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Article

Effects of Tide Dikes on the Distribution and Accumulation Risk of Trace Metals in the Coastal Wetlands of Laizhou Bay, China

by
Yuanfen Xia
1,
Xiaofeng Ling
1,
Yan Fang
1,
Zhen Xu
1,
Jiayuan Liu
2,* and
Fude Liu
2,*
1
State Power Environmental Protection Research Institute, Nanjing 210031, China
2
School of Environmental Science and Safety Engineering, Tianjin University of Technology, Tianjin 300384, China
*
Authors to whom correspondence should be addressed.
Water 2024, 16(22), 3230; https://doi.org/10.3390/w16223230
Submission received: 21 October 2024 / Revised: 7 November 2024 / Accepted: 8 November 2024 / Published: 10 November 2024
(This article belongs to the Special Issue Water Pollution Control and Ecological Restoration)
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Abstract

:
Tide dikes play a key role in preventing seawater intrusion in coastal regions; however, their effects on trace metal distribution and accumulation remain unclear. This study explored the distribution and enrichment of trace metals (As, Cr, Cu, Ni, Pb, and Zn) inside and outside tide dikes in Laizhou Bay. The accumulative risk of these metals in the two habitats was analyzed by combining their sources. The results show that the average enrichment factor, geological accumulation index, and potential ecological risk index of As in the outside habitat are significantly higher than those in the inside habitat (p < 0.001), which indicates that the tide dike effectively reduces the migration of As from outside to inside habitats. For other trace metals, no statistical differences were found between the two habitats. Based on principal component analysis and redundancy analysis of trace metals and their correlations with soil physicochemical properties, we speculated that Cr and Zn may derive from soil parent material and rock weathering. Cu, Pb, and Ni may be related to atmospheric nitrogen deposition resulting from nearby agricultural activities, and As may come from industrial wastewater or transport through seagoing rivers. The findings suggest that tide dikes effectively block exogenous trace metals but not those from natural sources.

1. Introduction

With rapid industrial and economic developments, many wetlands are filled with and polluted by trace metals and other organic pollutants, which threaten human health and destroy the balance of ecosystems [1,2]. Trace metals in wetlands can accumulate and spread through food chains [3,4]. Thus, the distribution and accumulation risk of trace metals has attracted the interest of ecologists [5,6]. Furthermore, the level of trace metal accumulation has become an important index in evaluating the ecological risk of ecosystems [7,8]. Correlated publications have greatly improved our understanding of trace metal behavior and toxicity in ecosystems.
Over the past two decades, the intensity of anthropogenic activity, the number of trace metal categories and their enrichment levels have increased [9,10]. Considerable research that focuses on the distribution and accumulation patterns [11,12], behavioral and toxic mechanisms [13,14,15], and risk evaluations [16,17] of trace metals in some ecosystems, such as rivers, estuaries, and coastal wetlands, have emerged. Such studies have suggested that the increasing level of trace metals in soils is mainly correlated with anthropogenic activities [18]. For instance, tillage can improve the mobility of trace metals (especially for Fe and Mn) and alter their accumulation conditions in soils [19,20]. Oil exploitation could bring As, Cd, and Pb into many oil fields [21,22]. Another study has reported that trace metals may carry freshwater inputs into restored wetlands, which elevates the concentration levels of As, Cd, Cr, and Pb in soils [23]. In addition, agricultural fertilization and/or other human activities, such as industrial plants, traffic activity, and harbor construction, may also cause the accumulation of trace metals in soils [16,24].
Different from those anthropogenic activities, the influences of natural factors on trace metal accumulation are weak. For instance, some geochemical elements, such as Al, Fe, and Mn, are stable and considered to originate from the weathering of minerals and rocks [25]. Generally, those trace metals, which are positively correlated with Al, Fe, and/or Mn, have similar sources and are less affected by external factors. However, in recent decades, air quality has been greatly influenced by fossil fuel burning, automobile exhaust, and industrial emissions on a global scale [26]. Thus, some natural events related to atmospheric conditions, such as rainfall, snowfall, and dry deposition, have conspicuous effects on the accumulation levels of trace metals in wetlands. In addition, Fe easily combines with particles in the atmospheric environment, which forms Fe (III) oxyhydroxide colloids and moves from one place to another with airflow and precipitation [27]. Undoubtedly, climate changes may confuse our understanding of the distribution and accumulation characteristics of trace metals at the regional and global scales.
Laizhou Bay receives input from over ten seagoing rivers, among which the Yellow Rivers serve as the primary aquatic pipeline for the transport of trace metals. Due to its vast water volume and extensive coverage of upstream industrial and agricultural areas, Laizhou Bay has become a typical example of trace metal enrichment [28,29]. Laizhou Bay is also a hub for aquaculture activities and is surrounded by key industrialized and urbanized cities, leading to significant trace metal pollution in this region [30]. Studies indicate that human activities have contributed to identified risks of Hg [31] and As [32] contamination in Laizhou Bay. Furthermore, evidence from dissolved organic matter in the overlying water highlights terrestrial inputs as one of the primary sources of ecological pressure in this area [33]. Fortunately, China’s Ministry of Ecology and Environment launched the “Bohai Comprehensive Management Action Plan”, which includes the construction of tide dikes in critical bays to address these environmental challenges [34]. From an ecological point of view, tide dikes play a key role in preventing seawater intrusion, resisting storm disasters, controlling marine pollution, and restoring coastal environments [35,36,37]. Meanwhile, the coastal zone is divided into two different habitats by tide dikes, and the outside habitat is flooded by tidewater. Although tide dikes effectively resist the threat of pollution from different aquatic pipelines to coastal areas and trace metal pollution from inside habitats, they also cut off the material exchange pathways between inside and outside habitats. Compared to the outside habitat, the importance of the inside habitat should also be emphasized, but most coastal development activities, such as farmland cultivation and grazing, are concentrated within the inside habitats [38]. These inconsistencies should be further investigated, as the differences in development intensity and management strategies between inside and outside habitats of tide dike may lead to markedly different patterns of trace metal accumulation and ecological impacts across these habitats. Based on this, we hypothesized that due to the barrier effect of the tide dike, the source of trace metals is expected to differ between the inside and outside habitats. Trace metals influenced by human activities or transported via river systems tend to accumulate in the outside habitat, while those metals from solid wastes in land or agricultural activities (such as fertilizer, irrigation, and pesticide application) might collect in the inside habitat of tide dikes. In addition, naturally sourced trace metals primarily enter soils through atmospheric deposition and geological weathering, processes that are typically more uniform. These metals tend to reflect a stable balance of natural inputs and long-term deposition rather than concentrated influxes [39]. Therefore, we further hypothesized that the tide dike effects are not clear for those metals that originated from natural sources.
Two habitats, the inside and outside of the tide dikes, were selected in the coastal regions of Laizhou Bay to test the hypothesis and identify the effects of tide dikes on the distribution and accumulation risk of trace metals in coastal wetlands. Therefore, this study mainly aims to (1) reveal the spatial distribution characteristics of As, Cr, Cu, Ni, Pb, and Zn in the inside and outside habitats of tide dikes, (2) evaluate the enrichment degree and risk status of trace metals in the inside and outside habitats of tide dikes, and (3) explore the trace metal sources and input pathways and then discuss the effect roles of tide dikes on trace metal distribution and accumulation risk in coastal wetlands.

2. Experimental Section

2.1. Study Area

This study was carried out in the National Marine Ecology Special Reserve (119°20′19″ E–119°23′49″ E; 37°03′07″ N–37°07′12″ N) of Laizhou Bay. In this study area, the main vegetation type is Tamarix chinensis, which covers a total area of 29.3 km2 [40]. The climate type is temperate and semi-humid monsoon continental. The annual precipitation is 580–660 mm, the annual evaporation is 1764–1859 mm, and the average temperature is approximately 12.9 °C. The extreme maximum temperature is 39.2 °C, and the extreme minimum temperature is −19 °C. The frost-free period is 195–225 days [40,41]. The ecological types in this study area are mainly composed of tamarisk shrubs, beach wetlands, and shallow seas. The soil types are mainly fluvo-aquic and saline. The vegetation types are mainly shrubs and herbs. The dominant shrub species is T. chinensis, and the dominant herbaceous plants are Suaeda salsa and Limonium bicolor. The tide dike lies in the nature reserve. Its north is named the inside habitat, and the vegetation type in this part is T. chinensis forest. The south of the tide dike is named the outside habitat, and it is located in a large tidal flat wetland and shallow sea.

2.2. Soil Sampling

In this study, soil samples were collected from both habitats of the tide dike in the wet season of the year. In the outside habitat, three sampling strips and 28 sampling points were set up. Two sampling strips and 20 sampling points were established in the inside habitat (Figure 1). The soil in the inside habitat is sandy soil, while that in the outside habitat is mainly silty soil under the influence of tides [42,43]. In this study, the distance between the two sampling points in each strip is 50 m, and a quadrat of 3 m × 3 m was performed at each point. In addition, five soil samples were randomly collected at depths of 0–10 cm, 10–20 cm, and 20–30 cm within the same quadrats using a manual soil sampler (JC 803B, JUCHUANGHB, Qingdao, China) and a steel tape measure, and then samples from each depth were then combined into a composite sample for subsequent analysis.

2.3. Soil Chemical Analysis

All the soil samples were air-dried and then ground into a fine powder for laboratory analysis. In this study, each soil sample was digested with 9 mL H2SO4 and 1 mL HF and then digested at 160 °C for 2 h. Finally, the digested solution was adjusted to 25 mL in a volumetric flask, and then the trace metals of As, Cr, Cu, Ni, Pb, Zn, and phosphorus (P) were measured using inductively coupled plasma optical emission spectrometry. Quality assurance (QA) and quality control (QC) were estimated using duplicates, method blanks, and national standard reference materials (GSB 04-1767-2004 [28], recovery efficiencies = 90.7~110.4%). During measurement, at least one control sample was spiked with each set of samples (1 bank for every 10 samples), and each measurement was replicated three times [23]. The test results were reliable if the repeat sample analysis error was below 5%. In this experiment, the soil organic matter was determined via 1/6 potassium dichromate titration [44]. The soil nitrogen (N) was analyzed using an elemental analyzer (2400II CHNS/O Elemental Analyzer, Perkin-Elmer, Waltham, USA) [45]. The soil pH was determined using a PHS-25C precision pH meter (soil/water = 1:5). The soil salt was measured using a conductivity meter (DDSJ 308A, LEICI, Shanghai, China) [45]. Specifically, the air-dried soil sample was added to deionized water (water/soil = 5/1), shaken for 5 min, and then allowed to stand for 30 min. The salt content of the solution was measured using a conductivity meter [45].

2.4. Sediment Quality Guidelines (SQGs)

Sediment quality guidelines (SQGs) have been widely used to assess the level of trace metal contamination in sediments and soils [15,46]. The probable effect level (PEL) is a method commonly used to assess the toxicity of sediments. The national standard of marine sediment quality [47], the SQGs of the US PEL Guide [48], and the background guidelines (Bg) for all trace metals in Shandong Province [49] were introduced in this study to assess the pollution status of the coastal wetlands in Laizhou Bay (Table 1).

2.5. Enrichment Factor (EF)

The enrichment factor (EF) was used to evaluate the external input for trace metals in soils [50]. The EF for each element was calculated using the following formula:
EF = ( C M C R ) sample ( C M C R ) background
where C M C R is the ratio of targeted elements (i.e., As, Cr, Cu, Ni, Pb, and Zn) and reference metal in soil samples and their background values. Here, we selected Al and Mn as the reference metals, which were proven to be suitable for trace metal contamination assessment in coastal wetlands [7,51]. The background values of Al and Mn in Shandong are 66,200 mg·kg−1 and 644 mg·g−1, respectively. The contamination categories were classified based on EF values: EF < 1, no enrichment; 1 ≤ EF < 3, minor enrichment; 3 ≤ EF < 5, moderate enrichment; 5 ≤ EF < 10, moderately severe enrichment; 10 ≤ EF < 25, severe enrichment; 25 ≤ EF < 50, very severe enrichment; and EF ≥ 50, extremely severe enrichment [52].

2.6. Geological Accumulation Index (Igeo)

The formula for calculating the geological accumulation index is as follows:
I geo = log 2 C n 1.5 B n
In the formula, Cn is the actual observation of the six target trace metals studied, and Bn is the local background value of the six trace metals. The introduction of constant 1.5 was used to eliminate the lithological changes in the sediment and the influence of human activities. The geological accumulation index (Igeo) was divided into seven grades: (1) Igeo < 0 is non-polluting, (2) 0 ≤ Igeo < 1 is a transitional stage from non-polluting to mild pollution, (3) 1 ≤ Igeo < 2 is moderate pollution, (4) 2 ≤ Igeo < 3 is a transitional stage from moderate pollution to intensity pollution, (5) 3 ≤ Igeo < 4 is intensity pollution, (6) 4 ≤ Igeo < 5 is a transition from intensity pollution to severe pollution, and (7) Igeo ≥ 5 is a serious pollution.

2.7. Potential Ecological Risk

In general, some of the pollution indicators for evaluating trace metals in soils are mostly slow, cumbersome, and computationally intensive to calculate. The potential ecological risk index (RI) was proposed and has been widely used to determine the ecological risk of trace metals in soils and sediments [53]. It is a relatively fast, simple, and standardized method. The toxicity risk index for combining multiple trace metals can be determined via the following formula:
R I = ∑ i = 1 m E r i = ∑ i = 1 m T r i × C r i = ∑ i = 1 m T r i × C i C n i
where T r i is the toxic-response factor for trace metals, as suggested by Hakanson, and the values of T r i for the trace metals As, Cr, Cu, Ni, Pb, and Zn are 10, 2, 5, 5, 5, and 1, respectively. Ci is the measured concentration of trace metals in soils. C n i is the background value for trace metals. E r i is the potential ecological risk for single trace metals. The five categories were classified based on E r i values: E r i < 40, low ecological risk; 40 ≤ E r i < 80, moderate ecological risk; 80 ≤ E r i < 160, considerable ecological risk; 160 ≤ E r i < 320, high ecological risk; and E r i ≥ 320, very high ecological risk. The contamination categories were classified based on RI values: RI < 150, low ecological risk; 150 ≤ RI < 300, moderate ecological risk; 300 ≤ RI < 600, considerable ecological risk; and RI ≥ 600, high ecological risk.

2.8. Statistical Analysis

In this study, a one-way analysis of variance was used to test the differences in trace metals and other soil properties between the two habitats of the tide dike. Pearson correlation analysis was also used to verify the relationship between physicochemical properties and trace metals in soils. In this study, principal component analysis (PCA) and redundancy analysis (RDA) were performed on the basis of the concentrations of all trace metals and physicochemical properties in soils. Here, RDA ws carried out in the Canoco 4.5 software package, and other statistical analyses were conducted in the software package of SPSS 19.0 for Windows.

3. Results and Discussion

3.1. Distribution of Trace Metals

Figure 2 shows the trace metal content and distinguishing index of our study site. The overall mean values of trace metals in the soils were shown to be in the order Zn > Cr > Pb > As > Ni > Cu in both habitats. For the same metal in different habitats, the concentration of As in the outside habitat in each soil layer was significantly higher than that in the inside habitat. While the mean concentrations of Cu, Cr, Ni, and Zn in the inside habitat were higher than those in the outside habitat, no statistical difference was found between the two habitats. Pb in the topsoil of the inside habitat was significantly higher than that in the soil of the outside habitat. This discrepancy may result from variations in soil physical properties shaped by different habitats. Previous studies have indicated that, due to different tidal influences, the soil particle size in the inside habitats was significantly coarser than that in outside habitats [42,43]. Smaller particle sizes typically have a larger specific surface area, which undoubtedly enhances the soil’s adsorption capacity for As, resulting in a longer retention time and higher accumulation potential for As in the outside habitat. Recent studies have also shown that As tends to accumulate in soil more readily compared with other metals [54]. In contrast, the coarse-grained soil in the inside habitat has a relatively smaller specific surface area and a lower adsorption capacity for metals such as Cu, Cr, Ni, Pb, and Zn, but these metals may still accumulate in this habitat through sedimentation processes [55]. Due to the inside habitat not being affected by tidal action, the soil structure and sedimentation processes are more stable, which is more conducive to the deposition and retention of these metals. Additionally, in the vertical distribution, the deep soil tends to have a high concentration of trace metals owing to historic human activities and leaching behavior [56]. Nevertheless, only the concentration of Pb in the topsoil was greater than that in the deep soils, while no clear differences were found for most trace metals between the different soil layers, which indicated that the vertical distribution of trace metals was affected by many factors. Here, it is reasonable to further speculate that the categories and sources of trace metals, land use and cover, and anthropogenic activities might all affect the distribution patterns of trace metals in soils.
Compared with the local (Shandong) background values, the average concentrations of Cr, Cu, Ni, Pb, and Zn in all soil layers were found to be lower than their respective background values, and even a few of the samples had higher Cu and Pb than their background values. Concerning the national standard of marine sediment quality (Table 1), the mean values of Cr, Cu, Ni, Pb, and Zn in all soil samples of the inside and outside habitats were lower than the specific values of Class I, suggesting that the sediments are not polluted by those trace metals.
In addition, the average value of As in the soils of the inside and outside habitats was higher than the local background value. Meanwhile, 79% of the observations in the topsoils (0–10 cm) were higher than the local background value. In addition, 74% of the observations in the second soil layers (10–20 cm) and 81% of the observations in the third soil layers (20–30 cm) were higher than the local background value. With reference to the national standard of marine sediment quality, roughly 50% of the soil samples were below the values of Class I, and another 50% of the soil samples lay between Classes I and II. This result demonstrated that the sediment in this region had been contaminated by As.
In previous studies, the PEL was commonly used to evaluate the ecological toxicity of trace metals in soils, which was higher than the possible toxic effects of the PEL on the ecosystem [23,57]. In the two habitats, the PEL values of Cu, Pb, Cr, Ni, and Zn were lower than the PEL threshold; hence, these trace metals do not potentially threaten the plants and animals of Laizhou Bay. However, one should note that 55.7% of the PEL values for As in the soils of the outside habitat were found to be higher than the PEL threshold, whereas 25% in the inside habitat were higher than the threshold, which indicates that As has a potential risk for aquatic organisms in both habitats. In the previous study, As was also found to be more ecologically toxic compared with other trace metals such as Pb, Cu, Cr, and Zn [58], which is consistent with our finding that As has a higher potential risk than other trace metals.

3.2. Enrichment and Accumulation of Trace Metals

In many previous studies, EF was used as an important index to evaluate the degree of pollution [51,59]. Here, no matter what normalization parameter was selected, the average EF values of As, Cr, Cu, Ni, Pb, and Zn decreased in the order of As > Pb > Zn > Ni > Cr > Cu, which indicated that As has the largest enrichment in the sediments of the inside and outside habitats, and the enrichment of Cu is the smallest. Comparing the EF of the same metal in the inside and outside habitats, we found a clear difference in As between the outside and inside habitats (Figure 3). As a petrogenetic element, Al is relatively stable in the Earth’s crust, so it is often used as a reference element to evaluate the enrichment and accumulation risk of trace metals [36]. In this study, when Al was used as the normalized element, the EF values of Cr, Cu, Ni, Pb, and Zn were less than 3 and greater than 1 in both habitats, which shows that the above five trace metals are slightly enriched in this area. The average EF value of As in the inside habitat is 4, whereas it is 6.82 in the outside habitat, indicating that As reaches moderate enrichment in the inside habitat and moderately severe enrichment in the outside habitat.
Although Mn is not a petrogenetic element, it is closely related to other trace elements and is often chosen as a reference element [51,60]. In this study, when Mn was used as a normalized element, the EF values of Cr, Cu, and Ni were found to be less than 1 in both sites, suggesting that the three types of trace metals are not enriched. However, the EFs of Pb and Zn in the inside and outside habitats are 1 ≤ EF < 3, suggesting that Pb and Zn are minor enrichment. The average EF value of As was 2.19 in the inside habitat, but it was 3.60 in the outside habitat, revealing that As is slightly in the inside habitat but moderately enriched in the outside habitat. In summary, regardless of whether the reference element is Al or Mn, the tide dike reduces the enrichment of As in the inside habitat. However, the effects of the tide dike were weak for the enrichment of Cr, Cu, Ni, Pb, and Zn in both habitats, which is possible because As had different sources and migration and diffusion processes compared with other trace metals.
Igeo is widely used in many studies to assess the degree of trace metal contamination in soil [21,61]. Figure 4 shows that the Igeo values of Cr, Cu, Ni, Pb, and Zn in the inside and outside habitats are less than 0; thus, none of these trace metals reached the degree of pollution. Furthermore, the tide dike has no significant effect on the distribution of those trace metals. The Igeo of As is less than 0 in the inside habitat, whereas it lies between 0 and 1 in the outside habitat; this indicates that As is not polluting the inside habitat, but it has changed from no pollution to moderate pollution in the outside habitat, which shows that the tide dike hinders the migration of As from the outside to inside habitats.

3.3. Soil Physicochemical Properties and Their Correlations with Trace Metals

Generally speaking, the salinity of soils in the outside habitat is much higher than that of soils in the inside habitat due to the blocking effect of tide dike [62]. In this study, the existence of the tide dike significantly reduced the salt contents of soils in the inside habitat (Table 2). There are insignificant differences in pH values between the inside and outside habitats, and both areas belong to the typical coastal alkaline soil, which ranges from 8 to 8.5 (Table 2). In addition, the concentrations of SOM and soils P and N in the soils of the outside habitat are much lower than those in the soils of the inside habitat, which suggests that the tide dike was effective in improving the nutrient conditions of the inside habitat because the tide dike can effectively prevent seawater intrusion, which may create a stable environment for plants in the inside habitat, thereby accelerating the soil nutrient mineralization and enhancing soil fertility [63]. When comparing different soil layers, the concentration of SOM decreased with the increase in soil depth, which may be due to the high decomposition rate of plant and animal residues in the surface soil [64]. In both habitats, the topsoil’s N concentration was significantly higher than that of the bottom soil, whereas an opposite trend was found for soil P (Table 2).
The relative abundance of trace metals in the sediment is controlled by various factors (natural and anthropogenic), but the relationships between them are often complex [61,65]. With reference to previous studies, the high correlations between trace metals in sediments may indicate their common sources [65,66]. In this study, a conspicuous positive correlation emerges between Cr, Ni, and Zn, which reflects that these metals may have similar geochemical processes in their source, migration, enrichment, and sedimentary environment. In addition, the positive correlation may also indicate that these trace metals influenced one another in their distribution. Moreover, other trace metals had no significant correlations with each other, indicating their different sources. Many studies have suggested that the mobility of trace metals is strongly affected by the physicochemical properties of soils [21,46]. Thus, the correlations between trace metals and soil physicochemical properties would help in judging the possible origins of trace metals. Here, the positive correlation between As and salt showed that this metal had a close relationship with seawater, suggesting that the concentration of As entering Laizhou Bay may increase with rising salinity levels. Numerous industrial cities are located around Laizhou Bay, including chemical production, metal smelting, and electronic manufacturing industries, which often use or release As and its compounds during production. When wastewater is inadequately treated, it is discharged directly or with minimal treatment through drainage systems or river networks into Laizhou Bay. Moreover, the aquaculture industry in the surrounding area is also one of the main reasons for the increase in As content in the bay area [32]. These external sources should be managed carefully, as the positive correlation between As and salt implies that seawater may serve as a transport medium for As, further exacerbating the risk of As pollution in Laizhou Bay. At the same time, we found that As had a significant and negative correlation with TN in soils. The element N is a limiting element for plant growth [67], and increases in N in soils may effectively promote root expansion and biomass increase in plants. To a certain extent, the absorption of plants may also reduce the concentration of As in the inside habitat. A significant positive correlation existed between Cr and TP (Table 3). Given that P was a sedimentary element [67], we speculated that Cr mainly comes from soil parent material and rock weathering. However, we were unsure of whether the trace metals of Ni and Zn have the same source as Cr, given that they were not significantly correlated with TP. In further studies on this topic, other quantitative ecological methods, such as PCA and RDA, should be used to explore the sources and input pathways of trace metals.

3.4. Sources and Ecological Risk of Trace Metals

In most cases, the correlation analysis does not completely reveal the exact sources of trace metals. Drawing on many previous studies, we found that the PCA method can effectively distinguish the sources and input pathways of trace metals [63,68]. In this study, PCA was carried out using the parameters of trace metals and physicochemical properties of soils.
In Figure 5, a biplot of the PCA loadings shows that Cr, Zn, Al, Mn, and TP are distributed in the positive semi-axis of the first principal component (PC1). Given that Al and Mn are part of the Earth’s crust, they are considered reference elements in many studies [7,51]. Furthermore, P is a sedimentary element, and it mainly comes from soil parent material and rock weathering [53]. Thus, PC1 can be considered a natural factor. In combination with correlation analysis, Cr and TP are significantly and positively correlated, whereas Cr and Zn share some degree of homology. We speculate that the main sources of Cr and Zn are also soil parent material and rock weathering. In addition, Cu, Pb, and Ni are distributed in the positive semi-axis of the second principal component (PC2) and are far away from the geological elements of Al and Mn, which indicates that the trace metals Cu, Pb, and Ni may be derived from anthropogenic sources or natural sources influenced by human activities. Here, combined with their proximity to TN and SOM, we speculate that Cu, Pb, and Ni may be related to nearby agricultural activities, industrial production, or atmospheric nitrogen deposition. In regions with frequent industrial and agricultural activities, nitrogen oxides are widely released into the atmosphere through fossil fuel combustion, agricultural fertilizer application, and other emissions. Nitrogen oxides are converted into nitric acid in the atmosphere, which then deposits on the surface through dry and wet deposition. This sedimentation process not only delivers nitrogen but may also carry trace metal elements introduced by industrial emissions [69]. Studies have shown that Cd, Pb, and Ni are associated with atmospheric particulate matter, which can carry trace metals into soil and water through the adsorption or acidification of nitrogen oxides, thereby affecting their distribution and accumulation in ecosystems [51]. Nevertheless, this study site is located in the nature reserve, and the inside habitat is surrounded by macrophanerophytes of T. chinensis, which have a barrier function against agriculture-related trace metals. However, on the outside habitat of the tide dike, frequent human activities (including industrialization, urbanization, and aquaculture) and the migration of trace metals with water flow have caused the occurrence of trace metal pollution events at the bay. This study also found that these metals and pH levels are located on the positive half-axes of PC2, which shows that Cu, Pb, and Ni may be most likely to come from atmospheric acid deposition (nitric acid). That is, some industrial emissions might affect the concentrations of Cu, Pb, and Ni in soils with rainfall or dry deposition types. Although the concentrations of these metals in both habitats are lower than the background value, we still pay attention to their enrichment in the inside habitat of the tide dike. The trace metal As is distributed in the negative semi-axis of the PC2. As has a significant positive correlation with salt, which suggests that As may come from industrial sewage. Industrial sewage enters the offshore waters through sewage pipes or other seagoing rivers, which increases the concentrations of As in the offshore waters and sediments. Given the blocking effect of the tide dike, the concentrations of As in the soils of the inside habitat are less affected by sewage discharge.
RDA was also conducted, as presented in Figure 6, which shows that the cumulative explanatory information of the first and second ordination axes on the relationship between soil physicochemical properties and trace metal is 78.9%, of which the first and second ordination axes account for 63.6% and 15.3%, respectively. Therefore, the first-order axis can better reflect the characteristics of soil physicochemical properties on trace metals. As shown in Figure 6, the scatters of the outside habitat are mainly located at the right of the RDA1 axis. However, all the samples collected from the inside habitat were scattered to the left part of the RDA1 axis and separated from other samples in the outside habitat (Figure 6), which indicated that the trace metals and physicochemical properties differed sharply between the two habitats of the tide dike. The result of the Monte Carlo test showed that the important values (F) of soil physicochemical properties decreased in the order of salt > soil N > pH > soil P > SOM. In this study, only the effect of salt is significant at the 0.05 level. Thus, we conclude that the tide dike significantly prevents seawater intrusion and reduces the accumulation risk of As in the inside habitat.
The RI value of trace metals in the outside habitat is 31.61, whereas that in the inside habitat is 24.47 (Table 4). Overall, both habitats have low ecological risk. However, the average value of RI in the outside habitat is significantly higher than that in the inside habitat, which indicates that the tide dike can reduce the potential ecological risk in the inside habitat. However, this protective effect is related to the sources of trace metals and their input pathways. In this study, the tide dike has clear effects on the distribution of As, which indicates that the exotic pollutants in seawater can be blocked by the tide dike, but those metals originated from natural sources (soil parent material decomposition and atmospheric deposition) and from land agricultural sources, which may not be affected by the tide dike. According to the ecological risk categories, the ecological risks of As, Cr, Cu, Ni, Pb, and Zn in both habitats are low. However, the contribution values of E r i -As in the inside and outside habitats are 56.39% and 70.28%, respectively. The waste discharge of As-related enterprises should be controlled in the surrounding area of Laizhou Bay.

4. Conclusions

In this study, the tide dike significantly reduced the ecological risk of the inside habitat, but the barrier effect was only evident in the distribution and enrichment of As. Moreover, we found that Cr and Zn may derive from soil parent material and rock weathering and that the accumulation risk of these metals can be neglected in both habitats. We also found that Cu, Pb, and Ni may relate to atmospheric acid deposition, which is easily affected by exhaust emissions. Although these metal concentrations are low in both habitats, we still considered their enrichment in the inside habitat of the tide dike. Finally, we found that As may come from industrial wastewater or river network transport even though the tide dike has barrier effects on this metal, and the discharge of waste from As-related enterprises should be controlled in the surrounding area of Laizhou Bay. This result implies that the barrier effect of the tide dike was significant for exogenous trace metals discharged directly into the sea but was not evident for metals that originated from natural sources. In further research, it will be essential to establish atmosphere deposition sampling points in this area and further investigate other forms (e.g., dissolved forms) of trace metals to accurately access pollutant transport mechanisms. In addition, it is also necessary to further conduct isotope analyses as a tracer tool for trace metal sources. This will help to more accurately identify the sources of pollutants and track the migration paths of metals inside and outside the tidal dike. Regardless, these findings can provide a theoretical basis and data support for the ecological protection and restoration of coastal wetlands.

Author Contributions

Conceptualization, F.L.; data curation, Y.X.; funding acquisition, F.L. and Z.X.; formal analysis, Y.X., X.L., Y.F., Z.X. and F.L.; methodology, Y.X.; investigation, X.L., Y.F., Z.X. and F.L.; project administration, F.L. and Z.X.; writing—original draft preparation, Y.X., J.L. and F.L.; writing—review and editing, Y.X., J.L. and F.L.; visualization, Y.X. All authors have read and agreed to the published version of the manuscript.

Funding

This work is financially supported by the Research on Ecological and Environmental Risk Management for CHN Energy (HBHZ2024Y01) and the Tianjin Research Program of Application Foundation and Advanced Technology (14JCYBJC23000).

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Acknowledgments

The authors are grateful to the Tianjin Key Laboratory of Hazardous Waste Safety Disposal and Recycling Technology, Tianjin University of Technology. The authors are thankful to all the participants for their help in the field and laboratory.

Conflicts of Interest

The authors declare no conflicts of interest.

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Water 16 03230 g001
Figure 1. Location map of our study site and the sampling points. The red box on the map of China represents the research area of this study.
Figure 1. Location map of our study site and the sampling points. The red box on the map of China represents the research area of this study.
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Water 16 03230 g002
Figure 2. The distribution of As (a), Cr (b), Pb (c), Cu (d), Ni (e), and Zn (f) in the inside and outside habitats of the tide dike.
Figure 2. The distribution of As (a), Cr (b), Pb (c), Cu (d), Ni (e), and Zn (f) in the inside and outside habitats of the tide dike.
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Figure 3. Box plots of enrichment factor (EF) of trace metals in the inside and outside habitats of the tide dike. (a) Al is a normalized element, and (b) Mn is a normalized element.
Figure 3. Box plots of enrichment factor (EF) of trace metals in the inside and outside habitats of the tide dike. (a) Al is a normalized element, and (b) Mn is a normalized element.
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Figure 4. Box plots of geological accumulation index (Igeo) of trace metals in the inside and outside habitats of the tide dike.
Figure 4. Box plots of geological accumulation index (Igeo) of trace metals in the inside and outside habitats of the tide dike.
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Figure 5. PCA for soil trace metals and physicochemical properties in the inside and outside habitats of the tide dike.
Figure 5. PCA for soil trace metals and physicochemical properties in the inside and outside habitats of the tide dike.
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Figure 6. RDA for soil trace metals and physicochemical properties in the inside and outside habitats of the tide dike.
Figure 6. RDA for soil trace metals and physicochemical properties in the inside and outside habitats of the tide dike.
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Table 1. Marine sediment quality and background values were used in this study.
Table 1. Marine sediment quality and background values were used in this study.
AsCrCuNiPbZn
Standards for marine sediment quality in China
Class I2080353460150
Class II6515010040130350
Class III9327020040250600
SQGs
PEL41.616010842.8112271
Background values
Shandong		9.3662425.825.863.5
Note(s): SQGs: sediment quality guidelines for the marine ecosystem; PEL: probable effect level.
Table 2. Soil physicochemical properties in the inside and outside habitats of the tide dike.
Table 2. Soil physicochemical properties in the inside and outside habitats of the tide dike.
Soil LayerSOM (mg·g−1)Soil P (mg·g−1)Soil N (mg·g−1)Salt (‰)pH
Outside habitat
0–10 cm9.83 ± 2.970.36 ± 0.050.43 ± 0.128.32 ± 0.348.06 ± 0.50
10–20 cm9.30 ± 1.920.37 ± 0.040.45 ± 0.147.72 ± 0.358.29 ± 0.39
20–30 cm8.18 ± 2.170.35 ± 0.050.39 ± 0.097.62 ± 0.418.29 ± 0.37
Inside habitat
0–10 cm10.32 ± 2.130.37 ± 0.030.53 ± 0.110.73 ± 0.078.13 ± 0.83
10–20 cm9.58 ± 2.200.36 ± 0.040.47 ± 0.070.62 ± 0.048.29 ± 0.75
20–30 cm7.72 ± 3.130.40 ± 0.070.47 ± 0.060.87 ± 0.068.44 ± 0.60
Table 3. Correlation matrix between soil physicochemical properties and trace metals in the inside and outside habitats of the tide dike.
Table 3. Correlation matrix between soil physicochemical properties and trace metals in the inside and outside habitats of the tide dike.
AsCrCuNiPbZnSOMTPTNSaltpH
As1
Cr−0.0931
Cu−0.2160.0881
Ni−0.1040.453 **0.1061
Pb−0.106−0.091−0.0710.121
Zn−0.0780.395 **−0.021−0.055−0.031
SOM−0.166−0.0410.168−0.042−0.050.0031
TP−0.0970.566 **0.1560.07−0.040.2030.0551
TN−0.315 *0.2480.0430.150.0030.1870.320 *0.1611
Salt0.530 *−0.384 **−0.123−0.330 *−0.01−0.268−0.084−0.098−0.432 *1
pH−0.109−0.2320.1910.1810.075−0.1760.222−0.1510.034−0.0571
Note(s): * indicates p < 0.05; ** indicates p < 0.01.
Table 4. Potential ecological risk index (RI) for all trace metals in the inside and outside habitat of the tide dike.
Table 4. Potential ecological risk index (RI) for all trace metals in the inside and outside habitat of the tide dike.
Habitat E r i RI
AsCrCuNiPbZn
Outside habitat22.220.942.212.463.180.6131.61
Inside habitat13.801.052.752.713.480.6924.47
Note(s): E r i is the potential ecological risk index for single trace metal.
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Xia, Y.; Ling, X.; Fang, Y.; Xu, Z.; Liu, J.; Liu, F. Effects of Tide Dikes on the Distribution and Accumulation Risk of Trace Metals in the Coastal Wetlands of Laizhou Bay, China. Water 2024, 16, 3230. https://doi.org/10.3390/w16223230

AMA Style

Xia Y, Ling X, Fang Y, Xu Z, Liu J, Liu F. Effects of Tide Dikes on the Distribution and Accumulation Risk of Trace Metals in the Coastal Wetlands of Laizhou Bay, China. Water. 2024; 16(22):3230. https://doi.org/10.3390/w16223230

Chicago/Turabian Style

Xia, Yuanfen, Xiaofeng Ling, Yan Fang, Zhen Xu, Jiayuan Liu, and Fude Liu. 2024. "Effects of Tide Dikes on the Distribution and Accumulation Risk of Trace Metals in the Coastal Wetlands of Laizhou Bay, China" Water 16, no. 22: 3230. https://doi.org/10.3390/w16223230

APA Style

Xia, Y., Ling, X., Fang, Y., Xu, Z., Liu, J., & Liu, F. (2024). Effects of Tide Dikes on the Distribution and Accumulation Risk of Trace Metals in the Coastal Wetlands of Laizhou Bay, China. Water, 16(22), 3230. https://doi.org/10.3390/w16223230

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