Daya Bay (22° 30′ - 22° 50′ N, 114° 29′ - 114° 49′ E) is located in the eastern part of Guangdong Province between Honghai Bay and Dapeng Bay and is a typical subtropical semi-enclosed shallow bay. There are more than 50 islands in Daya Bay, and a series of north-south oriented islands in the center of the bay (Central Island) divide the bay into two parts: the eastern entrance that is approximately 9.6 km wide and 19-20 km deep; and the western entrance that is approximately 5.4 km wide and 19 m deep (Xu, 1989). The exchange of seawater with external seawater mainly occurs through the mouth of the bay. The movement of seawater is mainly controlled by the tide wave and topography of the South China Sea, and the tidal type is an irregular semi-daily mixed tide.

Since the 1980s, Daya Bay has vigorously developed petrochemical-based industries, the good ecological environment has led to the rapid development of aquaculture, two nuclear power plants located on the west coast were put into operation in 1994 and 2002, and the first and second deep-sea outfall pipelines were put into operation in 2000 and 2017, respectively. These anthropogenic activities have led to heavy metal contamination of seawater, sediments and organisms in Daya Bay.

In November 2021, 42 seawater quality survey points, 21 sediment survey points, and 21 biological survey points were investigated in the study area; the specific sampling stations are shown in Figure 1 . All samples were collected and preserved according to the Marine Monitoring Code (GB17378.3-2007) (Ma et al., 2008). Seawater samples were collected using water quality samplers, and 500 mL of seawater samples were loaded into the sampling bottles and stored in low-temperature refrigerated conditions. When the water depth was less than 10 m, only surface samples were collected; when the water depth was greater than 10 m and less than 25 m, both surface and bottom water samples were collected; when the water depth was greater than 25 m, the surface, middle, and bottom water samples were collected. The depth of surface seawater was in the range of 0-1 m; that of the middle seawater layer was 10 m; and that of the bottom seawater layer was 2 m from the bottom. The pretreated elements Hg and As were determined by atomic fluorescence (BAF-2000), Cr by flameless atomic absorption spectrophotometry (PinAAcle 900T), and Cu, Pb, Zn, and Cd by anodic dissolution voltammetry (797VA).

Sediment samples were collected using a winch connected to a grab mud collector at the surface of the seawater bottom from 0 to 5 cm, packed into polyethylene bags, and stored under refrigeration at 0-4°C. Sediment samples were stored in a dry and ventilated environment, ground in an agate bowl, and then passed through a 160 mesh nylon sieve. After pretreatment, Hg and As were determined by atomic fluorescence method (BAF-2000), and Cu, Pb, Zn, Cd and Cr were analyzed by inductively coupled plasma mass spectrometry (ICP-MS DRCe). Organic carbon is determined by potassium dichromate redox volumetric method.

Fish, crustaceans and shellfish were caught in the trawl, but relevant research shows that shellfish are more seriously polluted by heavy metals. Therefore, this paper mainly studies the biological quality of Perna viridis and Ruddy Clam shellfish. Biological samples were trawled at a speed of 10 knots/h, and samples with a weight of approximately 1.5 kg were selected and rinsed on-site with seawater and subsequently frozen and stored. After dissecting the biological samples, 200 mg of muscle tissue samples were extracted by 24-h freeze-drying using HNO3-H2O2 (4:2) acidification, placed into an electric heating plate, and heated to 120-140°C. After pretreatment, elemental Hg was determined by atomic fluorescence spectrometry (AFS-8520), and As, Cu, Pb, Zn, Cd, Cr were analyzed by inductively coupled plasma mass spectrometry (7800 ICP-MS).

During the measurement of seawater samples, the standard solutions of Cu, Pb, Zn, Cd and Cr (GBW (E) 080040), Hg (GBW08617) and As (GBW (E) 080117) are used as calibration standards. The quality control shall be carried out with reference materials specified in China Offshore Marine Sediment Standard Material (GBW 07314) and China National Standard Material GBW 10024 (GSB-15). The error of parallel samples was less than 5%, and the recoveries of the standard substances were between 95 and 110%.

According to the Marine Monitoring Code (GB 17378-2007), when a few measured values (less than 50% of the total) were below the detection limit, the measured value was equal to 50% of the detection limit.

The results of the water quality evaluation for the sea area are presented in Table 3 and Figure 2 . As Cd is basically not detected, this paper will not analyze Cd in seawater. As presented in Table 3 , in 2021, except for Cu, the other six factors meet the standard of first-class water in Daya Bay; the maximum exceedance of Cu is 3.3-fold, which appears on W19 outside the bay. The exceedance rate of Cu in the surface layer of seawater in the bay was 15%, whereas those in the surface layer outside the bay were 18.2 and 16.7% in the middle layer and 19% in the bottom layer. This may be ascribed to the large amount of industrial wastewater discharged after the 2nd outfall line of the petrochemical zone of Daya Bay that was operational after 2017, which led to Cu exceedance at some points outside the bay; related studies also showed that the relatively high content of heavy metals at the mouth of Daya Bay may be due to the discharge at the two outfall lines (Liu et al., 2022).

According to the results of the coefficient of variation calculation, The variation coefficients of As in Daya Bay and Pb outside the bay are small, and the variation of other heavy metals is relatively high. Therefore, the seawater in Daya Bay may have originated from different pollution sources.

The comprehensive pollution index of heavy metals in the seawater of the nearshore area of Daya Bay is shown in Figure 3 . Figure 3 shows that the average values of the comprehensive pollution index of As, Hg, Zn, Pb, Cu, and Cr of the seawater inside and outside Daya Bay were less than 1. As presented in Table 1 , the seawater of Daya Bay is not polluted and the seawater is relatively clean.

The WQI of surface seawater in the bay ranged from 0.052 to 0.624, with a mean value of 0.173. The WQI of surface seawater outside the bay ranged from 0.052 to 0.71, with a mean value of 0.144., and that in the bottom layer ranged from 0.053 to 0.467, with a mean value of 0.141. The WQI shows that the pollution level of seawater inside Daya Bay was higher than that outside it, and the pollution level of the surface layer was higher than that of the bottom layer. Daya Bay is close to the Daya Bay Chemical Industry Park, and the productivity and life along the coast are more frequent. Effluent from enterprises in the petrochemical zone, except the sewage generated by some enterprises (China Shipping Shell Petrochemical Company Limited, CNOOC Huizhou Petrochemical Company Limited, Qingyuan Wastewater Treatment Plant, etc.), which are discharged through the sea discharge pipeline, and the wastewater generated by other industrial enterprises are mainly indirectly discharged into the sea through the rivers entering the sea, resulting in a large number of land-based pollutant sources to the bay. Additionally, the hydrodynamic diffusion conditions in near-shore waters were weaker and slower than those outside the bay. Therefore, the concentration of heavy metals in the bay was higher than that outside the bay.

According to the water quality monitoring data of near-shore waters released by Guangdong Province from 2015 to 2021 (http://gdee.gd.gov.cn/jhszl/index.html) (see Figure 4 ), Cu and Pb in the bay showed a fluctuating upward trend, increasing by 177.23 and 384.31%, respectively, in 2021 compared to 2015. Cd, Hg, and As showed a fluctuating downward trend, decreasing by 76.81, 82.43, and 76.70%, respectively (value of As in 2020 compared to that in 2015). The Zn value initially decreased and then increased, with a 6.15% increase in 2020 compared to 2015. The above data indicate a trend of intensified deterioration of Cu and Pb in Daya Bay, wherein the growth proportion of Pb is larger and should be controlled. Except for Cu and Pb, the remaining heavy metals showed an overall decreasing trend. This indicated that in recent years, owing to the continuous strengthening of ecological environmental protection, the emissions of heavy metals other than Cu and Pb have reduced.

The average concentrations of As, Hg, Zn, Cd, Pb, Cu, and Cr in this sea area were lower than those in other similar sea areas (Xiao et al., 2013; Zhang et al., 2017; Zhao et al., 2018; Lao et al., 2019; Nour, 2019). This revealed that the average levels of heavy metals in the nearshore sea area of Daya Bay are lower than those in other sea areas, indicating a lower environmental risk in Daya Bay.

The silt content of sediment in Daya Bay ranges from 53.28% to 84.14%, clay (mud) 12.56% to 29.93%, silt outside the bay is 66.44% to 81.98%, and clay (mud) 12.89% to 26.57%. The total content of silt and clay (mud) at the survey stations inside and outside the bay is basically more than 90%. The average grain size (Mz) of the sediment in the bay ranges from 0.01 to 0.07 mm. Except for S10 (0.07 mm) and S12 (0.03 mm), the average grain size (Mz) of other stations is 0.01 mm. Mz outside the bay ranges from 0.01 to 0.02mm. Except for W6 (0.02mm) and W9 (0.02mm), all other stations are 0.01mm. It shows that the sediments inside and outside Daya Bay are mainly silty sand or clayey silt, mainly silty sand, with a particle size of 0.01 mm.

The heavy metal contents in the sediments of the Daya Bay sea area are presented in Table 3 and Figure 5 . As presented in Table 3 , The average concentrations of seven heavy metals in and out of the bay are Zn>Cr>Pb>Cu>As>Cd>Hg from high to low. The average concentrations of Cr, Cu, Cd and Zn in the sediments in the bay are higher than those outside the bay. On the contrary, the concentrations of As and Pb are basically the same. Except for the Cu in S1, the heavy metals in the sediments of Daya Bay both inside and outside the sea area do not exceed the Class I standard, and the sediment quality is generally good. S1 station Cu exceeds Class 1 standard. This point is close to the petrochemical base of Daya Bay, and its heavy metal values may be influenced by the industry, resulting in a higher heavy metal content in the sediments. Based on the above phenomena, the heavy metal pollution levels of sediments in Daya Bay were higher than those outside the bay. Related studies have shown that the heavy metals in sediments in the near-shore waters of Daya Bay, such as Yaling Bay, offshore petrochemical areas, and nuclear power plants, are highly distributed but are still at an acceptable level (Qu et al., 2018; Liu et al., 2022).

The concentration of Cr and Zn in Daya Bay was the highest. As trace metals Cr and Zn are often used as feed additives in mariculture (Siano et al., 2017), resulting in high Cr and Zn contents in the sediment. Therefore, the sediment in Daya Bay was affected by mariculture, resulting in higher concentrations of Cr and Zn than other heavy metals.

The coefficients of variation of Cu and Cd in Daya Bay were 77 and 37%, respectively, which corresponded to high variability (CV > 36%), whereas those of seven heavy metals outside the bay were less than 36%, wherein the highest value was exhibited by Cr (32%). Thus, because Daya Bay is close to petrochemical parks and coastal living areas, each monitoring point is affected by different sewage discharges, resulting in the high dispersion of heavy metals in the sediment. Therefore, we inferred that Cu and Cd in the sediments of Daya Bay may have been influenced by anthropogenic sources, resulting in a high coefficient of variation. Related studies also suggest that high concentrations of Ni, Cu, and Cd in the sediments in the nearshore waters of Daya Bay are caused by human activities (Qu et al., 2018).

The potential ecological hazard index was used to evaluate the heavy metal content in sediments in the study area, and the results showed that ( Table 4 ) the potential ecological hazard index of seven heavy metals inside and outside Daya Bay was less than 40, which was a low hazard level. The average ecological hazard index in the bay was: Hg>Cd>As>Cu>Pb>Cr>Zn, and that outside the bay was: Hg>As>Cd>Pb>Cu>Cr>Zn; notably, the hazard levels of Hg, Cd, and As inside and outside the bay were relatively high. The ecological hazard index method mainly determines the occupancy rate of metals and the toxicity response coefficient of heavy metals. The occupancy rates of Hg and Cd were approximately 0.2, and that of As was below 0.4, which was lower than those of Cu (0.7), Zn (0.7), Pb (0.6), and other heavy metals. The higher hazard levels of Cd, As, and Hg were mainly due to the heavy metal toxicity response coefficients; the toxicity response coefficients of Hg (40), Cd (30), and As (10) were considerably higher than those of Zn (1) and Cu (5), thereby resulting in relatively high hazard levels of Hg, Cd, and As.

The E_RI ranged from 12.64 to 42.13 (mean of 25.43) in Daya Bay and 17.49 to 26.62 (mean of 23.56) outside the bay. Except S1 (E_RI 42.13) is greater than 40, while other survey stations are lower than 40, indicating that the potential impact of heavy metals in sediments on the ecosystem was relatively low. From each site, S1, S4, S5 and S2 are higher than other sites. From the perspective of spatial distribution, the four survey stations are located near the petrochemical area. This shows that the production activities in Daya Bay Petrochemical Area will increase the potential ecological risks of sediments in the bay. Relevant research also shows that the highest potential ecological risk index of Daya Bay sediment is close to the petrochemical base, which is mainly affected by land sources, and the risk mainly comes from Cd and As (Liu et al., 2018). Therefore, Daya Bay is mainly affected by the petrochemical base, and attention should be paid to the impact of Hg, Cd and As.

Notably, the results produced by the ecological hazard index method varied depending on the background values of the corresponding heavy metals. If the background values of heavy metals of sediments in the Daya Bay sea area calculated by Zhang (Zhang, 1991) and others are used for analysis and evaluation, Hg, Cd, and As values inside and outside the bay are at moderate or heavy ecological hazard levels. In this study, considering the current ecological and environmental management policies in China, and to better support local governments in implementing regional marine environmental management, the analysis and evaluation were conducted with reference to the primary standard of marine sediment quality (GB18668-2002).

The survey data of heavy metals in sediments for the past 10 years indicate that the heavy metal in the Daya Bay sediment shows an initial upward trend and a subsequent downward trend (Zhao et al., 2016; Liu et al., 2018; Tang et al., 2018; Liu et al., 2022). With economic and social development, the petrochemical production capacities of oil refining and ethylene in the Daya Bay petrochemical base are 22 and 2.2 million tons/year, respectively. Moreover, the scale of refining and chemical integration has ranked first in China; thus, the pollutant emissions have increased daily. However, the pollution prevention and control in recent years have improved the ecological protection requirements and reduced the pollutant emissions. Therefore, the trend of heavy metals in the Daya Bay sediments show an initial increase and subsequent decrease.

Compared to those of other offshore waters, the contents of most heavy metals in the sediments of Daya Bay are at an intermediate level, wherein the contents of Pb and Cr are higher than those in other waters (Zhang et al., 2007; Meng et al., 2008; Gao and Chen, 2012; Fu et al., 2013; Yu et al., 2016; Fan et al., 2022). This further indicates that the pollution of Pb and Cr in Daya Bay is more severe.

Two kinds of shellfish were obtained in autumn. The concentrations of As, Hg, Zn, Cd, Pb, Cu and Cr in shellfish are shown in Table 3 . The quality standards for shellfish in marine organisms are based on the marine biological quality (GB18421-2001).

The average concentrations of heavy metals inside and outside Daya Bay were 0.92 and 0.84 mg/kg (As), 0.15 and 0.09 mg/kg (Hg), 0.66 and 0.51 mg/kg (Zn), 0.31 and 0.30 mg/kg (Cd), 1.73 and 2.36 mg/kg (Pb), 0.12 and 0.16 mg/kg (Cu), 0.39 and 0.50 mg/kg (Cr), respectively.

The Pb in the shellfish inside and outside the bay exceeded the standards stipulated in the “Marine Biological Quality” (GB18421-2001). As exceeded the standard at S2, S4, S14 and S17 points in the bay, with the rate of exceeding the standard being 33%. Other heavy metal standards met the specified biological quality standards. The average concentration of Pb outside the bay was higher than that inside the bay, indicating that the exceedance of Pb in shellfish outside the bay was more serious than that inside the bay.

As presented in Table 5 and Figure 6 , the THQ and TTHQ of the seven heavy metals inside and outside Daya Bay are less than 1, indicating that there is no potential risk to human health from consuming shellfish from the Daya Bay. The order of THQ of all seven metals inside and outside the bay was As>Cu> Cr >Cd>Pb >Zn>Hg. THQ of As is the largest among the seven heavy metals, 0.28 and 0.25 respectively in and out of the bay, and the other six heavy metals are less than 0.1, indicating that mitigating the discharge of As should mainly be prioritized. This is consistent with Yang Liu’s conclusion that As is the highest carcinogen (Liu et al., 2022).

The TTHQ in the bay (0.319) was larger than that outside the bay (0.309), indicating that shellfish outside the bay are less hazardous to human health than those inside the bay. Heavy metal pollution in the bay is heavier than that outside the bay due to the influence of coastal chemical parks, such as those in Daya Bay. The BAF inside the bay is higher than that outside the bay, indicating that marine organisms in the bay are more likely to enrich heavy metals from seawater; therefore, heavy metals in marine organisms inside the bay are higher than those outside the bay, resulting in a higher risk inside the bay.

It is particularly noted that metal As is divided into organic forms, which are not toxic, and inorganic forms, which are more toxic. In this paper, the total amount of metal As is measured in the laboratory test analysis. The study proves that arsenic in seafood mainly exists in non-toxic or low-toxic organic form, while the most toxic inorganic arsenic only accounts for 1%-10% of the total arsenic content. Therefore, the carcinogenic risk of As will be lower if 10% of the total arsenic content is used as the inorganic arsenic content for calculation.

The weekly assessed intakes (EWIs) for Hg, As, Pb, and Cd in shellfish inside and outside Daya Bay were calculated to be 0.005 (0.003 outside the Bay), 0.59 (0.53), 0.112 (0.149), and 0.04 (0.038), all of which are lower than the WHO PTWIs for humans. This indicates that the probability of carcinogenic risk to residents for all heavy metals was below unacceptable levels. Overall, the health risks of seafood in Daya Bay were tolerable. With an appropriate reduction in frequency and dose of heavy metals, the carcinogenic risk can be reduced.