Annual and transient signatures of gas exchange and transport in the Castañar de Ibor cave (Spain)

Abstract

The large microclimatic stability is a basic characteristic of the subterranean karst systems and causes a high sensitivity to changesin environmental conditions. High-accuracy monitoring of Castañar de Ibor cave (Spain) determined the temporal evolution of theaerodynamic processes and ventilation rate by tracking CO2 and 222Rn levels over a twelve-month period. This cave is characterizedby a very stable microclimate, with high and relatively constant radon content (the mean value is 32200 Bq/m3, roughly, and thestandard deviation is 7600 Bq/m3) and a moderate and quite stable CO2 concentration (the mean value is 3730 ppm and the standarddeviation is 250 ppm). Beside the general patterns of cave microclimate throughout an annual cycle, some particular microclimaticprocesses are described with regard to the gas exchange between the cave and the outside atmosphere. There is a complexmicroclimatic functional relationship between the meteorological and cave microclimate conditions and the diffusion and flow of tracergases from the fractures and the pore system of soil and host rock to cave atmosphere. Transient variations of tracer gas on cave airare controlled by natural barometric fluxes and anthropogenic forced ventilation due to uncontrolled opening of cave entrance. Theshort-term fluctuations of gas levels on cave air reveal distinct patterns during the exhalation process of theses gases from the netof fissures and pores to the cave atmosphere, depending on the isolation effect of soil and host rock.

Keywords

• Carbon dioxide
• Radon
• Cave aerodynamics
• Microclimatic monitoring
• karst