基于钻孔抽水试验和示踪试验的岩溶地区含水层结构研究

Aquifer structure of karst areas derived from borehole pumping and tracer tests

岩溶含水层具有强异质性和各向异性特征,其空间结构的调查和认识是地下水流动和污染物迁移研究的重要基础。然而,岩溶含水层特殊的地质结构导致岩溶区水文地质调查非常困难。为了更好地掌握研究区岩溶含水层结构特征,本文在进行抽水试验的同时开展多示踪剂(荧光素钠、罗丹明、荧光增白剂)示踪试验。利用Theis标准曲线对比法分析抽水试验数据,计算试验场地渗透张量,根据示踪试验结果分析含水层结构性质。计算出研究区渗透张量主方向的导水系数分别为:0.297 m^2·h^(-1)和0.028 m^2·h^(-1),给水度为1.19E-04,优势方向为东偏南18.47°。抽水条件下示踪试验获得3种示踪剂的回收率分别为:荧光素钠97.98%,罗丹明B47.34%,荧光增白剂39.54%;平均流速分别为:0.378 m·h^(-1)、0.3 m·h^(-1)和0.14 m·h^(-1),反映研究区岩溶孔隙、裂隙较小,示踪剂运移慢、时间长。荧光素钠和荧光增白剂的BTC曲线表现为单峰型,罗丹明B的BTC曲线表现为多峰型,表明研究区既发育优势流裂隙,又发育相对均匀裂隙。同时,示踪剂最大回收率方向与抽水实验所得渗透张量主方向相符。研究结果为岩溶含水层精细结构的刻画,水文模型物理参数的确定提供了方法和依据。

Karst aquifers have strong heterogeneity and anisotropy.The investigation and understanding of their spatial structure are the important foundation for research of groundwater flow and contaminant migration.However,hydrogeology surveys are very difficult to conduct in karst areas due to their special geologic structure.To tackle this problem,this work conducted pumping tests and multiple tracer experiments(sodium fluorescein and rhodamine,fluorescent whitening agent)at the same.Using the Theis standard curve comparison,we analyzed pumping test data and calculate the permeability tensors of test sites.The structure properties of aquifers were determined by the results of the tracer tests.The calculated coefficient of transmissibility in the main direction of permeability tensor in the study area are:0.297 m^2·h^(-1) and 0.028 m^2·h^(-1),specific yield is 1.19 e-04 and predominant direction is east by south 18.47°.Tracer recovery rates in the tracer tests under the pumping condition for three kinds of traces are:fluorescein sodium 97.98%,rhodamine B 47.34%,and fluorescent whitening agent 39.54%.The average flow velocities are 0.378 m·h^(-1),0.3 m·h^(-1) and 0.14 m·h^(-1),respectively.They reflect the karst pores and fractures are small in the study area,tracer migration is slow and needs long time.The BTC curve of fluorescein sodium and fluorescent whitening agent is of unimode type,the BTC curve of rhodamine B is multi-peak,showing the development of single dominant fractures and relatively uniform fractures in the study area.At the same time,the direction of maximum recovery of the tracer is in accordance with the main direction of the permeability tensor from pumping tests.The results can provide a method and basis for precise description of karst aquifer characteristics and the determination of physical parameters of hydrological models.