# The Relationship between Radon and Geology: Sources, Transport and Indoor Accumulation
## 2 The Origin of Radon
### Rocks and Soil Migration Factors
#### 2.2.4 Other Factors  
Other factors, such as the presence of groundwater, climatic factors, or topography, can also influence the mobility of radon [56, 57, 58]:  
* Groundwater: The presence of water in the soil can act as a barrier, slowing down the migration of radon through the soil and rocks [6]. This occurs because water molecules tend to adhere to soil particles and create a film around them, which reduces the space available for gas migration [59]. However, if water is flowing through the soil or rock, it can carry dissolved radon to other areas [60]. Radon can dissolve in groundwater and be transported along with the water flow, potentially spreading the gas to other areas and increasing the risk of human exposure [61]. The concentration of radon in groundwater can vary greatly depending on the local geology and other environmental factors, and it is important to monitor the radon concentration in both groundwater and indoor air to assess the potential risk to human health [62]. Additionally, water can contribute to the accumulation of radon in indoor environments through the use of water sources such as showers and faucets, which can release radon gas into the air [63]. This was demonstrated in a recent study conducted by [PERSON] et al. (2023), who undertook an extensive survey of the Ria de Vigo catchment, a region that constitutes the most radon-prone area in the Iberian Peninsula [64]. The researchers sought to elucidate the environmental factors contributing to human exposure risk to radon during domestic water use. Their results revealed that continental waters in the area were considerably enriched in radon, with groundwater displaying levels one to two orders of magnitude higher than rivers. This was especially evident in the deeper fractured rock aquifers, where radon activity was an order of magnitude higher than in the heavily worn top regolith. Intriguingly, radon levels in the majority of the investigated waters virtually doubled during the dry season compared with the rainy season. This occurrence was related to seasonal water use, recharge cycles, and thermal convection. The authors warn that because of the high radon activity, home use of untreated groundwater results in a cumulative effective dose of radiation that is higher than the advised limit of 0.1 mSv\(\cdot\)y\({}^{-1}\). They advise the installation of radon remediation and mitigation measures prior to the extraction of untreated groundwater into residences, particularly during the dry season, given that over 70% of this dosage is caused by indoor water degassing and subsequent radon inhalation. * Climatic factors: Climatic conditions, such as temperature, humidity, and precipitation, also affect the transport of radon [65]. Temperature changes can affect the pressure in the soil, leading to variations in the mobility of radon [66]. Additionally, lower temperatures typically result in higher condensation, which can restrict the diffusion of radon. Soil moisture can affect the permeability and diffusion capacity of radon [67]. Wet soil can limit the mobility of radon, acting as a barrier to gas diffusion. Excessive precipitation can saturate the soil, making it difficult for radon to diffuse and increasing the likelihood that it will be transported by groundwater [7]. On the other hand, in drier climates, the diffusion of radon can be more efficient as soil moisture is lower. The research conducted by [PERSON] et al. (2008) investigated the influence of various meteorological parameters on soil radon levels in permeable glacial sediments in Kinsarvik (Norway), demonstrating the importance and influence of climatic and meteorological factors [68]. The authors continuously recorded soil radon concentrations, temperature, precipitation, wind speed and direction, air pressure, and soil moisture content throughout a ten-month period. The results showed that differences in air temperature had a significant seasonal and diurnal impact on soil radon concentrations.