Water quality in Beijing, China

1.     Water quality in Beijing, China

1.1 Current status and long-term changes in water quality

Rivers

Beijing's water quality monitoring data in recent years show a trend of continuous improvement in the quality of rivers and reservoirs. The Beijing Municipal Bureau of Ecology and Environment (BBE) 2023 monitored a total of 105 segments of rivers in the five major water systems, with a length of 2,551.6km. Among them, 71.3 % of the total number of river chiefs were of water quality categories I-III; there were no rivers of poor V category. Compared with 2019, the proportion of category I-III river chiefs increased by 16.2 percentage points, and the proportion of poor category V river chiefs decreased by 9.5 percentage points. The main pollution indicators of category IV and V rivers are chemical oxygen demand (COD), biological oxygen demand (BOD) and ammonia nitrogen (AN) (Beijing Municipal Bureau of Statistics, 2023).

As an example, the Beiyun River basin is the most populated, industrially concentrated and urbanised basin in Beijing (Li et al., 2014), and is a typical urban river. The Beiyun River The environmental quality evaluation index (QHEI) of the habitat of Beijing's Beiyun River system showed a trend of gradual increase from the middle reaches to the upper reaches and lower reaches; biochemical oxygen demand (BOD5) and total nitrogen (TN) were basically at a low level, and the overall condition was good; the benthic fauna Shannon-Wiener diversity index (B-H) was basically at a low level, and the overall condition was poor; dissolved oxygen (DO), plankton Shannon-Wiener diversity index (P-H and Z-H), bank slope erosion index (ISE), and water-friendly landscape comfort (DWLC) were relatively distributed spatially. (DO), the Shannon-Wiener diversity indices of plankton (P-H and Z-H), the indicator of slope erosion (ISE), and the degree of comfort of the hydrophilic landscape (DWLC) were spatially distributed in a relatively fragmented manner, with high and low levels, and greater spatial heterogeneity (Gu, 2018).

Similarly, the monitoring results of 17 sections of the Haihe River basin in Beijing in August 2024 showed that the ammonia nitrogen content ranged from 0.025 to 3.398 mg/L, the total phosphorus content ranged from 0.005 to 0.286 mg/L, and the total nitrogen ranged from 1.05 to 11.85 mg/L.

Lakes

The Beijing Municipal Bureau of Ecology and Environment (BBE) 2023 monitored a total of 22 lakes throughout the year, with a water surface area of 7,196,000 square metres. 58.3% of the total water surface area was of water quality class I-III, 41.7% of the total water surface area was of water quality class IV-V, and there were no lakes of inferior V class. Compared with 2013, the proportion of category I-III increased by 54.3 percentage points, and the proportion of poor category V decreased by 15.0 percentage points, and the proportion of category I-III decreased by 2.9 percentage points and the proportion of poor category V decreased by 2.7 percentage points compared with 2019. Compared with 2019, the proportion of Class I-III decrease by 2.9 percentage points, and the proportion of Poor V decrease by 2.7 percentage points. The main pollution indicators of Class IV and V lakes are total phosphorus and chemical oxygen demand (Beijing Municipal Bureau of Statistics, 2023).

The water quality of Beijing's lakes still needs to be improved. The main pollution indicators of the lake are total phosphorus and chemical oxygen demand (COD) (Wu et al., 2023).

For the Kunming Lake, in recent years there has been a trend of eutrophication in the waters of Kunming Lake, especially in the southern part of the lake, where there have been outbreaks of algae growth (Li et al., 2007).

The Tuan Cheng Lake, a Drinking Water Source, Tuancheng Lake regulating pool water source from the South-to-North Water Diversion Project, water quality is excellent, but in recent years there are high manganate index, chemical hydrogen demand and other indicators compared to the incoming water rise, excessive growth of algae phenomenon (Zhao, 2022).

The ‘Six Lakes’ is one of the most important scenic spots in Beijing, consisting of the Xihai Lake, the Houhai Lake, the Qianhai Lake, the Beihai Lake, the Zhonghai Lake and the Nanhai Lake. In recent years, with socio-economic development, the ecosystems of the six lakes have undergone serious changes as a result of insufficient water supply, man-made disturbances, destruction of aquatic systems, shallow depths of water, lack of mobility, poor water quality and frequency of waterwaters, and slow renewal of water bodies. These conditions impair the health of the six lakes and severely impact the function of the landscape. Poor influent water quality and non-point pollution (e.g., domestic sewage, storm water runoff, sediment releases, fishing, etc.) are the primary causes of water quality degradation in the six lakes (Zhang, 2008).

For the Tongzi River, due to consecutive droughts in Beijing, the amount of water replenished from the upper reaches has been reduced, the problem of eutrophication in the water body has become more and more prominent, and the phenomenon of water bloom has appeared in the summer and autumn seasons, which seriously affects the normal functioning of the water body. In order to effectively improve the water quality and enhance the landscape value of the water body under the condition of existing water resources, the water quality improvement project of the Tongzi River will be implemented in phases, and 54 underwater push-flow devices are to be installed to improve the status of the water body of the Tongzi River by increasing the flow of the water body.

The water quality environment of lakes in Taoranting Park is directly affected by the South Moat replenishment source, and its water body is mainly reclaimed water and a small amount of natural water body, the reclaimed water replenishment is Gaobeidian reclaimed water, which belongs to the quasi-IV quality of surface water (Wang et al., 2022).

Reservoirs

A total of 16 large and medium-sized reservoirs were monitored throughout the year, with an average total storage volume of 3.77 billion m³; water quality of categories I-III accounted for 100 % of the total storage volume. Compared with 2013, the proportion of Class I-III water increased by 12.3 percentage points. The proportion of Class I-III water increased by 12.3 percentage points compared with 2013, and the proportion of Class I-III water increased by 14.8 percentage points compared with 2019. The water quality of Miyun Reservoir and Huairou Reservoir stably maintains Class II, which is in line with the water quality standard for drinking water sources. The water quality of Guanting Reservoir is Class III, and the water quality continues to improve (Beijing Municipal Bureau of Statistics, 2023).

The Miyun Reservoir is one of the largest reservoirs in northern China. The upstream of Miyun Reservoir has a fragile ecological environment and serious soil erosion, which can damage the soil surface, reduce soil fertility, and silt up river channels (Wang et al., 2017). At the same time, water serves as a carrier, bringing large quantities of nitrogen, phosphorus and other nutrients to downstream water bodies, leading to excessive pollutants within the reservoir. Although the Miyun area has strictly limited the development and use of soil and water resources in order to protect the Miyun Reservoir and its surroundings, socio-economic activities still have a considerable impact on the environment around the reservoir, posing a threat to the supply of drinking water from the reservoir (Qiao et al., 2021).

1.2     Emerging pollutants in surface water in Beijing

In recent years, Beijing's water bodies have been threatened by a variety of emerging pollutants (EPs), mainly antibiotics, microplastics and persistent organic pollutants (POPs). These pollutants have become a focus of environmental research due to their difficult degradability and potential ecological risks.

Antibiotic contamination

Antibiotics, a class of drugs that kill or inhibit the growth of bacteria, have been recognised in recent years as an important emerging environmental pollutant. Antibiotic parent compounds and their metabolites have long-term stability in the environment and can bypass conventional water treatment processes and end up in water bodies (Kümmerer, 2009). Antibiotic pollution not only leads to the development of bacterial resistance, but also poses a threat to aquatic ecosystems and human health (Li et al., 2015; Jiang et al., 2011). The total amount of antibiotics used globally is 100,000 to 200,000 tonnes per year, and China uses more than 25,000 tonnes of antibiotics per year, making it one of the countries with the highest antibiotic use in the world (Jiang et al., 2011). The main sources of pollution from antibiotics include animal husbandry, aquaculture, medical wastewater treatment and domestic sewage discharges (Lu et al., 2019).

Antibiotic contamination of surface water is more serious in Beijing. A total of 22 antibiotics were found in urban surface water samples from rivers and lakes in Beijing in 2014, of which sulphonamide antibiotics (SAs) and quinolone antibiotics (FQs) were the main pollutants, with average concentrations of 132 ng/L and 136 ng/L, accounting for 42.8 % and 41.6 %, respectively, of the total amount of antibiotics detected (Wang et al., 2005). By 2021, a total of 33 antibiotics have been detected in 200 rivers, including 16 antibiotics with a detection rate of more than 50 %, and 100 % of sulfadiazine, methoxybenzamidopyrimethamine, quinclorac antibiotics and ofloxacin (Priyadarshini et al., 2021). A wide variety of antibiotics were found in the drainage water of Beijing sewage treatment plant, and the average concentrations of antibiotics such as sulfamethazine, sulfadiazine, and ofloxacin in the influent treatment of the sewage treatment plant were 1.20±0.45, 0.29±0.25, 0.048±0.012, 0.35±0.52, and 0.33±0.21 μg/L, and in the effluent treatment, they were 1.40±0.74, 0.22±0.19, 0.021±0.008, 0.22±0.21 and 0.01±0 μg/L at the effluent treatment (Chen et al., 2017). it is evident that the antibiotics in the effluent are mainly sulphonamide antibiotics.

Microplastic pollution

Microplastics are solid plastic particles less than 5 mm in diameter (UNEP, 2015). Microplastics can be divided into primary microplastics, which are tiny particles manufactured by industrial production, and secondary microplastics, which are formed by the decomposition of larger plastic objects (Issac et al., 2021). Microplastics pose a potential threat to ecosystems due to their small size and difficult degradation.

The Qinghe River, a typical urban river in Beijing, is being affected by discharges from sewage treatment plants. The study showed that a total of 18 microplastic polymers were detected in water samples from the Qinghe River and its outfalls along the river, with polyethylene (PE) and ethylene propylene rubber (EPR) accounting for the highest percentages, 39.47% and 67.47%, respectively (Wang et al., 2020). This further confirms that WWTP discharge is one of the major pathways for microplastics to enter water bodies.

Persistent Organic Pollutants（POPs）

POPs are a class of highly toxic, difficult to degrade, bioaccumulative organic compounds commonly found in industrial production and agricultural activities (Bao et al., 2012). POPs pollution in Beijing mainly includes polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and organochlorine pesticides (OCPs) (Wang et al., 2005).

Organochlorine pesticides (OCPs).

OCPs were detected in the sludge from 12 sewage treatment plants in Beijing in the concentration range of 38.0-143.3 ng/g dry weight. A total of 15 OCP compounds, including hexachlorobenzene (HCB), four hexachlorocyclohexane (HCH) isomers (a-HCH, b-HCH, c-HCH, and d-HCH), three dichlorodiphenyltrichloroethane (DDT) congeners, and 15 OCP compounds, of which HCB and p,p0-DDE were the main pollutants, accounting for 7.0%-71.7% and 16.0%-85.0% of the total OCPs, respectively (Li et al., 2019). In addition, the concentration of OCPs in Baiyangdian 2016 sediment samples ranged from 2.25 to 6.07 ng/g dry weight, and the concentration of OCPs in water samples ranged from 2.62 to 6.13 ng/L (Liu et al., 2013).

Polychlorinated biphenyls (PCBs).

PCBs, a widely used industrial compound, are commonly found in wastewater sludge. PCB28 was the most detected PCB congener in sludge samples from Beijing wastewater treatment plants, accounting for 41.1% of the total amount (Li et al., 2019). The results show that although PCB pollution levels in Beijing are still below the limits set by Chinese regulations, they still pose a potential threat to the environment.

Polycyclic aromatic hydrocarbons (PAHs).

PAHs are toxic organic pollutants produced by burning fossil fuels and industrial emissions. The Chaobai River Basin is heavily polluted by PAHs and PAHs, and the pollution mainly originates from the Beiyun River receiving wastewater from Beijing's municipal wastewater treatment plant, making the Beiyun River a major source of risk for pollution in the Chaobai River. As of 2017, the population density around the Chaobai River Basin is 100 people, which is about 1,500 people per square kilometre, and population density is also a key factor in PAH pollution in the Chaobai River (Yao et al., 2017). In addition, incomplete combustion of biomass and fossil fuels is one of the most important sources of PAHs and feedstock hydrocarbons in northern China (Qian et al., 2017).

The concentration of PAHs in sludge from Beijing sewage treatment plants ranged from 445.1 to 3586.4 ng/g dw, with an average concentration of 1550.5 ng/g dw, suggesting that the impact of industrial activities on the pollution of Beijing's water bodies (Li et al., 2019). In addition, OCP, HCH and DDT concentrations in sediments from Guanting Reservoir ranged from 8.48-24.40 ng/g dw, 1.11-7.73 ng/g dw and 2.97-10.52 ng/g dw, respectively, and PAHs were detected in the water bodies of Baiyangdian in 2016 ranging from 71.32 to 228.27 ng/L, further suggesting that the region faces a pollution challenge (Wang et al., 2005).

2.Pollution sources and effects of polluted water on ecological and human health

2.1 Cause of water pollution

Industrial emissions and their waste

Chemical substances and heavy metals discharged from industrial activities are one of the main sources of water pollution, posing a serious threat to water ecosystems. High environmental risk industries such as chemical, textile, and mining discharge large quantities of industrial wastes, both solid and liquid (Akhtar et al., 2021), accompanied by toxic chemicals, volatile organic compounds, etc (Trivedi et al., 2024), which are highly susceptible to sudden water pollution events. In addition, the extensive use of freshwater resources in industrial production significantly increases the risk of water pollution (Lin et al., 2022). About 20 % of the world's available freshwater resources are used for industrial production (Boretti et al., 2019), with the fossil fuel and nuclear energy sectors being particularly dependent on freshwater resources (Atimtay et al., 2011; Brook et al., 2014). This high dependency leads to increased concentrations of pollutants, which in turn have negative impacts on human health and ecosystems (Lin et al., 2022). Common heavy metal pollutants include lead, chromium, cadmium and nickel, mainly from metallurgical, chemical and battery manufacturing industries (Wuana et al., 2011; Brook et al., 2014; Dongre et al., 2020; Zhang et al., 2021; Sidhu et al., 2022). Therefore, the discharge of industrial waste has a significant impact on the accumulation of hazardous substances in water and has become a non-negligible factor in water pollution problems.

Highly concentrated industrial activities pose a serious threat to the ecological environment. Among the major river basins in China, the Haihe River Basin is the most intensively developed in terms of water resources and has the most serious water pollution problems. Pollutant discharges resulting from industrial agglomeration have greatly exacerbated the pressure on the water ecosystem. The Beijing-Tianjin-Hebei region, for example, accounts for 2.25 % of the country's land area, but contributes 5.75 % of the country's COD emissions and 5.83 % of its NH3-N emissions, making the problem of water pollution very serious (Ren et al., 2021). In addition, the ecological environment in the region has shown a trend of gradual deterioration, with the proportion of ‘category 5’ and poor category 5 water quality accounting for as much as 43 % of the region's surface water, the highest in the country (Wang et al., 2015).

Agricultural production

Pesticides, fertilisers and agricultural waste from agricultural production are important sources of water pollution. Nitrates, phosphates and pesticides enter water bodies through surface runoff during agricultural activities, leading to water quality pollution (Lu et al., 2015). In water-scarce areas such as China and India, untreated or partially treated wastewater is used for irrigation, further exacerbating contamination of farmland and foodstuffs (Lu et al., 2015). There has been a marked increase in the use of chemical fertilizers and pesticides in China, with the amount of fertilizer applied per unit of sown area rising from 86.72 kg/hm² in 1978 to 346.15 kg/hm² in 2010, and the amount of pesticides used rising from 5.12 kg/hm² in 1990 to 10.94 kg/hm², which has led to the entry of chemical residues into bodies of water and accelerated the contamination of both surface water and groundwater. Excess phosphates and irrationally used pesticides remain in water bodies for long periods of time, posing a continuing threat to ecosystems and human health (Rout et al., 2016; Priyadarshini et al., 2021). Studies have shown that pesticide use affects human health through drinking water and that a 10 % increase in pesticide use is associated with a 1 % increase in the medical disability index for people over 65 years of age (Onipe et al., 2021).

The Baiyangdian lake in Beijing's Qinghe River system is an example of the largest freshwater lake wetland in the North China Plain. The Baiyangdian receives a large amount of organic pollutants and nutrients due to its long history of agriculture and farming activities (Bian et al., 2012; Chaturvedi et al., 2016; Han et al., 2020). The number of rivers entering the lake was reduced from 9 in the 1960s to 6, and water mobility declined, coupled with the construction of water conservancy projects. The Baiyangdian was transformed from a natural over-water lake to an artificial storage lake, and its ability to dilute and degrade pollutants was significantly reduced (Tang et al., 2018). In 2005, 2006, 2014 and 2015, the water quality of the Baiyangdian has been reduced to the inferior V standard (Tang et al., 2018).

Natural environmental conditions and their changes

Natural conditions and climate change are also important influences on water pollution. In the Loess Plateau region, geological conditions lead to high concentrations of hexavalent chromium in groundwater (He et al., 2020). Climate change and natural disasters (e.g. volcanic eruptions, floods, etc.) can lead to large amounts of natural wastes entering water bodies and affecting the quality of freshwater (Pradhan et al., 2023). For example, between 2000 and 2022, there were 9,142 natural disasters globally, which not only affect water quality, but can also exacerbate pollution through mixtures such as sewage and municipal waste. In coastal Asia, rising seas and over-exploitation of groundwater have led to saltwater intrusion and further deterioration of groundwater quality (Yang et al., 2022).

National Policy Strategy and Industrial Layout

National development strategies and industrial layout have an important impact on the spatial distribution and intensity of water pollution. China's rapid industrialisation and urbanisation process, lagging infrastructure development and adjustments in industrial layout have exacerbated water pollution. Between 2008 and 2018, water pollution incidents in China showed a northeast-southwest distribution pattern (Ullah et al., 2021). For example, the implementation of the ‘Rise of Central China’ strategy and the ‘Yangtze River Delta Economic Zone’ development strategy in 2006 has promoted the clustering of industries in the east, which has increased the environmental pressure on the eastern region (Geng et al., 2022). However, the implementation of the ‘One Belt, One Road’ strategy has made the problem of water pollution in the south-western region increasingly prominent. In addition, in 2015, the Beijing-Tianjin-Hebei region began to implement industrial transfers in order to achieve coordinated regional development. Studies have shown that industrial transfers help to curb the intensity of industrial wastewater discharges and generate spatial spillover effects, which contribute to the mitigation of water pollution problems (Zhao et al., 2019). Therefore, promoting the normal flow of industry can alleviate the problem of industrial wastewater pollution to a certain extent. At the same time, it shows that the industrial structure has a significant impact on the intensity of industrial wastewater discharge, and that a high proportion of secondary industries is not conducive to reducing the intensity of industrial wastewater discharge (Xiao et al., 2022).

Expansion of population size

The risk of water pollution increases significantly with the size of the population. Population growth contributes to the expansion of production, while the centralised discharge of domestic wastewater exacerbates the pollution of water bodies (Geng et al., 2022). In China, for example, 89.5 billion tonnes of wastewater were treated in wastewater treatment plants in 2022, of which domestic wastewater accounted for 88.7% of the total volume treated. In large, densely populated cities, domestic sewage has become a major source of water pollution. For example, in Beijing, direct emissions and sewage treatment plant sources account for more than 60 % of the city's total COD and ammonia nitrogen emissions (Dine et al., 2015).

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