Study On The Characteristics Of Sediment Pollution And Nitrogen And Phosphorus Release In Rainwater Pipelines In Suzhou Urban Area

Suzhou is characterized by numerous rivers,densely populated and well-developed service industry.In Suzhou,the permeable ground area is small,and the flow rate of river is slow.After the implementation of"source control and pollution interception",the water quality of the river has been improved to a certain extent,but it is still necessary to continue to strengthen the control of river pollution.As one of the pollution sources of the river,the sediment in the rainwater pipe is washed away by the rainwater runoff and discharged into the river.This study collected 8 representative samples of typical functional areas in Suzhou during the rainy and dry seasons from2020 to 2021,namely living areas（A1）,commercial areas（A2）,historical and cultural protection areas（A3）,and cultural and educational areas（A4）.Pipeline sediment（PS）,street dust（SD）and overlying water（OW）samples were collected and evaluated for carbon,nitrogen,phosphorus and heavy metal content.The migration characteristics of the above pollutants in PS and SD with different particle sizes were analyzed.Taking a typical PS as an example,an indoor simulation experiment of nitrogen and phosphorus pollutants exchange between PS and SOW was carried out.Response surface methodology（RSM）was used to optimize the impact of environmental factors on exchange fluxes of nitrogen and phosphorus pollutants and their interaction.The content of nitrogen and phosphorus in PS is high,while the content of heavy metals is low.The overall distribution characteristics of carbon,nitrogen and phosphorus pollutants in PS and SD are:A2>A1>A3>A4.ω（TOC）,ω（TN）,ω（TP）andω（NH₄⁺-N）were significantly positively correlated with D50（median particle size）.Theω（TOC）,ω（TN）andω（TP）of PS are lower in the main pipe than in the branch pipe,whileω（NH₄⁺-N）is the opposite.The pollution loads of TOC,TP and NH₄⁺-N in PS are mainly concentrated on the particles with particle size≤75μm and 250～1000μm,TN load mainly comes from 250～1000μm,and the TOC,TN and TP loads in the rainy season are higher than those in the dry season.TP and TN were enriched in PS,especially TP with particle size≤75μm.The average values ofω（Cu）,ω（Zn）,ω（Pb）,ω（Cr）,ω（Ni）,ω（As）andω（Cd）of PS in Suzhou are 264.0,315.7,482.5,127.6,47.2,19.9 and 1.3 mg·kg^-1,respectively.Based on index of geo-accumulation（Igeo）,the pollution degree of heavy metals in PS and SD are Pb>Cd>Cu>Zn>As>Ni>Cr,where Pb and Cd are in grade 4（severe）pollution,Cu is grade 3（moderate）pollution,Zn is grade 2（moderate）pollution,Ni and As are grade 1（light）pollution,and Cr is rarely pollution.Based on efficient of potential risk（E_i）,the pollution degree is Cd>Pb>Cu>As>Ni>Zn>Cr.The comprehensive ecological hazard index（RI）value of heavy metals in PS ranges from208.0 to 1192.0,with an average value of 652.6,which is in the extreme hazard risk level.Pollutants exchange between PS and OW under the static conditions are monitored.Results show that the basic migration law as that the concentrations of COD,NH₄⁺-N,PO₄^3--P,TN and TP in the OW increased briefly,and then fluctuate repeatedly with time,showing a tendency of pollutants migration from PS to OW,while NO₃^--N tends to be consumed in OW.The rank of average exchange fluxes of pollutants is A2>A1>A3>A4,branch pipe>main pipe.Temperature,pH and flow rate have effects on pollutants migration.With the increase of temperature and flow rate,the fluxes of COD,TN,NH₄⁺-N,TP and PO₄^3--P increased,while NO₃^--N decreased.Based on RSM,the relationship between environment factors as temperature（X₁）,pH（X₂）,flow rate（X₃）and exchange flux of COD（Y₁）,TN（Y₂）,NH₄⁺-N（Y₃）,NO₃^--N（Y₄）,TP（Y₅）and PO₄^3--P（Y₆）are analyzed.Results show that all of them fit the quadratic polynomial regression model.It can be seen that the migration of COD between PS and OW is significantly influenced by the interaction of temperature and pH,while TN,NH₄⁺-N,TP and PO₄^3--P are influenced by the interaction of pH and flow velocity,in addition,NO₃^--N is influenced by interaction of temperature and pH as well as temperature and flow rate significantly.