**1. Introduction**

One of the eight largest freshwater lakes in China, Wuliangsuhai (WLSH) Lake is a type of furiotile lake. Since the mid-19th century it has been under the influence of geological movement and it is one of the most important and the largest lakes in the Yellow River Basin. WLSH Lake is an extremely rare large lake with abundant biodiversity and multi-environment functions in one of the world's desert or semi-desert areas [1,2]. Its environment and ecology has a significant impact on maintaining the ecological balance and protecting the diversity of species in northwestern China. WLSH Lake not only contains tremendous resources of aquatic plants, fisheries, birds, and tourism, but it is also an important ecological barrier in northern China [3,4]. During the dry season it is also a major water supply reservoir ensuring the continuous flow of the Inner Mongolia section of the Yellow River, and during the ice-flood season of the Yellow River or local rainstorms and flood seasons, it acts as a detention reservoir. It therefore has a huge and irreplaceable role in maintaining the water system of Yellow River. WLSH Lake is an extremely rare multi-function lake with high ecological benefits in a semi-desert area. At present, nearly half of the surface water of the lake is covered with reeds, and the rest with

**Citation:** Li, J.; Zuo, Q.; Feng, F.; Jia, H. Occurrence and Ecological Risk Assessment of Heavy Metals from Wuliangsuhai Lake, Yellow River Basin, China. *Water* **2022**, *14*, 1264. https://doi.org/10.3390/w14081264

Academic Editor: Bommanna Krishnappan

Received: 21 March 2022 Accepted: 9 April 2022 Published: 14 April 2022

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aquatic plants, making WLSH Lake a typical macrophytic eutrophic lake. The ecological security of WLSH Lake is an important for the stability of Hetao area. It accepts more than 90% of the farmland drainage of the Hetao area [5], and then discharges into the Yellow River after the biochemical processes of the lake, which play a key role in improving the water quality and regulating the water volume of the Yellow River, and controlling the salinization of the Hetao area. These processes have reduced the direct impact of farmland drainage on the water quality of the Yellow River.

In recent years, with the acceleration of industrialization and urbanization of Bayannaoer City, large quantities of industrial wastewater, urban domestic sewage, and agricultural waste water are being poured into the lake [6,7]. Meanwhile, the heavy metals carried by them are entering the aquatic ecosystem through a series of geochemical processes such as direct discharge, atmospheric deposition, surface runoff, and soil erosion. Considerable attention has been given to heavy metals in aquatic ecosystems because it is difficult for them to be degraded by microorganisms after entering the aquatic environment, and they can accumulate in the living body through food chains, destroying the normal physiological metabolic activities of aquatic organisms, and high concentration are a potential risk to human health [8–10]. The ecological environmental problems of WLSH Lake not only affect the function of the lake, but also directly affect the regional food security and threaten the safety of water supply in the middle and lower reaches of the Yellow River. Therefore, investigating and assessing the current of heavy metal pollution in WLSH Lake is important for ecological protection and high-quality development in the Yellow River Basin. Some scholars have studied different aspects of the heavy metals in WLSH Lake from [11–13]. However, there are little research on the comprehensive monitoring and assessment of heavy metals in this lake.

In aquatic systems, sediment serves as an important sink of heavy metals. After heavy metals enter the water, except for the small amount that dissolves in the overlying water, most can be absorbed into the sediment by complexation interaction with the organic matter, clay minerals, and sulfides in the sediment. Meanwhile, heavy metals in the sediment are more likely to be resuspended in the sediment interstitial water when environmental factors (e.g., wind–wave disturbance, pH, ORP, DO, and microbial activity) change [14]. Meanwhile, because of differential concentrations, heavy metals in sediment interstitial water can be re-released into the overlying water by diffuse flux [15,16]. Therefore, investigating the concentrations of heavy metals in the sediment interstitial water and surface sediments is of great significance for the study of lake water quality and aquatic organisms. Moreover, the vertical concentrations of heavy metals in the sediment cores have been used to study the degree of accumulation, exogenous sources, and further trends [17–20].

In addition, the total content of heavy metals in the sediment can provide information about the pollution level, but it cannot reflect the potential ecological risk. The environmental behaviors and ecological effects of heavy metals in different chemical fractions are also different [21,22]. The migration and transformation, toxicity, bioavailability and potential ecological risk of heavy metals in sediment greatly depend on their different chemical fractions. According to the BCR sequential extraction procedure [23], the heavy metals in the sediments can be classified into four chemical fractions. Under certain conditions, these fractions are easily released into the overlying water, thus causing secondary pollution [24,25]. Therefore, when evaluating the ecological risk of heavy metals in sediments, the chemical fractions, especially the bioavailable fractions, should be taken into consideration.

In order to provide more comprehensive information about the heavy metals of the WLSH Lake, this study (1) describes the contents of heavy metals in interstitial water and surface sediments of WLSH Lake, (2) quantified the percentages of chemical fractions of heavy metals at each sampling site; (3) investigated the vertical distribution of heavy metals in sediment, and (4) comprehensively assessed the pollution degree and ecological risk by the methods of enrichment factor (EF), geo-accumulation index (*Igeo*), toxic units (ΣTUs) and toxic-risk index (TRI), risk-assessment code (RAC), and potential ecological risk index (RI) and to calculate the modified potential ecological risk index (MRI) based on the chemical fractions.

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

#### *2.1. Study Area*

As the eighth largest freshwater lake in China, Wuliangsuhai Lake is the most typical shallow grassy lake in an arid area, and is also an important site for water conservation, storage, and water diversion in the middle and upper reaches of the Yellow River. Wuliangsuhai Lake (108◦430–108◦570 E, 40◦270–40◦030 N) belongs to the lake area of Neimenggu-Xinjiang Plateau, located in the city of Bayannaoer, in the Inner Mongolia Autonomous Region. It is also in the arid and semi-arid area of northern China. The area where the basin is located has strong solar radiation, rare rainfall, strong evaporation, a large difference between dry and wet periods, a large temperature difference between day and night, and frequent wind and sand weather. The region where the lake is located has four obvious seasons, and the temperature varies greatly. There is less rainfall and more evaporation in the lake basin.

This lake covers 285.38 km<sup>2</sup> , including a reed area of 118.97 km<sup>2</sup> and a clear water area of 111.13 km<sup>2</sup> . In the clear water area there is a 85.7 km<sup>2</sup> dense area of submerged plants and the rest is swamp area. The lake is long from north to south and narrow from east to west, with a length of 35–40 km, and a width of 5–10 km. The lakeshore is 130 km long and the water storage is 2.5 to 300 million m<sup>3</sup> . Most of the lake water depth is between 0.5 and 2.5 m, and the average annual water depth is 0.7 m.

#### *2.2. Sample Collection and Sample Preparation*

According to the characteristics of water system of WLSH Lake, the village distribution, land use, and administrative region, 23 sampling sites were selected (Figure 1). In August 2021, sediment samples were collected, and then eight sites were selected for collecting the core samples of the sediments. All surface sediment samples were collected using a Peterson grab bucket. At every sampling site, 5 cm of the surface sediment and four subsamples were collected. These were then mixed on the spot and put into a sterile sampling bag. The core samples (S1, S2, S5, S6, S8, S10, S16, and S18) were collected by column sampler (04.23 BEEKER, Eijkelkamp, Giesbeek, The Netherlands), and all the samples were segmented into 5 cm layers, with each layer being placed in the sterile sampling bag. At every core sampling site, three 3 sub-samples were collected and analyzed separately. All of the samples were kept at temperatures below −4 ◦C and transported to the laboratory within three days. In the laboratory, from each sampling site, approximately 250 g of well-mixed fresh sediment sample was dispensed into several 100 mL centrifuge tubes, and then centrifuged in 10,000 rmp for 10 min. Then, all the filtrate was mixed and filtered with 0.45 µm glass fiber filters. The obtained interstitial water was stored at −4 ◦C and analyzed within three days. After centrifugation, the sediment samples were dried by cold-drying and then the samples were sieved through a 40-mesh nylon sieve to remove rocks, organic debris, and other debris and a portion of each sample (about 50 g) was ground in a mortar and then sieved through a 100-mesh nylon sieve. The prepared samples were stored in self-sealing bags at 4 ◦C for further analysis.

#### *2.3. Analytical Methods*

Heavy elemental analyses were performed at the Zhengzhou Key Laboratory of Water Resource and Water Environment. As is metalloid, but for convenience of description, in this study, As is classified as heavy metal. Heavy metals in sediment interstitial water were digested by HNO3. The total content of heavy metals in the sediments was digested by a mixture of acid (6 mL HNO<sup>3</sup> + 3 mL HF + 1 mL H2O2) using a microwave digestion platform (Milestone, ETHOS UP). Chemical fractions of heavy metals in the sediments were extracted according to the BCR sequential extraction method, and the specific extraction steps are shown in Supplementary Materials. The digestion method of the residual fraction refers to the total content. Finally, the concentrations of all heavy metals were analyzed using inductively-coupled plasma-mass Spectrometry (ICP-MS, Agilent 8800, Agilent Technologies, Santa Clara, CA, USA) according to HJ 700-2014 of the Ministry of Ecology and Environment of China.

**Figure 1.** Locations of Wuliangsuhai Lake and sampling sites.

## *2.4. Quality Assurance and Quality Control (QA/QC)*

The containers used in the study were soaked in 20% HNO<sup>3</sup> solution (*v*/*v*) for 24 h before use, and then rinsed with ultra-water (Milli-Q, ρ > 18 MΩ·cm) to prevent container contamination. In order to ensure the accuracy and precision of the results, standard samples were used between every 20 samples. Each sample was determined three times in parallel, and the results expressed as the average value of the three parallel tests (error range < 5%). Soil reference materials (aquatic sediment standard materials (GSD7)) were used to control the quality during the determination process in the study, and the results of recovery of each heavy metal are shown in Table S1. During the experiment, blank samples were determined to eliminate the influence of background values.

#### *2.5. Pollution Assessment Methods*

#### 2.5.1. Enrichment Factor (EF)

The enrichment factor (EF) is an important index to quantitatively evaluate accumulation degree and pollution sources [26]. In order to reduce the influence of particle size on heavy metal pollution assessment, a standard metal is generally used to normalize the original data. Aluminum (Al) in the environment mainly comes from natural rock with poor mobility, and is often selected as the standard metal. The EF is calculated as follows:

$$\text{EF} = (\mathbb{C}\_i/\mathbb{C}\_{Al})/(\mathbb{B}\_i/\mathbb{B}\_{Al}) \tag{1}$$

where *C<sup>i</sup>* represents the measured concentration of metal *i*, *CAl* represents the measured concentration of Al, *B<sup>i</sup>* is the background value of metal *i*, and *BAl* is the background value of Al. EF > 1 indicates anthropogenic contamination. According to the calculated values, EF can be divided into five categories, which are shown in Table S2.

In this study, the background values of Henan Province soil were used to evaluate the pollution degree and ecological risk of heavy metals in WLSH Lake.
