Analysis and Research on the Pollution of Lakes in Beijing

This article analyzes the pollution status of 22 key lakes in Beijing in the past 10 years. The results show that from 2010 to 2014, the proportion of lakes in Beijing’s Class II water bodies was 17.4%, and the average proportion of lakes in Class III water bodies from 2015 to 2020 was 29.2%; the decline in the proportion of Class V water bodies from 2016 to 2020 was more obvious. Water bodies inferior to Class V have been basically eliminated, but in 2019, many lakes have been in a state of moderate eutrophication. At the same time, the analysis during the year showed that the proportion of Type II water bodies was low from June to September, the lowest value was 11.7% in August, and the proportion was relatively high in February and March, and the highest value was 40.5% in March; In August, the proportions of Category IV and V water bodies reached the highest values of 33% and 24% respectively, which is related to the increase in summer water temperature and the shortage of water resources that can supply lakes. The research results are expected to provide support for the improvement of urban lake water environment.

lakes that use the Jingmi Water Diversion Canal as the water source are Shichahai, Tongzi River, Zizhuyuan Lake, Honglingjin Lake, Olympic Lake, Liuyin Park Lake, Qingnian Lake, Tuancheng Lake, Kunming Lake and Exhibition Hall Houhu; Yongding River is used as a water source. The lakes whose canals are the water source include Yuyuantan Lake, Bayi Lake, Lotus Pond, Taoranting Lake, and Longtan Lake.
a Shichahai b Tongzi River c Yuyuantan Lake d Bayi Lake e Lotus Pond f Zizhuyuan Lake g Taoranting Lake h Longtan Lake i Red Scarf Lake j Olympic Lake k Liuyin Park Lake l Youth Lake m Tuancheng Lake n Kunming Lake o Exhibition Hall Houhu

.1. Interannual variability
The pollution status of lakes in Beijing from 2010 to 2020 is shown in Figure 2 and Figure 3. The number of lakes in Class II water bodies is on the rise, accounting for an average of 17.4% before 2015 and 29.2% after 2015. The percentage of lakes in category III water bodies varies from 5.9% to 64.7%. In 2013, the annual average value of category II water bodies was 16.5%, and the highest in 2019 was 34.7%. The overall trend was first down and then up, with an average of 26.6%. The proportion of lakes in Category IV and V water bodies showed a significant downward trend. From 2010 to 2016, the average proportions of Category IV and Category V water bodies were 28.2% and 19.2% respectively. From 2017 to 2020, and the proportions of Category IV and Category V water bodies were 22.3% and 12.5% respectively. According to the evaluation data of the nutritional status of 22 lakes in Beijing released by the Beijing Municipal Water Affairs Statistical Yearbook, a statistical analysis of the changes in the nutritional status of 22 lakes from 2016 to 2019, including Bayi Lake, Yuyuantan Lake, Beihai, Zhonghai, Nanhai, Tongzihe, Qingnian Lake and Olympic Lake have been in a state of mild eutrophication. Tuancheng Lake, Kunming Lake and Exhibition Hall Houhu Lake have better water quality, and they are all in a mesotrophic state. From 2016 to 2018, most of the 22 lakes will be monitored for mild eutrophication. In 2019, Taoranting Lake, Chaoyang Park Lake, Lianhua Pond and Liuyin Park Lake were in a moderate eutrophication state. According to research and analysis, the ecological environment water consumption in 2019 was the highest value from 2010 to 2019 of 1.466 billion m 3 , but the surface water resources were only 861 million m 3 , and the water transferred from the South-to-North Water Diversion Project was 985 million m 3 . Considering the evaporation and leakage of rivers and lakes, it is difficult for the upstream water to meet the needs of ecological environment, resulting in poor hydrodynamic conditions of the lakes, long replacement cycles, and overall deviations in the quality of the water environment.

Changes during the year
The annual changes in the pollution status of Beijing's lakes in the past 10 years are shown in Figure 3. The percentage of lakes with Type II water bodies was low from June to September, and the lowest value was 11.7% in August. The percentage was relatively high in February and February, and the highest value was 40.5% in March; the proportion of Category IV and V water bodies accounted for a relatively high proportion from June to September, and the highest values were 33% and 24% in August. After analysis, temperature, as an important physical feature of lakes, has an impact on the survival, growth and spatial distribution of lake organisms [5][6][7][8] . The increase in summer temperature promotes the decomposition of humus of submerged plants by heterotrophic microorganisms and causes sediment humus. The degree of chemistry increased [9] . At the same time, the increase in temperature has an important impact on water quality indicators. The increase in temperature will increase the release of phosphorus from the sediments. Studies have shown that when the water temperature increases by 3-4°C, the release of P will double [10] . At the same time, temperature will also participate in the whole process of algae recovery and growth. As the water temperature rises, algae will multiply, and lake eutrophication will increase [11][12] .

Analysis on the Causes of Urban Lake Pollution in Beijing
In order to preliminarily explore the response relationship between the pollution status of Beijing's urban lakes and hydrological water quantity, the annual precipitation, surface water resources, available water inflow from Miyun Reservoir, available water in Guanting Reservoir, and water inflow from the Southto-North Water Diversion Project are now calculated. The ecological environment water consumption, reclaimed water utilization and river and lake replenishment water consumption are counted, and the results are shown in Table 2 below.

Resource shortage
The clean water diversion project is an important measure to improve the quality of lake water environment and improve the self-purification ability of water bodies [13] . Beijing's urban lakes are mainly supplied by reclaimed water, Guanting Reservoir, Miyun Reservoir and South-to-North Water Diversion. Since 1999, Beijing has suffered continuous drought, with an average annual precipitation of only 475mm. Through the correlation analysis of the factors in Table 2 above, it is further found that the average annual precipitation in Beijing from 2010 to 2019 is significantly positively correlated with the amount of surface water resources ( P<0.01). The reason is that Beijing is located in the northern part of the North China Plain and belongs to a temperate continental monsoon climate. Summer precipitation is strong and fluctuating. Table 2 shows that the "7.21" heavy rain in 2012, with a maximum rainfall of 708 mm; however, the rainfall in 2014 The volume is only 439mm. At the same time, there is a serious shortage of surface water resources. Since 2014, the South-to-North Water Diversion Project has supplied water to Beijing, reducing the output of Miyun Reservoir by more than 50,000 m 3 every year [14] . In 2014, the usable inflow of Miyun Reservoir and Guanting Reservoir was reduced to 159 million m 3 and 46 million m 3 , respectively. Correlation analysis showed that the ecological environment water demand was significantly negatively correlated with the available water volume of Miyun Reservoir and the water transferred from the South-to-North Water Diversion Project (P<0.01), and the ecological environment water consumption was significantly positively correlated with the recycled water utilization (P<0.01). Almost all of the ecological environmental water demand is used for replenishment of rivers and lakes. Therefore, the replenishment of urban lakes in Beijing mainly comes from recycled water. The water resources of Guanting Reservoir and Miyun Reservoir are becoming increasingly scarce, and the demand for reclaimed water as an important source of water supply for Beijing's lakes is increasing year by year.

Single hydrodynamic conditions
A certain level of water fluidity is a necessary guarantee for the health of lakes. Insufficient water in the upper reaches of urban lakes leads to long water exchange cycles in lakes. Under suitable meteorological conditions, slower flow speeds slow the diffusion of local nutrients in the water body, which easily leads to eutrophication of the water body. The occurrence of chemical state [15] . At the same time, the low flow rate can promote the increase of water temperature and lengthen the hydraulic retention time, on the other hand, the living environment of the algae is relatively stable under the low flow rate [16] . Zhou Jing [17] found that low flow rate is more conducive to the growth of algae, and the growth of algae is inhibited at high flow rate. Zhao Ying [18] found that when the flow velocity is greater than 0.5m/s, the flow velocity has a significant inhibitory effect on algae.

Non-point source impact
In recent years, there have been many studies on the impact of heavy rains on lake water quality. On the one hand, the initial rainwater will carry a large amount of acid gases in the air, car exhaust and other polluting gases into the river, and it will also cause the lake's pH value to gradually drop. Threat [19][20][21][22] , and the lake pH value will also control the circulation of sediment phosphorus [23][24][25] . On the other hand, through the analysis of monthly precipitation and lake water quality in Beijing in 2012, there is a significant positive correlation between the number of lake water bodies in Category V and precipitation (P<0.05). It can be seen from Figure 2 that the analysis of the relationship between the monthly precipitation and lake water quality from 2010 to 2020 shows that the water bodies of Category IV and V accounted for a relatively high proportion during the flood season (June to September).
At the same time, extreme rainstorms will increase the concentration of water particles, affecting hundreds of square kilometers [26] . Nutrients carried by rainstorm runoff pose great challenges to the control of lake eutrophication [27] . On the other hand, in recent years, the amount of sewage discharge in Beijing has been increasing, and the combined rain and sewage pipelines still account for a large proportion. The problems of the dissolution of urban road garbage and dust and the overflow of sewage into the river are serious.

Improve relevant laws and regulations
The prevention and control of pollution in rivers and lakes not only requires a concentrated demonstration of public morality and responsibility from all walks of life in society, but also requires strong guarantees of national laws and regulations. Raise sewage discharge standards and strictly implement the penalty system; in accordance with the water quality requirements of different water function zones, the pollution-holding capacity of water bodies shall be verified, and different receiving standards shall be formulated for different rivers and lakes. Strengthen the government's ability to manage and control rivers, strengthen the responsibilities of the government and enterprises, and establish a sound management and control system.

Strengthen multi-departmental coordination and management
Implement the "river chief system" and "lake chief system", establish a water environment monitoring and evaluation system, implement multi-departmental joint management and control, coordinate reclaimed water and upstream water quality management ideas and guidelines; give full play to the "river chief + sheriff + inspector" linkage Mechanism, work together to rectify the problem of water pollution. Establish an interoperable river management platform and strengthen the close cooperation between the river management office and the government. Ensure that enterprises meet green emission standards, control agricultural non-point source pollution, and reduce the input of nitrogen and phosphorus in rivers.

Improve the treatment level of sewage treatment plants
With the rapid development of my country's industry and the continuous improvement of people's living standards, the composition of various sewage and wastewater has become more complex, and my country's comprehensive sewage discharge standard has also been upgraded from (GB8978-1996) to (DB11/307-2013). However, it cannot solve the contradiction between today's social development and the ecological environment. In order to better meet the requirements of ecological civilization construction and earnestly implement pollution prevention and control, it is necessary to strengthen the supervision and management of sewage treatment plants by government departments, and to Continue to increase efforts to upgrade standards, innovate sewage treatment technology, raise sewage discharge standards, and improve sewage treatment capacity.

Strengthen research on the mechanism of water eutrophication
The eutrophication of water body is affected by multiple factors. In the process of eutrophication research, it is necessary to comprehensively consider the interaction between hydrometeorological factors, hydrodynamic conditions, the influence of submerged plant adsorption and allelopathy, and the release of nutrients in sediments. Interaction relationship, in-depth multi-disciplinary research, to explore the mechanism of algal blooms, establish a water bloom early warning model, so as to do a good job in the prevention and control of water eutrophication.

Enhancing the diversity of water ecosystems
The achievement of water environmental governance standards should be based on the diversity of the water ecosystem as an assessment indicator, and the health of the water environment should be the ultimate goal. Therefore, on the basis of the overall treatment of source control and pollution interception and ecological restoration, precise measures are taken on polluted rivers, and on the basis of controlling the concentration of nitrogen and phosphorus in the upstream water, aquatic plant belts are constructed to increase the types of benthic organisms and create complex The aquatic ecosystem realizes the natural succession of algae and aquatic plants, while avoiding the overgrowth of a single species, and constructing a biological chain structure of "plankton-plankton-benthic organisms-fish" to improve the self-purification ability of rivers and lakes and realize shore green The situation is clear.

Conclusion
With the advancement of my country's ecological civilization construction and the implementation of environmental protection related policies, the pollution situation of urban lakes in Beijing has improved significantly in the past 10 years. The number of lakes in Class II water bodies has increased significantly, and the number of lakes in Class IV-V water bodies has decreased significantly. As of 2019, the monitored area of lakes in Beijing is 719.6hm 2 , the area that meets the water quality standards of II-III is 535.6hm 2 , and the area that meets the water quality standards of IV-V is 184.0hm 2 . However, in recent years, the number of moderately eutrophic lakes has been on the rise, and the pollution status of urban lakes is not optimistic. Therefore, comprehensive consideration of the direct or indirect factors related to lake health and the establishment of a sound water environment management system are of great significance to the improvement of the water environment quality of urban lakes.