岩溶水系统的水化学曲线及其在水文地质研究中的应用

Chemographs of Karst Water System and Its New Application in Hydrogeological Survey

为了认识水文地质条件与岩溶泉水化学之间的关系,以西南地区的6个典型岩溶水系统丫吉试验场S31号泉、毛村地下河、官村地下河、陈旗岩溶泉、青木关地下河以及金佛山水房泉为例进行研究。通过对比发现,在这些强烈非均质的山区裸露型岩溶水系统中,仪器能够实现自动化监测的电导率、温度和pH值等指标的水化学曲线存在差异。分析可知控制水化学变化的主要因素如下:首先是降雨补给引起系统的水量与碳酸盐岩溶解过程的变化;其次是CO2气体随降雨进入含水层,促进碳酸盐岩的溶解;最后是地表污染物的淋滤。当电导率、水温、pH曲线出现降低和碳酸盐岩矿物饱和指数下降时,反映的是岩溶水的稀释作用;当电导率曲线出现高峰时,反映的是岩溶水补给的CO2效应,此时水中二氧化碳分压(pCO2)升高;当场雨中水化学曲线的变化滞后于水文动态曲线时,反映的是岩溶管道的活塞流效应;与人类活动有关的NO-3等污染物质量浓度在降雨后出现高峰,反映土壤的降雨淋滤作用,并可能影响电导率的变化趋势。某个系统的水化学曲线趋向于经常出现某几种效应,并且彼此的类型不同,表明了降雨补给的面状渗流方式和集中灌入方式对岩溶水影响的强弱不同,以及系统在径流方式和调蓄机制上的差异。一般而言:以面状补给方式为主的系统,水化学曲线多表现为CO2效应,较少出现稀释作用;而岩溶发育强烈的系统,降雨补给受控于溶蚀裂隙和管道,其水化学曲线较多出现稀释作用,较少出现CO2效应;以管道为主要径流方式的系统易出现活塞流效应,而包气带厚度大和含水层储水能力强的系统水化学变化被减弱。

Six karst water systems in Southwest China are selected for comparison study,in order to find out the relationship between spring chemographs and their hydrogeological characters.The sixkarst springs are S31 in Yaji experimental site,Maocun subterranean river,Guancun subterranean river,Chenqi spring,Qingmuguan subterranean river,and Shuifang spring in Jinfo Mt.Some important results are summarized.The repeated chemograph changes in springs were found through continuous auto-monitoring of pH,temperature,specific electrical conductivity（SPC）and other indexes.There are some common differences in chemographs impacted by intrinsic and extrinsic properties of the system.Firstly,recharge from rainfall leads to changes in discharge and water-rock interaction.Secondly,the CO2 recharge with infiltration promotes the karst process.Finally,contaminate leaching from surface also has impact on water chemistry. When the SPC,pH,water temperature,and saturation index of calcite（SIc）decrease after rainfall recharge event,it indicates the occurrence of dilution process.When the SPC together with other indexes show peaks after rainfall recharge,it indicates CO2 effect caused by seepage recharge from epikarst water dissolved with CO2 in soil.When the chemographs are found lagging behind the change of hydrograph,piston flow effect occurs.Eluviation effect occurs when the ions related to human activities increase after rainfall recharge.This can be explained by the contaminant distinctly input from the surface washing,and probably have influence on variation trend of SPC.Chemographs of karst water system tend to have several effects frequently,and the mutual types are different,suggesting that different strong or weak impact on karst water by diffusion or concentration recharge from rainfall recharge,and the difference in runoff pattern and regulation and storage mechanism.In general,karst water system which is dominated by diffusion recharge,usually presents more CO2 effect in chemographs,and less dilution effect.In karst water system with strong karstification,rainfall recharge is controlled by solution fissure and conduits.Their chemographs presents more in dilution effect,while less in CO2 effect.Karst systems dominated by concentration recharge are easily to have piston flow effect.However,the variation of water chemistry is weakened in systems when they have big thickness of vadose zone and high storage capacity of aquifer.