Experimental Research On The Variation Of Permeability Of Aqueous Media In The Process Of Reclaimed Water Recharge

The reclaimed water recharge project has good results in preventing seawater intrusion,controlling land subsidence,utilizing aquifer energy storage,and solving regional water shortages.In recent years,with the continuous development and improvement of recharge techniques and methods,the type of recharge water tends to be diversified,and the quality of recharge water is also more complicated.During the process of entering underground environment through aquifer,reclaimed water will not only cause disturbance to the original hydrodynamic field and chemical field,but also water-rock interaction.Under different recharge conditions and recharge modes,these effects will have a certain degree of impact on the structure of aquifer.As a typical parameter describing the characteristics of aquifer,permeability coefficient is bound to change with the structure of aquifer.Therefore,researching the variation of the permeability of water-containing medium during the recharge process of reclaimed water has a leading significance for the study of pollutant behavior.Based on the field investigation,this paper carried out the aqueous medium medium of the floodplain at the junction of the Zaohe River and Weihe River as the research object,three typical samples of medium sand,fine sand and silt was selected,through the indoor one-dimensional sand column simulation experiment to research the permeability variation of typical aqueous media in the process of recharge.In view of the blocking behavior that leads to the decay of permeability coefficient,an in-situ one-dimensional sand column seepage experiment was carried out.By observing the blockage of suspended matter and the microbiological plugging test,the variation of the permeability coefficient of the aqueous medium caused by the two kinds of plugging behaviors were studied respectively.Hydrogeochemical simulation of PHREEQC software was used to simulate the law of chemical blockage behavior that may be caused by the water-rock interaction during the recharge process.The dissolution and sedimentation characteristics of representative inorganic minerals and heavy metal minerals under different environmental conditions and the changes in the form of heavy metal components during this process were studied.The research mainly leads to the following understanding:1.Permeability variation of typical water medium.Under the same conditions,the permeability of the three media was medium sand(0.200 cm/s)> fine sand(0.020 cm/s)> silty sand(0.008 cm/s).The permeability coefficients of the three media all show the same change rule: the permeability coefficient of the water-containing medium under the tap water recharge is basically stable,while the permeability coefficient of the water-containing medium under the reclaimed water retreat continues to decrease with time,and then tends to be stable.When the medium consists of medium sand: fine sand: silt = 3:2:1 and medium sand: fine sand: silt = 1:2:3,the attenuation coefficient of permeability coefficient was 54.55% and 68% respectively,and the attenuation coefficients were 0.107 and 0.136 respectively.The particle size composition of the sand body is different and the overall effective particle size changes.The larger the effective particle size,the greater the permeability coefficient.When the arrangement is “medium sand(top)fine sand(medium)silt(bottom)”and “medium sand(bottom)fine sand(medium)silt(top)”,the coefficient of permeability coefficient attenuation is 0.117 and 0.130,respectively.Three kinds of sand with different location arrangement,medium sand located at the bottom,when the recharge is stable,the overall permeability coefficient is much smaller than the permeability coefficients of the other two types of sand when they are in the bottom layer,which show that the recharge aquifer medium sedimentary structure are the main factors influencing the permeability change.2.Suspended solids blockage rule of experiment.During the recharge process,the permeability coefficient of the water-containing medium shows the same change rule over time: the initial period of recharge is “rapidly declining”,the medium-term “slowly decreasing”,and the final stage “tends to be stable”.In the early stage of recharge,more severe suspended solids were observed.Under the same conditions,the larger the particle size of the aqueous medium,the greater the permeability coefficient.When refilling is performed at the same time,the clogging of the medium with smaller particle size is more serious.When the clogging degree is the same,the medium with larger particle size needs longer time for recharging.The greater the concentration of suspended matter,the more severe the clogging,and the type of blockage is mostly surface clogging.3.Microbiological clogging rule of experiment.After the recharge test,the permeability coefficient of the five sand layers in the sand column dropped to the initial values of 0.09,0.11,0.54,0.78,and 0.82,respectively.Biological plugging is very violent near the water inlet in the sand column.The consumption of nutrients in the shallow layer restricts the development of microbial blockage to deeper layers.The suspended biomass in the sand column decreases as the infiltration distance increases.4.Chemical blockage rule of simulation.(1)PHREEQC software simulation shows that the proportion of reclaimed water and groundwater affects the dissolution and precipitation of representative minerals.For carbonate minerals,the precipitation/dissolution state of dolomite is greatly affected by the recharge of reclaimed water.For sulphate minerals,the tendency of gypsum precipitation decreases and the tendency of barite precipitation increases.For silicate minerals,the sedimentation tendency of albite and anorthite weakens,and quartz precipitation tends to increase.(2)Representative mineral precipitation/dissolution rules: Carbonate mineral saturation index increases with increasing temperature.For sulfate minerals,the saturation index of gypsum does not change,and the tendency of barite precipitation decreases.The saturation index of silicate minerals slowly decreases with temperature.When the mixed water is in an acidic solution,the carbonate minerals show a dissolving state.When the medium-alkaline solution is a carbonate,the minerals show a stable precipitation state.pH has little effect on sulfate minerals.The saturation index of silicate minerals increases linearly at pH<6 and remains constant in the range of 6-10.Oxidation-reduction potential does not have much effect on the precipitation and dissolution of the above types of minerals.(3)Heavy metal component forms and variation rules of metal minerals: The forms of components of Cd,Cu,and Pb are less affected by temperature,and the forms of components of Fe,Mn,and Zn are greatly affected by temperature.The pH change has a great influence on the form of the heavy metal components.In the acidic environment,various metals exist in free form or in the form of complex ions.In the neutral alkaline environment,the metals are mostly combined with carbonates.The morpholine and the OH complex product of the hydrolysis reaction exist.Both Fe and Cu are greatly influenced by Pe.Temperature changes have little effect on the dissolution and precipitation of heavy metal minerals.The change of pH and Pe had a great influence on the dissolution and precipitation of iron-manganese minerals.