Study on the Variation of Physical2Chemical Properties of Dripping Water in the Panlong Cave in Guilin and Its Significance
Abstract: Environmental factors affecting speleothem formation modern carbonate deposits such as air and water temperature as well as water properties at the surface and in the cave (pH, electrical conductivity , Ca2+, HCO3- ion concentrations and dripping rates) were measured during two years in order to reveal seasonal variations. Our monitoring at 8 sites of dripping water during the two hydrologic years in the Panlong cave of Guilin indicated that cave drip water originated mainly from meteoric precipitation and soil water , and is controlled by interactions between water , soil and carbonate rock. Cave drip water was of quick response to meteoric precipitation with features of seasonal variation. The physical and chemical characteristics of cave drip water are affected by the meteoric precipitation , the bedrock thickness of the cave roof above the cave , residence time of seepage water and fast or slow drip water rate. The temperature variation of cave drip waters was very constant from 19.4 ℃to 21.2 ℃ during the two years which is very close to the mean surface temperature (19. 4～19. 8 ℃). The pH values of cave drip waters vary between 7. 1 and 8. 3. The average pH values at all drip water sites in this monitor are 7. 85. The electrical conductivity , Ca2+ and HCO3- ion concentrations show a positive correlation with air temperature , drip water temperature and drip rates. In summer, meteoric precipitation is more or large, i. e. , the larger the soil humidity , the higher the partial CO2 ( PCO2 ) in soil water, the more the dissolved CaCO3, the stronger the karstification under the epikarst zone of percolation water. The electrical conductivity and Ca2+ and HCO3- ion concentrations in cave drip water tend to increase, and the modern carbonate deposits also increase. On the contrary, in winter season, meteoric precipitation is little or small, the smaller the soil humidity, the lower the partial CO2 ( PCO2 ) in soil water, the less the dissolved CaCO3, the weaker the karstification under the epikarst zone of percolation water. The electrical conductivity and Ca2+ and HCO3- ion concentrations in cave drip water tend to reduce, and the modern carbonate deposits also tend to reduce. It is seen that this is a processes from meteoric precipitation to soil water to cave drip water again. The secondary chemical carbonate deposits formed by cave drip water can record the information about karst environmental changes, which is very important information for studying the past environment variation recorded in karst areas.
Key words: dripping water; seasonal variation; water properties; Panlong cave; Guilin
[摘 要] 影响洞穴次生化学沉积物- 石笋的形成因素,包括降水、气温、水温和滴水的性质(如pH值、电导率、Ca2+、HCO3-浓度和滴率等)。通过对桂林盘龙洞8个滴水点的2个水文年的监测表明，洞穴滴水主要来源于大气降水(或土壤水)，受控于水-土-岩的相互作用。洞穴滴水对大气降水的响应较快，具有季节性的变化特征。洞穴滴水的物理、化学特性受降水、洞顶基岩厚度及渗入水的滞留时间、滴率的快、慢的影响。洞穴滴水的温度变化不大，大约为19. 4～21. 2℃。年平均pH值为7.85 。洞穴滴水的电导率、Ca2+、HCO3-浓度与气温、水温和滴水速率表现为正相关关系。在夏半年，大气降水量大，土壤湿度大，土壤中PCO2分压高，溶解的CaCO3多，表层带下渗岩溶水的岩溶作用强，滴水的电导率、Ca2+、HCO3-浓度增大，滴水中的碳酸盐沉积多；而在冬半年或冬季，其降水量小或少，滴水的电导率、Ca2+、HCO3-浓度降低，指示溶解的CaCO3少，表层带下渗岩溶水的岩溶作用弱，水中Ca2+离子含量低，滴水沉积物的沉积量或生长量减少。从大气降水到土壤水再到洞穴滴水这些过程可以看出，洞穴滴水形成的岩溶次生化学碳酸盐沉积物-石笋，可记录岩溶环境变化的信息，已成为研究岩溶地区过去环境变化记录的重要载体。