Highlighting the importance of transitional ventilation regimes in the management of Mediterranean show caves (Nerja-Pintada system, southern Spain)

摘要

This study shows the utilization of the air CO2exhaled by a very high number of visitors in the Nerja Cave as both a tracer and an additional tool to precisely evaluate the air circulation through the entire karst system, which includes non-touristic passages, originally free of anthropogenic CO2. The analysis of the temporal - spatial evolution of the CO2content and other monitoring data measured from January 2015 to December 2016 in the Nerja-Pintada system, including air microbiological controls, has allowed us to define a new general ventilation model, of great interest for the conservation of the subterranean environment. During the annual cycle four different ventilation regimes and two ventilation modes (UAF-modeandDAF-mode) exist which determine the significance of the anthropogenic impact within the caves. During thewinterregime, the strong ventilation regime and the airflow directions from the lowest to the highest entrance (UAF-mode) contribute to the rapid elimination of anthropogenic CO2, and this affects the whole karstic system. During thesummerregime theDAF-modeventilation (with airflows from the highest to the lowest entrances) is activated. Although the number of visitors is maximum and the natural ventilation of the karstic system is the lowest of the annual cycle, the anthropogenic impact only affects theTourist Galleries. The transitional ventilation regimes -springandautumn- are the most complex of the annual cycle, with changing air-flow directions (fromUAF-modetoDAF-modeandvice versa) at diurnal and poly diurnal scale, which conditions the range of the anthropogenic impact in each sector of the karst system. The activation of theDAF-modehas been observed when the temperature difference between the external and air cave is higher than 5°C.