Variation of CO₂ Concentration in Zhijin Cave, Guizhou Province Relative to Time Scale and Its Affecting Factors

Being an important factor affecting deposition of sediments in a cave, CO₂ concentration in the air of the cave influences significantly stability of the secondary sediment landscape and amenity of the tourist environment. Monitoring of CO₂ concentration, temperature and relative humidity of the environments inside and outside the Zhijin Cave of Guizhou Province hydrogeochemical indices, CO₂ concentration in the overlying soil, precipitation and flow of tourists was carried out consecutively for 18 months from January, 2015 to June, 2016, in addition to the 5 consecutive days of round-the-clock monitoring of the air environment in the cave during the national holiday period, 2015, to investigate spatio-temporal variation of CO₂ concentration in the air inside the cave and its controlling factors. Results show that spatial variation of CO₂ concentration from the entrance to the deep corner of the cave did exist in the Zhijin Cave at all temporal scales and CO₂ concentration rose nonlinearly and tended to level off with the cave going deeper, which might be the result of the joint effect of the airflow exchange in and out of the cave, tunnel structure, as well as variation of elevation inside the cave. Seasonally, CO₂ concentration in the air of the cave on the whole was significantly higher in rainy seasons than in dry seasons, which is attributed mainly to tourist activities, karstification that absorbs a large volume of CO₂ containing water drips from the overlaying soil layer, and degassing of the water inside the cave, while daily, CO₂ concentration varied sharply around the clock, especially during the National Holidays when the flow of tourists visiting the cave in day time increased drastically in volume, making CO₂ concentration much higher by day than by night and on the whole on a rising trend, with temperature and humidity being two factors affecting variation of air CO₂ concentration inside the cave, but relatively weak in effect. Generally speaking, variation of the CO₂ concentration in the air of the cave is regulated mainly by the joint effect of air exchange (ventilation effect), between in and out of the cave, tunnel structure, tourist activities changes in the natural environment and karstification. It is, therefore, essential to take into consideration all the aspects or factors listed above in protecting the cave environment, and adopt scientific management to promote sustainable development of cave tourism.