The level of mercury contamination in mariculture sites at the estuary of Pearl River and the potential health risk

Environmental Pollution xxx (2016) 1e8

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The level of mercury contamination in mariculture sites at the estuary of Pearl River and the potential health risk H.C. Tao a, K.Y. Zhao a, W.Y. Ding a, J.B. Li a, P. Liang c, S.C. Wu c, **, M.H. Wong a, b, * a

Key Laboratory for Heavy Metal Pollution Control and Reutilization, School of Environment and Energy, Peking University Shenzhen Graduate School, Shenzhen, PR China b Consortium on Health, Environment, Education and Research (CHEER), The Education University of Hong Kong, Tai Po, Hong Kong Special Administrative Region c School of Environmental and Resource Sciences, Zhejiang Agriculture and Forestry University, Linan, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 May 2016 Received in revised form 15 July 2016 Accepted 29 July 2016 Available online xxx

In the present study, the Hg contamination in mariculture sites located at the estuary of Pearl River was to investigate with an attempt to analyse associated health risks of dietary exposure to both total mercury (THg) and methyl mercury (MeHg) in cultured fish and shellfish. The highest total mercury concentration (7.037 ± 0.556 ng L
1) of seawater was observed at Zhuhai Estuary. The Hg concentrations of sediment in Guishan Island were significantly higher (p < 0.05) than in Daya Bay (away from the Pearl River). Besides, the both THg and MeHg levels in sediment at mariculture sites were higher (p < 0.05) than corresponding reference sites. It was attributed to the fact that mariculture activities increased Hg loading and promoted MeHg production. The vertical distribution of Hg in sediment cores demonstrated that mercury methylation mostly occurred at the sediment-water interface. Results of health risk assessments showed that fish consumption would impose a higher risk to children but less to adults, while shellfish produced in the studied area was safe for consumption. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Total mercury Methyl mercury Mariculture Health risks assessment

1. Introduction Mercury (Hg) is known as one of the most hazardous metals that can be found in the environment, such as marine and terrestrial environment, due to long distance transport and thereafter atmospheric deposition (Zhang and Wong, 2007). Hg exists in a variety of different forms (element Hg, inorganic mercury and methyl mercury) with a large varying properties that affect its complex distribution, biological enrichment and toxicity (Leermakers et al., 2005). Once discharged into the environment, mercury is converted rapidly into organic compounds through both abiotic and biotic pathways, mainly as methylmercury (MeHg) which is more toxic compared with inorganic and elemental forms of mercury (Cheng et al., 2013). Besides of consumption of Hg-contaminated rice, intake of

* Corresponding author. Key Laboratory for Heavy Metal Pollution Control and Reutilization, School of Environment and Energy, Peking University Shenzhen Graduate School, Shenzhen, PR China. ** Corresponding author. E-mail address: [email protected] (M.H. Wong).

contaminated fish has been considered the main pathway of MeHg for human exposure and is harmful to health. It has been well documented that the benthic environment beneath or close to mariculture zones are rich in organic matters owing to the deposition of (unconsumed fish feeds and fecal matters Through the role of sulfate or iron reducing bacteria under anaerobic conditions provided by the organic rich sediments, inorganic mercury can be converted to organic mercury (Liang et al., 2012). In addition, it is widely recognized that MeHg is more easily to be accumulated in fish than inorganic forms of Hg. Higher the trohpic position along food chains or web, higher biomagnifications level in the organisms (Cheng et al., 2013), resulting in extremely high concentrations in large carnivorous fish such as tuna, shark, grouper and swordfish, posing potential health risks to consumers (Man et al., 2014). The Pearl River Delta (PRD) is situated in Guangdong Province, South China, and geographically including Hong Kong and Macau. In this region, mariculture has a significant role in local fishery and the total aquatic products from freshwater aquaculture and mariculture reached 8 million tonnes (t) in 2014 (China Statistical Yearbook, 2015). Besides, the average personal consumption of aquatic product was about 15.88 kg in 2012 (China Statistical Yearbook, 2013). Due to the high demand of electricity of rapidly

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Please cite this article in press as: Tao, H.C., et al., The level of mercury contamination in mariculture sites at the estuary of Pearl River and the potential health risk, Environmental Pollution (2016), http://dx.doi.org/10.1016/j.envpol.2016.07.067

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H.C. Tao et al. / Environmental Pollution xxx (2016) 1e8

developed industries, it is envisaged that a great quantity of Hg would be released into the ambient air, especially from those substandard coal-fired power plants. However, so far little information has been available related to the Hg contamination level in Guangdong Province, which serves one of most important aquaculture base in PR China. It is known that over consumption of contaminated fish may raise levels of heavy metals, especially Hg in hair of children residing in coastal cities such as Hong Kong and Shanghai, linking with autism (Abdullah et al., 2012; Ko et al., 2012). Considering the human health risks of intake of MeHg via fish consumption, the present study focused on mercury methylation in mariculture ecosystem around the PRD, to investigate if the fish cultured would pose any health risks to the local consumers. The major aim of this study was to assess whether the aquaculture activities would result in higher Hg concentrations or methylation rate at the estuary of Pearl River, compared with those reference sites. More specifically, the objectives were to (1) investigate Hg (both THg and MeHg) contamination level in mariculture sites located at the estuary of Pearl River; and (2) assess the health risk of dietary exposure to both THg and MeHg in the maricultured fish and shellfish.

customized columnar sampler during October 2014 (first trip) and March 2015 (second trip) from the mariculture sites (MS) and corresponding reference sites (RS), located approximately 200e1000 m away from MS. The sediment cores were sectioned into 2 cm intervals from 0 to 10 cm and into 5 cm intervals from 10 cm to the end of the cores. Surface seawater samples (0e20 cm) were also collected at each site using acid-treated polyethylene bottles. There were triplicates for all the sediment and water samples collected from each site. Cultured fish including Indigo Hamlet (Hypoplectrus indigo), Orbfish (Ephippus orbis), yellow grouper (Epinephelus awoara), White Croaker (Genyonemus lineatus), and Yellow Grouper (Mycteroperca venenosa) (all carnivorous species) were chosen for the present study, due to their economic importance. Three to six replicates for each species were collected from the net cages using nylon nets. The lengths and weights of each fish sampled were recorded. Oyster (Ostreae concha), scallop (Chlamys nobilis), mussel (Mytilus edulis) and conch (Busycon canaliculatum) were purchased from the Zhuhai and Shenzhen local wet markets due to the unavailability on site. All samples were collected and then transported to the laboratory using ice boxes and stored at
20  C until analyses.

2. Materials and methods 2.2. Sample pretreatment 2.1. Sampling Three mariculture sites were chosen in the present investigation around the Pearl River Delta, (1) Zhuhai Estuary, (2) Guishan Island, between Zhuhai and Hong Kong, and (3) Daya Bay, far away and completely isolated from the Pearl River (Fig. 1). The detailed information for the mariculture sites is listed in Table 1. Surface sediment samples (0e25 cm) were collected using a

All sediment samples were freeze-dried, ground into powder and homogenized by using a stainless steel sieve of 0.154 mm. For THg analyses, about 0.2 g dry sediments were digested with 5 mL aqua regia at 95  C for 5 h. Mercury was oxidized into divalent mercury by BrCl before THg determination. For MeHg analyses, the sediments were extracted by HNO3/CuSO4/dichloromethane prior to stripping to water (Liang et al., 2004). To determine the level of

Fig. 1. Map of the Pearl River Delta showing the sampling sites (filled triangles): (1) Zhuhai Estuary, (2) Guishan Island, and (3) Daya Bay.

Please cite this article in press as: Tao, H.C., et al., The level of mercury contamination in mariculture sites at the estuary of Pearl River and the potential health risk, Environmental Pollution (2016), http://dx.doi.org/10.1016/j.envpol.2016.07.067

H.C. Tao et al. / Environmental Pollution xxx (2016) 1e8

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Table 1 Sampling site descriptions and samples collected from each site. Sampling sites

Locations

Descriptions

Samples

Zhuhai Estuary

22 190 2200 N 113 360 5000 E

Surface seawater (n ¼ 3), sediment (n ¼ 3);

Guishan Island

22 80 4000 N 113 480 5600 E

An area without mariculture activities but persistently collecting edischarges from different industrial and urban centers along the Pearl River over the past 30 years A small island of Zhuhai near the open-sea and with some contamination by urban discharges

Daya Bay

22 410 1600 N 114 330 800 E

A semi-enclosed bay situated in the east coast of Guangdong Province and is less contaminated by industrial or urban discharges

total mercury, water samples were added 0.5% (v/v) BrCl in order to oxidize all mercury into divalent mercury (Hg2þ) and then reduced to Hg0 by SnCl2. Water samples were detected for MeHg after distillation, aqueous ethylation, purge, and trap. Fish samples were dissected into five parts, including dorsal muscle, abdominal muscle, liver, gills, and skin. Edible parts of shellfish were isolated from shells and cleaned with distilled water. All biota samples were freeze-dried and ground into powder. Biota samples (0.2 g) for THg detection were digested using 10 mL H2SO4/HNO3 (3:7, v/v) at 95  C for 3 h. After cooling, the solution was diluted with 0.5 mL BrCl and deionized water to 25 mL. The samples for MeHg analyses were digested using KOH solution at 80  C for 4 h and diluted to 25 mL with deionized water.

Estimated daily intake; ðEDIÞ ¼

Surface seawater in MS (n ¼ 6), sediment in MS (n ¼ 6), Surface seawater in RS (n ¼ 4), sediment in RS (n ¼ 4), biota samples including indigo hamlet (Hypoplectrus indigo) (n ¼ 4), white croaker (Genyonemus lineatus) (n ¼ 5) Surface seawater in MS (n ¼ 6), sediment in MS (n ¼ 6), Surface seawater in RS (n ¼ 4), sediment in RS (n ¼ 4) biota samples including indigo hamlet (Hypoplectrus indigo) (n ¼ 3), orbfish (Ephippus orbis) (n ¼ 6), yellow grouper (Epinephelus awoara) (n ¼ 2)

2.5. Bioaccumulation factors and risk assessments of fish and shellfish consumption Bioaccumulation factor (BAF) indicates the accumulation capacities of biological creature for mercury. BAF can be obtained by the following equation:

BAF ¼ cb =ce where cb is THg or MeHg concentration in biota samples and ce is THg or MeHg concentration in environment. To measure the daily intakes of THg and MeHg by the PRD residents via the consumption of maricultured fish and shellfish, the following equation (USEPA, 2000) was used:



MeHg concentration mg g
1 ww  consumption rate g d
1 body weight ðkgÞ

2.3. Mercury determination The THg concentrations of sediment, water and biota samples were analyzed by Total Mercury Manual System from Brooks Rand (Brooks Rand Labs, USA) based on cold vapor atomic fluorescence spectrometry (CVAFS), following Method 1631 (USEPA, 2002). The MeHg concentrations of these samples were determined using aqueous ethylation, purge, trap and gas chromatography - cold vapor atomic fluorescence spectrometry (GC - CVAFS), following Method 1630 (USEPA, 2001a).

where MeHg concentration in fish muscle tissue and shellfish was in terms of wet weight, body weights for adults and children were 58 kg (Shao et al., 2013) and 21.8 kg (Leung et al., 2000), and the consumption rates were 43.5 g d
1 (China Statistical Yearbook, 2013) and 37.4 g d
1 (Tang et al., 2009), respectively in Guangdong Province. The oral reference dose (RfD) for MeHg of 0.1 mg kg
1 d
1 suggested by USEPA (2010) and the acceptable daily intake (ADI) for MeHg of 0.23 mg kg
1 bw d
1, established by the Joint Expert Committee on Food Additive (JECFA) were used (JECFA, 2006)

2.4. Quality assurance and quality control (QA/QC)

2.6. Data analyses

For the analytical quality control, method blanks, reagent blanks and 10% sample replicates were accompanied each sample (sediment, water and biota samples) batch in the analysis. The accuracy of THg and MeHg measurements was determined using the following certified reference materials: IAEA 433 (Marine sediment, International Atomic Energy Agency, Austria); GBW10029 (Fish tissue, National Research Center for Certified Reference Materials, China); GBW10024 (GSB-15) (Scallop tissue, National Research Center for Certified Reference Materials, China); NIST 1566b (Oyster tissue, National Institute of Standards and Technology, USA). The recoveries of the standard reference materials for THg and MeHg ranged from 91.2% to 103.1% and 92.4%e106.3%, respectively.

Data were analyzed using the Statistical Package for Social Sciences 18.0 for Windows. One-way ANOVA were used to examine any significant differences based on Hg concentrations in surface seawater, sediments and biota samples from sampling sites. Pearson's correlation test was adopted to identify the relationships between THg and MeHg in samples and environment. The level of significance was set at p < 0.05. 3. Results and discussion 3.1. THg and MeHg concentrations in surface water Table 2 shows the concentrations of both THg and MeHg

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H.C. Tao et al. / Environmental Pollution xxx (2016) 1e8 Table 2 Concentrations of THg and MeHg in surface water (mean ± SD, range). Sampling trips

Locations

Sites

Surface water THg (ng L
1)

First trip

Zhuhai Estuary Guishan Island Daya Bay

Second trip

Guishan Island Daya Bay

MS RS MS RS MS RS MS RS

7.307 1.449 1.548 1.270 1.647 2.076 3.153 2.045 1.999

± ± ± ± ± ± ± ± ±

0.556 0.073 0.268 0.058 0.690 0.486 0.999 0.430 0.307

MeHg (ng L
1) (6.758e7.871) (1.373e1.518) (1.359e1.738) (1.219e1.333) (1.159e2.135) (1.535e2.476) (2.447e3.860) (1.745e2.538) (1.782e2.216)

0.036 0.043 0.030 0.069 0.044 0.058 0.046 0.056 0.055

± ± ± ± ± ± ± ± ±

0.002 (0.034e0.036) 0.007 (0.037e0.050) 0.006 (0.026e0.034) 0.006 (0.062e0.074) 0.0002 (0.043e0.045) 0.011 (0.048e0.070) 0.003 (0.043e0.048) 0.005 (0.050e0.061) 0.005 (0.051e0.058)

two sites had fish culture activities). The results showed that no significant difference (p > 0.05) was observed in terms of THg and MeHg between two mariculture sites, respectively. Mariculture activities increased contents of organic matter in seawater, possible towing to the accumulation of unconsumed fish feed and fish excretion (Liang et al., 2011). Organic matter may increase methylation by triggering the activity of heterotrophic microorganisms, but could also inhibit MeHg production by binding Hg2þ, resulting in its unavailability to methylation reaction (Ullrich et al., 2001). Therefore, Hg methylation is a complicated process influenced by many factors. Due to the difference between sampling sites according to physical, chemical and biological properties, the identical phenomenon could not be found in all studied sites. Based on the results of one-way ANOVA, the THg level obtained in MS of Daya Bay in the second trip was significantly higher (p < 0.05) than that in the first trip. In contrast, the MeHg level obtained in MS of Daya Bay in the first trip was significantly higher (p < 0.05) than that in the second trip. Pearson's correlation analysis indicated a significant negative correlation (r ¼
0.431, p < 0.05, n ¼ 23) between THg and MeHg concentrations (Fig. S1).

3.2. THg and MeHg concentrations in sediments

Fig. 2. Concentrations of THg and MeHg in surface water. Columns with the same letter are not significantly different (p < 0.05) according to the results of Duncan multiple range tests. A: THg and MeHg concentrations of 5 sampling sites in first sampling trip; B: THg and MeHg concentrations of 4 sampling sites in the second sampling trip. ZE ¼ Zhuhai Estuary, GI ¼ Guishan Island, DB ¼ Daya Bay, MS ¼ Mariculture site and RS ¼ Reference site.

of surface water samples collected from the three sites during two sampling trips. The highest THg concentration (7.037 ± 0.556 ng L
1) in surface water was observed in Zhuhai Estuary during the first sampling trip (October 2014) while the highest MeHg concentration was 0.069 ± 0.006 ng L
1 observed in MS of Daya Bay. In general, MeHg concentrations in MS were higher (p < 0.05) than those in RS (Fig. 2), however no significant difference of THg between MS and RS was observed, except for Zhuhai Estuary.t For the second sampling trip (March 2015), only Guishan Island and Daya Bay were chosen as sampling sites (as only these

Concentrations of THg and MeHg detected in sediment are shown in Table S1. For the first trip (October 2014), the highest concentration of THg was recorded in RS of Guishan Island (165.83 ± 25.95 ng g
1 dry wt), whereas the highest MeHg was recorded in MS of Guishan Island (0.29 ± 0.12 ng g
1 dry wt). For the second trip (March 2015), the highest Hg concentrations were observed in MS of Guishan Island (THg: 130.34 ± 25.74 ng g
1 dry wt and MeHg: 0.19 ± 0.10 ng g
1 dry wt, respectively). According to the results of Duncan multiple range tests, THg concentrations in Zhuhai Estuary and Guishan Island were significantly higher (p < 0.05) than those of Daya Bay (Fig. 3A and B). Higher contents of THg in sediment from Guishan Island than Zhuhai Estuary may be attributed to the hydrodynamic in the estuary of Pearl River. Guishan Island located further than Zhuhai Estuary to the junction area of freshwater and marine water. It has been well-known, particles in freshwater river would be sedimented when mixed with marine water due to roles of charge neutralization and flocculation (Furukawa et al., 2014); and the finer the particulates, the further they can go. In addition, fine particles usuallyadsorb more Hg or other anthropogenic contaminants than coarse ones due to their higher surface area. In this case, the coarse particles with low Hg contents deposited at the sampling site of Zhuhai Estuary; on the contrary, fine particles with high Hg contents deposited at Guishan Island. This result is in line with Chen et al. (2013), who reported the similar pattern of the spatial distribution of Cu, Zn, Ni, and Pb at the estuary of Pearl River. Contents of MeHg in all MS

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H.C. Tao et al. / Environmental Pollution xxx (2016) 1e8 200

MeHg

A

-1

0.4

120

80

0.2

B

B

B

c

c

40

ZE

GI-MS

GI-RS

0.1

DB-MS

DB-RS

THg

MeHg

200

160

0.0

0.5

a

0.4

-1

a

-1

THg concentration (ng g dw)

0.3

B

0

B

MeHg concentration (ng g dw)

b

ab

THg

-1

THg concentration (ng g dw)

160

3.3. THg and MeHg concentrations in fish and shellfish

0.5

a

MeHg concentration (ng g dw)

A

120

A

0.3

b 80

AB

AB

0.2

B c

40

0

GI-MS

GI-RS

DB-MS

DB-RS

0.1

5

0.0

Fig. 3. Concentrations of THg and MeHg in surface sediments. Columns with the same letters are not significantly different (p < 0.05) according to the results of Duncan multiple range tests. A: THg and MeHg concentrations of 5 sampling sites in the first sampling; B: THg and MeHg concentrations of 4 sampling sites in the second sampling trip.

were significantly higher (p < 0.01) than CS in Guishan Island (Fig. 3A and B). This implies that mariculture activities could enhance MeHg production in the sediments. The physical, chemical and biological properties of the sediment beneath fish culture cages are altered by the deposition of organic matter from the food supply of fish (unconsumed feed) and fish metabolites. Furthermore, it has been reported that dissolved organic matter would enhance microbial mercury methylation under sulfidic conditions (Graham et al., 2012). The distribution of THg in sediment cores appeared to be related to the historical changes in the Hg inputs into the sampling sites (Muresan et al., 2007). The vertical distribution of THg concentrations in the sediments core ranged from 24.39 to 214.82 ng g
1 (dry wt), that were considered to be significantly diverse among different sites. In general, THg concentrations increased with increasing depth (Fig. 4A and B). The vertical distribution of MeHg profiles ranged from 0.043 to 0.787 ng g
1 dry wt. The concentrations of MeHg reduced prominently with increasing depth in the top 10 cm in 6 out of 9 sediment samples. Then there is a slight change in values beyond 10 cm (Fig. 4C and D). This showed that the sediment-water interfaces provided better conditions for mercury methylation. In fact, it was proved that MeHg production is most likely occurred in the top of core sediment (Rothenberg et al., 2008).

Two fish species were collected from Guishan Island MS, with higher Hg concentrations recorded in muscle of Indigo Hamlet (187 ± 38.8 mg kg
1 ww of THg and 121 ± 34.0 mg kg
1 ww of MeHg) than in muscle of White Croaker (148 ± 23.4 mg kg
1 ww of THg and 87.3 ± 41.6 mg kg
1 ww of MeHg, respectively). Three fish species were collected from Daya Bay MS, with the Hg concentrations measured in descending order as below: the muscle of Indigo Hamlet (THg: 135 ± 46.9 mg kg
1 ww, MeHg: 64.6 ± 4.97 mg kg
1 ww); Orbfish (THg: 123 ± 29.7 mg kg
1 ww, MeHg: 59.7 ± 12.3 mg kg
1 ww); Yellow Grouper (THg: 122 ± 4.24 mg kg
1 ww, MeHg: 43.9 ± 5.33 mg kg
1 ww) (Table S2). The THg concentrations determined in all fish samples were found to be complied with the USEPA 2010 health guideline of 0.30 mg kg
1 (USEPA, 2010), and China's National Standard of 0.5 mg kg
1 (w/w), which is acceptable for consumption. There was a linear relationship (r ¼ 0.885, n ¼ 20, p < 0.05) between THg and MeHg in different fish tissues according to Pearson's correlation analysis (Fig. S2). Similar results were observed in different fish species. The THg and MeHg concentrations in muscle were significantly higher (p < 0.05) than those in other fish tissues (Table S2). As to Indigo Hamlet collected both in two MS, the THg and MeHg concentrations in muscle of Indigo Hamlet in Guishan Island were significantly higher (p < 0.05) than those in Daya Bay. This was attributed to the fact that higher Hg levels were observed in seawater and sediment of Guishan Island than Daya Bay. In the present study, the average % MeHg in muscles was 55.6 ± 12.9%, ranging from 33.6 to 88.1%. In the aquatic environment, inorganic mercury can be microbiologically transformed into lipophilic organic compound ‘MeHg’. It is accumulated mostly in cysteine thiol complexes (Harris et al., 2003), and is excreted gradually with a half-life of about 400 d (Downs et al., 1998). Consequently, MeHg is the ascendant speciation of mercury accumulated in muscle. The liver, as a detoxification and storage organ, is capable of accumulating large amount of pollutants, notably Hg, by active involvement in the metabolism of these pollutants (Gonzalez et al., 2005). MeHg would be demethylated to Hg (II) in liver. Gills and skin have large surface areas and persistent exposure with the surrounding environment. The results showed that the gills and skin consisted comparatively high concentrations of Hg (II) than MeHg, that could be attributed to the fact that the majority of Hg existed in water was inorganic mercury. Table S3 shows the correlations between Hg concentrations in fish tissues and sediments. Data of shellfish were excluded in the calculations as the samples were purchased from wet markets, without knowing their sources. The relationship between Hg concentrations of fish and sediments would be influenced by varying factors, in particular Hg speciation in sediment and Hg bioavailability. According to Table S3, a significant correlation (p < 0.05) between MeHg concentrations in sediment and MeHg concentrations in dorsal muscle of Indigo Hamlet collected from Guishan Island was observed. In addition, a significant correlation (p < 0.05) between THg concentrations in sediment and THg concentrations in dorsal muscle of Indigo Hamlet collected from Daya Bay was observed. Among the fish species analyzed, Indigo Hamlet and White Croaker were generally larger than Orbfish (based on weight and length) (Table S2). MeHg concentrations in dorsal muscle of Indigo Hamlet and White Croaker were significantly higher (F ¼ 4.278, p < 0.05, SNK test) than those of Orbfish. Both THg and MeHg concentrations in abdominal muscle of Indigo Hamlet and White Croaker were significantly higher (F ¼ 13.653, F ¼ 11.651, p < 0.05, SNK test) than those of Orbfish. It is commonly observed that larger fish inclined to build up a

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H.C. Tao et al. / Environmental Pollution xxx (2016) 1e8

Fig. 4. Vertical distributions of THg and MeHg concentrations in core sediments of selected sampling sites. A: THg concentrations of 5 sampling sites in the first sampling trip; B: THg concentrations of 4 sampling sites in the second sampling trip; C: MeHg concentrations of 5 sampling sites in the first sampling trip; D: MeHg concentrations of 4 sampling sites in the second sampling trip.

higher mercury levels in their muscle (Simonin et al., 2008). However, significantly negative correlations (p < 0.01, Two-tailed test) was observed between THg and MeHg concentrations in abdominal muscle of Indigo Hamlet collected from Guishan Island and the length of fish (Table S4). This could be attributed to the relatively high growth rate of cultured fish making them accumulate less mercury from the environment during fast growth period (before being harvested). The rapid increase of weight may cause a dilution effect on Hg concentration measured in muscle (Jardine et al., 2009). The THg and MeHg concentrations in oysters, scallops, mussels, conches bought from Shenzhen and Zhuhai wet markets are displayed in Table S5. The average concentration of THg was 7.84 ± 3.83 mg kg
1, ranging from 2.96 to 16.9 mg kg
1 wet wt, while the average concentration of MeHg was 4.60 ± 2.85 mg kg
1, ranging from 1.69 to 10.7 mg kg
1 wet wt. It was noted that MeHg composed of a main proportion of the THg detected in shellfish, with an average of 62.4%, ranging from 17.4% to 93.1%. The results of Spearman's rho correlation test (two-tailed test) shows significant

correlations (p < 0.05, r ¼ 0.543) between THg and MeHg concentrations in different shellfish obtained from the two sites. However, no significant difference of THg and MeHg concentrations was obtained among different shellfish. The THg concentrations detected in all shellfish samples were found to be complied with the USEPA 2010 health guideline of 0.30 mg kg
1 (USEPA, 2010) and China's National Standard of 0.5 mg kg
1. Table 3 BAFs of all biological samples for THg and MeHg. The significance level was set at p < 0.05. Biological samples

THg

Indigo hamlet (GI) White croaker (GI) Indigo hamlet (DB) Orbfish (DB) Yellow grouper (DB) Shellfish (ZH) Shellfish (SZ)

3.4 1.6 4.6 2.8 3.5 1.9 1.3

      

MeHg 104e4.9 104e4.5 104e3.5 104e3.8 104e4.3 104e3.2 104e2.7

      

105 105 105 105 106 104 104

b b b b a c c

4.2 7.5 4.8 3.8 3.4 2.6 2.6

      

105e1.1 105e9.1 105e4.6 105e4.8 105e4.1 105e9.1 105e4.3

      

107 105 106 106 106 105 105

a c b b b c c

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H.C. Tao et al. / Environmental Pollution xxx (2016) 1e8

3.4. Human health risk assessment from consuming fish and shellfish Bioaccumulation factors (BAFs) of all biological samples for THg and MeHg are shown in Table 3. All BAFs in the present study matched those of Stein et al. (1996) who reported that BAFs of different fish species for mercury (both THg and MeHg) in aquatic systems were in the range of 104 to 107. In the present study, BAFs for MeHg are an order of magnitude higher than that for THg, which indicates that it is more easily for fish and shellfish to accumulate MeHg. The BAFs of fish for both THg and MeHg were significantly higher (p < 0.01) than shellfish. The BAF of White Croaker for MeHg was significantly lower (p < 0.01) than Indigo Hamlet collected from Guishan Island. This may be due to different fish species fish ages. In the aquaculture ecosystem, the population density of fish is overall higher than that of the natural environment, however the cultured fish would consume similar or larger amount of food. High population density would result in a weaker biomagnification trend (Cheng et al., 2011), not mentioning the rather simple food chain of the cultured fish. According to the present results, all fish species had greater BAFs for both THg and MeHg than shellfish. In general, carnivorous fish are on the higher trophic levels, and therefore they are able to accumulate higher mercury concentrations from the ambient environment and preys (Cheng et al., 2011). However, the sample size was small, and further research is needed before a further comprehensive conclusion to be drawn. Fig. 5 shows the EDI of MeHg through consumption of fish (from cages) and shellfish (from wet markets) by both adults and children in PRD. The results showed that consumption of 10% of the Indigo Hamlet collected from GI resulted a MeHg EDI go beyond the RfD (0.1 mg kg
1 bw d
1) for adults. The EDI of MeHg for adults for all White Croaker from GI and Indigo Hamlet, Orbfish, Yellow Grouper from DB were less than the RfD. With respect to children, higher EDIs of MeHg compared to RfD were found for all Indigo Hamlet from GI, White Croaker from GI and Indigo Hamlet from DB. EDIs of MeHg for all Orbfish and Yellow Grouper from DB by children were lower than the RfD. The EDI of MeHg for all shellfish by both adults and children were lower than RfD. Nevertheless, the EDI of MeHg for all samples (except for 5% Indigo Hamlet collected from Guishan

7

Island) did not exceed the ADI (0.23 mg kg
1 bw d
1) for both adults and children (Fig. 5). The present results were in accordance with our previous results that consumption of omnivorous and carnivorous fish (northern snakehead, mandarin fish, bighead carp, largemouth bass and grass carp) from the rather industrialized and urbanized environment in PRD, led to higher EDIs of MeHg than RfD for adults and children (Shao et al., 2011). Cheung et al. (2008) also showed that the consumption of freshwater and marine fish available around the PRD would result in higher EDIs of Hg than RfD for the local residents. It is noteworthy that the assessment results could be affected by many uncertained aspects such as sample quantity, mercury bioavailability and rate of fish consumption. Therefore further studies are required for assessing human health risk exposed to MeHg through seafood consumption. However, the current results still contribute some practical information indicating that cultured fish consumption at the mouth of Pearl River might impose adverse health risks, especially children and pregnant and nursing mothers who are more prone to MeHg. 4. Conclusion The extent of Hg (THg and MeHg) contamination of seawater, sediment and biota samples in mariculture sites at the mouth of PRD was investigated. The highest THg level (7.037 ± 0.556 ng L
1) in seawater was observed at Zhuhai Estuary, which persistently collected discharges from different industrial and urban centers along the Pearl River. Sediments collected from fish culture sites showed higher THg and MeHg concentrations than corresponding reference sites, and which indicated that the mariculture activities would enhance THg loading and MeHg production in the aquatic environment. The vertical distribution of THg and MeHg in sediments indicated that mercury methylation mostly occurred at the sediment-water interface. In fish samples, THg and MeHg were more easily to be accumulated in muscle than in liver, grills and skin. There were relatively lower mercury levels in shellfish. Risk assessments indicated that consumption of fish collected from Guishan Island and Daya Bay might impose no or less health risks to adults, but higher health risks to children. On the contrary, shellfish seemed to be safe for consumption. Further research is urgently needed for monitoring Hg, around PRD, one of the regions in China, with a very rapid socio-economic development. Acknowledgments Financial support from the National Natural Science Foundation (41373090) of China and Zhejiang Agriculture and Forestry University is gratefully acknowledged. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.envpol.2016.07.067. References

Fig. 5. Estimated daily intakes (EDI) of MeHg through consumption of fish and shellfish by adults and children in PRD. The consumption rates are 43.5 g d
1 for adults and 37.4 g d
1 for children, respectively. Each box displays interquartile range (25th and 75th percentile) of EDI of each fish species and shellfish. EDI ¼ estimated daily intake; ADI ¼ acceptable daily intake (0.23 mg kg
1 bw d
1); RfD ¼ reference dose (0.1 mg kg
1 bw d
1); IH, WC, OF, YG represent Indigo Hamlet, White Croaker, Orbfish, Yellow Grouper, respectively. GI means Guishan Island; DB means Daya Bay.

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Please cite this article in press as: Tao, H.C., et al., The level of mercury contamination in mariculture sites at the estuary of Pearl River and the potential health risk, Environmental Pollution (2016), http://dx.doi.org/10.1016/j.envpol.2016.07.067