Characterization Of Operation Effect And Optimization Of Structural Parameters Of Groundwater Circulation Well(GCW) Based On Numerical Simulation

Groundwater Circulation Well（GCW）is a kind of in-situ restoration technology which can be coupled with various restoration methods.It has the advantages of small disturbance to the site,easy maintenance and good repair effect.During the operation of GCW,a three-dimensional hydraulic circulating zone is formed and maintained in the aquifer around the GCW,and its range is directly related to its operation effect.It is difficult to directly observe the range of the circulation zone in the contaminated site,but the numerical simulation method can effectively solve this problem.However,the existing mature groundwater numerical simulation software has complicated modeling process and can not directly express the performance of GCW.In order to solve the above problems,the finite difference numerical method is adopted in this study,and relevant groundwater simulation programs in FloPy library are called by Python to build a numerical simulation system for GCW,and the characterization index of circulating well operation effect and the optimization of circulating well structural parameters are realized internally.On this basis,the variation characteristics of each characteristic index driven by GCW are analyzed,and the influencing factors of main index of GCW influence radius(R_to)and recovery efficiency（Pr）and their correlation are discussed in detail.Then,the structural parameters of circulating well in a demonstration site in Xi’an,Shaanxi Province are optimized,and the operation effects of circulating well before and after optimization are compared and analyzed.Through the above research,the following conclusions can be drawn:（1）The variation contour line of groundwater head can clearly represent the variation amplitude and range of groundwater head driven by GCW and the running state of GCW.During the operation of GCW,the distribution of contour lines of variation of head has the following rules:the closer the contour lines of variation of groundwater head are to the vicinity of the two sieve sections,the denser the contour lines of variation of groundwater head are,and the greater the variation range and hydraulic gradient of water head are,but the water head in the middle of the two sieve sections has almost no change.（2）The sum of GCW influence radio(R_to)can reflect the extent of circulating zone,while recovery efficiency（Pr）can quantitatively represent the amount of circulating water.The main influencing factors of R_to and Pr include flow rate（Q）,length of two screens（a）,spacing（L）,hydraulic gradient（I）,porosity（n）,feedwater degree（μ）,water storage ratio（Ss）,aquifer thickness（M）,and vertical to horizontal permeability ratio（vka）.The correlations between the influencing factors and R_to and Pr in descending order are as follows;for R_to,Q>M>vka>a>n>I>L>μ>Ss;For Pr,Q>L>a>vka>M>I>n>μ>Ss.（3）radius of contaminant migration（r_m）can directly reflect the size of the scope of repair,and pollutant removal efficiency（?）can be quantitative says circulation well repair effect.The migration of pollutants in groundwater driven by GCW mainly has the following characteristics:In the initial stage of GCW operation,the concentration of pollutants at the water injection screen decreases rapidly,which is manifested as a rapid increase in r_m,and inevitably pushes some pollutants to the surrounding area.In addition,the natural groundwater boundary hydraulic gradient and simulation area field under the condition of water pollutant concentration of 0,perpendicular to the flow direction of rm and?in a certain period of time to achieve a relatively stable state,which changes with the increase of time between the two values is very small;Parallel to the radius in the direction of flow upstream of pollutant migration r_m1 significantly less than the downstream r_m2,with circulation well run continuously r_m1 reached a relatively stable state,while r_m2 and?value will be increased with the increase of time.（4）Optimize the structural parameters of circulating well in a demonstration site in Xi’an,Shaanxi Province.The results show that the optimum position of the GCW pumping screen in the aquifer is 11-14m and 2-5m,respectively,with a maximum R_toof 44.52m when the restoration time is 180 days and the operating flow rate is 65 m3/d.The optimized scheme of R_to,Pr,r_m,r_m1,r_m2 respectively 10.61%,3.5%,22.9%,19.7%,18.71%,of the vertical and parallel to the flow direction profile on removal efficiency of pollutants?growth rate can reach 22.4%and 18.71%.The final result of this study is to construct a groundwater numerical simulation system specially used for GCW,and realize the quantitative and visual characterization of GCW operation effect indicators and the optimization of well structural parameters within the system,which can be used by technicians related to the restoration of contaminated sites.The research results can provide theoretical basis and reference for the later GCW site application,well structure parameter design and monitoring well layout scheme.