Effect of pollution load reduction on water quality in rural lakes in the shallow hill water network area Open Access

The water network area of Han River–Three Inland Rivers is a shallow hill water network area north of Han River, in the Jianghan Plain of Hubei Province, China. The ‘Three Inland Rivers’ are Hanbei River, Tianmen River and Fuhuan River. The water network originally belonged to the system of Diaocha Lake, but it was divided into two water systems and three water catchments after the implementation of Hanbei Water Project in the winter of 1969. Different basin water systems can be connected through river culverts and gates. For a long time, water-resource dispatching in the region was primarily driven by flood control and drainage, without comprehensive consideration of the water ecological environments of river and lake. Both the lake water quality and its ecological function in rural areas are deteriorating rapidly. It is urgent to systematically investigate the trend of the deterioration, and to provide valuable suggestions to alleviate the problem.

With accelerated human activity, water environmental problems have become increasingly prominent. Non-point pollution induced by rural production and life is the major source of rural lake water pollution (Nash et al. 2009). Nitrogen and phosphorus enrichment can cause algal blooms and cyanobacterial pollution that is harmful to the water quality of lakes (Chen et al. 2009; Liu et al. 2009). In recent years, with the harmonious concept of human–water symbiosis and the in-depth promotion of the river and lake chief guidelines, the water quality improvement of domestic rivers and lakes has drawn increasing interest (Yang & Zhang 2003; Song & Lu 2004). The water flow and pollutant diffusion in a lake can be quantified using mathematical models (Niedda et al. 2014; Tongal & Berndtsson 2014). For example, Zuo & Liang (2016) and Zuo & Li (2013) proposed a calculation model that was constructed through analysis of the river hydrological regime and its influence on the river ecosystem. Zhao et al. (2008) used an adapted ecological hydraulic radius approach to estimate the ecological water demand of a river. L. Chen et al. (2014a; 2014b) structured a hydrology–hydrodynamics–water quality coupling mathematical model to study the Huaihe River network. H. Chen et al. (2014) analyzed the spatial and temporal variation law of polluted river water quality under various operation conditions of a dam to explore the mechanism of the operation affecting the water environment. A model based on a spatial grid and combined with the verified loosely coupled model has been used to assess the flood risk and blue-algae bloom risk in the Hongze Lake (Huang et al. 2010; Liu et al. 2021). A Markov Chain model has been used in Baiyangdian to consider the guarantee degree of lake ecological water demand (He et al. 2020).

Water diversion greatly improved the fluidity of urban lakes and connection projects could improve the quality of lake water (Kang et al. 2012; Li & He 2015; Yang et al. 2018). In order to understand the actual situation of replenishing water in the water environment, a more suitable method is required for simulation and prediction. Zhai et al. (2012) used a SWAT model to evaluate non-point source pollution in Erhai Basin and distinguish the key factor of non-point source pollution control. A hydrodynamic and water quality model is a useful tool, and the integrated decision support system makes it simpler and more convenient for decision makers to make decisions efficiently and quickly (Liao et al. 2019). A hydrodynamic and water quality model successfully reproduced the complex water and pollutant exchange processes in the systems involving upstream rivers of the Poyang Lake and the Yangtze River (Li et al. 2018). If the ecological water demand of a single river or lake in a lake basin cannot meet the requirements of ecological water use, then the water relationship between river and lake needs to be comprehensively considered (Wang et al. 2021).

Generally, the majority of research on improving water environments has concentrated on rivers and urban lakes. However, less research has been conducted on a comprehensive evaluation system for the hydrodynamic force and water quality of rural lakes and rivers after the water system is connected. Even less study has been done on rural lakes in a shallow hill water network area. The primary purpose of this study was to develop a method for determining the ecological water demand based on the calculation results of the water quality improvement of a rural lakes group and the associated rivers as a whole in a plain water network area. To fulfil this purpose, we investigate rural lakes in the shallow hill water network area north of the Han River in Jianghan Plain, Hubei province, from the perspective of improving the water environment. Ten rural lakes across the river basin are used as objects for an analysis of the different water quality before and after pollution interception and control. Moreover, a coupled model of hydrodynamic and water quality of the river and lake system is constructed to simulate and compare the yearly changes in water quality in the present year and the planning year. The possibility of ecological water replenishment in each lake from the backbone river is also systemically examined. The results can provide a reference for rational allocation of water resources, sustainable utilization and ecological environment protection.

The Han River–Three Inland Rivers region is dotted with lakes, in total distributed as 112 large and small lakes, including 75 rural lakes and 37 urban lakes (29 in Wuhan, four in Tianmen and four in Xiaogan), with a total area of about 222.95 km². The main functions of the lakes are storage, irrigation and aquaculture. The aquaculture industry of lakes in the region is relatively developed, and the land around the lakes is favorable for agricultural cultivation, coupled with the insufficient conception of lake protection, so that the water quality of the lakes in the region deteriorates day by day. In this work, the MIKE21 model was used to simulate the two-dimensional hydrodynamics and water quality of the lakes in the region to determine the improvement of the water environment under the design hydrological conditions. The distribution of the river network in this study area is shown in Figure 1.

From the many lakes in the region, ten rural lakes such as Zhangjiada Lake and Shijia Lake were selected as typical lakes based on the principle that the lake surface area is larger than 1 km² and has typical functional characteristics and location economy. The basic information of the typical lakes is shown in Table 1.

We collected a series of hydrological data from each hydrological station in the area of Han River–Three Inland Rivers. Considering the water inflow, water supply and water recession in the region, the empirical frequency method was adopted to determine the low flow year with 90% guarantee rate as 2011. Subsequently, we took the design flow in the dry year as the designed hydrological conditions of ecological water demand of rivers and lakes in the target area based on the improvement of the water environment, and built a mathematical model to simulate and analyze the trend of hydrodynamics and water quality of the lakes from January 1 to December 31, 2011.

We measured the underwater topography of ten typical lakes, and then generated an unstructured triangular mesh applicable to complex boundaries, using the Beijing 1954 3 Degree GK CM 114E projection, integrating the topographic information from the lakes. The minimum allowable angle of mesh triangle is 26°, and the maximum number of nodes is 100,000.

Table 2 shows the number of nodes and grids generated. Figure 2 shows the underwater topography and the computational grids.

The boundary conditions of the model included rainfall, evaporation, runoff, wind field conditions, initial water quality, pollution load, model parameters, initial water level and flow rate, etc.

Because of the wide range of this simulation, different rainfall and evaporation stations were selected for different lakes to determine the rainfall evaporation of the lake. In this study, rain data for each lake in 2011 was selected from the corresponding rain station and evaporation station. The evaporation data was then calculated using the conversion coefficient of evaporation dish and large water surface, which is shown in Table 3.

For this simulation, the measured runoff data of the representative year (January 1 to December 31, 2011) was used: the runoff of Xiaogan City lakes was projected by the measured runoff of Tianmen station; the runoff of Tianmen City lakes was projected by the measured runoff of Huayuan station; the runoff of Wuhan City lakes was projected by the measured rainfall of Wujiashan station. See Table 4 for the monthly runoff of each lake.

The regional wind speed and direction were derived from the monthly average wind speed and direction for many years, as shown in Table 5.

The simulation started from January, in the non-flood period. Therefore, the lake non-flood period measured water quality according to the design of the representative year under the current pollution load of the initial water quality simulation of each lake. Table 6 details the initial water quality status of each lake.

As shown in Tables 7 and 8, the pollution load after taking pollution control measures in the current year and the planning year is presented in which point source pollution is discharged continuously into the lake through the outfall and non-point source pollution load is entered through the lake catchments following rainfall runoff. The pollution load statistical methods and original data are provided in the Supplementary Material.

Due to the inadequate number of hydrological and water quality monitoring stations in the Han River–Three Inland Rivers area, there is a lack of measured data of runoff and water quality in a series of years, making it difficult to determine the attenuation coefficient by model simulation.

Combined with relevant research results in Hubei Province, the TN attenuation coefficient is 0.025 (1/d), the TP comprehensive attenuation coefficient is 0.02 (1/d) and the COD comprehensive attenuation coefficient is 0.02 (1/d).

The initial water level of each lake is set to a constant water level (Table 1). The initial flow rate is set to 0. That is, the cold start mode is adapted.

Simulate the process of water quality changes in each lake when it encounters the hydrological conditions in 2011, the design representative year, under the current pollution control level.

Hydrological Conditions: runoff of the representative year 2011.

Pollution Load: current level of annual pollutants into the lake.

Simulate the process of water quality changes in each lake when it encounters the hydrological conditions in 2011, the design representative year, under the pollution control level after interception and pollution control.

Hydrological Conditions: runoff of the representative year 2011.

Pollution Load: planning level of annual pollutants into the lake (after interception and pollution control).

TP is an important indicator of eutrophication of lakes, and it is also the main indicator of water quality overflow of each typical lake, so this time it is used as the control indicator of the lakes' current working conditions and the pollution interception and control.

The simulation results of TP of each lake in scenario 1 are shown in Table 9. It is apparent from this table that the content of TP of all the lakes under the current pollution level exceeds class III, according to the Chinese Environment Quality Standard for Surface Water (GB3838–2002). The water quality of Longgu Lake, Dongxicha Lake, Wangmu Lake and Yezhu Lake is poor, with TP levels exceeding 0.1mg/L monthly. Longgu Lake and Wangmu Lake, in particular, have TP levels that exceed 0.15mg/L on a monthly basis. Zhangjiada Lake, Shijia Lake, Longgu Lake, Laoguan Lake, Longsai Lake, Wangmu Lake, Yezhu Lake and Tongjia Lake all have monthly increasing TP content from January to May with a peak in June; from July to December, the TP content of Zhangjiada Lake, Shijia Lake, Wangmu Lake, and Tongjia Lake decrease month by month, while Wangmu Lake and Yezhu Lake do not change much, with a slightly increased TP concentration in December; Longgu Lake, Laoguan Lake and Longsai Lake have small fluctuations from July to October, and the TP content increases significantly from November to December. Figure 3 presents the simulation results of TP in scenario 1.

Table 10 displays the TP simulation results for each lake in scenario 2. After the reduction of pollution load, the TP content of Diaocha Lake is the lowest among all lakes, and the TP content of Diaocha Lake, Zhangjiada Lake and Shijia Lake is less than 0.05 mg/L throughout the year. On the other hand, Laoguan Lake has the greatest TP level among them, with a maximum value of TP in July exceeding 0.1 mg/L, followed by Longsai Lake and Wangmu Lake. The TP content of Laoguan Lake, Wangmu Lake, Yezhu Lake, and Tongjia Lake falls dramatically from August to December after reaching a peak in July, whereas the TP trend of Diaocha Lake, Zhangjiada Lake, and Shijia Lake is not discernible in the whole year. The results of TP simulation for each lake in scenario 2 are shown in Figure 4.

The TP content of the aforementioned two working circumstances is simulated and computed based on the current pollution load and the pollution load after pollution interception and control, and it is presented in Figure 5.

When the changes in TP content in each lake between scenarios 1 and 2 are compared, the pollution load into the lake is lowered, the water quality of the lake is improved to a large extent, and the TP concentration of all lakes is significantly reduced, indicating that the pollution control project plays an important role in the improvement of the lake's water environment. After the cancelling of fishery breeding in the lake, the TP concentration of Diaocha Lake in the entire year is less than 0.05 mg/L, which meets the target requirement of the class III water quality standard; Zhangjiada Lake and Shijia Lake basically meet the target requirement of the class III water quality standard after the implementation of pollution interception and control, so there is no need for additional ecological replenishment; Longgu Lake, Laoguan Lake, Dongxicha Lake, Wangmu Lake, Yezhu Lake and other rural lakes, however, still have TP levels greater than 0.05 mg/L for the majority of the year after control, failing to meet the class III water quality standard.

As can be seen from the above analysis, after the implementation of the interception and pollution control project, except for Diaocha Lake, which meets the standard all year round, Zhangjiada Lake, and Shijia Lake, which basically meet the standard, the water conditions of the remaining seven lakes do not meet the standard, and ecological water replenishment is required. According to the analysis of water supply sources, ecological water supply needs to be diverted from the backbone rivers of the Han River–Three Inland Rivers. The TP concentrations of lakes and the corresponding river cross-sections of the Han River–Three Inland Rivers were compared and analyzed, respectively, on June 1 and December 1, the typical flood and non-flood periods, as shown in Table 11 and Figure 6.

Figure 6 reveals that the overall amount of change in TP in each lake and river channel in June and December is generally small, but the TP concentration in the backbone channels of the Han River–Three Inland Rivers in these two months is significantly higher than that in the lakes. In particular, the TP concentration in the main channel of Diaocha Lake is substantially higher than that in the lake. The TP concentration in the channel in June is four times higher than that of the lake, and it increases to 5.67 times in December; the TP concentration in the channel of Wangmu Lake in June is 3.1 times that of the lake, and 2.75 times in December; the TP concentration in the other eight river and lake channels is also 1.5–2.5 times that of the lakes.

• (1)

  At present, domestic research on the ecological water demand of rivers and lakes in the plain water network area is not mature. A method for determining the ecological water demand based on the calculation results of the water quality improvement of the rural lakes group and their associated rivers as a whole in the plain water network area is proposed for the first time.

• (2)

  The temporal and spatial variability of ecological water demand is studied, and the laws of different levels of ecological environmental water demand, representative years, spatial distribution and runoff characteristic values are clarified, which provides a solution for the prediction of ecological environmental water demand, and provides an important decision-making basis for the rational allocation of water resources, sustainable utilization and ecological environment protection.

• (3)

  TP is an important indicator of the eutrophication of lakes, and it is also the main indicator of water quality overflow of each typical lake. It is clearly shown that the concentration of pollutants in TP reached a peak in July for many lakes before and after the interception and control of pollution. Therefore, each lake should pay attention to the management of surrounding pollution sources especially during summer, strengthen the exchange with surrounding water bodies, prevent pollution sources from entering the water bodies, strengthen the flow of lake water bodies, and take a variety of measures to improve the water quality of lakes.

• (4)

  Through the comparison between current pollution load and the pollution load after pollution interception and control, obviously, pollution interception and management are particularly successful for improving lake water quality in rural lakes. A series of actions conducive to the protection of rural lake water quality, such as agricultural non-point source pollution control, livestock and poultry pollution control, garbage collection and transportation, domestic sewage collection and treatment, etc. should be carried out vigorously in rural areas to increase water quality.

• (5)

  Due to rivers and lakes adopting different water quality TP standards, the standard of river water quality is twice as high as that of lakes. If the water is diverted from the river, a flocculation and sedimentation tank must be set up after the gate to reduce phosphorus before it reaches the lake. In rural areas, however, flocculation and sedimentation tanks are expensive to construct, operate, and maintain, and rural lakes are mainly used as irrigation water sources with low requirements for water quality. Taking the above factors into consideration, it is prudent to use rivers for the ecological recharge of rural lakes.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.