Mitigating flood risk and environmental change in show caves: Key challenges in the management of the Las Güixas cave (Pyrenees, Spain)

A successful management of a show cave requires knowledge of cave dynamics and the main risk factors. Show caves close to the water table are prone to sporadic flooding, which can threaten visitor safety and result in severe economic losses. Las Güixas cave, located in the Collarada Massif (Pyrenees - Spain), is representative of a show cave close to the water table that is exposed to energetic flash floods. We conducted a five-year comprehensive cave monitoring study including air temperature, relative humidity, CO₂ concentration and water level. Additionally, we measured outside temperature and precipitation. Air temperature variations and ventilation dynamics occurring in most of the cave are controlled by the outside temperature due to entrances at different elevations, except in a non-ventilated area showing more stable hygrothermal characteristics and higher summer values of CO₂ concentration. The study also identifies distinct CO₂ sources related to the degassing of water and visitors' breathing. Monitoring data show that the rapid degassing of cave water during flooding may increase subsurface CO₂ concentrations to levels well above the exposure limits. However, the strong ventilation observed inside the cave rapidly removes CO₂ peaks produced by flooding and limits the anthropic CO₂ rise to ∼100 ppm. Hydrograph analysis revealed a response time of 8-12 h in the cave water levels to external rainfall/snowmelt events. Based on these results, a flood alarm system supports sustainable show cave management and the number of visitors is optimized according to the environmental conditions of the cave. This monitoring study has greatly contributed to our knowledge of cave dynamics, which can serve to improve flood risk management and increase the profitability of the show cave. Nonetheless, extreme floods remain a significant concern for potential economic losses in the future, considering current climate change scenarios. Hydrological studies together with a long-term monitoring will allow evaluating the impact of future changes in climate and environmental parameters.