# Radon Dynamics in Granite and Calcareous Soils: Long-Term Experiments in a Semi-Arid Context
###### Abstract  
Radon in soil poses a significant health risk when it accumulates inside dwellings. The estimation of radon potential is a difficult task due to the complex dynamics of radon within soil and its relations with the weather. This research focuses on the variability of radon activity, driven by environmental changes, assessed in two loam soils (loamy sand-granite soil and silty clay loam-calcareous soil) with different radium contents. We conducted an experiment with team soil columns in a semi-controlled outdoor laboratory, in a warm semi-arid climate. We also examined the consequences of abundant rainfall on radon activity through artificial soil water content (SWC) experiment conditions. Statistical analyses reveal that SWC is the most significant parameter influencing radon activity in these experiments. Radon is proportional to SWC and inversely proportional to temperature, evapotranspiration, and pressure in both soils, while wind is negatively related only in the loamy sand soil. Based on our findings, we modelled radon potential considering different soils and climatic contexts. SWC influences radon potential by changing radon emanation, activity, and permeability, depending on the local soil texture and radium concentration. e +
Footnote †: journal: Applied sciences  
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where \(C_{Rn}\) is the measured radon activity in soil air (Bq m\({}^{-3}\)) and \(pk_{g}=-\log k_{g}\) (\(k_{g}\) is gas permeability in m\({}^{2}\)). The dynamics of radon in soil are complex and conditioned by several factors, such as parent material, mineralogy, porosity, organic matter, texture, water content, and gas permeability [13; 14; 15; 16; 17]. On the one hand, the higher the uranium content of the parent material, the higher the radon activity. However, it is also necessary to consider the microscopic-scale distribution of uranium within the mineral grains resulting from the consecutive phases of pedogenesis [18]. On the other hand, soil physical properties such as pore size and soil water content (SWC) affect the emanation and diffusion coefficients of radon, and therefore soil gas permeability in unsaturated systems [19; 20; 21; 22]. Therefore, changes in soil gas permeability have a key role in reducing or enhancing radon migration outwards, while soil type is a decisive factor when characterising radon in soil [23; 24; 25; 26; 27]. In addition, weather parameters play a fundamental role by affecting the soil parameters mentioned above, as well as conditioning the development of the soil itself. A similar soil parent material may derive into different soils due to, for instance, more humid or arid climates [28]. Also, this difference in climatic context conditions the daily and seasonal weather, affecting geogenic radon potential [29; 30]. In the short term, barometric pressure, temperature, wind, and evaporation are weather parameters associated with changes in radon dynamics [31; 32; 33; 34]. Generally, a local high wind speed, a decrease in barometric pressure, and/or an increase in temperature lead to a decrease in radon activity [12; 35; 36]. Furthermore, soil and environmental variables, such as water content, also modify CO\({}_{2}\) production and migration within the soil [37]. A great variety of studies in geosciences, including vulcanology and GHG soil emissions to the atmosphere or cave sites, among others, use CO\({}_{2}\) together with radon as tracer gases [38; 39], even though they have different origins. Organic CO\({}_{2}\) is mainly formed in soils either by the proliferation and subsequent decomposition of microorganisms, or by the respiration of plants through their roots [40]. Many studies have been carried out to understand the relationship of radon with fractures and fault areas, and weather variables [41; 42; 43; 44; 45] with a closer perspective under controlled conditions--e.g., [PERSON] et al. [46], [PERSON] et al. [47], and [PERSON] et al. [48]--but not deep enough to study the radon potential at a local scale.