# Radon Dynamics in Granite and Calcareous Soils: Long-Term Experiments in a Semi-Arid Context
## 3 Results
### Artificial Soil Water Content Experiment  
The results obtained in the previous analyses highlighted the relevance of water content on soil radon activity in the studied soils. For this reason, we forced soil water conditions with an experiment that was developed to better understand the relationship between soil water content and soil radon variations. In granite soil, soil temperature showed natural daily temperature variations, while the SWC changes were entirely consequences of the artificial addition of water (Figure 4). Water content was first detected at the upper sensor 2 min after the first water addition. It flowed downwards by water infiltration, and it was detected in the lower measurement point 4.03 h after the first water addition. The different SWC peaks (0.230, 0.230, and 0.246 m\({}^{3}\) m\({}^{-3}\) in the upper layer; 0.144 and 0.145 m\({}^{3}\) m\({}^{-3}\) in the lower layer; Figure 4) are a consequence of the different water additions. As the soil becomes saturated, the water infiltration is constant from the upper to the lower point of the soil column. Radon activity at the beginning of the experiment (3301 Bq m\({}^{-3}\)) was below the average measured in continuous monitoring (4215 Bq m\({}^{-3}\); Figure 2). With the addition of water, radon activity sharply increased, reaching up to 9170 Bq m\({}^{-3}\) (Figure 4). This increase occurred in a period ranging from 14 to 22 h from the beginning of the experiment. Afterwards, radon activity remained stable for approximately 4 days, from which it decreased and returned closer to its initial values (2266 Bq m\({}^{-3}\)). After the fifth day  
Figure 4: Evolution of soil and weather variables in the artificial soil water content (SWC) experiment of the granite soil. The measured variables from bottom to top are as follows: radon exhalation rate (Bq m\({}^{-2}\) s\({}^{-1}\)); discrete, continuous, and smooth continuous records of \({}^{222}\)Rn activity in the lower soil column layer (Bq m\({}^{-3}\)); discrete and continuous records of CO\({}_{2}\) concentration at the upper and lower layers of the soil column (ppm); continuous records of SWC at the upper and lower layers of the soil column (m\({}^{3}\) m\({}^{-3}\)); soil gas permeability, in pk\({}_{6}\) terms, at the lower layer of the soil column (-); and continuous records of the Soil T at the upper and lower layers of the soil column (\({}^{\circ}\)C). The period of water addition is highlighted in blue. of the experiment, the continuous measurement of radon was replaced by discrete daily measurements (Figure 4) that confirmed the reduction in its activity over time. Concerning CO\({}_{2}\), a similar pattern is described, with an increasing concentration in response to the addition of water to the soil. Both gases reached maximum values as a fast response to water addition and showed a drop nearly four days after the beginning. Maximum concentrations of CO\({}_{2}\) (greater than 14,000 ppm) were not measured by the continuous probes but by taking discrete samples. The radon exhalation rate shows a decrease with respect to the value measured before the addition of water (first measurement of the series; Figure 4). As the soil porous network was filled with water, radon could not migrate through the soil pores towards the atmosphere. Consequently, exhalation values recorded on the soil surface are the lowest with maximum water contents. Before conducting the artificial soil water content experiment, pkg was 11.00 (kg = 10\({}^{-11.00}\) m\({}^{2}\) is considered high gas permeability according to [PERSON] et al. [7]). After the second water addition, soil gas permeability decreased to its minimum (pkg = 11.46). This coincided with the growth of radon activity and its highest recorded level. Gas permeability has a clear variation throughout the experiment (from 10\({}^{-11.00}\) to 10\({}^{-11.46}\) m\({}^{2}\)).