Variabilidad de la concentración de radón-222 como gas trazador de procesos geodinámicos en ambientes subterráneos

Abstract

This Doctoral Thesis has focused on the study of radon (222Rn) as a trace gas for diverse geodynamic processes in subterranean environments. An extensive sampling of 8 different caves have been carried out (Castañar de Ibor, Altamira, El Sidrón, El Pindal, Rull, Ardales, Ojo Guareña y la cueva Pintada de Gáldar) although the main results come from Castañar Cave as it is considered to be the Spanish cave with the highest radon concentration (> 30 kBq/m3). The study period is from 2011 to 2013, although it includes temporal series up to 2015. Radon is a radioactive noble gas chemically inert and essentially non-reactive. These characteristics allows him to be a potential trace gas as it is not influenced by any biological processes being, therefore, very useful to determine the gas exchange fluxes between (exterior atmosphere, soil and underground atmosphere in the karstic ecosystems. In this research it has been analyzed in detail the factors that favors radon gas accumulation in soils and caves, including: 1) lixiviation and deposition mechanisms of radioactive elements from the host rock to the karstic system due to water-rock interaction processes, 2) the air density ratio between the underground and external atmospheres and 3) the moisture variations in the external soil and condensation processes on the porous system of the rock surfaces inside cave, which favor the caves isolation. Moreover, the processes involved on the degasification of the underground air have been analyzed (diffusion versus advection) using temporal series and spatiotemporal maps of the thermo-hygrometric conditions and CO2 concentrations from the three subsystems (exterior atmosphere-soil-underground environment). In addition, it is included a comprehensive study relating the role of caves as active methane (CH4) sinks with the high air ionization degree of underground air, which is possibly, derived from the high radon content of subterranean atmospheres. Finally, an exhaustive spatiotemporal monitoring of radon activity in Castañar cave was conducted aiming to assess the effective dose of radiation that workers (guides and researchers) and tourists would have received. This study has resulted in setting the maximum number of hours that visitors and workers could be spend inside the cave, the effective doses received and optimal paths recommended