Thoretical Investigation And Numerical Simulation Of Solute Transport In Heterogeneous Porous Media Near A Pumping Or Injection Well

Solute transport is usually considered to be radial near a pumping or injection well in the study of pollutant transport mechanism.The theory of radial solute transport has been extensively applied to tracer injection tests,cross strata pollution of groundwater,groundwater remediation and so on.For example,single-well push-pull(SWPP)tests are used to estimate hydrogeological parameters,such as regional groundwater flow velocity,porosity,dispersivity by fitting the observed breakthrough curves(BTCs).The process of this test can be summarized as follows: a tracer is injected into a target aquifer where the mixed solution is pumped out from the same location;large number of existing geological drilling and supply wells generally used for mineral and groundwater resources exploration can act as preferential conduits for flow and contaminant migration.This results in cross strata pollution of groundwater;besides,groundwater circulation well is investigated as a less expensive and more efficient remediation technology,and this technology has been an important application of radial solute transport in situ remediation of groundwater pollution.Following a careful review of relevant literature locally and internationally,we observed that despite the successes that have been recorded based on current theories and works on radial solute transport,many scientific questions are yet to be addressed.For example,the impacts of skin on radial solute transport generally have been neglected due to extensive well development resulting in anomalous hydrogeological properties.Additionally aquifer heterogeneity usually shows dual-permeability,leading to abnormal solute transport phenomena,such as double peaks in BTCs which has received only cursory attention.Similarly,biochemical clogging results in a change in hydraulic conductivity,effective porosity and dispersivity near a pumping well.Overall the issues highlighted,i.e.,skin effect,dual-permeability porous media and bioclogging are collectively called the heterogeneous problem near a pumping well in this study.Factoring these issues radial solute transport increasing the complexity of conditions.Consequently,the issues mentioned above,as well as radial tracer tests and cross strata groundwater pollution are considered as the research backgroundwhile analytical method,numerical method and parameter inversion are used to elucidate the mechanisms of solute transport in heterogeneous porous media near a pumping or injection well in this study.(1)Using tracer test with some monitoring wells,we investigated the radial solute transport mechanism considering skin effect and presented a new Non-Fickian model of radial solute transport which used to simulate a radial dispersion experiment in a heterogeneous aquifer.Traditional tracer tests usually neglect skin effect near a pumping well,due to extensive well development resulting in anomalous hydrogeological properties and transport parameters.A semi-analytical mobile-immobile model was developed to analyze the effects of skin on BTCs.The Laplace transform and numerical inverse Laplace methods were used to solve the model.The following conclusions were drawn:(1)an abrupt change in the spatial concentration distribution was observed at the interface between the skin and aquifer formation zones due to variation in dispersivity in the skin zone,where smaller dispersivity results in steeper concentration curves in the skin zone;(2)variation in porosity whitin the skin zone does not change the slopes of concentration curves at the interface between the skin and aquifer formation zones;(3)larger solute transfer coefficient results in the decrease of peak values of BTCs and a longer tail;(4)sensitivity analysis of skin parameters show that the BTCs are the most sensitive to the effective porosity of the skin zone.Therefore,the impacts of porosity on injection tests are significant and should be taken into consideration in future studies.This study presented a Mobile-Mobile model(MM)of radial solute transport considering different boundary conditions.The Laplace transform,linear diagonalization and numerical inverse Laplace methods were used to solve the model.The following conclusions were drawn:(1)MM model is consists of two advection-dispersion equation considering solute transfer between two domains,where BTCs exhibit double peaks.Hence,our MM model can be used to investigate double peaks phenomenon in solute transport studies particularly in heterogeneous aquifer;(2)larger transfer coefficient results in a weaker double peak phenomenon which was notably more obvious when there was greater variation in pore velocities between the two domains;(3)our comparative assessment of the three models(convection-dispersion equation(ADE),the mobile-immobile model(MIM),and mobile-mobile model(MM))showed that MM can be used to better describe the anomalous breakthrough curves in heterogeneous aquifer adequately.(2)Given the problem of flow and solute transport during SWPP test,this study systematically developed a new radial solute transport model which considers skin effect as well as regional groundwater flow using a numerical approach resulting in a new methodology for estimating regional groundwater velocity using SWPP test.Generally,it is known that the skin can have strong impact on SWPP test,including different structure types(uniform or non-uniform skin).Consequently,we used the numerical model to investigate the impacts of uniform and non-uniform skin on SWPP test interpretation.For the case of a non-uniform skin,an effective way would be to integrate the product of depth-dependent concentration and velocity as the weight to obtain the average value of concentration at the well.Several important results were obtained:(1)for the case of a uniform skin,smaller effective porosity in the skin zone results in smaller values of the breakthrough curves.A smaller dispersivity also means smaller values of the breakthrough curves;(2)for the case of a non-uniform skin,the variation in hydraulic conductivity,dispersivities and effective porosities whihin the skin zone results in concentration changes along the well screen in the z-direction.Although the change in hydraulic conductivity in the skin zone can result in fluctuation of the concentration curve versus depth,however,it does not affect the flux-averaged concentrations inside the well;(3)detailed configuration about a non-uniform skin may rarely be obtained,hence,we recommend utilizing the average concentration over the entire screened section with an equivalent effect of uniform skin for a SWPP test interpretation.Meanwhile,this study systematically presented theory of radial solute transport considering skin effect and regional groundwater flow,and developed a numerical model for a SWPP test to estimate regional groundwater velocity,defining the impacts of skin property and regional groundwater velocity on radial solute transport.Other important findings include:(1)regional groundwater velocity results in two types of BTCs,the first type shows a monotonically decreasing trend with time due to relatively low regional flow Darcy velocities,and the second type of curve has a rising limb at early stage followed by a falling limb at late stage because of relatively high regional Darcy velocities;(2)the model of Leap and Kaplan(1988)underestimates the regional groundwater velocity,and the main error of the parameter estimates is that the peak time is not always corresponding to the center of mass;(3)lager hydraulic conductivity of skin results in a lower concentration for breakthrough curves at the early stage while a lager thickness in skin strengthens the influence of skin on SWPP test.(3)Looking at the problem of cross strata groundwater pollution,we developed a theoretical model for cross strata pollution based on multifactor and multiphases criteria to simulate groundwater flow and reactive solute transport in deep confined aquifer.On effects of bioclogging on hydraulic conductivity near an injection well,we developed a numerical model for cross strata pollution with multi-species(microbe,oxidants and reductants),multiphysics field(flow field,solute field,biochemical field),multiphases(dissolved and adsorbed phases)and variable hydraulic conductivities(the dynamic skin due to bioclogging).Several important results were obtained:(1)microbial growth is mainly concentrated around the well,resulting in a decrease in porosity and hydraulic conductivity around the well;(2)bioclogging results in decrease of flow rate due to the decrease in hydraulic conductivity;besides,a higher concerntration of dissolved organic carbon in the wellbore means higher degree of bioclogging and consequently smaller flow rates at middle and later stage;(3)while bioclogging results in decrease in flow rate,on the other hand,microbes also participate in the reactive consumption of pollutants,which has positive impacts on protection of deep groundwater resources.Cross strata pollution is impacted by multiple factors,resulting in exponential decrease of flow rate.Therefore,this study presented an exponential function to discrible the variation of flow rate during cross strata pollution,and developed a semi-analytical model of radial solute transport considering vaiable rate and skin effect.The Laplace transform and numerical inverse Laplace methods were used to solve the model.The following conclusions can be drawn:(1)the exponential function can discrible the law of flow variation due to physical,biological,chemical clogging around a pumping well;(2)decrease of flow rate results in decrease of radial flow velocity,which can impact radial solute transport;it can be found that a larger decay constant of the exponentially variable flow rate will result in lower breakthrough curves;(3)a smaller flow rate of ultimate steady stage means a smaller radial flow velocity,resulting in slower solute transport.Therefore,exponential variable flow rate has a positive influence on rotection of deep groundwater resources.In summary,this stuty investigated the laws of radial solute transport near a pumping or injection well in heterogeneous aquifer from four aspects(skin effect,Non-Fickian transport,bioclogging and vaiable rate),and used three methods(analytical method,numerical method and parameter inversion)to investigate the impacts of heterogeneous structure around a pumping or injiecton well on tracer test and cross strata pollution.The results indicate that skin can change the hydrogeological properties and transport parameters near a pumping or injection well.Therefore,artificial disturbance results in heterogeneous zone around wellbore,leading to impact the the inversion accuracy of tracer test;for abnormal solute transport,a new Non-Fickian model was developed to simulate a radial dispersion experiment,and the results show that the new model can describe the double peaks phenomenon of radial solute transport in heterogeneous aquifer adequately;in addition,on the one hand microbes participate in biochemical reaction,which promotes the growth of microbes and results in bioclogging,thus bioclogging change hydrogeological properties near a pumping or injection well,on the other hand bioclogging can result in decrease of flow rate and inmpacting the law of radial solute transport.Therefore,bioclogging has positive impacts on protection of deep groundwater resources.In summary,the research results of this study have certain guiding significances for improving the precision of hydrogeological parameters inversion and protecting groundwater resources.