水-岩-气相互作用引起的水化学动态变化研究——以桂林岩溶试验场为例

Study on the hydrochemical variations caused by the water-rock-gas interaction ——an example from the Guilin Karst Experimental Site

利用多参数自动记录仪对桂林岩溶试验场的降水量、水位、水温、pH值和电导率进行了监测,数据采集间隔根据参数变化的程度由2min到1h不等。结果发现,岩溶裂隙水在洪水期间pH值呈降低趋势,而电导率呈升高的不寻常变化。与此相反,对于岩溶管道水,同样是在洪水期间,它的pH值是升高的,而电导率呈正常的降低。考虑到Ca2+和HCO-3分别为地下水中主要的阴阳离子(>90%),及它们与电导率的线性关系,计算得到了洪水期间方解石的饱和指数(SIc)和水的CO2分压(Pco2)的变化情况。发现洪水时裂隙水的Pco2高于正常情况的Pco2,而它的SIc值比正常情况低。与此相对,对于管道水,尽管同一洪水期间其SIc降低,但Pco2也降低。从这些结果,可以推断,至少有两个关键的过程控制着洪水期间的水化学变化。一个是雨水的稀释作用,另一个是水-岩-气的相互作用。然而,对于裂隙水来说,后者的作用可能更重要,即在洪水期间,高浓度的土壤CO2溶解于水中,则更具侵蚀性的水能溶解更多的石灰岩,从而增强水的电导率。而对于管道水,雨水的稀释作用更重要,因为研究区较高的pH和低电导率的雨水能更快地通过管道流出,所以,要了解岩溶系统水化学的变化,仅考虑水-岩相互作用是不够的,我们还必须重视CO2气体对岩溶系统中水化学变化的影响。总之。

High resolution measurements of stage, pH, conductivity, temperature, and carbonate chemistryparameters of groundwater at two adjacent locations within the peak cluster karst of the Guilin Karst Experimental Site in Guangxi Province, China, were made using data loggers recording with two minute resolution. While waters from a large, perennial spring represent the exit for the aquifer's conduit flow, a nearby well measures water in the conduitadjacent, fissured media. During flood pulses, the pH of the conduit flow water rises while the conductivity falls. Inversely, and at the same time, the pH of groundwater in the fissures drops, while conductivity rises. As Ca2+ and HCO-3 were the dominant (>90%) ions, we developed linear relationships (both r2>0.91) between conductivity and those ions, respectively, and in turn calculated variations in the calcite saturation index (SIc) and CO2 partial pressure (Pco2) of water during flood pulses. It was found that the Pco2 of fissure water during flood periods is higher than that at lower flows, while its SIc is lower. Simultaneously, Pco2 of conduit water during the flood period is lower than that at lower flows, and its SIc is also lower. From these results we conclude that at least two key processes are controlling hydrochemical variations during flood periods: ① dilution by precipitation, and ② water/rock/gas interactions. To explain hydrochemical variations in the fissure water, the water/rock/gas interactions may be more important. For example, during flood periods, soil gas with high CO2 concentrations dissolves in water and enters the fissure system, the water which has in turn become more highly undersaturated dissolves more limestone, and the conductivity increases. Dilution of rainfall is more important in controlling hydrochemical variations of conduit water, because rainfall with higher pH (in this area apparently due to interaction with limestone dust in the lower atmosphere) and low conductivity travels through conduit system rapidly. These results illustrate that to understand the hydrochemical variations in karst systems, considering only water/rock interactions is not sufficient, and the variable effects of CO2 on the system should be evaluated. Consideration of water/rock/gas interactions is thus a must in understanding variations in karst hydrochemistry.