Source of cave CO2 and its spatial attributive characteristics of Zhijin Cave in Guizhou Province

In order to explore the source of cave CO2 and its spatial distribution in karst plateau gorge area，a monitoring program on monthly basis was conducted at Zhijin Cave, Guizhou province, from January to July in 2015 for the monitoring of CO2 concentrations in cave and soil, cave water, soil moisture, rainfall and in the spring water above the cave. The results show that: (1) CO2 concentration in the soil above Zhijin cave is 11-17 times of atmospheric CO2, and 4-7 times of the cave CO2, respectively. The major sources of Zhijin cave CO2 come from air exchange and tourists respiration in horizontal direction; in vertical direction they are mainly from the respiration of plant root extending into the bedrock at the top of the cave, overlying cave foundation in karst fissure, soluble tubes entering inside the cave, the atmospheric CO2 ,and the freed CO2 due to the cave calcium carbonate deposition via groundwater degassing and dripping water in the cave. (2) Zhijin Cave is a multiline cave, the interpolation of space distribution of CO2 content showed characteristics of low ends and high in the middle, at the same time,the low value range of 800-1,000 ppm appears in the range of 1,200-1,400 ppm. In the whole cave, CO2 concentration tends to increase from both ends to the middle of the cave, along with an elevated altitude, which reaches a maximum value in Lingxiao Palace. (3) The groundwater in cave and soil water outside of the cave are both HCO3--Ca2+ type, while the atmospheric precipitation and spring water are SO42--Ca2+type.In the process of vertical migration, the chemical composition (hardness,Ca2+/Mg2+、HCO3-/SO42-、PCO2、SIc) in different parts of water from rain-spring-soil water-cave water is different.