Study On Migration And Transformation Of Atrazine In Porous Media During The Groundwater Recharge By The Yellow River Water In The Yufuhe River

Yellow River water is an important source of water in Shandong province,and the Yellow River water has high sediment content,low organic matter content,and serious pollution.Sediment has adsorption and adsorption coprecipitation effects on pollutants discharged into Yellow River,which can be used as a carrier of pollutants into the groundwater system and affect the normal operation of the recharge system.As one of the effective measures,managed aquifer recharge(MAR)is adopted to improve the Yellow River water quality.Atrazine is one of eight priority controlled trace organic pollutants in Yellow river water.Aiming at the associated-colloid atrazine transport problem during the groundwater recharge by the Yellow River water,the Yufuhe River strong leakage zone is selected as the study area,which is located in the western part of Jinan Spring Basin,Shandong Province,China.The lab sand column comparison experiments with different velocity and porous media and batch equilibration test are conducted to study the migration of atrazine in heterogeneous porous medium(non-uniformity coefficient of 25.2)with saturated and steady flow.And the model of colloid-associated atrazine transport is simulated at laboratory scale.The observed data of breakthrough curves are numerically simulated using convection-dispersion equation(CDE)and CXTFIT 2.1 software.The conclusion of this research are as below.(1)The batch equilibration test showed that the adsorption effect of washed sand,raw sand and montmorillonite to atrazine reached equilibrium state within 24 hours.In 0-5 hours,the adsorption rate is relatively large,especially in 0-2.5 hours,the adsorption capacity is very large,reaching 42%,56%,73%of the maximum adsorption capacity,respectively.Compared with the washed sand and raw sand,the adsorption capacity of montmorillonite to atrazine is an order of magnitude increase,and the Freundlich model can better fit the adsorption curve with the R~2>0.95.Meanwhile,the desorption experiment indicated that the more clay mineral content is,the weaker desorption capacity is,and the desorption of montmorillonite to atrazine is the weakest,raw sand the second,washed sand the third.(2)For the tracer experiment,under the condition of washed sand,the breakthrough curves of NaCl are basically symmetrical and without the phenomenon of double peak and trailing,and the time for salt solution to the peak is approximately 55min.There are slightly trailing at the end of the curve under the condition of the raw sand,and the time for salt solution to the peak is approximately 85min.At the same time,the results of F(t)curve and E(t)curve are consistent with the experimental results.The time(tm)for the outlet solution to the peak is also calculated by E(t)with the t_m of 67.3min(washed sand)and the t_m of 83.3min(raw sand),which further verify the accuracy of the experimental results.(3)Under the condition of different media,the Breakthrough curves obtained are asymmetric and has obvious tailing characteristics.In the washed sand with montmorillonite,atrazine transport fastest with the time to the peak of 60min,and the peak concentration is the lowest,accounting for 84%of the initial atrazine concentration.In the raw sand,the time to the peak is approximately 135min,and the peak concentration is the lowest,accounting for 90%of the initial atrazine concentration.In the washed sand without montmorillonite,the time to the peak is approximately 105min,and the peak concentration is the largest,accounting for 95%of the initial atrazine concentration.Compared with washed sand with or without montmorillonite,the result showed that montmorillonite with larger specific surface area and more adsorption sites is useful for adsorbing atrazine to facilitate the migration of atrazine in the porous media.Compare washed sand with raw sand,the bigger porosity of washed sand provides a convenient path for atrazine migration.And the retardation factor(Rd)calculated indicted the washed sand media has less retardation effect on atrazine transport with R_d=3.15,and the retardation factor of raw sand is 6.75.Compare raw sand with washed sand with montmorillonite,the result showed atrazine migrate faster in the washed sand with montmorillonite.Although the raw sand contains 17%clay minerals,these clay minerals are randomLy attached to the sand layer,resulting in the decrease of porosity in different degrees.Moreover,compared montmorillonite with clay minerals contained in the raw sand,montmorillonite has a smaller particle size,is nano-scale,which has a larger specific surface area and more adsorption sites.Multiple adsorption sites and small particle size and are more conducive to adsorb more atrazine and facilitate the migration of pollutants in the pores.(4)Under the condition of different velocity,the Breakthrough curves obtained are asymmetric and has obvious tailing characteristics.With an increase in flow rate,in washed sand,the peak concentration has no obvious change,the time to peak appears earlier and earlier,and the degree of tailing is smaller and smaller.The result showed that the increase of flow rate accelerates the migration of atrazine,resulting in shorter and shorter time to the peak.In the washed sand with montmorillonite,the increase of flow rate reduces the contact time between the pollutants and the medium,casing insufficient adsorption,which eventually leads to an increase in pollutants in the liquid phase,the increase of peak concentration,the decrease of peak time and tailing degree.(5)The BTCs of NaCl are fitted by equilibrium CDE model.The simulation results are in good agreement with the experimental results,but the BTCs obtained by the simulation results have a more obvious tailing phenomenon in the end.The atrazine BTCs are fitted by the non-equilibrium two-site CDE model,and the actual concentration in the early stage of migration is greater than the simulated value.The result shows that the equilibrium CDE model and the non-equilibrium two-site CDE model can better fit tracer BTCs and atrazine BTCs(R~2>0.9).The results of this study are helpful to fully understand the transport characteristics of trace organic pollutants related to colloids,and have important guiding significance for possible preventive measures to protect the groundwater system and the long-term operation of MAR.