Study On Effect Of Dredging And Law Of Adsorption And Release Of Nitrogen And Phosphorus In Cenozoic Sediments

In order to further improve water quality,sediment dredging has been widely used as an effective means to remove endogenous pollution,and the environmental problems caused by the sediment dredging project have also been widely concerned.This paper analyzes the impact of sediment dredging project of Yihe River in Yishui on the water quality,sediments,benthose and microorganisms both macroscopically and microcosmically.In addition,i this paper explored the impact of sediment dredging on the adsorption/release characteristics of Cenozoic sediments in the Yihe River,and constructed and verified the migration and transformation model of nitrogen and phosphorus in the“overlying water-surface sediments”of the Yihe River.The main conclusions are as follows:（1）The environmental effects of sediment dredging project of Yihe River1)Water environment effect.Sediment dredging can significantly reduce the content of COD_Cr and TP in the water body of Yihe River dredging area,with the reduction rates of 27.33%-71.25%and 4.01%-94.20%,respectively.But they will return to the previous level after 38 months and 50 months,respectively.However,the content of NH₄⁺-N and TN can not be effectively reduced.Moreover,the water quality indices in Yihe River downstream has not been significantly improved due to the interception of rubber dam.2)Environmental effects of Cenozoic sediments.Sediment dredging has not significantly removed TN and TP in the dredging area of Yihe River,and the TP content of the Cenozoic sediments in the dredging area increases sharply caused by downstream drainage operations.The TOC content of Cenozoic sediments is higher than that of the area that has not been dredged at the beginning of sampling,but,it then becomes lower than that of the undredged area within 38 months after dredging.And the C/N in the Cenozoic sediments have gradually decreased as time goes.It indicates that the degradation difficulty of organic matter in the dredging area of the Yihe River gradually decreases,which is beneficial to reduce the nutrient load in the Cenozoic sediments and the recovery of benthos and microbial communities of the dredging area.3)Biological effects.A total of 8 benthos will be collected in this study,belonging to 8 species and 5 families.Sediment dredging significantly reduces the biomass of the Tubificidae in the dredging area,and then recovers within 50 months after dredging.The high-throughput sequencing of microorganisms shows that the sediment dredging of Yihe River has little effect on microbial community structure（phylum level）in sediments.Moreover,since 33 months after dredging,the abundance and diversity of microorganisms in the Cenozoic sediments in the dredging area are higher than those in the undredged area,of which the abundance indices ACE and Chao1 increase by13.26%-31.40%and 14.35%-28.40%,respectively.The diversity indices Simpson and Shannon increase by 0.01%-11.58%and 0.20%-36.70%,respectively.（2）Adsorption/release characteristics of nitrogen and phosphorus in Cenozoic sediments of Yihe River1)The adsorption of NH₄⁺-N and PO₄^3--P by Cenozoic sediments in Yihe River dredging area is mainly chemisorption adsorption;The adsorption of NH₄⁺-N and PO₄^3--P by sediments in Yihe River undredging area is mainly Physical adsorption,and all of them are mainly surface adsorption.Sediment dredging does not significantly change the adsorption mechanism of Cenozoic sediments in Yihe River dredging area,but it can effectively improve the adsorption rate and quantity of NH₄⁺-N and PO₄^3--P by Cenozoic sediments,and reduce its release quantity.Within a period of time after dredging,the equilibrium adsorption quantity and maximum adsorption quantity of PO₄^3--P will increase by 1.80%-26.13%and 20.69%-81.07%,respectively.The maximum adsorption quantity of NH₄⁺-N will increase by 7.81%-32.92%,while the release quantity of NH₄⁺-N and PO₄^3--P will decrease by 13.77%-39.30%and 0.04%-18.31%,respectively.However,The maximum adsorption quantity of NH₄⁺-N and PO₄^3--P will lower than that of the undredged area within 50 months after dredging,while the release quantity of NH₄⁺-N and PO₄^3--P will higher than that of the undredged area.2)When temperature,pH,velocity of flow,overlying water concentration and adsorption time reach 17.35℃,8.26,0.71 m/s,2.84 mg/L,34.08 h and 15.80℃,6.85,0.71 m/s,0.71 mg/L,26.92 h,respectively,the adsorption quantity of NH₄⁺-N and PO₄^3--P in the Cenozoic sediments of Yihe River reaches the maximum value,8.20 mg/kg and 26.72 mg/kg,respectively.When the above values remain 16.04℃,5.83,0.29 m/s,1.18 mg/L,33.95 h and 24.20℃,8.26℃,0.71 m/s,0.29 mg/L,13.91 h,respectively,the adsorption quantity of NH₄⁺-N and PO₄^3--P reaches the minimum value,-64.16mg/kg and-6.32 mg/kg,respectively.3)According to TN and available phosphorus content in Cenozoic sediments of Yihe River and the central composite experimental data determine the adsorption quantity of NH₄⁺-N and PO₄^3--P in Cenozoic sediments of Yihe River and regression equation of environmental factors.It is concluded that temperature,overlying water concentration and TN content of Cenozoic sediment can significantly affect the adsorption quantity of NH₄⁺-N by Cenozoic sediment;and temperature,overlying water concentration,effective phosphorus concentration of Cenozoic sediment and adsorption time can significantly affect the adsorption quantity of PO₄^3--P by Cenozoic sediments of Yihe River.（3）Migration and transformation model of nitrogen and phosphorusBased on the migration and transformation law of nitrogen and phosphorus in the"overlying water-surface sediment"system,the effect of suspended particles and nitrogen and phosphorus morphology in different media is comprehensively considered in this paper,and a one-dimensional model of nitrogen and phosphorus migration and transformation suitable for Yihe River will be established with the mathematical software Matlab.And the Direct Search Method will be used to solve and optimize the parameters in the model.It is verified that the total relative errors of the optimized nitrogen and phosphorus model in the study area are 20.83%and 14.10%,respectively,with high accuracy.