4.1. Assessment of Heavy Metal Contamination
presents the changes in heavy metal content and pH in sediment, overlying water, and vegetation. From Figure 2
a, it can be seen that the pH range of sediment was 7.7–8.4, indicating that it was weakly basic. This result is consistent with the results of research on high water table mines in eastern China [18
]. The slightly high pH of sediment can be attributed to many factors, including climatic characteristics, parent material of the soil, soil cations, and organic acids released during organic matter accumulation and decomposition [21
]. Specifically, Pei County is located north of the Yangtze River, where precipitation is low, leaching is weak, and soil basic ion content is high. In addition, the parent material of the soil is fluvo-aquic soil, in which organic matter content is low and free calcium carbonate content is high. The existence of carbonate soils causes the pH to be basic and the precipitation in the form of metal oxides. Therefore, the soil of this region tends to be basic.
In the sediment, the AN, AP, AK, and SOC concentrations were 0.2–10.2 mg kg−1, 4.3–15.8 mg kg−1, 5.2–27.8 mg kg−1, and 1.22–7.56 g kg−1, respectively. AN, AP, AK and SOC concentrations in the sediment could act as an important indicator of the health of the ecological system.
In the sediment, the Cu, Zn, Cr, Cd, Hg, As, and Pb concentrations were 12.54–45.21 mg kg−1
, 55.86–123.00 mg kg−1
, 39.44–84.34 mg kg−1
, 0.08–1.17 mg kg−1
, 5.92 × 10−3
–77.88 × 10−3
, 6.02–14.70 mg kg−1
, and 14.50–32.40 mg kg−1
, respectively. By referencing the “Soil Environmental Quality in the Agricultural Land Soil Contamination Risk Management Control Standards,” it can be seen, with the exception of Cd content in the control region exceeding the limits, the levels of the other heavy metals were below the soil contamination risk management threshold. The assessment results were not encouraging; however, when the soil background values of Jiangsu Province were used as the basis for assessment. In the sediment, Cu, Zn, Cr, Cd, Hg, As, and Pb exceeded limits by 75%, 100%, 42%, 92%, 50%, 58%, and 67%, respectively. This indicates that the soil heavy metal content in Pei County is relatively low, and heavy metal content exceeding limits is the outcome of long-term exogenous input [23
]. The reason Cu and Zn exceeded the limits is that high levels of Cu and Zn are currently present in feed additives, and long-term use, therefore, increases heavy metal content in the sediment. In addition, a large number of studies have shown that mining, the application of chemical fertilizers, and wastewater discharge may lead to heavy metal contamination [18
]. According to reports, chemical fertilizers with poor calcium superphosphate contain trace amounts of As and Cd [26
], while some pesticides contain Pb and Hg, which will enter the natural environment after application.
In water bodies, Cu, Cr, and Cd levels were below the detection limits, while Zn, Hg, As, and Pb levels were 0.01–0.22 mg kg−1, 0.12 × 10−3–28.39 × 10−3 mg kg−1, 0.01–0.10 mg kg−1, and 0.001–0.01 mg kg−1, respectively. Among all heavy metals detected, the levels of Zn, Hg, and Pb were below the Class III limit of China’s Environmental Quality Standards for Surface Water (GB 3838–2002), while As content slightly exceeded the limit. This was mainly due to high As content in water from the Longgu development zone wastewater treatment plant that was collected in the western water inlet A4 of the Anguo wetland and exceeded the limit by a factor of 9.8.
In vegetation from all sample plots, Cr and Cd from some sample plots were below the detection limits, while Cu, Zn, Hg, As, and Pb levels were 4.01–14.00 mg kg−1
, 40.29–127.95 mg kg−1
, 1.18 × 10−3
–3.51 × 10−3
, 0.43–0.61 mg kg−1
, and 0.30–1.92 mg kg−1
, respectively. From the National Food Safety Standard Contaminant Limits in Food (GB2762-2017), it can be seen the vegetation Hg level was within the clean range. Pb content, however, severely exceeded standard limits, particularly in the control region and the Anguo wetland water inlet, where it exceeded the limit by factors of 19.21 and 17.00, respectively. This is mainly because Pb is easily transported into plants, and high levels of Pb are absorbed by the root system into vegetation. In addition, studies have revealed that root absorption is not the only pathway by which heavy metals are enriched in vegetation, with migration and transformation of heavy metals at the air–plant interface also playing important roles in heavy metal enrichment [27
]. Sediment, overlying water, and vegetation were considered as a whole, with Csum
representing the sum of different heavy metals in sediment, overlying water, and vegetation. The Csum
ranges of Cu, Zn, Cr, Cd, Hg, As, and Pb were 17.66–55.21 mg kg−1
, 115.80–224.29 mg kg−1
, 39.44–84.34 mg kg−1
, 0.12–1.17 mg kg−1
, 9.37 × 10−3
–81.24 × 10−3
, 6.51–15.16 mg kg−1
, and 15.00–34.11 mg kg−1
4.2. Heavy Metal Spatial Distribution
We used the Anguo wetland outlet A9 to represent wetlands for comparison with the aquaculture pond B, fishery–solar hybrid project region C, and control region D. There are two main reasons for using A9 to represent wetland. The first reason is that A9 represents the enrichment level of heavy metals in sediment after constructed wetlands have received wastewater. The second reason is to maintain consistency with the analysis of heavy metals in water. The following text will compare the heavy metal pollution levels in water at the A9 sampling point after wetland treatment. As shown in Figure 2
b–c, the Cu and Zn content in sediment from different sample plots exhibited identical relative relationships: B > D > C > A9. This indicates that Cu and Zn may have identical pollution sources, and Cu and Zn levels are higher in the aquaculture pond. These findings are consistent with the results of [28
]. Studies have determined that long-term usage of bait in aquaculture will increase Cu and Zn content in sediment, causing these heavy metals to exceed limits.
From Figure 2
e–h, it can be seen that Cr, Hg, As, and Pb displayed identical patterns, indicating that the pollution sources of these heavy metals may be identical. These distribution patterns are insufficient to demonstrate that the pollution sources are indeed the same, and further study is required to determine whether these heavy metals are associated in order to confirm that they share the same pollution sources. Cr, Hg, As, and Pb levels in the Anguo wetland A, aquaculture pond B, and fishery–solar hybrid project region C were all lower than those of control region D, demonstrating the advantages of these three coal mining subsidence pond reutilization methods. The sources of heavy metals in coal mining subsidence regions may be due to coal mining, pesticide application, domestic sewage, and industrial wastewater discharge [18
]. First, although the Anguo wetland A receives water from the Longgu development zone and the Pei County development zone wastewater treatment plant, the heavy metal content in the water from these zones was not high, and its effects on sediment heavy metal accumulation are minimal. Second, a large number of reeds were planted in the Anguo wetland, which exerts significant absorption effects on Cd and other heavy metals [30
]. This type of environmental intervention also decreases heavy metal content in wetland sediment. Some heavy metals in aquaculture pond B will enter aquatic animals via food chains [31
]. Therefore, sediment heavy metal content in the aquaculture pond is lower than the control region. In the fishery–solar hybrid project region B, solar power is considered a clean energy source. Under normal circumstances, solar-powered batteries do not pollute the environment and will not increase heavy metal accumulation in sediment [33
]. Since control region D is low-lying, heavy metals produced from coal mining and tailing accumulation will enter into water bodies through surface runoff [35
]. In addition, the application of large amounts of pesticides in farmlands surrounding subsidence ponds is also an important source of heavy metals that cannot be ignored. Given that heavy metals are highly stable and difficult to degrade, heavy metals in water bodies will be enriched in sediment. Therefore, the Cr, Hg, As, and Pb levels in control region D were higher than in other regions.
With the exception of sample plots that were lower than the detection limits, the heavy metal content in the corresponding plants of various sample plots exhibited patterns identical to those of sediment. This is mainly because heavy metals in vegetation originate in sediment, from which they are absorbed and enriched by root systems into vegetation [36
]. To some extent, the higher the heavy metal content in sediment, the higher the heavy metal content in vegetation [38
]. During application, some heavy metals will enter the soil. In overlying water, heavy metal content was significantly lower than heavy metal content in sediment and vegetation, which is consistent with the results of many related studies. For example, [39
] studied heavy metals in the sediment, water bodies, and water grasses of Pulicat Lake, India, finding that the Cd and Cr levels in sediment are far higher than the heavy metal content in water bodies. [40
] studied the distribution characteristics of heavy metals in Uluabat Lake, Turkey, discovering that the heavy metal content in the overlying water was significantly lower than the heavy metal content in the sediment. The main reason for this phenomenon is that heavy metals in overlying water will be adsorbed to suspended matter through complexation. In addition, the results also revealed that heavy metals in overlying water did not exhibit patterns identical to those of sediment and vegetation. Studies found that as flow occurs in a water body, heavy metals in overlying water will be adsorbed within a distance of 10 km under normal circumstances [41
]. In contrast to heavy metals in sediment and vegetation that are the result of long-term accumulation, heavy metal content in overlying water tends to undergo real-time changes. Therefore, it is not difficult to describe the differences between heavy metal content in overlying water versus that in sediment and vegetation.
When Csum was used as a study subject, the variation trends of Cu, Zn, and Cd in the system consisting of sediment, overlying water, and vegetation were identical, and the variation trends of As, Cr, Hg, and Pb were identical. This proves that the two types of heavy metals may have different pollution sources.
In the two independent wetland systems of the Anguo wetland, the heavy metal content of sediment, overlying water, and vegetation displayed identical trends, while Cu, Zn, and Cd levels manifested the following relative pattern: secondary surface flow region > sediment > primary surface flow region. Meanwhile, As and Pb levels exhibited the following pattern: sediment > secondary surface flow region > primary surface flow region. In addition, the heavy metal content in the overlying water of the Anguo wetland outlet was lower than that of the water inlet, indicating that the joint effects of wetland surface flow and aquatic plants can effectively purify the 50,000 tons of tailings that enter the Anguo wetland daily from the Pei County and Longgu development zones.
4.5. Pearson’s Correlation, PCA, and HCA
lists the Pearson’s correlation coefficient of heavy metals and other physicochemical properties. Since soil pH is one of the important physical characteristics of soil and strongly affects heavy metal adsorption sites and adsorption stability, many studies have reported the impact of pH on heavy metals [45
]. Imoto et al. [47
] employed multivariate regression to study the effects of pH on Cd and Pb adsorption and derived an adsorption formula. Their results revealed that soil pH greatly affects heavy metal adsorption. Bang et al. [48
] demonstrated that soil pH changes the surface charge of heavy metal adsorbents, which alters the adsorption of heavy metals by soil organic matter. In this study, the pH of the aquatic ecosystem was found to be negatively correlated with most heavy metals. These results are consistent with previous studies [49
]. The specific reason may have to do with the basic pH of the study site. In a basic soil solution, heavy metals in the soil will form insoluble hydroxides through complexation, and the metallic ion concentration in the soil solution will decrease. Within a certain pH range, the higher the soil pH, the greater the solubility of soil organic matter and the stronger the metal complexation ability. This causes a large number of heavy metal ions to be in a more stable bound state in the form of hydroxides [51
]. Due to the electron structure of heavy metal ions, however, heavy metal ions and hydroxide ions exist in the form of water and ions. The higher the soil pH, the more favorable the hydrolysis reaction, which decreases heavy metal concentration in soil solution [52
There is a strong interaction among sediments elements in the natural environment. The results showed that AN, AP and AK have strong influences on the concentrations of As and Cr via physical sorption and precipitation. SOC was positively correlated with Cu and Zn, which is similar to the studies reported by Liao J [53
] and Katalin Juhos [54
In the aquatic ecosystem, the correlation coefficients of Cu-Zn, Cu-Cd, Cu-Hg, Zn-Cd, Zn-Hg and Cd-Hg were 0.90, 0.53, 0.17, 0.55, 0.19, and 0.64, respectively, while the correlation coefficients of As-Cr, As-Pb, and Cr-Pb were 0.91, 0.18, and 0.33, respectively. These results show that Cu, Zn, Cd and Hg may have identical pollution sources, and As, Cr, and Pb may have identical pollution sources, which is consistent with the analysis results of the preceding section.
The Kaiser-Meyer-Olkin (KMO) value (0.517) of the PCA and Bartlett test results indicate that principal component analysis can be carried out on the data. Four principal components were extracted based on eigenvalues (eigenvalues >1), which explained 80.89% of the total variance. PC1 accounted for 33.70% of the total variance and displayed strong positive correlations with As, Cr, AK, as seen in Figure 4
a. PC2 explained 19.81% of the total variance and exhibited strong positive correlations with Pb, AN, AP and pH. PC3 explained 14.61% of the total variance and showed a strong positive correlation with Cu, Zn, Cd, and Hg. PC4 explained 12.76% of the total variance and exhibited strong positive correlations with SOC. Based on the above results, we can deduce that Cu and Zn have a common source and As and Cr have a common source. Figure 4
b presents the HCA results. Evidently, all indicators clustered into four categories: Category 1, consisting of As, Cr, AK; Category 2, consisting of Pb, AN, AP and pH; Category 3, consisting of Cu, Zn, Cd, and Hg; and Category 4, consisting of SOC. This result is identical to the Pearson’s correlation and PCA results.
In combination with the distribution patterns of heavy metals, it can be seen that Cu and Zn, was the most prevalent in aquaculture pond B, while As and Cr predominated in control region D. It is well known that heavy metal sources are both natural and anthropogenic. Natural sources include rock weathering and soil erosion, while anthropogenic sources include coal mining, agriculture, and industrial production. For example, waste heaps are eroded by wind to form dust, which is suspended in the air and deposits on the land surrounding the waste heap. Coal gangue undergoes erosion and leaching, causing heavy metals to enter the soil via surface runoff caused by precipitation. In addition, domestic sewage, the application of pesticides and chemical fertilizers, aquaculture, and tourism will also result in heavy metal accumulation [55
]. Thus, aquaculture can be interpreted as the main source of Cu and Zn, including long-term bait usage. Meanwhile, As and Cr mainly originate from atmospheric deposition, coal mining, and leaching.