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Review

Occurrence and Speciation of Pollutants in Guilin Huixian Wetland: Nutrients, Microplastics, Heavy Metals, and Emerging Contaminants

by
Hang Gao
1,
Hao Chen
1,
Yue Jin
2,
Ruoting Gao
2,
Chunzhong Wei
3,
Chunfang Zhang
4 and
Wenjie Zhang
1,*
1
Guangxi Key Laboratory of Theory and Technology for Environmental Pollution Control, Guilin University of Technology, Guilin 541006, China
2
Collaborative Innovation Center for Water Pollution Control and Water Safety in Karst Area, Guilin University of Technology, Guilin 541006, China
3
Guangxi Beitou Environmental Protection & Water Group, Guangxi Engineering Research Center for Smart Water, Nanning 530029, China
4
Institute of Marine Biology and Pharmacology, Ocean College, Zhejiang University, Zhoushan 316021, China
*
Author to whom correspondence should be addressed.
Water 2024, 16(19), 2816; https://doi.org/10.3390/w16192816
Submission received: 16 August 2024 / Revised: 20 September 2024 / Accepted: 26 September 2024 / Published: 3 October 2024
(This article belongs to the Special Issue Water Treatment Technology for Emerging Contaminants)
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Abstract

:
The Huixian Wetland is a natural ecosystem of immense ecological value, providing crucial ecosystem services such as water purification, water regulation, and a habitat for the region’s flora and fauna. Its karst peak forest landforms and surrounding environment also possess unique ecological and landscape value. However, with the ongoing socioeconomic development, including the rise of industrial, agricultural, and aquaculture activities in the wetland area, the nutrient composition of the Huixian Wetland has been altered. This paper reviews the current status of nitrogen, phosphorus, heavy metals, emerging pollutants, and biodiversity in various environmental media of the Huixian Wetland. It synthesizes the literature to identify the factors influencing these changes and projects future research directions for the wetland. This work is of significant practical importance, providing scientific foundations for the restoration and protection of the Huixian Wetland.

1. Introduction

Wetlands are considered the most valuable natural ecosystem type, serving as major habitats for wildlife. They provide food, water, and shelter for fish, birds, and other wildlife [1], and offer important ecosystem services, including water quality improvement, flood regulation, and microclimate stabilization (such as reducing greenhouse effects) [2]. It is estimated that wetlands cover a total area of 12.1 million km2, accounting for 40.6% of the global ecosystem services (ES) value. In China, wetlands contribute 54.9% of the national ecosystem service value, indicating their significant role in maintaining global ecosystem functions [3]. To ensure international cooperation on wetland protection and management, many countries signed an international treaty, the Ramsar Convention, in 1971. According to the Ramsar Convention’s classification system, wetlands can be broadly categorized into nine types: coastal wetlands, estuarine wetlands, lacustrine wetlands, riverine wetlands, marshes, peatlands, artificial wetlands, and complex wetlands. China has designated 49 sites as wetlands of international importance [4], with natural wetlands covering 5.58% of the country’s land area [5], primarily consisting of marshes, rivers, and lakes. Huixian Wetland, a comprehensive natural wetland encompassing marsh, river, and lake types, is notable for its karst region characteristics [6]. Huixian Wetland is located in a subtropical monsoon climate zone, with an average annual temperature ranging from 16.5 °C to 20.5 °C. The highest recorded temperature reaches 38.8 °C, while the lowest is −3.3 °C [7]. The annual average rainfall is 1915.2 mm, with distinct wet and dry seasons, during extreme drought years, the lakebed can completely dry up [8]. Additionally, Huixian Wetland is situated in a multi-directional inclined basin, a unique geological structure that provides the region with advantageous hydrogeological conditions. The large-scale fold-fault composite basin forms an enclosed karst reservoir system [9]. The wetland’s northern terrain is characterized by peak-cluster valleys, while the east and west consist of isolated peak plains. The central and southern regions are defined by residual hill plains, primarily located in the area between Phoenix Mountain, Dulong, Mudong, and Doumen, with an elevation ranging from 150 to 160 m, forming the wetland’s core area [10]. Overall, the wetland’s terrain is higher in the north and south, with a lower, depressed center, flanked by low hills and mountains on the east and west. Surface and groundwater flow are influenced by the terrain, with the main catchment areas including the Mudong River and Fenshui Pond, which flow into the ancient Guiliu Canal [11]. The water sources for these catchment areas are derived from the mountainous regions to the north and south. A north-south watershed exists in the center of the wetland, dividing the surface and groundwater flow. Surface water flows eastward into the tributary of the Lijiang River, the Liangfeng River, while groundwater flows westward into the tributary of the Liujiang River, the Xiangsi River [12]. The primary lithology of Huixian Wetland includes limestone, dolomite, mixed limestone/dolomite, and impure carbonate rocks containing clay or other silicate minerals. From a hydrogeological perspective, factors such as groundwater recharge, storage, runoff, discharge, water quality, and quantity play critical roles in the wetland’s hydrological cycle [13]. The karst peak forest landform of Huixian Wetland and its surrounding environment possess unique ecological and scenic value, contributing to enhancing Guilin’s tourism appeal and helping to establish it as a world-renowned travel destination [14]. However, due to a lack of awareness regarding wetland conservation, coupled with rapid economic development and urbanization, the natural wetland area of 25 km2 that Huixian Wetland had in the 1950s has drastically shrunk. As of March 2022, the natural wetland area had diminished to 4.9359 km2 due to human activities and ongoing global climate change [15]. Huixian Wetland, primarily classified as a lake wetland, faces threats such as declining water quality and reduced ecological benefits. It is, therefore, essential to analyze and assess the current state of water quality, as this is the foundation for understanding the wetland’s water environment and managing water quality. The migration characteristics of water-rock interactions and the geochemical evolution of common soluble ions in the water can be inferred by measuring soluble ions such as Ca2+, Mg2+, Na+, K+, Cl−, HCO3−, and SO₄2− [16]. In addition, trace elements such as Al, Ti, Cu, Pb, Zn, and Hg, among others, can be analyzed for environmental assessment and source tracing of the water body [17]. Moreover, the impact of various nutrients on Huixian Wetland does not operate in isolation within a single environmental medium but through an interconnected network of water, soil, and sediments. Wetland soils and sediments serve as important reservoirs of nutrients, playing a critical role in the deposition and release of nutrients, which in turn is one of the key factors influencing changes in water body nutrients. The release of nutrients from soil and sediments directly threatens the growth of aquatic plants and microorganisms. Therefore, in addition to assessing and analyzing the current water quality of Huixian Wetland, it is also necessary to monitor and trace the indicators of soil and sediment to prevent potential harm from nutrient release. In 2012 [15], Huixian National Wetland Park was included in the national pilot wetland park construction initiative, and since then, the local government has launched large-scale wetland restoration projects. These efforts include converting farmland back into wetlands, controlling harmful species, restoring ancient canals, dredging operations, comprehensive environmental remediation, and building rural wastewater treatment facilities. To protect the wetland ecosystem, the “Wetland Protection Law of the People’s Republic of China”, the first national law specifically for wetland protection, officially came into effect on 1 June 2022. On 1 January 2023, the “Regulations on the Protection and Management of Huixian Karst National Wetland Park in Guilin” were also implemented [18]. After restoration efforts, the water quality of Huixian Wetland has now reached Class III or better. This article reviews the current state of nitrogen, phosphorus, heavy metals, emerging pollutants, and biodiversity in the different environmental media of Huixian Wetland. Based on literature analysis, the paper summarizes the factors affecting changes in the wetland and provides an outlook on future research directions. The study is of significant practical value, providing scientific evidence for the restoration and protection of Huixian Wetland.

2. Materials and Methods

A systematic review of existing literature on Huixian Wetland was conducted to gain a comprehensive understanding and assess the pollution status of the wetland. Various methods exist for conducting systematic literature reviews. For instance, Khan et al. (2003) proposed the following five steps: (1) formulating review questions, (2) identifying relevant studies, (3) assessing study quality, (4) summarizing the evidence, and (5) interpreting the findings [19]. This review follows a similar methodology approach.
Keywords such as “Huixian Wetland”, “Huixian Karst Wetland”, “Guilin Huixian Wetland”, and “Huixian Karst Wetland” were used to search the Web of Science and CNKI databases. The search retrieved original research articles, review papers, and research reports related to the topic. This study primarily focuses on the environmental assessment of Huixian Wetland. Relevant information from the retrieved articles, such as titles, authors, publication years, and abstracts, was collected for further evaluation. The relevance of each article was determined based on the authors’ keywords, problem-solving approaches, and the research domain, which had to focus specifically on the environmental status of Huixian Wetland. Articles that did not meet these criteria were excluded.

3. Results

3.1. Water Chemistry Conditions

Due to the previous lack of awareness regarding the protection of Huixian Wetland, the available hydrochemical data is incomplete. Therefore, the more complete hydrochemical data from the rainy season of 2018 and the dry season of 2019 were selected as references to reflect the seasonal variation in the hydrochemistry of Huixian Wetland. Table 1 presents the average hydrochemical values for the wetland during the 2018 rainy season and the 2019 dry season. The pH of the wetland water generally ranges from 6.53 to 9.66, indicating slightly alkaline to moderately alkaline conditions. Well water, compared to other samples, tends to be closer to neutral or slightly acidic. The temperature of the water samples varies significantly with the seasons, and these temperature changes may impact the hydrochemical environment of Huixian Wetland. Due to the karstic nature of the wetland, the primary hydrochemical type is HCO₃−Ca water, with calcium (Ca2+) being the dominant cation, followed by magnesium (Mg2+). The concentrations of Ca2+ and Mg2+ in the water samples reflect this characteristic, though ion concentrations vary greatly between samples. Specifically, the concentrations of Ca2+ in cave drip water and well water is significantly higher than in other samples, suggesting that the water source for the wells is primarily karst groundwater. Additionally, the total dissolved solids (TDS) in the well water are notably higher than in other samples, indicating that the groundwater has undergone more extensive dissolution processes. Moreover, Chen et al.’s study [9] on the source analysis of soluble ions in Huixian Wetland also confirmed its karst wetland characteristics. Their findings indicated that Ca2+, Mg2+, and HCO3− in the water samples mainly originated from the dissolution of carbonate rocks, while SO42− primarily came from the oxidation of pyrite and the dissolution of gypsum. NO3− was primarily sourced from chemical fertilizers, with well water showing significantly higher concentrations of NO3− compared to other samples, likely due to more intensive human activities near the wells. In addition, Na+, Cl−, and K+ concentrations were also higher in the well water samples, further suggesting a connection between these ions and human activity. Li Jun et al.’s study [20] on the temporal characteristics of major hydrochemical ions divided the data into three periods: the wet season, the normal season, and the dry season. The concentration order of the major ions in groundwater across these periods was: normal season > wet season > dry season. Using the Gibbs model, they analyzed the dominant factors controlling the ions and water-rock interactions. The proportions of HCO3−Ca in water samples during the wet, normal, and dry seasons were 77.78%, 77.78%, and 88.89%, respectively. They further noted that K+, Na+, SO42−, NO3−, and Cl− were partially sourced from atmospheric precipitation, while Na+ and Cl⁻ also originated from local residential activities, and K+ was associated with the application of potassium fertilizers in agriculture.

3.2. Spatiotemporal Distribution Characteristics of Nutrients

The accumulation of nitrogen and phosphorus nutrients in water bodies can trigger algal blooms, which consume dissolved oxygen and lead to the mass death of aquatic organisms, ultimately causing eutrophication and disrupting the ecological balance of the aquatic environment [21]. However, nutrients are widely present in various environmental media and influence the surrounding environment. Therefore, understanding the distribution characteristics and transformation of nutrients in surface water, sediments, and riparian soils can provide crucial data for preventing eutrophication in Huixian Wetland. Figure 1 summarizes the spatial and temporal distribution of nutrients in the sediment-water-riparian soil system of Huixian Wetland in 2019, which represents the most recent comprehensive analysis of nutrient spatiotemporal distribution in the wetland. The data offer valuable insights into eutrophication trends. The figure clearly illustrates the temporal variations in nutrient levels across different media. Notably, nitrogen concentrations are highest in surface water and sediments during the wetland water period, while riparian soils have the highest nitrogen content during the low-flow period. Conversely, nitrogen levels are lowest across all media during the wet-season high-flow period. The larger the range of error bars in the figure, the greater the spatial variability in water quality across different sampling points, indicating localized areas with higher nutrient concentrations that affect the overall water quality. This suggests that strengthening point source control is critical to improving the water quality of the wetland. The spatiotemporal distribution of nutrients also exhibits seasonal patterns [22]. According to Han Junlei et al. [23], the total nitrogen (TN), soil organic matter (SOM), and nitrate nitrogen content in river sediments, drawdown zones, and farmland soils are significantly affected by seasonal changes, with marked differences between seasons but minimal influence from interannual variation. The SOM content in river sediments is highest in spring and lowest in autumn, largely influenced by terrestrial input. In contrast, the SOM content in drawdown zone soils is lowest in summer and highest in winter. These seasonal variations in nutrient levels may be related to rainfall, biological growth cycles, and agricultural activities. The evaluation of water body eutrophication typically employs methods such as the Trophic State Index (TSI), Grey Clustering Method, and Comprehensive Trophic State Index (CTSI) [24,25,26]. Before the restoration of Huixian Wetland, its waters experienced eutrophication. From 2009 to 2010, the overlying water in the central area of the wetland during the dry season exhibited eutrophic conditions, ranging from Class IV to Class V [27]. In 2017, Li Luxiang et al. [28] collected surface water samples and applied the Eutrophic Index (EI) to assess the wetland system. The results indicated that the wetland as a whole was in a state of eutrophication, with distinct temporal and spatial characteristics. Temporally, the degree of eutrophication was higher during the dry season than the wet season; spatially, eutrophication levels were ranked as Canal > Fen Shui Pond > Mudong Lake. In October 2017, Li Luxiang et al. [29] measured an annual average TP (total phosphorus) of 0.16 mg/L and an annual average TN (total nitrogen) of 2.55 mg/L. In 2019, Liu Xiaoning [30] reported an annual average TP of 0.09 mg/L and TN of 1.36 mg/L.

3.3. Heavy Metal Content and Pollution Status

Guangxi is rich in mineral resources and is one of China’s top ten regions for non-ferrous metal production. Metal elements can be classified into essential elements and toxic elements. Carcinogenic metals among the toxic elements have been classified as highly hazardous substances by the U.S. Environmental Protection Agency (EPA), which has heightened awareness about the risks of using such metals. However, some toxic elements still pose potential ecological and health risks through food chain transfer, accumulation, and biomagnification [31,32]. Heavy metal pollution occurs when the concentration of heavy metals in the environment exceeds normal levels or when toxic elements enter environmental media.
Due to the unique geological background of the Cambrian limestone bedrock in Huixian Wetland, high concentrations of heavy metals have been detected in the soil [33,34]. However, the highest number of elements exceeding standard values are found in river sediments, followed by lakes and marshes [35]. Research indicates that approximately 85% of heavy metals eventually settle and accumulate in the surface sediments of aquatic environments [36]. The concentration of heavy metals in soils varies with land use types. In 2015, Xu Li et al. [37] analyzed the soil in Huixian Wetland and found that Ni, Cr, and As levels were primarily influenced by natural sources with minimal anthropogenic impact. In contrast, Zn, Cu, Hg, and Cd pollution mainly originated from agricultural activities, industrial wastewater, and domestic sewage, while Pb was mainly influenced by coal burning and transportation pollution. Table 2 summarizes recent studies on heavy metals in Huixian Wetland. In recent years, targeted management of the wetland basin based on health risk assessments of heavy metals has led to a decreasing trend in their concentrations. Figure 2 summarizes the comparison of heavy metal content in the surface water-sediment system of Huixian Wetland from May to July and August to October in 2018. The heavy metal measurement methods for sediments refer to the research by Chen Jing [9]. The data show that heavy metal concentrations are generally higher in summer than in autumn and higher in sediments than in surface water.
In 2018, the concentrations of heavy metals in surface water, including As (<0.05 mg/L), Cr (<0.005 mg/L), Ni (<0.002 mg/L), Cu (<1.0 mg/L), Zn (<1.0 mg/L), and Pb (<0.005 mg/L), met the Class III water quality standards. However, the concentration of Hg slightly exceeded the standard (>0.001 mg/L). In the same year, Li Jun et al. [37,38] conducted a temporal and spatial analysis of heavy metal elements in the wetland water body and reached similar conclusions. They measured Hg concentrations of 1.08 µg/L in well water, 0.78 µg/L in surface river water, and 0.47 µg/L in pond water, all of which exceeded national standards. Only the underground river water samples did not show exceeding metal concentrations. Hg presents a high potential risk in rivers and is a major metal affecting surface water quality, thus warranting strict control. Currently, heavy metal concentrations have all reached or exceeded the Class III water quality standards, and the water quality at all sampling points meets the requirements for agricultural irrigation.

3.4. Pollution Status of Emerging Contaminants

3.4.1. Antibiotic Pollution Status

  • Antibiotics are released into the environment in their original form or as metabolites through feces and urine. In Huixian Wetland, antibiotics have been detected in varying degrees in soil, sediments, and water. Although the concentrations of antibiotics in the environment are generally low and have a weak direct impact on the ecological environment and human health, they can increase bacterial resistance, which poses a risk to human health [40].
  • Xia Feiyang et al. [41] detected four types of antibiotics in surface water and sediments of Huixian Wetland. The average concentrations of these antibiotics during the summer and autumn seasons were as follows: tetracyclines > fluoroquinolones > sulfonamides > antimicrobial enhancers. This distribution was suspected to be influenced by surrounding aquaculture activities. Plants can accumulate antibiotics from surface water sediments, and the average concentrations of the four types of antibiotics in plant tissues were ranked as follows: antimicrobial enhancers > fluoroquinolones > sulfonamides > tetracyclines.
  • In 2018, a total of 12 typical antibiotics were detected in the wetland, with 10 types found in aquaculture water and 8 types in surface water, while 5 types were detected in the soil. Among these, sulfa-dimethoxine (SMD) was the most frequently detected in aquaculture water, posing a high potential ecological risk, while sulfa-chloropyridine (SCP) had the highest concentration in surface water, representing a moderate potential ecological risk. The increase in antibiotic concentrations in water bodies is attributed to external pollution and the development of aquaculture within the wetland [42]. The conversion of most natural lakes and wastelands into fish farms has impacted the water quality and the original hydrological conditions of Huixian Wetland. Livestock farming and fish farms discharge sulfonamides into the environment. To mitigate the ecological and health risks posed by sulfonamides, Qin et al. [43] investigated the presence and spatial distribution of sulfonamides in four main aquatic environments of the wetland (including aquaculture water, ditch water, wetland water, and groundwater). Monitoring results showed that the concentration of sulfonamides in surface water ranged from 0 to 1281.50 µg/L, while in groundwater it ranged from 0 to 20.06 µg/L. The highest concentration was found in ditch water, and the lowest in groundwater. Ecological and human health risk assessments using green algae indicated that sulfamethoxazole and sulfamethoxypyridazine posed high ecological risks in both surface and groundwater, with sulfamethoxazole being particularly risky in ditch water. Therefore, sulfonamides pose an ecological risk to the wetland system and require effective pollution control measures.

3.4.2. Organochlorine Pesticides (OCPs) Content and Pollution Status

  • Agriculture is the primary economic source around the Huixian Wetland. Organochlorine pesticides (OCPs), known for their high efficiency and low cost, are widely used in agricultural activities. However, the extensive use of these pesticides affects OCP concentrations in water, soil, and the atmosphere [44,45]. Additionally, atmospheric transport and deposition processes, such as wet and dry deposition, also play a significant role in influencing OCP levels in the study area [46].
  • Research on OCPs in the water and soil of Huixian Wetland has already been conducted. Table 3 Range of Organochlorine Pesticides (OCPs) Concentrations in Different Environmental Media of Huixian Wetland. In 2016–2017, Fu Xin et al. [47] conducted the first study on OCPs in the water bodies of Huixian Wetland. The average total OCP residues in lakes, ditches, and shallow groundwater were 137 ng/L, 137 ng/L, and 38.6 ng/L, respectively. Analyzing the concentration ranges and standard deviations, the results indicate that OCP pollution in surface water is at a high level, while in shallow groundwater, it is at a moderate level. Hexachlorocyclohexane (HCHs) was found to be the dominant pollutant, accounting for over 61.7% of the total OCPs. The risk assessment results revealed that OCPs posed medium to high risks to the aquatic ecosystem.
  • Additionally, the temporal and spatial analysis showed that OCP concentrations were highest during the summer in agricultural areas, likely due to the concentrated planting of rice during this period. As agricultural land is the primary land use type surrounding the wetland, it is essential to evaluate the ecological risk of OCPs in the soil. Cheng Cheng et al. [46] collected soil samples from different land use types and confirmed that agricultural activities significantly contributed to elevated OCP levels. Component analysis showed that the main OCPs in agricultural soil samples were DDTs and MXC (methoxychlor), accounting for 39.2% and 30.5%, respectively. Fortunately, the overall OCP levels in the surface soils of the study area were relatively low, and the ecological risks were also minimal.
  • Yu Yue et al. [48] conducted a health risk assessment based on Monte Carlo simulations. The results indicated that while OCPs in water posed a potential health risk to humans, the risk was within an acceptable range, and the residual levels were insufficient to cause non-carcinogenic harm. Currently, OCPs pose a low to moderate ecological risk; however, potential risks still exist. Therefore, proper management of OCPs usage in agricultural areas is necessary to mitigate further risks.
Table 3. Range of Organochlorine Pesticides (OCPs) Concentrations in Different Environmental Media of Huixian Wetland.
Table 3. Range of Organochlorine Pesticides (OCPs) Concentrations in Different Environmental Media of Huixian Wetland.
Sampling TimeMediumConcentration RangeAverage ConcentrationReference
August 2018–January 2019Wetland LakeND–182.59(ng/L)11.633(ng/L)[49]
August 2018–January 2019Agricultural Ditch WaterND–146.636(ng/L)16.813(ng/L)[49]
November 2019Surface Soil3.56–69.7(ng/g)14(ng/g)[46]
December 2019 (Dry Season)Water4.33–47.30(ng/L)12.2(ng/L)[48]
August 2020 (Wet Season)Water3.17–92.50(ng/L)21.15(ng/L)[48]
December 2019 (Dry Season)Surface Sediment2.99–219.52(ng/g)39.25(ng/g)[48]
August 2020(Wet Season)Surface Sediment1.12–56.16(ng/g)10.80(ng/g)[48]
August 2020Well Water (Farmhouses)5.76–15.4(ng/L)10.58(ng/L)[48]
Note(s): ND: No detection.

3.4.3. Microplastic Pollution Status

  • Microplastics are emerging pollutants, defined as plastic fragments and particles with a diameter of less than 5 millimeters [50]. In recent years, microplastics have been reported in various environments, including oceans [51], lakes [52], terrestrial areas [53], and wastewater [54]. Microplastics can enter the environment through atmospheric deposition, tides, aquaculture, wastewater treatment plant discharges, and agricultural waste. They accumulate in organisms and pose risks to them through biological magnification via the food chain. The transport of microplastics in aquatic environments is influenced by factors such as density, shape, size, and hydrodynamics of the water system. The most common polymers found are polyethylene (PE), polypropylene (PP), and polystyrene (PS), followed by polyethylene terephthalate (PET) and polyamide (PA) [55]. In Huixian Wetland, the predominant microplastic polymers are PE and PP. Due to their lower density, PE and PP are more likely to migrate in surface water, resulting in higher proportions of these polymers in surface water compared to sediment.
  • Table 4 summarizes the distribution of microplastics in Huixian Wetland. The abundance of microplastics in the wetland is spatially correlated with population density. Areas like Hehuatang Wharf, Maojia Wharf, and Qixing Wharf, which have higher population densities due to concentrated economic activities and better transportation, exhibit significantly higher microplastic abundance. High-intensity human activities, whether in urban or rural areas, tend to create hotspots for microplastics. Tourist areas, fishing and agriculture-intensive zones, and densely populated villages have higher microplastic abundances compared to other regions.
  • Chen Yan [56] found that the temporal characteristic of microplastics in Huixian Wetland shows higher abundance during the dry season compared to the wet season. The abundance of microplastics in sediments varies between different water periods, primarily influenced by hydrological factors. Frequent rainfall during the wet season disrupts riverbed sediments, releasing microplastics from sediments into the water, thereby reducing the sediment microplastic abundance. In the water and sediments, transparent microplastics constitute 32.02% and 39.35% respectively, followed by black (17.97% and 17.75%), with white, red, blue, purple, green, and yellow in descending order of abundance. Feiyang Xia [57] found that underwater exchange processes capture or retain more particles smaller than 100 µm in sediments, while microplastics sized 100–500 µm are more prevalent in surface waters. Additionally, the microplastics’ surfaces contain inorganic non-metallic elements like C, O, B, Si, and various metallic elements such as Al, Fe, Ca, Zr, and platinum. Huixian Wetland acts as a sink for microplastics, with primary sources including rural domestic wastewater discharge, aquaculture, agricultural production, and tourism activities. Point source emissions are the major contributors of microplastics to the wetland. To protect the wetland ecosystem, controlling the emission of microplastics from their sources is essential.
Table 4. Distribution of Microplastics in Huixian Wetland.
Table 4. Distribution of Microplastics in Huixian Wetland.
Sampling TimeMediumAbundancePhysical CharacteristicsPolymer TypeReference
October 2019
(Dry Season)		Surface Water16.5 ± 4.4 − 89.0 ± 14.2
items/L		Primarily ranging from 50 to 500 µm; consisting of fibers, films, and fragments; surfaces exhibit fractures and cracksPE:37.6%
PP:24.7%
PVC:15.3%
PA:11.8%
PS:10.6%		[57]
October 2019
(Dry Season)		Sediment(16.8 ± 14.0) × 103 − (52.8 ± 5.1) × 103
items/kg		Primarily ranging from 50 to 500 µm; consisting of fibers, films, and fragments; surfaces exhibit fractures and cracksPE:32.2%
PP:23.0%
PVC:18.4%
PA:14.9%
PS:11.5%		[57]
May 2021
(Wet Season)		Surface Water5466.7 − 24,333.3
n/m3		0.45 μm to 0.5 mm in diameter: 29.43%
Fibrous: 55.59%
Film-like: 24.37%
Fragmented: 19.81%
Granular: 0.23%		PE:37.99%
PS:14.44%
PP:25.12%
PET:17.43%		[56]
October 2021
(Dry Season)		Surface Water12,713.3 − 34,906.7
n/m3		0.45 μm to 0.5 mm in diameter: 34.80%
Fibrous: 52.63%
Film-like: 26.14%
Fragmented: 21.06%
Granular: 0.17%		PE:42.78%
PS:17.85%
PP:22.55%
PET:14.23%		[56]
May 2021
(Wet Season)		Sediment3380.0 − 14,533.3
n/kg		0.45 μm to 0.5 mm in diameter: 36.02%
Fragmented: 43.05%
Film-like: 33.11%
Fibrous: 28.28%
Granular: 0.36%		PE:26.84%
PS:21.51%
PP:16.80%
PET:29.05%		[56]
October 2021
(Dry Season)		Sediment11,866.7 − 42,486.7
n/kg		0.45 μm to 0.5 mm in diameter: 40.41%
Fragmented: 38.25%
Film-like: 33.18%
Fibrous: 23.67%
Granular: 0.10%		PE:30.50%
PS:19.76%
PP:13.94%
PET:32.37%		[56]

3.5. Biodiversity Characteristics

The ecological functions of wetlands are paramount, offering optimal habitat conditions for a wide range of flora and fauna, particularly serving as valuable habitats for wildlife and contributing to biodiversity conservation. Surveys from 2006 to 2009 recorded 150 bird species across 29 families and 39 fish species across 16 families in the wetland. However, follow-up surveys from 2017 to 2018 revealed a decrease in the encounter rate of protected bird species [46]. Additionally, the extensive vegetation in wetlands plays a crucial role in regulating local climates, and the carbon sequestration abilities of plants and microorganisms are significant aspects of carbon sink research. Huixian Wetland has also made progress in carbon sink studies [58,59,60,61,62]. Yan et al. [61] investigated the carbon sequestration capacities of microalgae and carbonic anhydrase (CA) in situ, laying the foundation for studying the wetland’s carbon sink potential. Their results showed that microalgae convert inorganic carbon to relatively stable organic carbon at an average rate of 4207.5 t C/a. The main environmental factors influencing microalgae’s carbon sequestration capacity include water CA activity, light, temperature, total phosphorus, and total nitrogen.
The wetland vegetation predominantly consists of emergent and submerged plants, with high plant diversity and growth, often achieving coverage of 80%–95%. Key species include Common Reed (Phragmites communis), Chinese Tallow (Cladium chinense), Hornwort (Ceratophyllum demersum var. oryzelorum), and Limnophila (Limnophila sessiliflora) [63]. Different plant species exhibit varied physiological responses, leading to diverse plant community characteristics [64]. However, the wetland’s ecological structure is degrading annually, with some nationally protected plants such as Water Shield and Wild Rice becoming rare. Many plant communities have become fragmented, which could further impact the wetland’s biogeochemical cycles. Therefore, studying the bacterial community structure and diversity in wetland soils and sediments from a microbial perspective is crucial [65,66,67,68]. Tu Yue et al. [66] analyzed bacterial communities associated with different plant rhizospheres, identifying 10 major bacterial phyla, with Proteobacteria being the dominant phylum, averaging 42.4%. The dominant class within this phylum is β-Proteobacteria, averaging 13.5%, and the most prevalent genus is Bacillus, accounting for 3.8% of the total genera. Winter showed the highest bacterial richness and diversity in Vallisneria, while Canna had the lowest in autumn. Overall, bacterial diversity and richness in soil are higher in winter compared to autumn.

3.6. Protection Measures

In 2012, the Huixian National Wetland Park was included in the national pilot program for wetland parks, prompting the local government to initiate large-scale wetland restoration projects. To ensure that the surface water and the water in the ancient Guigui and Liuyun Canals meet the required water function zoning standards, several measures were implemented:
  • Livestock and Poultry Farming Pollution: All livestock and poultry farming enterprises within the wetland area were required to relocate within a set timeframe. Enterprises on both sides of the Guigui and Liuyun Canals without environmental approval were shut down in accordance with the law. Those with environmental approval were required to manage waste following principles of reduction, resource utilization, and harmlessness. Strict measures were enforced to prevent livestock wastewater from entering the Guigui and Liuyun Canals.
  • Agricultural Pollution: The government encouraged and promoted the use of biological pesticides or highly effective, low-toxicity, and low-residue pesticides. Integrated pest management, biological control techniques, and scientific fertilization were advocated, including soil testing and formulation-based fertilization. Farmers were also encouraged to recycle agricultural film.
  • Solid Waste Management: The sale and use of single-use foam food containers and non-degradable plastic bags were banned within the wetland area. A comprehensive cleanup of waste was conducted, and necessary disinfection was carried out in areas with severe waste pollution to prevent disease spread.
  • Biodiversity Restoration: Efforts were made to restore aquatic biodiversity by utilizing the existing micro-topography and landforms of lakes and rivers. This included planting submerged, floating-leaved, emergent, and wetland plants to restore and reconstruct a complete aquatic plant community structure. The goal was to restore the wetland ecosystem’s functions by leveraging the physical, chemical, and biological interactions among substrates, aquatic plants, and microorganisms to purify the water through filtration, interception, adsorption, sedimentation, ion exchange, plant absorption, and microbial decomposition.
As a result of these efforts, the water quality of Huixian Wetland has improved year by year. By September 2020, the water quality had reached above Class III standards according to the Surface Water Environmental Quality Standard (GB3838-2002) [69].

4. Discussion

The primary water chemistry type of the Huixian Wetland is HCO3−Ca, with Ca2+ being the dominant cation, followed by Mg2+. The main anion is HCO3−, with SO42− being secondary. The concentration of major ions in groundwater varies seasonally, following the order: normal water period > flood season > dry season. The concentrations of total nitrogen, organic matter, and nitrate nitrogen in river sediments, drawdown zones, and agricultural soils are significantly influenced by seasonal changes, which may be related to seasonal rainfall, biological growth cycles, and agricultural activities. Heavy metal concentrations are generally higher in summer than in autumn, with sediments having higher levels than surface water. Currently, the water quality of the Huixian Wetland meets or exceeds Class III standards according to the Surface Water Environmental Quality Standard (GB3838-2002). Moreover, conventional monitoring indicators, emerging pollutants such as antibiotics, organochlorine pesticides (OCPs), and microplastics are also of concern. Twelve typical antibiotics have been detected in the wetland, with sulfonamides posing the highest ecological risk. The highest OCP concentrations are found in agricultural areas during the summer sowing season. Currently, OCPs pose a low ecological risk and their residual levels are insufficient to cause non-carcinogenic risks to humans. Microplastics in the wetland are primarily composed of polyethylene (PE) and polypropylene (PP), with abundance correlating with population density. Sewage discharge is the main source of microplastics. Extensive vegetation on the wetland surface helps regulate the local climate. Microalgae convert inorganic carbon into relatively stable organic carbon at an average rate of 4207.5 t C/a, with water CA activity, light, temperature, total phosphorus, and total nitrogen being the main environmental factors influencing this process.
In recent years, research on the ecological environment of the Huixian Wetland has increased, focusing on microbial community characteristics and diversity, the relationship between soil and microbial activity, and the carbon sequestration capabilities of aquatic plants. Carbon sequestration is a crucial aspect of the transition between source and sink functions in wetlands, with its capacity significantly surpassing that of terrestrial and marine systems. The potential for carbon sequestration in the Huixian Wetland requires further exploration, especially regarding anaerobic ammonia-oxidizing bacteria, which have been successfully applied in wastewater treatment. The carbon sequestration potential of these bacteria in wetlands remains largely unexplored. Therefore, future research on the Huixian Wetland should focus on the community characteristics of carbon-sequestering bacteria in different environmental media, the carbon sequestration potential of these bacteria, and the potential of anaerobic ammonia-oxidizing bacteria for carbon sequestration in wetlands.

Author Contributions

Conceptualization, H.C. and Y.J.; methodology, H.C.; software, H.G.; validation, H.C., C.W., R.G. and C.Z.; formal analysis, R.G.; investigation, H.G.; resources, W.Z.; data curation, H.C.; writing—original draft preparation, H.C.; writing—review and editing, H.C. and W.Z.; visualization, H.C. and W.Z.; supervision, W.Z.; project administration, W.Z.; funding acquisition, W.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This study is funded by the National Natural Science Foundation of China (Grant No. 52360004), Guangxi Engineering Research Center of Comprehensive Treatment for Agricultural Non-Point Source-Pollution, Modern Industry College of Ecology and Environmental Protection, Guilin University of Technology.

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy.

Conflicts of Interest

The authors declare no conflicts of interest.

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Water 16 02816 g001
Figure 1. The spatiotemporal distribution of nutrients in the sediment-water-riparian soil system of the Huixian Wetland in 2019: (a) The spatiotemporal distribution of nutrients in the groundwater system; (b) The spatiotemporal distribution of nutrients in riparian soils; (c) The spatiotemporal distribution of nutrients in sediments.
Figure 1. The spatiotemporal distribution of nutrients in the sediment-water-riparian soil system of the Huixian Wetland in 2019: (a) The spatiotemporal distribution of nutrients in the groundwater system; (b) The spatiotemporal distribution of nutrients in riparian soils; (c) The spatiotemporal distribution of nutrients in sediments.
Water 16 02816 g001
Water 16 02816 g002
Figure 2. Summarizes the comparison of heavy metal content in the surface water-sediment system of Huixian Wetland from May to July and August to October in 2018: (a) Comparison of heavy metal concentrations in the surface water; (b) Comparison of heavy metal concentrations in the sediment system.
Figure 2. Summarizes the comparison of heavy metal content in the surface water-sediment system of Huixian Wetland from May to July and August to October in 2018: (a) Comparison of heavy metal concentrations in the surface water; (b) Comparison of heavy metal concentrations in the sediment system.
Water 16 02816 g002
Table 1. Average Water Chemistry Values of Huixian Wetland during the Rainy Season of 2018 and the Dry Season of 2019.
Table 1. Average Water Chemistry Values of Huixian Wetland during the Rainy Season of 2018 and the Dry Season of 2019.
GrouppHTemperature(mg/L)
DOK+Na+Ca2+Mg2+Sr2+NO3−Cl−SO42−HCO3−TDS
Rainy
Season		1n = 19.737.015.94.51.215.91.70.010.027.211.440.262
2n = 117.428.43.83.41.451.98.30.035.986.738.1143.1187
SD0.21.11.60.90.527.315.40.012.860.884.656.4149
3n = 37.729.88.32.91.451.83.60.037.536.410.7152.1160
SD0.10.02.50.50.31.60.20.000.420.40.43.94
4n = 18.021.38.50.00.280.10.10.030.084.09.8225.3207
5n = 137.321.54.314.75.988.017.30.0735.1912.492.5220.2376
SD0.41.61.217.24.449.839.50.0330.616.0232.991.6329
Dry
Season		1n = 18.99.310.62.11.150.54.40.0326.076.810.7152.5184
2n = 117.69.77.83.41.751.17.00.0415.356.727.2148.2191
SD0.30.61.41.51.712.05.00.029.792.532.538.456
3n = 37.610.77.53.11.276.35.30.0416.466.414.4220.6238
SD0.00.10.60.40.23.10.60.001.660.40.88.73
4n = 17.913.510.2NDND77.00.30.030.171.19.6221.8199
5n = 137.515.14.813.96.694.126.10.0732.1712.8160.0239.5473
SD0.41.71.617.55.563.865.30.0324.2610.6460.092.7589
Note(s): Group 1: Fishpond Water; Group 2: River and Lake Water; Group 3: Shiziyan Underground River; Group 4: Cave Drip Water; Group 5: Well Water; n: number of samples; SD: standard deviation; ND: No detection.
Table 2. Research related to heavy metal pollution in the Huixian wetland.
Table 2. Research related to heavy metal pollution in the Huixian wetland.
Sampling YearCollection SiteAverage Concentration Order of Metal ElementsPollution AssessmentTemporal/Spatial Scale CharacteristicsHealth Risk AssessmentReferences
2018,
2019		Important well points in the central wetland areaMn, Fe, Zn, Al, Hg, Cr, Cu, Cd, As, PbFlood period: Mn pollution level VI, Cd, Al, Zn, Fe pollution levels all III; Normal water period: Hg pollution level VI, Al pollution level IIIDry period water quality is better than flood and normal water periods; Hg and Pb concentrations have obvious temporal characteristicsCr is a major carcinogenic metal; control Mn, Hg, and Cr to some extent[37]
2019Well water, surface river water, underground river water, and karst spring waterAl, Mn, Zn, Cr, Ni, As, Hg, Cu, Pb-Well water and underground river water quality are generally better than surface river water and karst spring waterCr is a major carcinogenic metal; control Hg and Cr to some extent[38]
-Soil samples from corn fields, rice fields, rapeseed fields, vegetable fields, and forests; Sediments from rivers, swamps, lakes, farmland ditches, and fish pondsCd, Ni, As, Pb, Cr, Hg, Zn, CuCd is the highest pollution level in the area; rivers, swamps, lakes, and rapeseed fields have moderate pollution, fish ponds, corn fields, rice fields, and farmland ditches have slight pollutionRivers and rapeseed fields have the highest ecological risk; forests have the lowest ecological risk, and other land use types have moderate ecological riskCd poses considerable potential ecological risks in rivers, swamps, lakes, fish ponds, corn fields, rice fields, and rapeseed fields; Hg has higher potential risk in rivers[34]
2018–2019River surface waterAl, Cu, Pb, Zn, Cr, Cd, Ni, Mn, As, HgOverall water quality is good, but some sampling points have seasonal pollution, with pollution levels ranging from mild to moderate, and the highest pollution level in flood period surface waterMost metal ions show seasonal variation as flood period > normal water period > dry period; Hg shows normal water period > flood period > dry period-[39]
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Gao, H.; Chen, H.; Jin, Y.; Gao, R.; Wei, C.; Zhang, C.; Zhang, W. Occurrence and Speciation of Pollutants in Guilin Huixian Wetland: Nutrients, Microplastics, Heavy Metals, and Emerging Contaminants. Water 2024, 16, 2816. https://doi.org/10.3390/w16192816

AMA Style

Gao H, Chen H, Jin Y, Gao R, Wei C, Zhang C, Zhang W. Occurrence and Speciation of Pollutants in Guilin Huixian Wetland: Nutrients, Microplastics, Heavy Metals, and Emerging Contaminants. Water. 2024; 16(19):2816. https://doi.org/10.3390/w16192816

Chicago/Turabian Style

Gao, Hang, Hao Chen, Yue Jin, Ruoting Gao, Chunzhong Wei, Chunfang Zhang, and Wenjie Zhang. 2024. "Occurrence and Speciation of Pollutants in Guilin Huixian Wetland: Nutrients, Microplastics, Heavy Metals, and Emerging Contaminants" Water 16, no. 19: 2816. https://doi.org/10.3390/w16192816

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