# Radon Dynamics in Granite and Calcareous Soils: Long-Term Experiments in a Semi-Arid Context
## 2 Materials and Methods
### Soil Properties  
The soils studied were chosen by their clear differences in terms of mineralogy and, therefore, differences in terms of geogenic radon potential: a silt soil from a calcrete, common in the Alicante region (Spain; identified henceforth as calcareous soil) that does not belong to a priority action area or a high-RP area according to the radon potential map of Spain [10]; and a second soil of weathered granite origin (granite soil hereafter) which belongs to the central area of Spain, classified as a priority action area. Additionally, they also represent two very different soil typologies that are at the same time representative of the context of the Spanish country.  
Soil samples were taken, for both soils, from horizon A and horizon B separately. Horizons represent soil layers with different characteristics. They are arranged vertically in a soil profile. After that, columns of 50 cm diameter and 110 cm depth were prepared. The soil profile differs in both soils: calcareous soil has an AC profile, while the granite soil has an ABC profile. Therefore, the calcareous soil column only had horizon A, while the granite soil was prepared with horizon A (layer 1) to complete the first 55 cm, and the remaining 55 cm with horizon B (layer 2), as at the original location.  
To characterise both soils, soil samples were prepared by removing gravel and root debris, followed by sieving. Soils were oven-dried at 40 \({}^{\circ}\)C for 24 h to determine dry bulk density (ratio of the dry weight to the volume of a sample). The grain density (particle or real density) was determined using an AccuPyc 1330 Helium pycnometer (Micromerities Instrument Corporation, Norcross, GA, USA). Total porosity was obtained from grain and bulk densities. Soil particle size distribution was determined by hydrometer analysis according to the Spanish standard [50], and the soil texture type was obtained by the United States Department of Agriculture (USDA) classification. The granite soil was classified as a sandy loam and loamy sand type (horizons A and B, respectively), while the calcareous soil was classified as silty clay loam soil. Their initial characteristics are summarised in Table 1.  
The determination of \({}^{226}\)Ra and \({}^{238}\)U radionuclides, performed by gamma spectrometry, demonstrates the higher uranium and radium content of granite soil compared to calcareous soil. The mineralogical composition, obtained with powder X-ray diffraction with a Bruker D8-Advance diffractometer (mirror Goebel, CuK\(\alpha\) radiation--40 kV, 40 mA--26: 3-60\({}^{\circ}\), step size of 0.05\({}^{\circ}\), scan step of 3 s; Bruker, Billerica, MA, USA), showed that the  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{3}{c}{**Granite Soil**} \\ \cline{3-4}
**Initial Parameters** & **Calcarous Soil** & **Layer 1** & **Layer 2** \\  & & **Horizon A** & **Horizon B** \\ \hline Bulk density (g cm\({}^{-3}\)) & 1.25 & 1.68 & 1.55 \\ Porosity (\%) & 52 & 35 & 42 \\ Soil texture: & Silty clay loam & Sandy loam & Loamy sand \\ (\%) sand & 18 & 72 & 80 \\ (\%) silt & 51 & 22 & 14 \\ (\%) clay & 31 & 6 & 6 \\ \({}^{238}\)U (Bq kg\({}^{-1}\)) & 21.7 & 53.0 & 48.0 \\ \({}^{226}\)Ra (Bq kg\({}^{-1}\)) & 16.9 & 39.7 & 44.4 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Summary of the characteristics of the soils under study.  
calcareous soil is dominated by calcite, with a minor amount of quartz, dolomite, feldspar, and clay minerals. The mineralogical composition of horizons A and B of the granite soil is similar, composed of quartz, plagioclase, and feldspar. Horizon A presents higher amounts of clay minerals--e.g., kaolinite--derived from the alteration of a leucocratic granite, whereas horizon B shows a higher amount of muscovite.