First assessment on the air CO2 dynamic in the show caves of tropical karst, Vietnam

Abstract

In this study, air, water, and host rock in show caves in a Vietnam’s karst region was monitored and analyzed to identify the ventilation regime and track the cave air CO₂ sources. In general, the studied caves are well ventilated. In dynamic – multiple entrance caves, air ventilation is described with the use of U shape model. In static – single entrance cave, air circulation is explained by cold air trap model. Both ventilation models suggest that air is more circulated in winter than in summer. Seasonally, the cave air CO₂ increases from early spring to summer. Value in the deepest part of the single-entrance cave is approximately 1,000 ppmv and 8,000 ppmv in early spring and summer, respectively. In multiple-entrance and wet caves, CO₂ level is fairly constant all over the show section, increasing from 500 ppmv in early spring to 2,000 ppmv in summer. Data of microclimate, CO₂ content, and particularly δ¹³C show that cave air, particularly in single entrance cave, has higher CO₂ concentration during summer due to a stagnation of cave air circulation and an elevated CO₂ input from soil and epikarst. The cave air CO₂ increase is also observed after intense rainfalls. A factor that increase cave air CO₂ in show caves during the festive days could probably be huma n exhaling but the extent of human factor in these studied cave systems should be further investigated. Cave waters including cave pools and streams mediate CO₂ level in wet caves. Above all, the atmospheric fraction of CO₂ is always dominant (>60%) in all cave sections.