重庆芙蓉洞洞穴水水文地球化学指标的时空变化

通过对重庆武隆芙蓉洞5个滴水点和2个池水点多个水化学指标(pH、HCO3-、EC、SO24-、Ca2+、Mg2+)以及滴水滴量和现代沉积物进行连续监测,发现芙蓉洞滴水中的物质主要来源于上覆岩层和土壤,其地球化学指标有一定的时空变化特征。由于芙蓉洞上覆土壤岩层较厚,所以5个滴水点滴量与降水量相关性不明显,洞穴滴水多数时间未能及时反映洞外当季降水量的变化。夏季上覆土壤生物活动强烈,释放更多的CO2,导致滴水pH值在夏季较低,而HCO3-含量和电导率(EC)较高,冬季则相反。洞穴滴水中SO24-含量的变化受到土壤生物过程以及基岩岩层差异的重要影响。滴水的方解石饱和指数(SIc)和pH值变化大致同步,滴水点下接收到的现代沉积物沉积速率和饱和指数有很好相关性,冬季沉积速率最快。滴水中Ca2+、Mg2+浓度变化受控于水—土—岩的相互作用,其中水源差异、活塞效应、水岩相互作用时间对其产生重要影响。洞穴水中Mg/Ca主要受Ca2+含量的影响,能够指示外界温度和降水的变化。此外,洞穴内不同分布位置的滴水,其地球化学指标空间差异主要受到各点的渗流通道、流经途径以及上覆基岩及土壤成分等差异的影响。

Spacial and Temporal Variation of Hydrogeochemical Indices of the Cave Water in Furong Cave,Chongqing

By continuous monitoring at five drip water sites and two pool water sites in Furong cave,Chongqing,China from September 2009 to December 2010 covering a series of hydrochemical indices namely pH value,concentration of HCO-3,EC,SO2-4,Ca2+ and Mg2+,combined with monitoring of drip water discharge and active deposits under drip water sites,it was concluded that the content of drip water mainly depended on the overlying soil and bedrock,with the geochemical indices showing obvious temporal and spatial variation characteristics.Because of the hundreds meters thick stratums overlying the Furong cave,there was no direct correlation between cave drip water discharge and precipitation outside of the cave.The relatively lower pH value,higher HCO-3 concentration and EC value of drip water during summer months should be attributed to the increasing CO2 release from the enhanced bioactivities in soils.Concentration of SO2-4 in drip water varied with drip sites and seasons,which was generated from the difference of biological processes in overlying soil and different bedrock through which the karst water transferred.There was a similar change between the SIc and pH values for drip water through the monitoring period and the depositional rate for active deposits under drip water sites,which had the highest values in winter,in coincidence with the SIc.Additionally,concentration of Ca2+ and Mg2+ in drip water depended on the interaction of water-soil-bedrock.What’s more,water sources,piston effect and residence time of water in bedrock also influenced the concentration of Ca2+ and Mg2+.The Mg/Ca ratio of cave water was mainly controlled by the content of Ca2+,which could reflect the variation of temperature and precipitation outside the cave.For drip water distributed in different position of the cave,the geochemical variation mainly came from the difference of seepage route,transfer cranny and component of bedrock.