| In recent years,the pollution problem of antibiotics is increasingly serious.Researches showed that a large number of oxytetracycline is discharged into the Weihe river,not only caused water pollution,but also can pass through the vadose zone and get into the aquifer,even caused the pollution of groundwater.As the connection of surface water and groundwater,the vadose zone plays a vital role in behavior of transformation and migration about pollutants.Therefore,the research on the migration behavior of antibiotics in the vadose zone is a very important significance for the protection of the groundwater environment.Based on the field investigation,this paper carried out the research on the interchange of the Weihe river and Fenghe river and selected oxytetracycline as the research object.Then through the analysis of oxytetracycline in experiment of three kinds?medium sand,fine sand,silt?in vadose zone,such as medium adsorption kinetics experiment,isothermal adsorption experiment,microbial natural degradation experiments,a one-dimensional diffusion experiment of conservative ion and oxytetracycline experiment,the article get the relevant parameters.Combined with the actual situation in the study area,the study established oxytetracycline solute transport model in the vadose zone.Then with the hydrus-1d software,on the basis of experience,combining with the calculating parameters in experiments,the paper simulated and predicted the regulation of transport in the vadose zone.Under the condition of divulgence,rainfall and evaporation,it carried out distribution study of oxytetracycline in vadose zone,and revealed that the migration regularity of oxytetracycline in vadose zone,and evaluate the reason of a groundwater pollution.Get the following conclusions:?1?Studies showed that adsorption equilibrium time of three kinds of vadose medium is about 24 h,and the sequence in adsorption rate and equilibrium adsorption amount of oxytetracycline is silt > fine sand > medium sand.When the concentration of oxytetracycline is 1-15 mg/L,three model?the linear model,the Freundlich model,the Langmuir model?conforms to the isothermal adsorption model.Among the three kinds of adsorption,modellinear model fitted best,and the correlation coefficient of the three medium :R2 > 0.986.?2?Microbial degradation of static experiment: oxytetracycline degradation process of three kinds of medium conformed to the first-order kinetic model,the degradation coefficient of medium sand,fine sand and silt coefficients was 0.0124d-1,0.0143-1,0.0168d-1,the correlation coefficient is 0.9461,0.8686,0.9122,respectively.The half-life of degradation is 41.3-55.9 d.?3?One dimensional dispersion experiment:The penetrating time of chloride ion?conservative ions?in three soil column?medium sand,fine sand,silt?is 27 min,605min,714 min,respectively;Oxytetracycline penetration experiment:the penetrating time of oxytetracycline in three soil column is 45 min,5d,9d,respectively.Namely penetration rate of oxytetracycline in three kinds of soil decreases with the decrease of the particle size.With the point of pore volume?V0?,Experimental determination of three soil column pore volume were 21 ml,200 ml,40 ml,respectively.The volume of oxytetracycline passed through three soil column were 6V0,170V0,406V0,respectively.Meanning that the required volume of oxytetracycline penetrating in the silty sand column was maximum,the time was the longest,the effect of adsorption and degradation was strongest.?4?According to the experimental results,this paper built the concept model of vadose zone structure in the interchange of Feng river and Weihe river,calculated the parameters,and established migration model of oxytetracycline in vadose zone,simulated and predicted with hydrus-1d software.With simulation results,it can be seen that both instantaneous leakage and leakage in a row,oxytetracycline will penetrate the vadose and polluted groundwater.Under the condition of continuous leaking,oxytetracycline firstly pass through the vadose zone on the 200 th day,in 900 th days the concentration basically stable at 0.225 mg/L,the vadose zone of oxytetracycline reduction rate was 77.5%.Instantaneous leakage firstly pass through the vadose zonestart at the 150 th days,and get to peak concentration on 380 th days,which is 0.9 x 10-4 mg/L,then begin to decay,it is zero concentration after 1000 th days.Due to the adsorption and degradation,concentration become small with the increase of buried depth.Because of oxytetracycline amount is small,although it can pass through the vadose zone,but the concentration of groundwater is very low,and gradually reduced to zero over time.The results showed that in the case of a continuous infiltration pressure,high concentrations of oxytetracycline give the greatest threat to groundwater.?5?Under the condition of rainfall and evaporation,oxytetracycline is hard to get through the vadose zone,most of oxytetracycline trapped in the surface layer of sand in 20 cm,mostly will migrate to the 60 cm.In the absence of water,evaporation and adsorption degradation hinders the down of oxytetracycline,it is very difficult to penetrate of the vadose zone and pollute underground water,but can cause the pollution of vadose zone.Thus under the condition of fertilization,rainfall and evaporation cause a smaller threat to groundwater.?6?The different distribution of vadose zone structure have different impact on the migration behavior of oxytetracycline.When the initial concentration of oxytetracycline was 1 mg/L,the structure changes of the different profile was simulated.From the time point of view,the balance time of the model 1?from fine to coarse?and model 2?from coarse to fine?were 900 d,500 d,respectively.From the point of equilibrium concentration,when the oxytetracycline fully penetrate,the concentration of model 1 and model 2 were 0.015 mg/L,0.72 mg/L.It Showed that the concentration of oxytetracycline reduced little with the structure?from coarse to fine?,but the concentration of oxytetracycline reduced more with the structure?from fine to coarse?,and adsorption degradation became stronger. |
PDF Full Text Request