Heavy Metal Contamination and Ecological Risk Assessments in Urban Mangrove Sediments in Zhanjiang Bay, South China

Abstract

With the acceleration of industrialization and urbanization, increasing attention has been paid to the problem of heavy metal pollution in mangroves and its ecological restoration. Urban mangroves can be used to measure the impact of human activities on the urban ecological environment because mangroves are sensitive to human activities. However, studies on the evaluation of heavy metal elements in urban mangroves are still limited. Consequently, this study selected the urban mangroves in a central commercial area of Zhanjiang Bay as a case study to investigate the content and distribution of the heavy metals (Co, V, Cu, Pb, Ni, As, Cd, and Hg) in mangrove surface sediments. Risk levels and possible sources of heavy metals were evaluated based on multivariate statistical analysis methods and pollution indices. The results showed that the average concentrations of heavy metals for Co, V, Cu, Pb, Ni, As, Cd, and Hg were 2.91, 29.96, 18.24, 20.07, 7.86, 5.0, 0.20, and 0.09 mg/kg, respectively. Cd, Cu, and Hg were most prominent within the Zhanjiang Bay mangrove sediments, whereas other metals showed a low contamination factor of therm. Cd displayed a high potential ecological risk followed by Hg and Cu. The sampling site, the sewage outlet sampling site, exhibited the highest pollution degree followed by the surrounding area of the sewage outlet sampling site. Those polluted heavy metals could arise from anthropogenic sources, including domestic sewage and automobile exhaust emission. Correlation analysis between the heavy metals and physicochemical properties indicated that fine particles and organic matter play a key role in controlling heavy metal enrichment.

1. Introduction

Mangrove is a woody plant community that grows in the coastal intertidal zone of the transitional area between land and sea, which is dominated by evergreen trees or shrubby mangrove plants. It plays an important role in preventing wind and waves, purification of seawater, and carbon sequestration and storage.¹ However, rapid urbanization, industrialization, and industrial and agricultural production activities in coastal areas in recent decades have generated a large number of heavy metal pollutants, which flow into the ocean along with surface runoff and directly discharge nearshore, causing serious destruction to mangroves.²⁻⁴ The mangrove wetland ecosystem has a strong interception, adsorption, and fixation effects on heavy metals due to its unique habitat characteristics, and thus, it often becomes a pollution sink for heavy metal pollutants.⁵⁻⁷ Heavy metal pollutants have been a hot topic in research since the heavy metals have diverse sources, strong resistance to decomposition, long residual time, high toxicity, etc., and may even cause great harm to biological and human health through the food chain or other migration pathways.^8,9 Up to now, numerous studies have been conducted on the pollution of heavy metals in mangrove ecosystems in diversified regions of the world.^2,10,11 However, previous studies are principally concentrated in industrial areas and ecological protection areas, and few studies have been carried out on heavy metal pollution of mangroves in the urban central business district. Therefore, this study selected the Zhanjiang Bay mangroves in the central business district of Zhanjiang City, Guangdong Province as a research object, covering the shortage of data for heavy metal pollution research.

The Guangdong Zhanjiang Mangrove Nature Reserve is the largest natural distribution area of mangroves in mainland China.¹² With the speeding up of industrialization and urbanization in Zhanjiang City, aquaculture, industrial pollution, vessel pollution, and domestic pollution in the bay increased by years, resulting in a rapid accumulation of heavy metal pollutants in mangrove sediments, although the mangrove wetlands have a certain tolerance for heavy metal pollutants. As a result, the coastal mangrove wetland in Zhanjiang City has increasingly severe environmental pressure.^13,14 In this study, the heavy metal content in 25 surface sediment samples from Zhanjiang Bay mangroves in the central urban area of Zhanjiang City in South China has been measured using ICP-MS, elemental analysis, and grain size analysis, aimed at acquiring a better understanding of mangroves in a central urban area: (i) status and characteristics of heavy metal pollution; (ii) comprehensive evaluation of ecological environment quality and source-to-sink analysis of heavy metal pollutants; (iii) to lay a theoretical foundation for pollution prevention and control, more specifically, for reasonable prevention and control of heavy metal pollution in mangrove wetland sediments.

2. Materials and Methods

2.1. Research Area

The Zhanjiang Bay mangrove forest is located on the northwest bank of Zhanjiang Port, back against the downtown area of Zhanjiang City. This area has a mild climate, fertile water, and abundant plankton. It is subtropical with an average annual temperature of 24.1 °C, which is well suitable for mangrove growth. This region is an artificially planted mangrove forest, including Sonneratia apetala, Kandelia candel, and Avicennia marina, and it is close to the center of Zhanjiang City (Figure 1).

Figure 1.

Study area and sampling sites.

2.2. Sample Collection and Analysis

Mangrove surface sediment samples of Zhanjiang Bay were collected in January of 2020. Twenty-five sampling sites were set up in four sections parallel to the ocean from east to west to clearly understand the distribution characteristics of heavy metals from sea to land in mangroves in this region (Figure 1), of which the JA sample was collected from a sewage outlet. The samples were collected in the surface soil of 0–5 cm thick after clearing away the surface debris during sampling. Each sample was mixed up homogeneously before being sealed in a clean sample bag.

2.2.1. Measurement of Heavy Metal Elements in Mangrove Sediments

Varian 820 ICP-MS was used for the determination of the trace element concentration in surface sediments. The experimental procedure is as follows: 0.04 g samples were weighed accurately and put into a Teflon cup before adding 1.5 mL of hydrofluoric acid and 0.5 mL of nitric acid. The cup was then sealed and placed in an oven at 180 °C for 12 h. Evaporation of acid was performed on an electric hot plate kept at 150 °C after the cup was taken out from the oven and cooled down to room temperature. Similarly, the cup was again added with 1 mL of nitric acid and 1 mL of water before being sealed in the oven at 150 °C for 12 h. Finally, the cup was taken out and cooled down to room temperature. The samples were weighed and diluted to 40 g (at a dilution ratio of about 1000) for the determination of trace elements through ICP-MS. The analytical accuracy and precision were usually within ±10% (RSD) of the certified values, and the concentrations were calibrated against several U.S. Geological Survey (USGS) and Chinese certified reference standards (BHVO-2, BCR-2, GBW07314, GBW07315, and GBW07316).¹⁵ Previous studies have proved that the microwave digestion method has high precision and accuracy in the determination of heavy metals.¹⁶

2.2.2. Elemental Analysis

Each sample of 0.5 g was put into the centrifuge tube, and 5 mL of hydrochloric acid was added for a chemical reaction of 8–12 h, during which it was mixed up two to three times on a vortex mixer. The supernatant was carefully sucked out with a dropper, and 5 mL of deionized water was added to wash the sample after the supernatant was centrifuged for 6 min (2000 r/min) and discarded. The above procedure was repeated five to six times until the pH value of the supernatant was close to neutral pH. The acidified samples were freeze-dried in a vacuum for 48 h. All the dried samples were taken out, ground, and mixed, before being placed in a stainless steel crucible fired at 450 °C, to remain dry and preserve for measurement. The organic carbon and nitrogen elements in the sediments were determined using an Elementar varioMAXCN elemental analyzer with errors of ±0.02 and ±0.005%, respectively.¹⁷

2.2.3. Physical Parameters

pH values of sediment samples were determined using a glass electrode pH meter (E-201-L, pH composite electrode). For grain-size analysis, ∼0.2 g samples were pretreated using the 30% H₂O₂ and 1 mol/L HCl to remove organic matter and carbonates, respectively, and then dispersed by ultrasonication in a 10 mL 10% 0.05 mL/L (NaPO₃)₆ solution. A grain size analyzer (PSA, Coulter, model L 230) was used to measure the grain size of the soil sediments with an error of less than 3%.¹⁸⁻²⁰ Particle size was divided and named using Sheppard and Foker trigonometry.

2.2.4. Statistical Analysis

Excel was used for data processing, Surfer was used for spatial distribution analysis, and SPSS was used for correlation analysis, principal component analysis, and cluster analysis.

2.3. Evaluation Methods

The sediment quality used the geological accumulation index, contamination factor (CF), and potential ecological risk index to evaluate the degree of heavy metal pollution. They are defined as follows in Table 1.

Table 1. Pollution Indices Used in the Present Study and Their Classifications.

pollution indicators	procedures of calculation and classifications
geoaccumulation index (Igeo)21−23	Igeo = Log2(Ci/(1.5 × Bi))
	Ci is the measured value of heavy metal (i) in the sediments, Bi is the geochemical background value of the heavy metal (i) in sedimentary rocks, Bi is using the background value of soil inorganic elements in Guangdong Province during “The 7th Five-Year Plan” (Co = 7.0 mg/kg, V = 65.3 mg/kg, Cu = 17 mg/kg, Pb = 36 mg/kg, Ni = 14.4 mg/kg, As = 8.9 mg/kg, Cd = 0.056 mg/kg, and Hg = 0.078 mg/kg),24 and the 1.5 is the coefficient to decrease the effects of variations in the background values; Igeo can be divided into seven grades:
	Igeo ≤ 0	0 < Igeo ≤ 1	1 < Igeo ≤ 2	2 < Igeo ≤ 3	3 < Igeo ≤ 4	Igeo > 4	Igeo > 5
	unpolluted	unpolluted to moderately polluted	moderately polluted	moderately to heavily polluted	heavily polluted	heavily to extremely polluted	extremely polluted
contamination factor (CF)23,25,26	CF = Ci/Cn
	Ci is the heavy metal (i) content measured in sediments, and Cn is the reference value of heavy metal (i) in the study using the background value of soil inorganic elements in Guangdong Province during “The 7th Five-Year Plan”; CF values can be divided into four groups:
	CF < 1	1 ≤ CF < 3		3 ≤ CF < 6	CF ≥ 6
	low contamination	moderate contamination		considerable contamination	very high contamination
potential ecological risk index (RI)25,26
	Eri is the potential ecological risk coefficient of single heavy metal (i), pointing to the ecological risk level of a single pollutant, and Tr is the response factor of the heavy metal toxicity coefficient in the order Co = Cu = Pb = Ni = 5, V = 2, Cu = 5, Pb = 5, Ni = 5, As = 10, Cd = 30, and Hg = 40. The degree of ecological risk can be classified into the following levels:
	Eri < 40	40 ≤ Eri < 80	80 ≤ Eri < 160	160 ≤ Eri < 320		Eri ≥ 320
	low risk	moderate risk	strong risk	very strong risk		extreme risk
	RI < 150	150 ≤ RI <300	300 ≤ RI < 600	RI ≥ 600
	low risk	moderate risk	strong risk	very strong risk

3. Results and Discussion

3.1. Physicochemical Properties of Mangrove Surface Sediments

The main sediment characteristics are shown in Table 2, which represent the granulometry, total organic carbon, total nitrogen, and pH in surface sediments from Zhanjiang Bay. The results showed that the sediments were predominantly composed of sand and silty soil, which account for means of 54.86 and 36.18%, respectively. Both clay and gravel contents were low, with averages of 8.72 and 0.24% respectively. Additionally, the high-value areas for the sand content were mainly distributed in the southern and western regions of the study area, whereas the high-value areas for the silty and clay contents were primarily distributed in the northern and eastern regions (Figure 2). This may be related to tide direction since the tide was entering the study area from the east.

Table 2. Physicochemical Properties (%) and Heavy Metal Concentrations (mg/kg) of Mangrove Surface Sediments in the Study Area^a.

site	gravel	sand	silt	clay	TOC	TN	pH	Co	V	Cu	Pb	Ni	As	Cd	Hg
J1	0.00	51.38	42.69	5.93	0.86	0.08	6.62	2.20	26.24	10.38	16.64	5.47	5.51	0.14	0.04
J2	0.00	42.72	47.47	9.81	0.75	0.08	6.49	3.04	32.82	13.88	19.52	7.06	4.98	0.18	0.06
J3	0.01	53.27	37.93	8.79	0.38	0.04	6.81	2.34	15.48	11.88	13.47	4.70	2.69	0.09	0.04
J4	0.00	62.17	30.34	7.50	0.37	0.04	6.67	2.08	21.70	5.61	14.60	4.30	4.66	0.07	0.04
J5	0.04	44.00	43.97	11.99	0.55	0.05	6.43	2.77	30.60	7.27	19.12	6.50	5.89	0.09	0.05
J6	0.01	34.48	52.46	13.05	2.45	0.19	6.62	4.62	64.73	19.13	33.99	14.77	10.36	0.24	0.09
J7	0.01	22.80	62.68	14.51	1.55	0.20	6.21	4.66	56.94	19.37	31.53	13.83	8.13	0.27	0.10
J8	0.00	20.44	62.11	17.47	4.51	0.35	6.37	6.65	87.71	27.05	42.67	21.88	11.23	0.38	0.13
J9	0.00	59.08	33.05	7.87	1.37	0.12	6.45	1.87	25.51	6.60	16.34	5.88	3.82	0.09	0.04
J10	0.01	57.01	33.99	8.99	1.33	0.15	7.30	3.38	25.65	24.61	22.31	9.40	3.75	0.37	0.24
JA	0.00	24.92	62.75	12.33	4.94	0.64	7.41	5.12	51.88	58.95	36.24	18.17	5.51	1.01	0.58
J11	0.00	70.40	23.48	6.13	0.45	0.05	6.34	1.33	13.70	6.48	10.55	3.13	2.58	0.07	0.04
J12	0.02	79.75	15.29	4.94	0.24	0.04	6.62	1.76	12.26	4.05	9.95	2.96	2.85	0.06	0.03
J13	0.03	54.96	35.59	9.42	1.11	0.12	6.96	2.18	22.48	13.57	15.55	5.84	3.88	0.11	0.05
J14	0.00	59.17	32.39	8.44	1.04	0.11	6.47	2.02	21.50	11.20	17.13	5.35	4.01	0.12	0.05
J15	0.02	51.31	38.21	10.46	2.26	0.22	6.21	3.15	37.73	20.43	25.26	9.71	6.52	0.18	0.08
J16	0.00	59.01	32.23	8.77	1.10	0.13	6.39	2.57	28.12	14.28	21.35	10.02	4.86	0.14	0.06
J17	0.01	68.72	24.41	6.86	0.83	0.09	6.26	2.56	23.42	10.38	18.82	6.31	4.61	0.11	0.04
J18	0.00	62.52	29.55	7.94	0.88	0.08	7.08	2.85	24.20	9.97	18.13	5.89	6.18	0.09	0.04
J19	0.24	77.64	19.66	2.46	0.14	0.03	6.80	2.67	14.09	14.06	12.41	3.72	2.73	0.07	0.03
J20	0.00	74.85	19.96	5.19	0.21	0.02	6.82	2.31	15.11	5.62	11.68	3.26	3.49	0.07	0.03
J21	1.09	70.92	22.51	5.48	0.29	0.04	6.50	1.95	16.33	40.56	15.49	4.05	3.02	0.10	0.03
J22	0.05	59.01	32.61	8.33	0.49	0.06	7.32	2.32	21.55	23.36	16.77	6.31	4.7	0.14	0.12
J23	4.35	75.47	15.84	4.34	0.28	0.03	6.67	1.33	11.92	43.49	10.62	3.24	2.31	0.27	0.06
J24	0.00	35.57	53.32	11.11	2.74	0.27	6.94	4.91	47.30	33.90	31.58	14.82	6.62	0.42	0.11
mean	0.24	54.86	36.18	8.72	1.24	0.13	6.67	2.91	29.96	18.24	20.07	7.86	5.00	0.20	0.09
CV	3.68	0.31	0.39	0.38	0.99	1.02	5.00	0.44	0.61	0.73	0.43	0.63	0.45	1.00	1.27

^aNote: gravel >2 mm, sand is 0.063–2 mm, silt is 0.004–0.063 mm, and clay <0.004 mm;^28,29 CV stands for the coefficient of variation.

Figure 2.

Spatial distribution of grain sizes: (a) gravel, (b) sand, (c) silt, and (d) clay.

The total organic carbon (TOC) content in surface sediments varies from 0.14 to 4.94%, with an average of 1.24%, and the total nitrogen (TN) content ranged from 0.02 to 0.64%, with a mean of 0.13%, both of which the highest values were located at the sampling site of JA. The large coefficient of variation for the TOC and TN contents indicates that the distribution of TOC and TN is greatly affected by the external environment; the sampling site of JA is located at the sewage outlet, so it may be greatly influenced by human activities. The sediment in the Zhanjiang mangrove had a natural pH of 6.21–7.41, consistent with most of the wetlands.²⁷ This could be due to the frequent tidal flooding preventing acidification in reducible conditions.²⁷

3.2. Contents of Heavy Metals in Mangrove Surface Sediments

The contents of eight heavy metals (Co, V, Cu, Pb, Ni, As, Cd, and Hg) in sediments from the Zhangjiang Bay Mangrove are shown in Table 2. The average contents of all heavy metals are ranked in descending order: V (29.96 mg/kg) > Pb (20.07 mg/kg) > Cu (18.24 mg/kg) > Ni (7.86 mg/kg) > As (5.00 mg/kg) > Co (2.91 mg/kg) > Cd (0.20 mg/kg) > Hg (0.09 mg/kg). Only three heavy metals of Cu, Cd, and Hg display contents higher than their respective background values with the high variation coefficients of Cd and Hg, which were probably influenced by human activities. The spatial distribution of eight heavy metals within the studied area shows high contents in some individual samples without a consistent trend (Figure 3). Co, Pb, and Ni shared the same distribution pattern, especially in recording high concentrations in samples J6, J7, J8, JA, and J24. V and As recorded high contents in samples J6, J7, and J8, whereas Cu recorded high contents in samples JA, J21, and J23. Cd and Hg exhibited a high level in sample JA (sewage outlet). Higher concentrations of heavy metals are generally those with a high proportion of silty sand and clay contents, probably suggesting that sediment grain size is an important factor affecting the enrichment of heavy metals.

Figure 3.

Spatial distributions of heavy metal concentrations: (a) Co; (b) V; (c) Cu; (d) Pb; (e) Ni; (f) As; (g) Cd; (h) Hg.

Table 3 shows the comparisons of heavy metal contents between our sediments in Zhanjiang Bay and other various studies. The results showed that the heavy metal content of mangroves in Zhanjiang Bay was slightly lower than those of Donghai Island,³⁰ Dongzhai Harbor,³¹ and Southern Vietnam,³² whereas it is significantly lower than those of the Maowei Sea,² Futian,³³ Qi’ao Island,³⁴ Nansha,³⁵ Pearl River,³⁶ Maipo,³⁷ and Saudi Arabia.³⁸ This discrepancy could be due to the local economic development and population density differences. Comparatively, the mangroves in France³⁹ and Singapore⁴⁰ have much lower average heavy metal concentrations presumably due to better management of anthropogenic inputs. Except for Cd and Hg, the degree of heavy metal contamination is not high when compared with other regions. The high concentrations of heavy metals (e.g., Cd and Hg) observed in this region required further evaluation of environmental risk.

Table 3. Comparison of the Heavy Metal Content in Surface Sediments of China and International Mangroves^a.

location	Co	V	Cu	Pb	Ni	As	Cd	Hg	references
Zhanjiang Bay	2.91	29.96	18.24	20.07	7.86	5	0.2	0.09	this study
Donghai Island, Zhanjiang	/	/	12.5	27	17.2	12.5	0.04	0.07	(30)
Dongzhai Harbor, Hainan, China	/	/	19.51	20.52	30.4	8.52	0.56	/	(31)
Maowei Sea Guangxi, China	20.1	/	61.9	48.9	50.7	/	0.79	/	(2)
Beihai of Guangxi, China	/	/	3	7	<3.00	<3.00		<0.04	(41)
Nansha,South China Sea	/	/	113	55.3	48.4	/	0.78	/	(35)
Futian of Shenzhen, China	/	/	82.6	105	117	/	5.7	/	(33)
Qi’ao Island, Zhuhai, China	/	/	81.5	70.6	50.4	/	9.5	/	(34)
Pearl River Estuary, China	/	/	321.48	49.89	56.7	/	2.26	/	(36)
Maipo, Hong Kong	/	/	42.8	52.6	36.4	/	1.05	/	(37)
Southern Vietnam	19.6	/	27	21	53	/	0.1	/	(32)
Senegal, West Africa	0.9	14.3	3.5	2.4	2.5	/	0.03	0.01	(8)
Gulf of Khambh, India	0.25	/	11.64	7.14	34.66	2.79	0.09	0.12	(42)
Saudi Arabia	3.94	759.15	209.8	4.4	81.05	23.75	1.67	1.98	(38)
Sungei Buloh, Singapore	/	/	7.06	12.28	/	/	0.18	/	(40)
Sinnamary, French Guiana	0.32	/	0.28	0.13	0.54	/	/	0.41	(39)

^aNote: “/” represents no data.

3.3. Ecological Risk Assessment of Heavy Metals and Possible Sources

The I_geo values are shown in Table 4. The average I_geo values in the investigated sediments were Cd > Cu > Hg > Pb ≈ As > Ni > V > Co. Among the investigated heavy metals, the I_geo values of Pb, As, V, and Co indicated little heavy metal contamination at all sites (I_geo < 0). Cd showed an unpolluted level at sites J4, J11, J12, J19, and J20 (I_geo < 0), and other sites showed mildly polluted to highly polluted (I_geo, 0.1–3.59). For Hg, sites J8, J10, JA, and J22 indicated values falling into mildly to middle-level pollution (I_geo, 0.04–2.31). Cu exhibited mildly polluted to moderately polluted at sites J8, JA, J21, J23, and J24 (I_geo, 0.09–1.21). The results indicated that sites J8 and JA were polluted by more heavy metals than other sites.

Table 4. Maximum, Minimum, and Average Values of Different Pollution Indices in This Paper.

	Igeo			CF			Eri			RI
metals	min	max	aver	min	max	aver	min	max	aver	min	max	aver
Co	–2.98	–0.66	–1.98	0.19	0.95	0.42	0.95	4.75	2.08	55.96	878.63	168.78
V	–3.04	–0.16	–1.92	0.18	1.34	0.46	0.37	2.69	0.92
Cu	–2.65	1.21	–0.83	0.24	2.56	1.07	1.19	17.34	5.37
Pb	–2.44	–0.58	–1.55	0.28	1.19	0.56	1.38	5.93	2.79
Ni	–2.87	0.02	–1.70	0.21	1.52	0.55	1.03	7.60	2.73
As	–2.53	–0.25	–1.55	0.26	1.26	0.56	2.60	12.62	5.61
Cd	–0.49	3.59	0.88	1.07	18.04	3.49	32.14	541.07	104.57
Hg	–1.96	2.31	–0.92	0.38	7.44	1.12	15.38	297.44	44.72

A contaminant factor (CF) was used to determine the contamination status in the surface sediment of Zhanjiang Bay, and the results are shown in Table 4. The result for the average contaminant factor of eight heavy metals is as follows: Cd > Hg > Cu > Pb ≈ As >Ni > V > Co, consistent with the I_geo index. Cd showed a considerable contaminant factor, whereas Hg and Cu were moderate contaminant factors. The mean CFs of Pb, As, Ni, V, and Co were 0.56, 0.56, 0.55, 0.46, and 0.42, respectively, suggesting a low contaminant factor of them. Meanwhile, in combination with the I_geo index, Cd, Cu, and Hg were most prominent within the Zhanjiang Bay mangrove sediments.

The potential ecological risk assessment of eight heavy metals in the surface sediments of the mangrove wetland in Zhanjiang Bay is listed in Table 4. The potential ecological risk (E_rⁱ) of Cd is between 32.14 and 541.07, indicating a low to extremely strong ecological hazard level. The E_r of Hg is between 15.38 and 297.44, suggesting an ecological risk level of low to very strong. The results of Co, V, Cu, Pb, Ni, and As exhibited slight ecological risk levels. Thus, Cd has the highest level of potential ecological risk in the studied sediments followed by Hg. The potential ecological risk index (RI) value of the Zhanjiang Bay mangrove wetland ranges from 55.96 to 878.63, suggesting a low to very strong comprehensive ecological risk level. The highest potential ecological risks of Cd and Hg for single heavy metals and comprehensive potential ecological risks are located at JA (sewage outlet) followed by sampling sites around JA (Figure 4). This indicates that the heavy metal pollution in the study area is mainly affected by human sewage discharge, consistent with the previous study.⁴³

Figure 4.

Spatial distributions of potential ecological risk of heavy metals in the study area: (a) E_rⁱ Cd ; (b) E_r Hg ; (c) RI

The correlation analysis between various heavy metals can reveal whether they have homology or not as heavy metals with a strong correlation may share the same source, and heavy metals with a weak correlation may have multiple sources.⁴⁴ Correlation analysis indicated that Co, V, Pb, Ni, and As were strongly correlated, whereas Cu, Cd, and Hg showed a strong correlation (Table 5). These results demonstrated that the sources of heavy metals in the study area can be broadly grouped into two categories: group I (Co, V, Pb, Ni, and As) and group II (Cu, Cd, and Hg). Group II may represent heavy metals from anthropogenic sources, such as domestic sewage and automobile exhaust emission. On the one hand, because the study area is located both in the densely populated residential and commercial areas in the central business district that are adjacent to high traffic roads, those heavy metals of Cu, Cd, and Pb may have been originated from traffic pollution such as automobile exhaust emission, tire wear, and ship pollution.^36,45−47 Additionally, the sewage outlet sampling site exhibited the highest pollution degree followed by the surrounding area of the sewage outlet sampling site. Group I represented natural sources of heavy metals originating from weathering and diagenetic processes because Co, V, Pb, Ni, and As exhibited unpolluted contents. Cr originates mainly from parent rocks, and its distribution pattern may depend on local hydrodynamic conditions.^48,49

Table 5. Correlation Matrix of Studied Heavy Metals, Grain Size, TOC, TN, and pH Values in Sediments^a.

	Co	V	Cu	Pb	Ni	As	Cd	Hg
Co	1
V	0.939**	1
Cu	0.436*	0.341	1
Pb	0.960**	0.964**	0.482*	1
Ni	0.959**	0.951**	0.507**	0.988**	1
As	0.839**	0.937**	0.139	0.872**	0.828**	1
Cd	0.668**	0.556**	0.801**	0.694**	0.734**	0.330	1
Hg	0.535**	0.408*	0.704**	0.565**	0.606**	0.196	0.947**	1
gravel	–0.287	–0.247	0.458*	–0.259	–0.236	–0.296	0.042	–0.082
sand	–0.859**	–0.895**	–0.358	–0.897**	–0.882**	–0.807**	–0.623**	–0.510**
silt	0.848**	0.873**	0.343	0.881**	0.865**	0.784**	0.627**	0.521**
clay	0.835**	0.903**	0.244	0.884**	0.869**	0.846**	0.491*	0.400*
TOC	0.863**	0.852**	0.570**	0.911**	0.931**	0.691**	0.824**	0.723**
TN	0.779**	0.723**	0.650**	0.831**	0.858**	0.528**	0.922**	0.863**
pH	0.089	–0.125	0.376	0.006	0.054	–0.184	0.437*	0.548**

^aNote: **, correlation is significant at the percentile level.

Table 6 shows the results of principal component analysis (PCA) for heavy metal concentrations. The first two principal components accounted for 93.9% of the total variance. The first principal component (PC1) was predominantly associated with Co, V, Pb, Ni, and As and accounted for 72.9% of the variance. PC1 could be interpreted as an indicator of natural source because the heavy metals of Co, V, Pb, Ni, and As have high positive loadings on PC1. The second principal component (PC2, 21% of total variance) exhibited a positive factor loading with Cu, Cd, and Hg, which could be better represented as an anthropogenic source. Additionally, the cluster analysis results show that the heavy metals in Zhanjiang Bay mangrove sediments can be divided into two categories, with the first category of Pb, Ni, Co, V, and As and the second category of Cd, Hg, and Cu (Figure 5a). Thus, the studied heavy metals can be divided into two groups: group I (Co, V, Pb, Ni, and As) and group II (Cd, Hg, and Cu), and each group shared the same source. The sampling sites in the study area can be divided into three categories: category 1: J1, J2, J3, J4, J5, J6, J7, J9, J11, J12, J13, J14, J15, J16, J17, J18, J19, J20, J21, J22, and J23, category 2: J8, J10, and J24, and category 3: JA (sewage outlet) (Figure 5b). In the first category, the sediments consisted mainly of sand with a low heavy metal content and slight to moderate total potential ecological risk. The second type is located near the sewage outlet. Those sediments are mainly composed of silt and clay and showed a high heavy metal content and strong total potential ecological risk. The third category included only one sample JA (sewage outlet), having a high proportion of silt and clay, and the JA sample showed a high heavy metal content and extremely strong total potential ecological risk.

Table 6. Principal Component Analysis Matrix of Heavy Metals in Mangrove Sediments.

component	Co	V	Cu	Pb	Ni	As	Cd	Hg	initial eigenvalue	variation contribution/%
PC1	0.95	0.92	0.61	0.98	0.98	0.79	0.82	0.70	5.84	72.9
PC2	–0.20	–0.36	0.65	–0.18	–0.13	–0.57	0.55	0.63	1.67	21

Figure 5.

Dendrogram of trace (a) heavy metal and (b) different sampling sites.

3.4. Correlation Analysis between the Physicochemical Properties and Heavy Metals

The Pearson correlation analysis was carried out between heavy metal concentrations and physicochemical parameters as the physical and chemical properties of mangrove sediments and other environmental factors can greatly influence the accumulation and distribution of heavy metals.^5,7,11,50,51 The correlation results are shown in Table 5. Of these, Co, V, Pb, Ni, and As are significantly negatively correlated with sand and positively correlated with silt and clay, indicating that they are easily absorbed by fine particles (<63 μm, silt, and clay). Similar but slightly weaker correlations were observed between the heavy metals of Cd and Hg with fine particles, suggesting less influence of grain size on Cd and Hg metals. Cu showed no relationship with sediment particle size. This is also supported by the spatial distributions of heavy metals and grain size.

TOC and TN exhibited a positive correlation with heavy metals including Co, V, Pb, Ni, Cd, and Hg and showed a relatively weaker correlation with the Cu and As. The results showed that organic matter plays a critical role in controlling the fate of these heavy metals, which were consistent with the fact that the sampling sites with high TOC and TN contents were accompanied by high heavy metal contents as the organic matter has strong adsorption properties for heavy metals through adsorption, complexation, and precipitation effects.⁵²⁻⁵⁵ The other heavy metals, except Hg, showed a weak correlation with pH, possibly because of the strong mobility of heavy metals in the study area.⁵⁶

4. Conclusions

• (1)By measuring heavy metal contents of Co, V, Cu, Pb, Ni, As, Cd, and Hg in the mangrove surface sediments of Zhanjiang Bay, the results showed that the average contents are 2.91, 29.96, 18.24, 20.07, 7.86, 5.0, 0.20, and 0.09 mg/kg, respectively. Cu, Cd, and Hg are higher than the Guangdong soil environmental background values. The large variation coefficient of Cd and Hg indicates that they are seriously affected by human activities.
• (2)Based on the results of the statistical analyses, the studied heavy metals can be divided into two groups, namely, group I of Co, V, Pb, Ni, and As and group II of Cu, Cd, and Hg. The potential ecological risk analysis of individual heavy metals demonstrated that the risk degree of Co, V, Cu, Pb, Ni, and As was slight, Cd was slight to extreme strong, and Hg was slight to very strong. Comprehensive potential ecological risk analysis shows that the risk degree of the study area is slight to very strong, and the pollution degree of the JA (sewage outlet) sampling site is the highest followed by the surrounding area of the JA sampling site.
• (3)Most heavy metals were positively correlated with silt, clay, TOC, and TN. The high value of the heavy metal content and risk is located in the high value of the TOC and TN content in silt and clay. Compared with most mangroves in previous studies, Cd showed high pollution in the study area. Overall, a certain degree of heavy metal pollution exists in the surface sediments of the mangrove forests in Zhanjiang Bay, with the heaviest pollution of Cd followed by Hg. Further investigation is needed in future works, such as the distribution characteristics of heavy metal forms.

The authors declare no competing financial interest.

Notes

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