**1. Introduction**

Rivers are an integral part of urban ecosystems [1]. Rapid urbanization and economic development have caused urban river pollution globally, and human activities directly or indirectly lead to changes in river environments. Under natural flow conditions, river water can self-purify, while during the process of urban expansion, the water quality of the river is more or less affected, and the river cannot fully play its original function [2]. In the last decade, the number of hydraulic structures such as sluices and dams has increased with rapid urbanization. It is expected that by 2025, these structures will be present in 70% of rivers around the world [3,4]. The small size of urban rivers and the excessive construction of sluices will undoubtedly affect the material transport movements in rivers and then the river environment [5]. The factors affecting the water quality of the regulated rivers are more complex and are susceptible to the influence of water diversion and storage by sluices and dams [6]. They can largely result in the degradation of water quality in rivers [7]. The water quality of rivers in cities directly or indirectly affects human health, and thus the health of regulated rivers and the attainment of water quality standards are essential to the high-quality development of the urbanization process [8].

According to the 2020 statistics of the World Commission on Dams (WCD), the number of dams worldwide has reached 59,071, of which the number in China reached

**Citation:** Lan, F.; Haisen, W.; Yan, Y. Spatial–Temporal Variations of Water Quality in Urban Rivers after Small Sluices Construction: A Case in Typical Regions of the Taihu Lake Basin. *Int. J. Environ. Res. Public Health* **2022**, *19*, 12453. https:// doi.org/10.3390/ijerph191912453

Academic Editors: Alban Kuriqi and Luis Garrote

Received: 21 August 2022 Accepted: 26 September 2022 Published: 29 September 2022

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23,841, accounting for about 40% of the world [9]. Urban rivers play a significant role in water extraction, navigation, and stormwater drainage, and also in urban domestic sewage as well as part of the industrial wastewater discharge. Furthermore, the impact of sluices and dams on river environments and ecology is increasing for watersheds with high population density, relatively concentrated production and life, and more serious water pollution [10]. Especially in some areas with abundant stormwater, various flood control sluices and dams have been built in urban rivers to prevent urban waterlogging. These sluices and dams inevitably affect the hydrological status of the rivers, thereby affecting the diffusion and distribution of pollutants in urban rivers. Studies have shown that the accumulation of nitrogen and phosphorus pollutants without treatment will bring great risks to the water environment [11].

Currently, there are many studies focusing on the impact of sluice and dam construction on the water quality of rivers. Dou et al. [12] analyzed the effect of sluice operation on water quality in the Shaying River and developed a hydrodynamic model incorporating sluice operation and a water quality transport and transformation model that incorporated the release of endogenous loads and identified that the influent concentration, size, and the number of sluices were the main factors affecting water quality. Wang et al. [13] reported the effect of sluice operation around Poyang Lake on water quality and found that sluice operation slowed down the water flow rate and increased the risk of water eutrophication. Young et al. [14] found that the opening of the Arase Dam resulted in a significant decrease in the concentrations of As, Zn, Pb, and the total sulfur in the mudflat sediments of the Kuma River and the aquatic environment improved. Tang et al. [15] demonstrated that the construction and operation of a large number of sluices in the Yangtze River Basin changed the natural transport rhythm of the runoff, suspended solids and nutrients, and reduced flow velocities, resulting in the decline of water exchange, the narrowing of the connectivity between rivers and lakes, and the accumulation of nutrients and SS, which led to water eutrophication. Soukhaphon et al. [16] concluded that the sluices in the Mekong River Basin affected fish migration, river hydrology, and sediment transport and consequently had a negative impact on regional food economic security. Obviously, the impact of sluices and dams on the river water environment is multifaceted. However, the environmental effects of large dams or sluices are universally known, but those of these small sluices or dams (≤15 m or ≤<sup>3</sup> × <sup>10</sup><sup>6</sup> m3) have rarely been considered [17]. In view of the proliferation of flood-prevention dams in the world's river systems, the challenge appears as to their cumulative impacts on water environments. An endeavor to evaluate these flood-prevention facilities' cumulative environmental impacts suggested that a large number of small dams or sluices may have an immeasurable impact on energy generation than that of large ones [18]. Thus, there is an urgent requirement to understand the multiple environmental impacts of small flood-prevention development and to understand how these dams or sluices might be better developed and managed.

This study focuses on urban regulated rivers in Wuxi, and the water quality data of rivers within the city are compared and analyzed to explore the differences in the changes of major pollutant concentrations upstream and downstream of urban rivers when the sluices are closed, and then to explore the impact of small sluices construction and operation on urban rivers water quality. The harmonious balance between the urban water environment and ecology is a critical basis for sustainable urban social and economic development; however, the interaction between them is extremely complex [19]. The target of China's water resources management has been changing from "water quantity management" to "water quality management" [20]. The results of this study can provide a basis for the management of water resources in urban rivers, ecological regulation, and the construction of small sluices and dams and promote the coordinated development of socio-economic and urban rivers.

#### **2. Materials and Methods**

#### *2.1. Study Area*

Wuxi is one of the cities with numerous rivers in China, and it is located on the north shore of Taihu Lake in the Yangtze River Delta [21]. There are more than 3100 rivers in Wuxi, with a total length of 2480 km. The total length of the rivers in the city is approximately 150 km, with the volume of the water body being 8 million m3 during the flat water period. Wuxi is relatively rich in surface water and is well recharged by external water sources. The storage capacity of the city is 63.49 million m3, and the annual recharge is 64.53 million m3 [22]. Wuxi has now built more than 1200 large and small sluices because of rapid industrialization and urbanization. In 2020, Jiangsu Province invested about 5.9 billion dollars focusing on the implementation of flood control projects in the Taihu Lake Basin, which has greatly improved the flood control capacity of Taihu Lake [23]. The construction and operation of these new sluices and dams, as well as the regulation of existing ones, will have a direct impact on the water quality of Wuxi's rivers and Taihu Lake.

Currently, about 50% of rivers in Wuxi fail to meet the requirements of Class III "Water Environmental Quality Standards of China" (WEQSC) (GB3838-2002). Among them, the ones in Wuxi's downtown were the most seriously polluted, and the main pollution indicators are total nitrogen (TN) and total phosphorus (TP) [24]. The pollution sources in Wuxi are mainly municipal solid waste, industrial pollution, and agricultural irrigation and fertilization. In this study, we selected 8 major rivers with a total length of 159.34 km (Figure 1). Among them, Bodugang River, Xibei Canal, Jiuli River, and Liangtang River, and their water quality are required to meet the Class III of WEQSC (TN ≤ 1.0 mg·L<sup>−</sup>1; TP ≤ 0.2 mg·L<sup>−</sup>1). Meanwhile, the water quality of the Xicheng Canal, Beijing–Hangzhou Grand Canal, Beixingtang River, and Ancient Canal is required to meet Class IV (TN ≤ 1.5 mg·L<sup>−</sup>1, TP ≤ 0.3 mg·L<sup>−</sup>1) (GB3838-2002).

**Figure 1.** Research area and sample points.

A new "Flood Control Plan of Wuxi" was issued at the end of 2001, which would protect a 136 km2 region. In May 2003, the construction of flood control facilities began and was completed at the end of 2008 [22]. This project contained eight flood control stations: Yandaigang flood control station, Beixingtang flood control station, Jiuli River flood control station, Bodugang flood control station, Limin Bridge flood control station, Xianli Bridge flood control station, Liangtang flood control station, and the Jiangjian flood control station. These stations are applied not only to prevent floods in Wuxi but are also used for ship navigation [25]. The floodgates are controlled by the relevant government departments according to the annual precipitation and total water quantity of Wuxi City.
