Abstract
Understanding the fluctuations in cave air concentrations and their climatic control is substantial not only to preserve the quality of indoor atmospheres but also to avoid the risk related to the presence of hazardous substances. In this study, we investigated the most influential factors affecting 222Rn and CO2 concentrations, the nature of their dynamics, and their coupling with climatic variations. For this purpose, we combined a set of mathematical methods that included a statistical and wavelet analysis of a 6-year time series in Rull Cave (Spain). Generally, the 222Rn and CO2 dynamic in cave air showed similar patterns. However, the obtained results show that these gases have a different frequency response. Thus, the annual component of 222Rn and CO2 is controlled by the relationship between external and internal temperatures. At low frequencies, both gases are affected by the same variables when the cave atmosphere reaches a minimum concentration. However, when the cave atmosphere is isolated from the outdoors, 222Rn and CO2 behave differently and disturbance caused by the visitors is evidenced in terms of the CO2 concentration; the latter observation was confirmed by the wavelet analysis at high frequencies. In contrast, the 222Rn concentration shows important variations following rainfall, which was weakly identified in the CO2 concentration.
Highlights
The study of microclimate and gas composition in underground environments is critical in many investigations such as global carbon cycle, paleoclimate, geological and parietal art conservation, and health risk for guides and visitors
The key factor to understand the influence of each variable is to consider that the gas concentrations of 222Rn and CO2 are the result of the interaction of different components that prevail under different periodicities [17, 20, 57, 58]
Results from principal component analysis (PCA) allow establishing gas concentration from the most influential variables calculated for each stage
Summary
The study of microclimate and gas composition in underground environments is critical in many investigations such as global carbon cycle, paleoclimate, geological and parietal art conservation, and health risk for guides and visitors. Understanding gaseous fluctuations in indoor environments is of great importance when analyzing the possible existent health risks related to the presence of humans in these locations. Tourist caves accumulate sufficient features to be considered potential locations to be managed as indoor gas concentrations can be significant [3, 4]. Centration of 31.9 kBq m-3 in Castañar Cave, the highest radon gas concentration in a Spanish cave. In Postojna Cave (Slovenia), Gregorič et al [6] noted maximum radon and CO2 concentrations greater than 37 kBq m-3 and 4700 ppm, respectively. Fernandez-Cortes et al [7] demonstrated that up to 5000 ppm of CO2 were stored in the Ojo Guareña Karst
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