to rainfall, as [PERSON] et al. [63], [PERSON] et al. [64], and [PERSON] et al. [65] described. In the calcareous soil, the occurrence of maximum values of radon peaks following rainfall events was delayed when compared to the granite soil, likely due to its loamy and clayey texture (Table 1). Both soils exhibit different responses to rainfall events related to SWC variations closer to the surface and deeper layers. Close to the soil surface, SWC achieved higher and more variable values, compared to the more stable SWC in the lower layer, less influenced by weather conditions [30; 31]. In the case of calcareous soil, the SWC of the deeper layer was less affected and remained nearly constant after rainfall events, which lasted from a few hours to days until the water reached a peak in the lower SWC layer.