Spatiotemporal Variations In Water Quality And Nitrogen And Phosphorus Pollution Sources:A Case Study Of Gaoyou Lake And Its Inflowing And Outflowing Rivers

Lakes are one of the most important sites to maintain regional water balance,protect biological resources,and preserve biodiversity.Gaoyou Lake is an important freshwater lake in Jiangsu Province,under the jurisdiction of four districts,with many rivers flowing into the lake having varying degrees of impact on the water quality of the lake area.Nitrogen and phosphorus pollution are important factors leading to eutrophication of lakes,and Gaoyou Lake has long been plagued by excessive total phosphorus.Therefore,enhancing water quality management in the watershed of Gaoyou Lake,especially controlling the concentration of nitrogen and phosphorus pollutants,is an extremely important environmental management task.This paper explores the differences in water quality of Gaoyou Lake and its inflowing and outflowing rivers over the course of four years of continuous monitoring.Regarding the excessive total phosphorus in Gaozyou Lake,different types of nitrogen and phosphorus pollution in inflowing rivers and lakes were identified.The contribution rates of various pollution sources to nitrogen and phosphorus in the lake area were calculated.The impact of endogenous pollution and fisheries aquaculture on the water environment quality of Gaoyou Lake was also explored through the monitoring of sediment and net cage aquaculture area water quality.Finally,based on the relevant research results,the actual conditions and management status of Gaoyou Lake were considered,and several pollution prevention and control measures related to water quality management of Gaoyou Lake were proposed,providing theoretical support for subsequent water quality improvement and protection.The main conclusions of this study are as follows:(1)In several inflowing rivers and the central area of Gaoyou Lake,the vast majority of pollutants,such as permanganate index,dissolved oxygen,and total nitrogen,have reached the surface water environmental quality standards.Total phosphorus can also meet the surface water total phosphorus standard in most months,and even the lake and reservoir total phosphorus standard in some months,but there is non-compliance in July to October due to high total phosphorus levels.In 2021,Wangqiao River had the most severe total phosphorus pollution with an average annual concentration of 0.13 mg/L,followed by Tonglong River.In 2022,Baita River had the most severe total phosphorus pollution with an average annual concentration of 0.12 mg/L.From the perspective of the regions where the inflowing rivers are located,Tianchang district has the most severe river pollution,followed by Baoying district,and the pollution level in Jinhua district is lighter.(2)The water quality of Gaoyou Lake is greatly affected by the water quality of the inflowing rivers.Specifically,the changes in total nitrogen and total phosphorus concentrations in Gaoyou Lake are basically consistent with the changes in total nitrogen and total phosphorus in the inflowing rivers.The correlation coefficient between the mean total phosphorus of inflowing rivers and the monthly concentration of total phosphorus in the lake center region is 0.29,with a significance level of 98%,indicating a significant positive correlation.Among all inflowing rivers,Baita River has the greatest contribution rate to the total phosphorus concentration in the lake center region.(3)Based on the results of the absolute principal component-multiple linear regression analysis,the pollution sources in Gaoyou Lake can be categorized into five known sources:domestic,agricultural,industrial,urban,and meteorological sources.The representative pollutants for these five sources are total phosphorus,total nitrogen,COD,ammonia nitrogentotal phosphorus,and dissolved oxygen-pH.The pollution sources also include an unknown source,identified as internal pollution.Further analysis through multiple linear regression has revealed that in 2019,internal pollution had the greatest contribution to water pollution in Gaoyou Lake,accounting for 75.08%of the contribution to total nitrogen and 44.17%of the contribution to total phosphorus.In the following three years,nitrogen and phosphorus pollution were mainly influenced by a combination of agricultural and domestic sources.Agricultural sources contributed more to total nitrogen,whereas domestic sources contributed more to total phosphorus.In 2020 and 2021,agricultural sources contributed 53.19%and 62.39%to total nitrogen,respectively,while domestic sources contributed 44.59%and 56.69%to total phosphorus during the same period.Meteorological factors and industrial sources were not the primary contributors to total nitrogen and total phosphorus,but they still had some influence on these pollutants.(4)According to the results of absolute principal component-multiple linear regression analysis,the known sources of pollutants in Gaoyou Lake can be simply summarized as follows:The sources of nitrogen and phosphorus pollution in Gaoyou Lake can be divided into the following five categories:domestic non-point source,agricultural non-point source,urban non-point source,meteorological factor and sediment.The representative components of these five types of pollution sources are respectively total phosphorus,total nitrogen,COD,ammonia nitrogen-total phosphorus,dissolved oxygen-pH.In addition,the source of pollutants also includes an unknown source,which is identified as internal pollution based on the actual situation and literature research results.It is found that agricultural non-point source pollution has a great contribution rate to total nitrogen and total phosphorus index,reaching nearly 70%.Meteorological factors contribute more to total nitrogen,urban non-point sources contribute more to total phosphorus,and domestic non-point sources and sediment also contribute to total nitrogen and total phosphorus to a certain extent,but the impact is less than the above three.It is found that agricultural non-point source pollution has a great contribution rate to total nitrogen and total phosphorus index,reaching nearly 70%.Meteorological factors contribute more to total nitrogen,urban non-point sources contribute more to total phosphorus,and domestic non-point sources and sediment also contribute to total nitrogen and total phosphorus to a certain extent,but the impact is less than the above three.(5)Due to the temporal and spatial variations of water pollution in the Gaoyou Lake basin,as well as the complexity and diversity of nitrogen,phosphorus and other key pollutants sources,it is recommended that the environmental management department integrate the existing monitoring sections and establish a long-term mechanism for monitoring the water environment in the lake area.In addition,it is suggested to establish a multi-department joint management system to standardize the management of the lake basin,and increase the governance of nitrogen and phosphorus key pollution sources,and strictly control the total amount of pollutant emissions.