The Migration And Transformation Mechanism Of Disinfectant By-products In The Process Of Managed Aquifer Recharge

In recent years,a series of environmental geological problems such as areal underground funnel,ground fissure and saltwater intrusion due to the overexploitation of groundwater.Managed aquifer recharge is effective to restore the underground water level and control the deterioration of environmental geological problems.However,the groundwater chemical composition differences between recharge water and water in the target recharge aquifer and hydrodynamic field change influence groundwater quality.The influence of wide application of managed aquifer recharge on groundwater quality caused wide attention of scholars both domestic and abroad.The disinfectant in recharge water reacts with nature organic matter and produce disinfection by-products(DBPs)that cause groundwater quality safety risk.But for a long time,the use of disinfectants in the recharge water is for the lack of understanding on the influence of the groundwater.Lacking of system research on disinfection by-products pollution for artificial recharge process in our country,and the migration and transformation mechanism of disinfection by-products are in the blank state.So,the research of disinfection by-products formation mechanism,the migration and transformation mechanism under the condition of managed aquifer recharge provides necessary theoretical basis in order to ensure the security of the process of managed aquifer recharge.Based on the project "Shouguang Mi River recharge and underground reservoir construction feasibility study" as the backing,this paper selected an recharge sites in the Shouguang,on the basis of analysis of the geological and hydrogeological conditions in the recharge sites,the sodium hypochlorite was choose as disinfectant,humic acid was used as simulated groundwater organic matter,trichloromethane was selected as research object because of its highest detection rate and harmfulness,formation,batch and column experiments reveal the disinfection by-products formation of chloroform,the migration and transformation process,through the establishment of analytical model of chloroform,predict the migration and transformation rules of chloroform,determine the disinfectant concentration threshold in recharge water source.The research results show:1.Chlorine reactive with organic humic acid generated chloroform is not instantaneous,increases with the extension of exposure time and finally tends to be stable.When the concentration of organic matter is constant(TOC=5 or TOC=10mg/L),available chlorineconcentration within a certain range(0.125?4 mg/L),chloroform and disinfectant supply is a linear relationship,chlorine concentration continued to increase and generation tends to be stable.The formation of CHCI3 related to the water quality conditions,the production increased with higher pH and lower ionic strength,and divalent cation's inhibitory effect is more apparent.2.Adsorption kinetic experiment results show that the pseudo-second order kinetic is well fitted,the adsorption capacity were 2.22,1.57 and 1.40?g/g;isothermal adsorption experiment results show that the isothermal adsorption of chloroform in accordance with the Freundlich model,the adsorption capacity order is silt,fine sand and medium coarse sand.3.The biodegradation process by porous medium can be described by Monod equation.The aquifer size order of attenuation rate of chloroform is silt,fine sand and coarse sand,with the increase of particle size,the decline of the organic content and microbial,biodegradation rate of chloroform decreases.Besides,the biodegradation rate of chloroform increased with the increasing of the initial concentration.4.Through the column experiments,the fate and transport of chloroformwas studied:(1)The influence of porous media on the fate and transport chloroformFine sand and coarse were used in the column experiments and results showed that the transport of chloroformin the coarse is greater than the fine sand,it is mainly due to the greater surface area provides more adsorption sites;besides,under the same darcy flow,pore velocity decreased with the increase of the porosity,made the shear force reduces,the adsorption was abate and transport was enhanced.(2)The influence of hydrochemistry conditions on the fate and transport chloroformHydrogen ion concentration increased in low pH,competitive adsorption increased,with the increased of pH,adsorption was enhanced and adverse to the transport of chloroform;with the increasing of IS,cation can neutralize more negatively charged,electrostatic force is abate and transport of chloroformwas inhibited.In addition,divalent cation has greater effect,it can form bridge with porous medium,adsorption site increased and promoted the adsorption.(3)The influence of hydrochemistry conditions on the fate and transport chloroformThe experimental results show that the migration ability increases with the increase of flow rate,on the one hand,the contact time reduced at high flow rates and adsorption reduced;on the other hand,the shear stress increased with the increase of flow rates.5.According to the hydrogeological condition and experiments results built the migration and transformation numerical model and analytical model in the recharge site,predicted CHCl3 concentration in the monitor well and results shows that,when the concentration of disinfection is 20mg/L,the CHCl3 may exceed national drinking wawter standard under high pH,low inoic strength and high flow rate conditions,determined the initial concentration of CHCl3 when the peak concentration of CHCl3 less than 60 ?g/L,according to the law of the formation,forecast available chlorineof recharge water concentration limit.