| Agricultural non-point source pollution has become an important source of water pollution in China,which pesticide non-point source pollution problem is very serious,the high toxicity and high residue of pesticides poses a significant threat to aquatic ecosystems and human health.Therefore,it is necessary to explore the application of non-point source pollution ecological restoration technology.Ecological pond and ecological ditch repair technology is more suitable for rural areas with low cultivated land,because they can use existing land resources.At present,the ecological pond is used to deal with nitrogen,phosphorus and other pollutants in rural non-point source pollution,but lack the study of pesticides and other toxic organic pollutants.At the same time,there is no research about ecological ditch to remove pesticides pollutants.In recent years,foreign studies have confirmed that the ecological ditch has active removal effects on pesticides,but the mechanism of migration and transformation of pesticides in the system is not clear.Especially in the rain or irrigation runoff erosion conditions,the interception and mechanism of the plant on pesticides runoff need to be proved.Therefore,we established a Hydrilla verticillata-Hydrilla verticillata pond,and evaluated the effect of submerged vegetation restoration on the control of typical pesticides.Established vegetation(Canna indica?Thalia dealbata?Typha latifolia and Juncus effuses)and unvegetation ditch system,investigated the intercept capacity of four plants on pesticides runoff,and Screening plants with strong contaminant interception ability,and clarify the dynamic change of pesticides in ditch system.By exploring the removal of atrazine and diuron in the ditch,revealed the migration and transformation of the two pesticides in the ditch system.(1)Constructed Hydrilla verticillata-Vallisneria natans pond system,studied the purifying effect of seven pesticides and conventional pollutants,and the control of pesticides ecological risk.Result showed that the construction of vegetation could effectively control the concentration of mixed pesticides in water,and reduce the ecological risk.The average removal efficiencies of chlorpyrifos,fenvalerate,procymidone,phorate and acetochlor in water were over 75%,the removal efficiencies of malathion and dimethoate were around 50%.The ecological risk of procymidone varied from medium risk to low risk,chlorpyrifos,acetochlor and dimethoate varied from high risk to medium risk,and the risk quotient value of the other three pesticides were significantly decreased(P<0.05).Moreover,All conventional water quality indicators were decreased significantly,and the water quality was improved from worse than grades ? to grades ? or ? according to state water quality standards for surface water.(2)Established vegetation(Canna indica?Thalia dealbata?Typha latifolia and Juncus effuses)and unvegetation ditch system,estimated the dynamic changes of atrazine and diuron and the interception of plants in ditch systems in the simulation of the flushing-standing-reflushing-restanding runoff events.Result showed that at the first runoff event(0~5 h),the two kinds of pesticides were flushed into the ditch,and the concentration declined rapidly dueing to adsorption of soil.Followed by the standing stage(5~48h),the concentration decreased slowly.During the second runoff stage(48~53h),the concentration of pesticide in each system decreased drastically.From the total removal rate(after 7 days),in high concentration test,the removal rate of atrazine was 47~56% and 27% in plant and non-plant groups,and the removal rate of diuron was 49~53% and 37% respectively.In the low concentration test,the removal rates of atrazine were 34~43% and 18% in plant and non-plant groups,and the removal rates of diuron were 58~73% 49% respectively.The presence of plants significantly increased the removal rate of atrazine and diuron(P <0.05),but there was no significant difference between plant species.In the second runoff scouring process,the release rate of atrazine and diuron from the highest to the lowest was in the order of no plant group,J.effuses,Typha latifolia,T.dealbata and C.indica,plant groups and non-plant group had significantly difference(P <0.05),indicating that the plants played a significant interception effect on the pesticides in the ditches,but there was no significant difference among the plant species.(3)In the stationary stage of the simulated runoff event(5~48 h),the kinetics of removal process of atrazine and diuron in ditch was studied.Result showed that the pesticides could degrade in the system,and the concentration change process was in good agreement with the first order reaction kinetic equation C=C0e-kt(R2>0.7).In the high concentration test,the degradation k values of atrazine and diuron in each system were 0.001~0.004 h-1 and 0.003~0.007 h-1,respectively.In the low concentration test,the degradation k values of atrazine and diuron in each system were 0.003~0.009 h-1 and 0.006~0.013 h-1,respectively.For these two pesticides,the degradation rate k of the plant group was higher than that of the non-plant group(P <0.05),indicating that the presence of the plant increased the biodegradation rate of the pesticide in the ditch system.In the high and low concentration experiments,the degradation rate constant k of the atrazine was higher than that of the diuron,which may indicate that the physicochemical properties of the pesticide could affect its degradation in the ditch system,the higher log kow value of the pesticide,the greater degradation rate in the ditch system.(4)Based on the principle of mass balance,the distribution of atrazine and diuron in the ditch was studied in the whole simulated runoff event,and the removal pathway and mechanism of the two pesticides were clarified.Result showed that,in the high concentration test,the contribution rates of biodegradation to atrazine and diuron were 20~48% and 19~39%,the contribution rates of soil adsorption to atrazine and diuron were 6~10% and 12~18%,the contribution rate of biodegradation to atrazine and diuron were 15~ 8% and 36~62%.In the low concentration test,the contribution rates of biodegradation to atrazine and diuron were 6~10% and 12~18%,the contribution rates of soil adsorption to atrazine and diuron were 15~38% and 36~62%,respectively.This suggests that the two relatively hydrophilic pesticides(logkow <3.0),the soil adsorption were relatively weak,and not the most important way to remove them.Although the direct absorption of plants were not obvious,but there indirect effects were significant,and they could contribute to microbial degradation than the non-plant system by about 0.5 times(diuron)and about 1 times(atrazine),thereby reducing the residual rate of the two pesticides in the system.In summary,the study of ecological pond and ecological ditch restoration can effectively reduce the non-point source pollution of pesticides,revealed the behaviors of typical pesticides and clarified the important role of plants in the simulation of the flushing-standing-reflushing-restanding runoff events,which provides the theoretical and practical basis for the optimization design of ecological ditch technology for the control of pesticide pollution in actual agricultural non-point source,and has innovation and significance. |
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