Experimental And Numerical Simulation Research Of Impact Factor Of Heavy Tail Of The Solute Transport

Heavy tail has been the hotspot and difficulty of research in solute transport in porous media,which is one of the most important research fields in groundwater science.It has been widely acknowledged that heavy tail is caused by the heterogeneity of groundwater flow rate.The reasons for this are likely to be twofold.For one thing,spatial structure like macropore,lenticle lens and immobile domain in porous media lead to the heterogeneity.For another,density flow,which is caused by the variation of density as a consequence of changeable temperature and concentration,is also responsible for this.In this thesis,both laboratory column experiments and numerical simulation,which select NaCI as a tracer,are conducted to study the effects of the temperature,concentration and immobile donain on the transport of NaCl in saturated porous media.The main conclusions are as follows:(1)With the increase of solute concentration(2mmol/L,10mmol/L and 50 mmol/L),the breakthrough curve of NaCl changes from normal distribution to skewed distribution and displays tailing in sand column.This can be attributed to the density flow stimulated by the increase of concentration,which results in the heterogeneity of flow rate.The change of temperature will have little impact on the solute breakthrough curve under the condition of low concentration.Distribution of heavy tail appears and the peak decreases with the increase of temperature(5?,20?and 40?)under the condition of high concentration.This is because the impact of temperature on density and viscosity is very small under the condition of low concentration.However,the impact is greater under the condition of high concentration.Numerical simulation results show that the coupled model of traditional flow equation and advection-dispersion equation fits very well with the measured data under low concentration.However,the coupled model of variable-density flow equation and advection-dispersion equation fits better than the coupled model of traditional flow equation and advection-dispersion equation under high concentration,further confirming the existence of density flow.(2)In the experiments with immobile domain,the breakthrough curve of NaCI has an apparent skewed distribution with a delayed breakthrough time,lower peak and significant heavy tail.The main causes of this are the fact that the solute will get into the immobile domain in the transport path and can only escape from the immobile domain by means of molecular diffusion afterwards.Higher concentration leads to more significant skewed distributions of breakthrough curves because the solution with high concentration forms a pool in immobile domain.The volumes and quantities of the immobile domain also have significant effects on the solute transport.More volumes and quantities of immobile domain results in more delayed breakthrough time,lower peak and nore significant heavy tail.Among all these factors that have influences on the heavy tail of solute transport in this study,the most important one is immobile domain,followed by concentration and temperature.Numerical simulation results show that the coupled model of traditional flow equation and advection?dispersion equation is difficult to fit with the experiment data under low concentration,but the coupled model of traditional flow equation and dual-domain model does well because the dual-domain model includes the solute exchange process between the immobile domain and mobile domain which advection-dispersion equation does not have.And the coupled model of traditional flow equation and dual-domain model is difficult to fit with the breakthrough curve under high concentration,but the coupled model of variable-density flow equation and dual-domain model does well due to the density flow which is caused by the great differences of density between solution and water under high concentration.While the results obtained from the coupled model of traditional flow equation assumes that the stream field is unchangeable and not affected by density.This thesis focuses on the effects of temperature,concentration and immobile domain on the heavy tail of solute transport breakthrough curve from mechanism and simulation through the combination of the laboratory experiment and numerical simulation.The formation mechanism of breakthrough curve heavy tail has been studied from the aspects of heterogeneity of media and properties of fluid,enriching the research of solute transport theory.Moreover,this study compares and analyzes the differences and application of different numerical simulation models,also improves the application accuracy of numerical simulation under varied hydrogeological conditions,to provide accurate information for the prevention,management and remediation of groundwater contamination.