Research On Terrestrial Nutrient Pollution Source Apportionment And Water Quality Objectives Of Coastal Waters In Tianjin

The total pollutant discharge control system of specific sea areas is an important system clearly required by the marine environmental protection law and an important means to effectively curb the deterioration trend of water environmental quality in coastal areas.At present,the total amount control framework of land and sea coordination needs to be further improved.Under the condition of limited stations,hydrological and water quality monitoring data,it is difficult to establish a watershed pollution source apportionment model based on mechanism process,especially in plain urban areas with complex river networks,the model needs to be adaptively modified.In addition,the water quality objectives of the sea area determined according to the marine or environmental functional zoning sometimes have too large differences between the adjacent functional areas.Therefore,this study focused on the shortcomings of the two key technical links,proposed the total amount control framework of land and sea coordination,improved the methods of pollution source apportionment and water quality objectives formulation,and carried out the application verification in Tianjin,by adopting the whole chain research idea of"basic theoretical study-key technological innovation-integrated application and demonstration"and comprehensively using the methods of literature research,mechanism analysis,field investigation,spatial analysis and model simulation.The main research contents and conclusions of this paper are as follows:1)Based on the SPAtially Referenced Regression On Watershed attributes(SPARROW)model and export coefficient model(ECM),a watershed pollution source apportionment model(ECM-SPARROW)was constructed.Firstly,based on the systematic analysis of the principle and structure of SPARROW model,the sources and emission paths of terrestrial nitrogen and phosphorus pollutants,the model pollution source index system composed of mandatory indexes and alternative indexes is constructed.The calculation method of total nitrogen(TN)and total phosphorus(TP)emissions of industry,sewage,fertilizer application,livestock and poultry breeding,aquaculture and rural communities based on ECM model was established.It was proposed to unify the nitrogen and phosphorus pollution sources to the"emission"node,and then merge the pollution sources with the same or similar transportation process to reduce the pollution source parameters required in the model.Then,according to the fact that different types of land carry different socio-economic activities,the spatial analysis method of pollutant emission was studied and established,and the coupling and integration of ECM model and SPARROW model was realized.Finally,taking 2013 as the benchmark year,the model application verification was carried out.TN and TP emissions from different sources were calculated using ECM model,and TN and TP fluxes at 26 river sections were calculated applying multiple linear regression model(LOADEST)and time flux method,constructing the ECM-SPARROW model of TN and TP in Tianjin.The results showed that the TN and TP models have good simulation results,the correlation coefficient R~2 was both above 0.99,and the root mean square error(RMSE)was 0.189 and 0.229,respectively.Using the pollution source merge method and the ECM-SPARROW coupling integration model can reduce the number of measured stations for modeling and expand the application scope of the SPARROW model.2)According to the application in plain urban area,the improvement of ECM-SPARROW model was studied.Firstly,in view of the unreasonable watershed division results caused by the crisscross river networks,the principles and methods of basin division in plain urban areas were established.Secondly,according to the situation of river dams,the confluence structure of river network in the model was improved,and a pseudo reservoir was added in the dam area to simulate the retention of nutrients.In addition,in order to identify the contribution of upstream input and locally generated pollutants into the sea,avoiding whole basin simulation and reduce modeling workload,a pseudo sub-basin was set at the entry section,which was regarded as a point source and added to the river network confluence structure.After the above three improvements,the model was successfully applied in Tianjin,established a massive list of pollution sources,clarified the sources and spatial distribution of pollutants,and defined key regions and sources.In 2013,the total amount of TN and TP entering the sea in Tianjin was 21320 t and 1504 t,respectively,of which the contribution rate of upstream(Hebei and Beijing)input accounted for 39%and 33%,respectively.The contribution rate of TN and TP in local Tianjin to the sea from the sewage discharge was larger,accounted for 34%and 21%,respectively.The influencing factors of pollutant transport in land and water areas were quantitatively analyzed.The TN retention rates in land and water areas in Tianjin were 16.7%and 50.4%,respectively,and TP retention rates in land and water areas were 34.5%and 58.8%,respectively.The average attenuation efficiency of TN and TP in the river reach with dams was 16.3%and 17.9%,respectively.It was suggested to reasonably dispatch water resources in the basin,enhance the water cycle between rivers and wetlands,and make full use of the retention and attenuation of pollutants in land and water areas to reduce pollution to sea areas.3)Based on the generalized additive model(GAM),the"classifying,zoning,staging and grading"setting method of water quality targets in coastal waters was established.Taking the application in Tianjin as an example,firstly,based on the data of nutrient concentration and precipitation in Tianjin coastal waters from 2007 to 2018,the GAM model of water quality was established to quantitatively analyze the change trend of dissolved inorganic nitrogen(DIN)and soluble reactive phosphate(SRP)concentration,and separate the contribution of precipitation to the change trend of water quality.Among the 12 stations in the coastal waters of Tianjin,DIN generally showed a downward trend,with an average decline of 13.19%,of which the contribution of precipitation was 5.89%.SRP showed an overall upward trend,with an average increase of 7.01%,which was almost unaffected by precipitation.Finally,based on the trend analysis and prediction of DIN and SRP concentrations,and the water quality target setting method established in this study,it was proposed that the control target of excellent water(Grade I andⅡ)quality in the 14th Five-Year Plan reached 75%,and analyzed the rationality of this target.The coastal waters of Tianjin was divided into 7 areas,and it was suggested to implement zoning control according to the control level.The method established in this study overcomes the shortcomings of determining the sea water quality target mainly based on marine or environmental function zoning of coastal waters,and provides a scientific basis for accurate pollution control.