Recharge Experiment And Numerical Simulation On Aquifer Physical Clogging

Artificial Recharge, which uses urban storm-water to supply the groundwater, is a powerfultechnology to regulate and control groundwater storage. However, clogging problems (especiallyphysical clogging caused by suspended particles) during the process of recharge seriously affectthe recharge effect, thus limiting the use and popularization of recharge projects.In this study, a laboratory simulation test is conducted to understand the physical cloggingmechanism of aquifer during storm-water recharge under constant head, with the primary aim ofinvestigating how particle concentration and hydrodynamic condition impact on cloggingoccurrence and development. Besides, a model of particle transition-deposition in porous media isestablished to predict how clogging behaves during recharge. This model gives an overallconsideration of the particles’ attachment and detachment in the media, and relationships betweenhydraulic conductivity, porosity and concentration of particles in retention, thus figuring out theclogging level. In the meanwhile, clogging predictions about Qingdao Dagu River undergroundunconfined aquifer during surface flooding irrigation and deep well injection are carried outrespectively by means of the afore-mentioned mathematical model, while taking the actualrecharge conditions into account. Conclusions from the research are summarized as follows.(1) Under the condition of surface infiltration, the relative permeability K’ of aquiferexperiences a large-scale decline at the initial period of recharge, and gradually becomes steadierlater.(2) Particle concentration of recharge water is a mean factor that influences the aquiferphysical clogging level. Clogging is much more serious with higher concentration. Particles areinclined to deposit on the surface of media grains and to some extent, suppress their downwardmigration. As a result, the permeability of deep sand layer is slightly affected. Experiments underthe condition of different particle concentration are conducted, respectively25mg/L,100mg/Land200mg/L. After100h recharge, relative permeability of sand layer OB (0~11cm) is reduced to0.75,0.32and0.12under each concentration, while the whole sand column drops to0.85,0.66and0.56. (3) Seepage velocity is also an important clogging impact factor that should be considered.With higher velocity, particles will easily get off the pore wall because of gravity andhydrodynamic shear stress, and migrate to the deeper layer, leading to the decrease of the internalpermeability. Regardless of seepage velocity, relative permeability of the surface sand layershowed a similar changing trend, with its final value equals to0.10; At the end recharge, relativepermeability of sand layer CF (19~34cm) falls to0.44and0.54, with a water head difference of40.00cm and105.00cm respectively, while the whole sand column declines to61%and55%,compared with the initial permeability.(4) Particle attach-detach phenomenon in porous media is in a dynamic changing process,which is actually influenced by seepage velocity. With lower velocity, particles can only beattached to the surface of the medium grains, and then particles will gradually leave the grainsurface when the velocity becomes higher. That is the moment when particle retention andre-migration phenomenon coexist. The attachment-detachment process will cause particledeposition, eventually occupy the pore space and clog the aquifer. This phenomenon can bequantitatively expressed by mathematical relationships between seepage velocity and particleattachment, detachment parameters.(5) According to particle attachment and detachment process, physical clogging model ofparticle migration-deposition in porous media can be proposed, with which the clogging situationbe quantitatively described and prediction of clogging level under different recharge conditions bemade. Through the model sensitivity analysis, it is proved that particle concentration is the mostdistinct impact factor.(6) By coupling the groundwater well flow models during surface flooding irrigation andwell injection, and considering the actual site conditions of research area and the recharge waterquality, the afore-mentioned physical clogging model can be used to forecast how clogging takeplace and develop in porous aquifer under different recharge modes. The calculation results showthat clogging meanly happens in the aquifer closed to the bottom of recharge pool (surfaceirrigation) and within the limits of10cm away from the well wall (well injection) both in the wetseason and dry season, and permeability of aquifer far away from the recharge facilities isuninfluenced.(7) Pre-treatment of recharge water resource (such as filtering) is an effective measure to prevent aquifer physical clogging. And different clogging solving methods can be taken depend onthe recharge mode. For surface irrigation, frequent dredging of recharge pool is valid; for wellinjection, periodic backwash of injection well is of great necessary to effectively prevent clogging.