Hydrogeochemical Process Of Cave Drip Water And Its Reponse To Karst Desertification

There are some important achievements for speleothems in reflecting surface climate and ecological environment. As the main material sources of speleothems, cave drip water must also carry some important information of external environment, however, the hydro-geochemical process of drip water may blur or hide the information interpretation to ground surface environment in different degrees, therefore, in the purpose of discussing the response mechanism of drips’ information to karst desertification environment, we combined major issues of the national science and technology support program in 12 th Five-Year “Comprehensive management technology and demonstration of karst desertification in Karst Plateau Canyon” and scientific research program of Southwestern University “Temporal and spatial variation characteristics of cave drips’ elements and their karst desertification effect”, and took the theory of earth system science, karst dynamics and the law of distribution and evolution on karst desertification as the guiding ideology, we had monitoring 6 caves（Chaoying Cave, Hanjiachong Cave, Benzhai Cave, Shijiangjun Cave, Daxiao Cave and Rongfa Cave） of different karst desertification degrees in Bijie, Anshun and Huajiang in Guizhou Province dynamically over a period of 1 year from July 2014 to June 2015, drip rate, pH, elements and other indicators of drip water were tested by ICP-OES, Multi350 i and other instruments, and with these indicators as the research object, we discussed the spatial and temporal variation characteristics and material sources of drip water, focusing on the hydro-geochemical process and the response characteristics to karst desertification environment, and the conclusions are as follows:（1） Drip rates during rainy season are larger but smaller in dry season, drip rates fluctuate intensely in rainy season, and amplitude of drip rate can reflect the change of dry and wet seasons in general. The closer to cave inside, the more stable drip rate is for Shijiangjun Cave, there is no obvious variation for the other caves on space; drip rate is controlled by seasonal variation, thickness of soil layer and bedrock fracture. The factors such as opening of cave entrance and cave structure may influence the water temperature and its amplitude; seasonal variation of pH has the different change trend, seasonal fluctuation of EC and elements for drip water is larger, elements in most of the selected caves are higher in rainy season but lower in dry season, the spatial variation of each indicators is not obvious, it is necessary to study further on the variation law of drip water by combining different time scales.（2） Elements like Mg, Sr, Ba in drip water mainly come from the mixture of soil and surrounding rock which proportion is different. Soil contributes to Mg²⁺ of SJJ-W3 and SJJ-W4 more than the surrounding rocks; the latter is the main source of elements for most drip water. Hydro-chemical indicators of cave drip water are affected by rainwater dilution, Prior Calcite Precipitation effect, piston effect and other hydro-geochemical process, among of them, Shijiangjun Cave, Daxiao Cave and Rongfa Cave affected by PCP effect more obviously. Material sources and hydro-chemical process of drip water lead to the fuzzy interpretation on rocky desertification environment directly, it needs to be concerned that exclude the interference of these factors reasonably in the following work.（3） Drip rates of three study areas in the condition of potential-mild karst desertification degree are all less than those in moderate-intense karst desertification area, however, there is no this law on EC, pH and element content. Difference of karst desertification environment may more reflected on the indicators’ amplitudes, hydro-chemical process like PCP and piston effect obscures the response to karst desertification environment for drip water in a certain extent. By comparing the indicators of drip water between moderate-intense karst desertification area and non-desertification area, we find that fluctuations of EC, pH, element concentration of drip water in moderate-intense karst desertification area are smaller, and relatively lower for EC, Ca²⁺, Mg²⁺ and higher for pH and Mg/Ca than those of non-desertification area at the same time.（4） EC and Ca content are larger in summer for moderate-intense karst desertification region which is under control, however, pH, Mg²⁺ and Mg/Ca represent higher in potential-mild karst desertification area. Drip rate and pH are larger in summer and autumn in Anshun study area which is not under control, but the high value of EC and Ca appear in winter, variation of Mg/Ca and its amplitude in potential-mild rocky desertification area are smaller than those of moderate-intense rocky desertification area, but pH, EC and element concentrations are larger. There is certain effect for seasonal variation on the short-term fluctuations of ground surface, so it needs to strengthen the seasonal contrast on each indicators of drip water.（5） Drip rates and EC of four caves are on the increase in the two years which may indicate that karst effect turns stronger after karst desertification control; Ca²⁺、Mg²⁺ and Mg/Ca of drip water show a decreasing trend, the indicators are influenced by various hydrogeochemical process, which indication to the interannual variation of surface environment is not obvious. It is not according with the change characteristics of "higher in rainy season and lower in dry season" for pH of drip water in the latter year after the water dripping in Shijiangjun Cave, it is opposite to the previous research results, it is necessary to make a further observation on the annual record characteristics of drip water.