Study On The Application Of Pre-dam Technology Of Small Watershed In The Upper Reaches Of Dongjiang River And Sediment Variation In Reservoir Area

With the effective control of point source pollution such as industrial wastewater and municipal sewage in China,the problem of non-point source pollution has become the most important source of water pollution.Agricultural watersheds are generally the source of rivers,lakes,reservoirs and other waters.The rainfall runoff carrying pollutants will flow to rivers,lakes and reservoir water quality deterioration,especially nitrogen,phosphorus and other nutrients which usually causes water eutrophication.With the advantages of small investment,easy operation and management,ecological restoration and landscape effect,there are many successful cases of Pre-dam mainland China and abroad,which is the recommended ecological engineering technology.In this study,it is supported by the National Water Pollution Control and Treatment Science and Technology Major Project in the Dongjiang Upper Catchment Water Environment Risk Control Technology Integration and Comprehensive Demonstration Research Project.Based on the analysis of the characteristics of non-point source pollution and the load of major pollutants,the demonstration research of the complex ecosystem of the pre-dam is carried out.Through the study on the purification of river pollutants and the distribution characteristics of major pollutants in sediments,the mechanism of the interception of the pre-dam and the removal mechanism of the main pollutants such as phosphorus and pesticide were analyzed.The application prospect of Gaopu Creek prefabricated reservoir in the pollution control of river in small watershed was comprehensively evaluated.And the applicability of estuarine prefabrication technology to small watershed was discussed to provide theoretical basis and technical method for water pollution prevention and control in Dongjiang River Basin?The main conclusions of thesis are shown as follows:(1)The removal rates of CODCr,TP,TN and SS were 42.8%,23.39%,13.03% and 68.95%(rainy)in the pre-dam system.TP and SS removal rate is more stable,precipitation is the main removal mechanism.Gravel river bed on CODCr,NH3-N and sunny low SS concentration of river water purification effect is significant,and TP,TN purification effect is not obvious,high rain on the rainy days,the SS concentration of river water purification effect is limited;The sedimentary area(K1)of the prefabricated reservoir has a good sedimentation effect on the granular nitrogen and phosphorus,organic matter and sediment,but this part of the pollutants is more transfer of the water phase to the mud phase,so the regular dredging of the sediment is very important.(2)The annual amount of pollutant reduction is shown as follows: CODcr = 203.85 t/a,TP = 3.59 t/a,TN = 47.30 t/a,SS = 736.64 t/a.The operating cost is low,the effect is obvious,and the small watershed non-point source pollution load can be effectively reduced.(3)The sediments of the prefabricated sediments are mainly composed of silt and grit,and the content of organic matter is higher than that of Gaopu River sediments,and it is decreasing along the flow direction of river,and the sampling point of the into river channel is minimum.The phosphorus content removed from the prefabricated reservoirs is mostly settled in the sediments.The phosphorus in the sediments of the pre-dam is mainly in the form of IP,and the predominantly IP is mainly Na OH-P.The TP content in the sediments of the prefabricated reservoir ranges from 467.81 mg / kg to 3719.44 mg / kg,with an average value of 1357.43 mg / kg,of which the IP content is 70.2%.The correlation equation between TP and IP is Y = 0.7789X-88.508.The results showed that the sedimentation capacity of mud sediment in different hydrological periods are shown as follows: low flow period is 221.81 t,average flow period is 202.78 t,high water flow period is 3809.18 t,and the total amount of dredging is 4233.77 t.Sedimentation in the sediment area is 71.74 t in the low flow period,66.40 t in the level water period and 2285.81 t in the high water period,and the total dredging rate is 2423.64 t.(4)The distribution of total pesticide content in the surface sediments of the study area is from the following aspects: high flow period>average flow period>low flow period,the sediment source of the pesticide is affected by the sediment scouring effect,and has the characteristics of non-point source pollution.The pesticide content of the sediments in the prefabricated reservoirs is decreasing along the direction of the process flow,and the sediments in the prefabrication reservoir are the important destination of the pesticide.Sediment organic matter content and particle size distribution play an important role in the content and distribution of sediments.The pre-dam mainly removes the pesticide organic pollutants in the water through the sedimentation of the particles,and the removal effect of the pesticide is not obvious.The rate of migration of pesticides to water in the sediments is roughly equal to the rate of degradation of pesticides in the pre-library,and the sediment pesticides are less risky to overlying water as secondary sources.