Indoor Simulation Study Of Karst Fissure-pipeline Hydrodynamic Dispersion Characteristics

Geological field work was largely limited by the specialness of karst aquifer system,that laboratory simulation experiment has became an effective means to study karst fracture-conduit flow theory and solute transport problem.In order to reveal the hydrodynamic characteristics and solute transport law in fracture-conduit aquifer system,this study design and manufacture a 3D fracture-conduit physical model used to simulate the flow and solute transport in laboratory.Before the design,we established a conceptual model of karst fracture-conduit aquifer system,it include all components of recharge,runoff and discharge features.The design simultaneously considered both the conceptual model and the similarity of the fracture-conduit geometry in karst aquifer.The physical model are made up of precipitation simulator,epikarst box,intersected fractures and conduits network.The precipitation has high raindrop uniformity and controllable intensity.The epikarst filling material is dismountable,and it is convenient to us to switch the valve of fracture and conduits.This study plans the schemes of hydrology process simulation based on different precipitation conditions and solute transporting through saturated and unsaturated flow in fracture-conduits,fracture and conduits,respectively.The experiment results demonstrated that:?1?the spring discharge process is affected by the factors of precipitation intensity,epikarst and aquifer geometry.The flow curves can be divided into three stages:flow increase stage,flow stable fluctuation stage and flow decline stage.?2?The fitting goodness of the fractional polynomial in spring recession process nonlinear fitting is very high,but the physical meaning of the fitting parameters needs further exploration.In traditional recession equation,the initial flow fitted by quadratic is closer the measured data than the value by the exponential recession function.?3?Breakthrough curves'symmetry in unsaturated flow is better than the saturated flow,initial time and peak time are also earlier.There are multi-peaks in BTCs corresponding multi-points injection.?4?The factors affecting the longitudinal dispersion coefficient are the linear distance?x?between the injection point and the receiving point,the initial time?t₀?,the peak time?t_m?,the peak concentration?c_m?,the tracer transport velocity?V?,and aquifer voidage?p?.?5?Gaussian multi-peak fitting is a good predictor of the overall shape and trend of peaky BTCs.The research results have important reference significance in the experimental research field of fracture-conduits hydrodynamic characteristics and karst groundwater solute transport.At the same time,it can provide reference for the design of field tracer test.