Distribution Characteristics Of Nitrogen And Analysis Of Pollution Sources In Miyun Reservoir

Miyun reservoir is an important centralized drinking water source in the capital,and water quality safety is very important.In recent years,the relevant departments have strengthened the environmental management measures of the reservoir,and the water quality of the reservoir has been improved,but the total nitrogen index has always exceeded the class II standard.At present,the reservoir is in the state of mesotrophic,while has the trend of eutrophication.Taked Miyun Reservoir and its inflow river as the study area,then collected water and sediment samples.The nitrogen,p H,water temperature,dissolved oxygen and other indicators water samples were determined.This paper has measured the nitrogen forms and distribution characteristics of nitrogen in sediment of Miyun reservoir and estimated the nitrogen diffusion flux of nitrogen at sediment boundary by Fick’s first law.The stable isotope technology and SIAR model were used to analyze the source of nitrogen pollution in water body qualitatively and quantitatively.Meanwhile,the source of organic matter in sediment is studied to understand the source of nitrogen in water reservoir further.The main conclusions are as follows:（1）NO₃^--N is the main form of nitrogen in Miyun reservoir.The concentration of nitrogen nutrients in the east of the reservoir is higher than that in the west,and the concentration at the river inlet is higher than that in the middle of the reservoir.The main inflow river of Miyun reservoir,Baihe River,inputs 26.86t/month of total nitrogen,and Chaohe River inputs 34.84t/month of total nitrogen.The inflow river of the reservoir causes greater nitrogen load on the reservoir.（2）The nitrogen content in the surface sediments of the reservoir is characterized by surface enrichment.The total nitrogen content in the surface sediments of the reservoir is804.89～3151.32mg/kg.Organic nitrogen is the main form of total nitrogen in the sediments,and the nitrogen content in the upper sediments is higher than that in the lower sediments.There is a significant correlation among ON,NH₄⁺-N and NO₃^--N,which indicates that ON,NH₄⁺-N and NO₃^--N in sediments may have similar sources.（3）The reservoir is at a potential risk of endogenous NH₄⁺-N release.NH₄⁺-N is the main component of sediment interstitial water.NH₄⁺-N is released from the sediment to the overlying water at the sediment water interface of the reservoir,and the diffusion flux ranges from 1.11 mg/m²·d to 17.17 mg/m²·d.NO₃^--N mainly diffuses from the overlying water to the interstitial water,and the diffusion flux ranges from-2.67 mg/m²·d to0.71mg/m²·d（4）Stable isotope and SIAR model were used to analyze the source of nitrogen pollution.The results show that the main sources of nitrogen pollution in the reservoir area are domestic sewage and manure,with the contribution rate ranging from 22.27%to39.95%.The sources of water pollution in the reservoir area include domestic sewage and manure and agricultural fertilizer,with the contribution rate ranging from 20.64%to29.38%,and the contribution rate of nitrate nitrogen fertilizer ranging from 18.29%to23.81%.（5）The main source of organic matter in reservoir sediments was studied by carbon and nitrogen isotope method.The endmember diagram shows that the main sources of organic matter in sediments are composed of aquatic vascular plants and soil organic matter.The organic nitrogen in sediments is affected by both internal and external factors.