Source Apportionment Of Nitrogen And Phosphorus Pollutants And Management Strategy In Beishahe Sub-catchments

The Beishahe River,which is in the northwestern suburb of Beijing,flows into the Shahe Reservoir and then enters the Beiyunhe River.The water quality of Beiyunhe River is majorly affected by the migration and transformation of nitrogen and phosphorus pollutants across the Beishahe sub-catchments.The study of migration and transformation include non-point source pollution and internal pollution of river channels,making the research process of nitrogen and phosphorus pollutants complicated.And since the Beishahe sub-catchments lacks perennial hydrological monitoring,study of nitrogen and phosphorus pollutants becomes even more challenging.This study explored the migration and transformation of nitrogen and phosphorus pollutants from both external and internal sources to solve the problems of Beishahe sub-catchments.The study of external pollution includes non-point source pollution migration and point source pollution emission.For the estimation of non-point source pollution,this paper proposes a method based on Arc GIS coupled SCS-CN model and RUSLE model.This study calculated the base flow to estimate the point source since secondary effluent is the major water source for Beishahe sub-catchments.The study of internal pollution includes nitrogen and phosphorus estimation of the potential release from both the dead aquatic plants and sediment.For the estimation of plant biomass,this study took reed release as the the model plant.The results show that the main pollution source of the Beishahe sub-catchments was external pollution which accounted for more than 92%of the total pollution.Among the dissolved parts of the external pollution,the total nitrogen pollutants from the point source accounted for 3 1.32%of the total dissolved nitrogen,and the total phosphorus pollutants from the point source accounted for 14.61%of the total dissolved phosphorus.Dissolved nitrogen pollutants accounted for 63.37%of non-point source nitrogen pollutants,and adsorbed nitrogen pollutants accounted for 36.63%.Dissolved phosphorus pollutants accounted for non-point source phosphorus pollution.40.98%of the non-point source phosphorus pollutants and the adsorbed phosphorus pollutants accounted for 59.02%.Therefore,non-point source pollution is the most important part of the external pollution of the Beisha River.Dissolved nitrogen pollutants and adsorbed phosphorus pollutants are the main sources of non-point source nitrogen and phosphorus pollution,respectively.In the internal sources,the potential release of decaying aquatic plants is nearly 10 times that of sediment.The source analysis of non-point source pollutants shows that the dissolved nitrogen in non-point source pollutants is mainly from cultivated land,followed by urban construction land,grassland and rural residents,and the contribution of forest land is the smallest;dissolved phosphorus is mainly from rural residents.The arable land,garden land and urban construction land are second,the contribution of forest land is also the smallest.The amount of migration of dissolved pollutants is highly correlated with runoff.Adsorbed non-point source pollutants mainly come from areas with severe soil erosion-mountain and hilly areas,and 99%of the study area is slightly soil erosion.For the transformation of pollutants,both aquatic plants and sediments can be "source" and"sink" of pollutants.Aquatic plants can absorb nitrogen and phosphorus during the growing period and thus remove nitrogen and phosphorus pollutants from the water.However,the potential release of nitrogen pollutants from dead aquatic plants is half of the point source emissions compared to the point source nitrogen emissions.The potential release of phosphorus pollutants is comparable to the discharge of point source,the amount is similar.The sediment was a sink of external non-dissolved pollutants.In this study,it was found that the pollutant was released into the water under the effects of the disturbance.In addition,total phosphorus pollutants could be re-adsorbed by the sediment,while total nitrogen pollutants were not.Among the factors affecting the release of nitrogen and phosphorus from the sediment,there was no significant effect with pH,and dissolved oxygen and temperature were negatively correlated with the release amount.The conclusions and research methods in this study can be applied to nutrient management in hydrological data-scarce sub-catchments with similar natural geography and socio-economics.