Models Of Groundwater Circulation Well And Their Applications To In Situ Remediation Of Aquifer Contamination

Among the multiple technologies for in situ remediation of aquifer,groundwater circulation well（GCW）has a considerably high application value due to its convenience and feasibility to implement different kinds of technologies in microbial,physical and chemical respects.As a special kind of double-screen well system,GCW can pump water out from and inject water into aquifer simultaneously.In aquifer remediation,vertical circulation well（VCW）,which is a special kind of GCW and holds both injection and extraction screens in the same single wellbore,has the ability to estimate the in situ parameters involving the reactive transport of solutes in aquifer,especially in respect of the parameters of vertical hydraulic conductivity,sorption/desorption,reaction rate and mass transfer,among others.Besides,with proper technical operation,GCW can also produce its flow field to drive reagent to designated spot for the degradation of pollutant substances.Either VCW or multi-well GCW can be utilized in the remediation of aquifers with different hydrogeological parameters or pollution situation.When performing GCW technology,the quantitative description of groundwater seepage and solute transport is critical to aquifer parameter interpretation or in situ remediation.Through literature review,it is found that although the mechanisms of groundwater seepage,solute transport and contaminant degradation have been widely investigated theoretically and experimentally,there still exists extensive scientific problems that need further study:1)Quantitative estimation of the effects of aquifer factors on the accuracy of dipole flow tracer test.The existing tracer test with VCW often overlooks common aquifer factors such as regional groundwater flow and skin effects around wellbore.2)Solving methods of multi-species reactive transport model in the multidirectional flow field induced by VCW.Due to strong divergence and convergence in the flow field induced by VCW,the solution to the solute transport in this multi-direction flow field is precision limited;meanwhile,the semi-analytical-semi-numerical solution cannot incorporate multicomponent reaction.Therefore,a more accurate solution is needed for the multi-direction,multicomponent reactive transport in VCW-induced flow field.3)The applications of flow and solute transport models in GCW system.When parameters of reactive transport in the preliminary stage of GCW remediation engineering are unavailable,it is essential to study how to quantitatively estimate the circulation efficiency based only on groundwater flow field;besides,most existing studies focus on the influences of some hydrogeological and operational parameters on groundwater seepage and solute transport in the GCW system,it is essential to combine both modeling studies and field test to investigate the optimization scheme of technological parameters with respect to specific hydrogeological constraint conditions.Considering the abovementioned problems,this thesis conducts a series of modeling studies of groundwater seepage and solute transport in GCW system as well as in situ GCW remediation case,to elaborate how relevant models of GCW can be applied in aquifer remediation.Main research contents and conclusions are as follows:（1）Developing the modes of VCW tracer test considering regional groundwater flow and skin effects,respectively;proposing quantitative characterization index to indicate how regional groundwater flow affects reactive tracer test with VCW;investigating the potentials of VCW tracer test to identify the basic properties of skin around wellbore.Regional groundwater flow and skin effects exist widely around VCW due to undulating terrain and drilling process,respectively.Existing VCW tracer test models that overlook these two factors probably lead to errors of parameter estimation.It is of great research and application value to investigate how to quantitatively evaluate the influences of regional groundwater flow on parameter inversion accuracy,and whether the existence and properties of skin can be identified by VCW tracer test.For this purpose,this thesis firstly develops reactive transport model accounting for regional groundwater flow,which is solved by finite element method.The results indicate,the regional groundwater flow will wash part of the solute that should have been recovered by the VCW system to the downstream,resulting in less solute collected by the extraction screen and lower breakthrough curve（BTC）value,which further leads to overestimation of the first-order reaction coefficient.In addition,the larger the regional groundwater hydraulic gradient is,the lower the accuracy of the evaluation value of the reaction coefficient.Besides,based on the considerations in previous dimensionless expressions and a large number of numerical tests,a new dimensionless index’composed by pumping/injection rate,sealed section length,screen length,horizontal hydraulic conductivity and regional hydraulic gradient is proposed to mathematically measure the overestimation degree of reaction rate under the influences of regional groundwater flow.This negative correlation between the overestimation degree and’tells whether the influences of regional groundwater should be taken into consideration.For example,if one needs to keep the overestimation degree of reaction rate through VCW tracer test under 50%,then the value of’should not exceed 3.7%.Additionally,this thesis also develops the model of solute transport in VCW-induced flow field,which is solved by finite element method.The results indicate,positive skin impedes the water and solute interactive processes between wellbore and aquifer,which leads to later frontier and peak of BTC in the extraction chamber as well as lower peak value.Negative skin has totally opposite performance compared with positive one.The results also indicate,the BTCs in VCW tracer test affected by skin effects will represent four kinds of line types including two single-peak types（early-high type,late-short type）and two dual-peak types（high front and low back type,low front and high back type）,among which early-high type BTC implies a negative skin close to the permeability of aquifer while late-short one implies positive skin.As for dual-peak BTCs,high front and low back type indicates negative skin with low anisotropy ratio of hydraulic conductivity while low front and high back type indicates negative skin with high one.The indicative effect of various BTCs on skin properties reflects the potential function of VCW tracer test in determining the existence and basic properties of skin.（2）Aiming at the necessity of quantitative description of circulation efficiency in the preliminary stage of GCW remediation technology,this thesis utilized particle tracking method and stream（potential）function method based on groundwater seepage model,to propose quantitative measure of circulation efficiencies of VCW and tandem circulation well.In the preliminary stage of GCW technology implement,in order to avoid the interference with the contaminated field,the parameters involving reactive transport are often too late to be obtained,one often has to quantitatively measure the circulation efficiency of GCW base only on groundwater seepage model.This thesis firstly derived analytically the seepage velocity distribution in both radial and vertical directions in the GCW-induced flow field,then particle tracking method is revisited to calculate the recovery ratio of a certain number of particles departing from injection chamber.Through calculating the temporal recovery ratio,the influencing mechanisms of aquifer anisotropy,pumping/injection ratio and screen locations on circulation efficiency are revealed.The results indicate,increasing aquifer anisotropy,decreasing the sealed section length between screens and increasing pumping/injection ratios can increase the recovery ratio of particles.Based on the newly proposed recovery ratio,an optimization instruction manual for the preliminary stage of VCW remediation is designed,which provides theoretical foundation for the technological parameter design in early stage.On the other hand,as for the tandem circulation well with one well for injection and the other one for extraction,when accounting for regional groundwater flow,the positions of stagnation points in the flow field are determined by stream and potential functions,which further leads to the volumetric ratio（1）)of injected water recovered to the total water extracted.The results indicate,giving a constant distance between pumping and injection wells,the larger the intersection angle between the injection-pumping well connecting line and the regional groundwater flow direction,the higher value of 1);giving fixed positions of pumping and injection wells,the higher hydraulic gradient of regional groundwater flow,the lower value of 1)while the higher pumping/injection ratio,the higher value of 1).Similar to the recovery ratio in VCW-induced flow field,this 1)can be utilized as a quantitative index to measure the circulation efficiency in the preliminary stage of tandem circulation well remediation.（3）Considering the difficulties in solving the multi-direction and multicomponent reactive transport model in terms of VCW-induced flow field,this thesis developed node-dependent finite difference method to solve this complicated model,revealing the influencing mechanisms of aquifer anisotropy and some technological parameters on the VCW degradation performance.The multi-direction flow field in VCW system brings difficulties in solving the solute transport model either analytically or numerically,the conventional finite difference method used in well hydraulics leads to numerical dispersion or oscillation.The semi-analytical-semi-numerical solution cannot incorporate multicomponent reaction.Therefore,this thesis made an improvement on the basis of conventional finite difference method by designating all directions of solute transport at a specific node,making the solution overcome the numerical dispersion or oscillation while able to incorporate multicomponent reaction among microorganism,electron acceptor and electron donor.The results indicate,firstly,increasing a limited order of magnitude of anisotropy ratio can significantly affect the shape of groundwater flow field but has weak influence on the total mass of pollutant degradation;secondly,compared with interval injection mode,one-time injection mode leads to larger degradation area but has worse degradation performance at a local site,especially in the vicinity of wellbore,therefore,using multiple VCW with reasonable time interval injection mode can promote the degradation efficiency;finally,in the case where the reactive rate cannot match transport rate,a smaller sealed section length or a greater pumping/injection rate can lead to more“unreacted”reagent,in the opposite case,a moderate reduction of sealed section length or increasing of pumping/injection rate can promote the circulation efficiency as well as the remediation performance.（4）A series of studies involving models and their applications were carried out in the implementation case of multi-well GCW in a halogenated solvent contaminated site,the hydrogeological constraints of the site were analyzed,the optimization scheme of technological parameters of multi-well GCW implemented in actual aquifer was designed,and the reactive transport process of the important components in the GCW system was predicted.In order to further explore the practical application of the seepage and reactive transport model of GCW,the aquifer polluted by halogenated solvents in an industrial city was selected in this study.On the basis of fully understanding the basic situation of the site,particle tracking method,well hydraulics equation and solute reactive transport model are used.On one hand,modeling results are employed to optimize the technological parameters including well locations,range of pumping/injection rate as well as the comparison between“one extraction well with three injection wells”and“one injection well with three extraction wells”in terms of degradation area and energy consumption;on the other hand,modeling results can indicate the transport of dissolved oxygen（DO）and dissolved hydrogen（DH）,the numerical results are compared with the observed ones to illustrate the performance of models.The results indicate,firstly,as for the GCW system composed of“one extraction well with three injection wells”,keeping any of the pumping/injection well connecting lines parallel to regional groundwater flow direction while placing the other two injection wells at the downstream,then the circulation efficiency reaches highest,in this case the pumping rate Q_抽is suggested to range within 3～7.8 m³/day while the injection rate Q_注should range within 1～2.9 m³/day;secondly,the isoline range of DH concentration at 0.1ug/L is larger than the isoline range of DO concentration at 1mg/L,which implies that there probably exists a range of anaerobic degradation zones suitable for H₂ dechlorination outside the aerobic degradation zone,indicating that aerobic and anaerobic degradation can be designated in specific regions according to the model results;additionally,the observed DO concentrations in No.G05 and No.G07 wells have great difference from the calculated results,which implies the existence of preferential flow due to aquifer heterogeneity;finally,“one extraction well with three injection wells”has more adjustable range of pumping/injection rate than“one injection well with three extraction wells”,but in the actual field of this study,both GCW modes are allowed,so if taking both degradation performance and energy consumption into account,“one injection well with three extraction wells”has more benefits than the other one.To sum up,this thesis mainly investigates the functions of GCW in terms of aquifer parameter estimation and in situ remediation,the types of GCW involved in this study includes VCW,tandem circulation well and multiple well system,by means of numerical modeling,analytical calculation and field test,the models and their applications in aquifer remediation are deeply investigated.The research results in this thesis not only enrich the theoretical research on the seepage and solute transport models of GCW,but also excavate the application value of various related models of GCW in practical remediation engineering.