Self-purification ability of tourist caves on a short-term scale: An example of the Dafeng cave in Suiyang county

The Shuanghedong cave system lies in Shuyang county, Zunyi City, northern Guizhou Province, which is the primary source of the Furong river that is the first-order tributary of the Wujiang river. This cave system is 257.4 km long, ranked as first in Asia and fifth in the world now. Around the cave, strata trend is mainly NNE with highly variable dip angles. The exposed rocks include dolomites of the Cambrian middle-upper Loushanguan formation (∈2-3l) and the Ordovician lower Tongzi formation (O1t),of which the gray dolomite and porphyrite and muddy dolomite are dominant. Overall this area belongs to a mid-subtropical monsoon climate. The Dafeng cave is a branch of the Shuanghedong cave system. Its tunnel is complex, where there are many beautiful secondary calcium carbonate sedimentary landscapes. The purpose of this work was to explore the self-purification ability of the air environment in the Dafeng cave and its response mechanism. Temperature, humidity, CO2 and concentration at three sites, 1# (Shiguangsuidao), 2# (Yemingzhu), 3# (Shenquanyulu),in the cave and temperature, humidity, rainfall and other indicators outside the cave weare monitored continuously and automatically for 10 days from September 30th to October 9th, 2017. Combined with the amount of tourists and local rainfall during the monitoring period, a mathematical statistics was made to the measurement data above. The results show that the self-purification ability of the cave air environment mainly depends on the intensity of the air exchange in the cave, especially in the extreme weather (rainy days in summer and autumn). The temperature outside the cave makes the ventilation mode change, thus strengthening air exchange between the inside and outside of the cave. Airflow exchange improves the self-purification ability of the cave environment. Nevertheless such ability remains limited. Besides the temperature and humidity, when the CO2 concentration generated by the tourists exceeds the threshold value of self-purification capacity of the cave air environment, the cumulative effect of CO2 in the cave occurs. Otherwise, the CO2 concentration in the cave returns to the background value of the cave environment. At the same time, the self-purification ability reaction time of the cave air environment will also differ due to varied cavity volume and tunnel structure, for instance such reaction time at monitoring sites 1#, 2#, and 3# of the cave is 15, 18, and 20 hours, respectively.