Mechanism Study of Transport and Distributions of Trace Metals in the Bohai Bay, China

This study shows that there are two regions with high trace metal concentrations in the Bohai Bay, China. The numerical hydrodynamic model coupled with geochemical analysis was applied to understand the mass transport and sedimentation in the bay. The modeling results show that the two regions are located within the residual current vortexes. Results from the particle-tracking model indicate that the trace metals released from the land sources enter the regions and take a relatively long residence time in the vortexes. The sediment radionuclide data indicates that the two regions experienced continuous and high sedimentation, and trace metals are prone to deposit in the regions. The correlations among trace metals, residual currents and radionuclides data suggest that the tides are the governing factor controlling the distributions of the trace metals in the bay. The consistence among these results also supports the reliabilities of the numerical simulation results of water and trace metal transport in this study.


Introduction
A large portion of trace metals enters the ocean from the anthropogenic source such as wastewater outlets. In aquatic environment, most trace metals are scavenged by fine sediments and transported with the particles under the drive of currents. In addition, other processes can also affect distributions of trace metals. For example, the distribution can be controlled by sediment deposition and resuspension, which are closely related to the local hydrodynamic conditions. Therefore, the hydrodynamic conditions in the ocean are considered as the predominant factors that determine the transport and the distributions of trace metals. In certain circumstances, the elevations of the trace metal concentrations in the overlying seawater often result in the rising of the concentrations in seabed sediments and the distributions of trace metals in the seabed sediments record the information of hydrodynamic conditions in the ocean (Cundy et al., 2003;Jha et al., 2003;Xu et al., 2009;Qiao et al., 2017), which can provide valuable information on resolving the source and sink of trace metals.
Generally, the transport of fine sediments in the ocean can be represented by the water flows that can be well studied by the hydrodynamic model. However, an accurate computation of the sediments transport may be complicated because the mechanism and processes of sedimentation and resuspension have not been understood thoroughly. Several numerical models (e.g., ECOM-SED, FVCOM, EFDC and Delft3D) have been developed to study the sedimentation and morphodynamic process. However, the modelling results could be very different from the real situations. For example, Ganju et al. (2009) pointed out that the accuracy of the sediment model was limited by various conditions such as the initial status of sediments and boundary conditions which were difficult to be determined. In addition, Ganju et al. (2009) indicated that the model they developed could not be used to study the sedimentation and erosion in some spe-cific locations, even though it could be well applied to the bathymetric changes in large temporal (decades) and spatial (basin) scale.
On the other hand, radionuclide with different half-lives has been proven to be useful tracer to track the sediment transport and fate, and to the estimate sediment deposition rates (Feng et al., 1998;Woodruff et al., 2001;Su and Huh, 2002;Feng et al., 2010;Qiao et al., 2017). Cochran and Masqué (2005) and Cochran et al. (2006) reviewed the application of radionuclide to the sedimentation processes. In general, the radionuclide profiles in sediments can reflect local sedimentation patterns caused by physical, geomorphological and/or biological processes on seasonal to centennial time scales. For example, 7 Be has a relatively short half-life (t½= 53.3 d) while 210 Pb has a relatively long half-life (t½=22.3 y), which make them useful tracers for short-and long-term variations in the transport and deposition of sediments. In this sense, the radionuclide profiles offer an alternative approach to examine the local hydrodynamic conditions and sedimentation in the ocean, and to study the transport and distributions of trace metals.
The Bohai Sea is the largest inner sea in China. With the fast development in the surrounding regions, the Bohai Sea, especially the Bohai Bay, is facing with many environment-al problems, one of which is the enrichment of trace metals in the seabed sediments (Li et al., 2015;Zhu et al., 2017). It is necessary to have a good knowledge of the mechanism of the transport and distributions of trace metals to assess the environmental risk in the future. The current study aims at studying the mechanisms that control the transport and distributions of the trace metals in sediments in the Bohai Bay with the combination of numerical models and geochemical analysis.

Study area
As a semi-closed sea in China (Fig. 1a), water in the Bohai Sea is quite shallow with an average depth of 18 m. The deepest area is approximately 86 m in the north end of the Bohai Strait. The semi-diurnal tidal component M 2 is the dominant tidal constituent in the Bohai Sea with an average amplitude of 2 m.
The Bohai Bay is one of the bays in the western part of the Bohai Sea. Coastal regions around the Bohai Bay are highly industrialized and heavily populated (Figs. 1a and 1b). Rivers flowing into it are often contaminated (Duan and Li, 2017;Xiao et al., 2017). With a series of economic stimulus policies, the economy in this region is being expected to bloom. In the meantime, economy development is putting more threats on the environment in this area. During the past years there have been several environmental issues in the Bohai Sea, including discharge of wastewater which exceeded the criteria implemented by the government from the wastewater outfalls around the Bohai Bay, as reported by the State Oceanic Administration of China (SOAC) (SOAC, 2009(SOAC, , 2010. According to the reports, the main contaminant sources around the Bohai Bay were the Jian River, the Yongdingxin River, the Ziyaxin River and the Dakou River (Fig. 1). The background trace metal concentrations in the sediments in the study area have been reported in previous studies (Wu and Li, 1985;Wei et al., 1991), which gave 3.62% for Fe, 0.13 mg/kg for Ag, 0.14 mg/kg for Cd, 49 mg/kg for Cr, 25.3 mg/kg for Cu, 36.1 mg/kg for Ni, 20.5 mg/kg for Pb, and 71.2 mg/kg for Zn, respectively. However, the land-based anthropogenic impact on the coastal environment does exist. Recently, petroleum and shipment industries have been well developed in the Bohai Bay, and coastal constructions arise with them. Constructions of several world class ports have been launched and made progress at a fast pace with a huge scale of land reclamation. All those constructions as well as the land reclamation have changed the environment and contributed to the contaminants entering the Bohai Bay.
The environment quality in the Bohai Bay is attracting more attentions from the public, the environment authorities and the scholars. To protect the environment of the Bohai Sea, it is necessary to well understand the mechanisms that govern the transport and the distributions of trace metal contaminants in this area.

Field investigation and measurement
Two field investigations were conducted in the western Bohai Bay during May and November 2012 to study the sediment quality in the Bohai Bay. Surface (~2 cm in thickness) or core (approximately 16-30 cm long) sediments were sampled at 18 sites along three transections (Fig. 1c). Surface layer of the seabed sediments was collected using a grabber sampler and sediment cores were collected using a gravity corer designed and manufactured by ourselves. After recovery of the samples, the surface sediment samples were placed in plastic bags and kept at 4°C in a refrigerator immediately on board, while the sediment cores were sectioned at 1 cm interval on board immediately, also placed in plastic bags, and then kept in the fridge. As soon as the research vessel arrived at the port, all the samples with the fridge were shipped to the laboratory at Hebei University of Engineering. In the laboratory, the sediment samples were measured for Cr, Cu, Pb and Zn concentrations using an inductively coupled plasma mass spectrometry (ICP-MS Model X-II manufactured by Thermo Fisher Scientific). All the sampling processes and measurements conform to the National Standards on Specifications for Oceanographic Survey in China (GB17378.5-2007) published by Standardization Administration of China (SAC) ( SAC, 2007).

Numerical models
Because the water is relatively shallow, the movement of seawater and contaminant in vertical direction in the Bohai Bay is much weaker than that in horizontal directions, and thus can be neglected. Therefore, two-dimensional (2D) models can be applied to predict the water flow and mass transport in the Bohai Bay. Previously, we developed a 2D hydrodynamic model (Sun, 2007;Li et al., 2016) and a particle-tracking model . These two models were applied to simulating the transport of water and the fine sediment-associated metal contaminants.
The boundary between the computational area and open ocean is defined as an open boundary. In this study, the open boundary is located between Dalian in Liaoning Province and Yantai in Shandong Province to eliminate the boundary effect on the area of interest (Fig. 1). Along the open boundary, a time series of water level calculated by the eight main tidal constituents (K 1 , O 1 , P 1 , Q 1 , N 2 , K 2 , M 2 , S 2 ), is given at every node.
The particle-tracking model  is based on the Lagrangian viewpoint which studies water and contaminant movements by tracking every single particle that represents a certain amount of the water parcel or the contaminant. With this model, the tracks of the water and the contaminant parcels can be directly studied. It provides detailed information of the transport of water and contaminants discharged from the outfalls. Without numerical dispersion and dissipation, the particle-tracking model is especially useful in cases with a sharp concentration gradient such as the situation near the outfalls.
In this study, the hydrodynamic model covering the entire Bohai Sea ran on the grids with a resolution of 1.5′×1.2′ (Longitude×Latitude). The time step of the hydrodynamic model and the particle-tracking model was set at 3 min. The hydrodynamic model and particle-tracking model run simultaneously. At the end of each time step of the hydrodynamic model, the water levels and velocities at individual grid node, which were the outputs of the hydrodynamic model, were immediately transferred to the particle-tracking model as input values. The instantaneous water depth and velocities at the point of the individual particle were calculated using a linear interpolation method. Under these conditions, the advection of the particles in the particle-tracking model was driven by the velocities calculated by the hydrodynamic model. The positions of these particles at the end of the current time step was calculated based on the position at previous time step and the velocities, considering the random walks of the particles that represent the diffusion of the water mass and contaminants. Fig. 2 shows the trace metal concentrations in the surface sediments at each station during the two field investigations in 2012 (Fig. 1c). It is seen that the distributions of Cr, Cu, Pb and Zn in seabed sediments have the similar trends along the three transects. The metal concentrations at the north and the south ends of the transects tend to be lower than those in the middle of the transects. Moreover, the concentrations of the trace metals along Transect A tend to be higher than those of the other two transects. The metal concentrations along Transect D tend to be lower. For all the four trace metals, the lowest concentrations appear at Station D6 located in the south of the bay. The result suggests that there are two regions with higher trace metal concentrations, one is in the west and the other in the center of the Bohai Bay.

Trace metal distributions
The four trace metals (Cr, Cu, Pb and Zn) in the Bohai Bay investigated in this study have similar distribution patterns and are correlated significantly (p<0.05) ( Table 1). In the past decade, there have been several large-scale pollution investigations in the Bohai Bay (Peng et al., 2009;Zhan et al., 2010;Zhang et al., 2011aZhang et al., , 2011bZhou et al., 2013;Sun et al., 2017;Zhu et al., 2017;Li et al., 2018). Previous studies showed that Cr, Cu, Pb and Zn may have the same source input (Zhang et al., 2011a(Zhang et al., , 2011bZhu et al., 2017;Li et al., 2018). The results from the field surveys in 2012 are consistent with those found in these studies.

Eulerian residual currents
Tides are the dominant force driving the movements of water, contaminants and suspended fine sediments in the Bohai Bay. Water level goes up and down accompanied by the alternative tidal flows that cause oscillation movements of suspended sediments and associated contaminants. However, the long-term transport of water and contaminant is determined by the residual circulation driven by the tides. According to a previous study (Delhez, 1996), the Eulerian residual transport calculated from the depth-integral transport was used as Eulerian residual current and to examine the trend of the mass transport in the Bohai Bay in this paper.
Large-scale land reclamation during recent decades has significantly altered the shoreline of the Bohai Bay, making the shoreline in 2003 and 2010 different. Fig. 3 shows the Eulerian residual currents in 2003 and 2010, respectively, which indicates that there is no significant change in residual currents in the offshore region due to the shoreline changes. However, differences do appear mainly near the reclamation areas. The results suggest that the residual current pattern in the Bohai Bay has not changed significantly during the past several years due to land reclamation. Therefore, the distributions of the trace metals in the seabed sediments in the Bohai Bay should be related to residual currents due to the tides.
It is seen in Fig. 3 that water coming from the Liaodong Bay and the central part of the Bohai Sea flows into the Bohai Bay along the north bank. However, this current cannot go far enough. Once it enters the Bohai Bay, part of it turns south toward the region southeast off Caofeidian and then flows back to the Central Basin of the Bohai Sea passing through the mouth of the Bohai Bay, forming an anticlockwise vortex near the mouth of the bay. Another relatively larger vortex was found south off Caofeidian. This vortex is an anticlockwise one that can reach Tanggu on the west bank of the bay. Along with the vortex, water flow entering the Bohai Bay along the north bank can reach the coast near Tanggu. Afterwards, it flows along the west bank to the south of the bay where part of it turns northeast and goes back to the region near Caofeidian, forming an anticlockwise vortex in the center of the bay. The rest continues flowing eastward and merges into a current coming from the Yellow River estuary near the mouth of the bay. Then, the current directs to the northeast, comes out of the Bohai Bay near the central part of the mouth, and joins a current from the northwest. In the region east off Tanggu, another branch of current from Caofeidian turns north and enters the harbor in the northwest of the Bohai Bay. This current travels along the shoreline and goes back to the west off Caofeidian where it converges to the flow from the east and then forms a clockwise vortex in the northwest of the Bohai Bay.
Moreover, the residual currents seem to move slowly in offshore region and fast along the coast. In the regions near the south bank and off the east coast of Caofeidian, the speed of the residual currents can reach more than 0.1 m/s with an average of 0.05 m/s. In the centers of the vortexes, however, it can be as slow as 0.001 m/s, which indicates a weak hydrodynamic condition in these regions. Even though it is a small region in the northwest of the bay where low speed of residual currents occurs, it is seen that this region is located in the center of the vortex and the speed of the residual currents in the center of the vortex is only approximately 0.005 m/s. Generally, the water circulation in the Bohai Bay is anticlockwise. Water from the Central Basin of the Bohai Sea enters the Bohai Bay along the north bank and flows out of the bay through the middle and south of the bay mouth. It is found that there are three vortexes in the Bohai Bay (Fig. 3). In the center of these vortexes, the residual currents are relatively weak.

Particle-tracking simulations
Although the trace metal distribution is correlated with the residual currents, the transport process of trace metals from riverine outlets also controls the trace metal distributions. It has been reported that wastewaters are discharged into the Bohai Bay from several riverine outlets around the bay. The Yongdingxin River, the Jian River, the Ziyaxin River and the Dakou River are four typical sources of wastewater containing trace metals (SOAC, 2009). Referring to the residual currents calculated previously and the locations of the four riverine outlets (Fig. 1), we infer that the areas with high trace metal concentrations in the northwest and the center of the Bohai Bay are related to the discharges from these outlets. To confirm this hypothesis, four cases with contaminants being discharged into the Bohai Bay through the four outlets are simulated separately using the particle-tracking model (Table 2). However, as most of the outlets are controlled by water-gates in the estuaries and the data of flow rates with time are not easily available from the local facilities, it is difficult to get access to the data. Moreover, the discharge rates of the outlets are rather small and they would not have obvious effects on the water circulations in the Bohai Bay. As a basic exploratory study, using a constant rate of wastewater discharge will not significantly reduce the certainty of the conclusion. In this study, a constant wastewater discharge rate is represented by a constant number of particles discharged at every time step. Fig. 4 shows the particle distributions after two-year simulations, which presents transport trend of contaminants coming from the riverine outlets.
Because the outlets are located in the northwest of the Bohai Bay, Cases 1 and 2 have the similar contaminant transport patterns. In Case 2, the contaminants are discharged from the Yongdingxin River. A plume of contaminants turns toward left after being discharged, and flows northward along the shoreline and then into the harbor in the northwest of the Bohai Bay where the trace metal concentrations are high. After reaching the north end of the harbor, the flow carrying contaminants continues to flow southward along the shoreline and get into the west off Caofeidian. Then, a branch of it flows to the southeast and gets into the region near the Yellow River estuary. A portion of particles stays in the region near Caofeidian. The rest of the particles converge into the flow from the east and go back to the region near Tanggu, where the particle flow goes separately with one branch flowing northward forming a clock-  wise vortex in the northwest of the Bohai Bay and the other branch flowing southward along the west bank of the bay. It is found that the particles falling into the vortex spend a relatively longer time in the area before escaping, and the particles concentrate in the center of the vortex, implying high contaminant concentrations in the region. The particles escaped from the vortex in the northwest of the bay converge into the flow from the east and continue to travel southward along the west bank to the south of the Bohai Bay. However, the density of the particles discharged from the Yongdingxin River and the Jian River in this branch is much weaker than that staying in the northwest vortex of the Bohai Bay. Similar to the situation in the northwest of the Bohai Bay, particles entering the region southeast off Caofeidian also spend a long time in it. The relatively long residence time of the particles in this area is not only due to the vortex, but also due to the low speed of the residual currents (Fig. 3). The particles accumulating there lead to more particles in another region.
Cases 3 and 4 show similar situations. The particles discharged from the Ziyaxin River flow southward along the west bank of the bay along with the residual current from the region near Tanggu. When the particles get to the region not far away from the outlet of the Dakou River, most of them turn left and go northeastward to the center of the Bohai Bay along with a flow that is part of the vortex in the center. Near the center of the vortex, many particles are trapped and hover, resulting in condensed particles in another region. In this region, the particles may go to two different directions. One branch goes to the west with the westward flow that enters the Bohai Bay along the north bank, forming an anticlockwise circulation in the central-west of the bay. The other branch goes southeastward along with the flow from southeast off Caofeidian and gets to the north off the Yellow River estuary. Then some of the particles turn eastward and go back to the Dakou River estuary along the south bank, forming a clockwise circulation in the south of the bay. The other particles may continue flowing southward and finally reach the region east off the Yellow River estuary.
In general, the simulation results of the four cases from the particle-tracking model conform to the Eulerian residual current pattern based on the result of the hydrodynamic model. The particles such as trace metal contaminants can be transported to the regions with high trace metal concentrations that were found in this study and trapped in centers  of the vortexes as found in this study. The results indicate that the high trace metal concentrations in the two regions are due to the discharge of river outlets and the existence of vortexes there. However, the results from the particle-tracking model also show a slight difference from the Eulerian residual currents calculated from the results of the hydrodynamic model. The results from the particle-tracking model show a clockwise circulation on the west of the Yellow River Estuary, which is absent in the results from the hydrodynamic model. Zhao et al. (1995) suggested that there may exist a clockwise vortex in the south of the bay and the flow from the Yellow River Estuary can reach the west bank of the bay. The conclusion is partially supported by the results of the above numerical simulations, but the Yellow River plume cannot reach the west bank of the bay based on the numerical modeling results in this study.

Discussion
To further confirm the distribution patterns of the trace metals and the correlation with water circulations, data in year 2007 from the previous studies by other researchers (Feng et al., 2011;Zhang et al., 2011) were further analyzed. The spatial distributions of Cr, Cu, Pb and Zn in the seabed sediments in the Bohai Bay from these studies are summarized in Fig. 5. It shows that there are three zones with relatively high trace metal concentrations. The highest concentrations are found in the northwest of the bay and the area near the outlets of the Yongdingxin River. In the cen-ter and at the mouth of the bay, there are also two zones with relatively high metal concentrations. Overall, these results coincide with the conclusions based on our field investigations in 2012. Recently, Duan and Li (2017) also have confirmed the existence of high trace metal concentrations in the center of the Bohai Bay. All the facts support the credibility of the conclusion drawn from this study.
Several studies indicate that the fine sediments (silt and clay) are predominant in the Bohai Bay (Qin and Liao, 1962;Zhou et al., 2014;Chen et al., 2017). Owing to large specific surface area, fine sediments tend to absorb more trace metals from the seawater. It means that the transport of fine sediments can be used to represent the movement of the trace metals in the bay. In general, fine suspended sediments are more faithful to follow the water current than coarse suspended sediments because of less gravitation. These suggest that the tidal driven residual currents are usually consistent with the transport trend of the trace metals in the Bohai Bay. According to the results of the numerical models and geochemical analysis, we can hypothesize that the contaminants discharged from the four outfalls can flow to the northwest and center of the Bohai Bay. The trace metals can be trapped in the two regions for a long time due to the vortexes and the low speed in their centers. However, it does not result in the high trace metal concentrations in the seabed sediments without steady-state sedimentation of the particles and associated metals. Therefore, it is necessary to study the sedimentation conditions in these regions to clarify the reasons of the high trace metal concentrations in the seabed sediments of these regions.
Herein, 210 Pb activities in the Bohai Bay sediments from other studies were reviewed and used to estimate the sedimentation rates. Generally, with the steady-state fallout of 210 Pb from the atmosphere and the continuous sedimentation, the activities of the excess 210 Pb in the sediment cores typically show exponential decay profiles with the increasing depth (Feng et al., 2010). Therefore, the exponential decay profile of excess 210 Pb implies relatively stable and weak hydrodynamic condition. Fig. 6 shows the profiles of excess 210 Pb activities in these sediment cores (the location of the sediment cores are shown in Fig. 1c). Decaying profiles of excess 210 Pb activities can be found in overall depth of sediment cores at Stations B1, B2, B4, C0710, C0711 and C0712 in the northwest of the Bohai Bay and Stations C4 and BS5 in the west and the center of the Bohai Bay in the past decades with some occasional disturbances. Actually, when compared with the trace metal distributions shown in Fig. 2, it can be seen that, except Station B4, these stations are located in the regions with relatively high trace metal concentrations, where there are vortexes and low speed of residual currents. In the northwest of the Bohai Bay, the upper 5 cm of sediment core at Station B3 and upper 15 cm of the sediment at Station B5 show a continuous excess 210 Pb decay, which indicate a steady-state sedimentation (Fig. 6). Stations B3 and B5 are located near the navigation channel to the Port of Tianjin. In this region, there were intensive construction and anthropogenic activities, which may affect the sedimentation in this region. Stations C0707, C0710, C0711 and C0712 are located in the intertidal zone in the northwest of the Bohai Bay. Except Station C0707, steady-state decay profiles of excess 210 Pb activities were found at varying depths at these stations. Like the stations in the offshore region northwest of the Bohai Bay, these profiles suggest steady-state sedimentation.
As to the stations located near the west bank of the bay, Station C4 shows a relatively steady-state decay of excess 210 Pb activity with obvious disturbance at the depth of 60-80 cm. Meng et al. (2005) considered that it might be re- lated to the coarse sediments supply caused by the flooding events during the corresponding period. The profiles of excess 210 Pb activities at Stations C1 and C3 show non-steady state sedimentation. With little influence of the particle size (Meng et al., 2005), such profiles of excess 210 Pb activities indicate that there is slow sedimentation in these areas and the regions near these two stations may experience relatively strong hydrodynamic conditions considering that the region is located on the path of residual current.
As to the center of the Bohai Bay, Station BS5 presents a relatively steady-state decay of the excess 210 Pb activity. This suggests that this area may experience relative steadystate sedimentation during the past decades. However, no decay profile of excess 210 Pb activity was found in the sediment cores at other three stations in this region (Stations BS1, BS2, and BS10). Since the three stations are far from the shoreline and there are few anthropogenic activities in this region, the disturbed profiles of excess 210 Pb activities may be the result of relatively strong hydrodynamic conditions of these three stations. Table 3 summarizes the results of the regression analysis of the excess 210 Pb activities in sediments and the sedimentation rates at the stations. Stations B1 and B2 are located in the center of the vortex and the region with higher trace metal concentrations in sediments. Relatively high sedimentation rates are found at these two stations ( Table  3). The similar situations are also found at Stations C0707, C0710, C0711 and C0712. These results suggest that the seabed in these regions was less disturbed by the currents.
The p-values in Table 3 indicate the significance of regression based on a radionuclide decay profiles in the sediments and reliability of the estimated sedimentation rates at the sampling stations. Statistically, p<0.05 indicates a significant correlation and hence, a reliable estimation on the sedimentation rate. The p-values for Stations B4 and C4 (p<0.05) suggest that the radionuclide profiles at the two stations may indicate a relatively steady-state sedimentation. However, there is no vortex center found at Stations B4 and C4 in this study. The simulations of the models indicate that the trace metals from the outfalls do not accumulate and hover near them. Station B4 is located near Tanggu, one of the busiest ports in China, and Station C4 is located near Nangang, an industrial area with large-scale of land reclamation. The relatively steady-state sedimentation at Stations B4 and C4 should be related to the smaller-scale circulations caused by local topography, which cannot be resolved in this study due to the coarse grids used. In contrast, continuous sedimentation was not found at other stations near Stations B4 and C4, such as Stations B3, B5, C1 and C3. Located in the center of the Bohai Bay, Station BS5 also presents smaller p-value. Because of its location in the center of the vortex, the smaller p-value indicates that the lessdisturbed sediment deposition and higher sedimentation rate contribute to the high trace metal concentrations near it.
The excess 210 Pb activities in the sediment cores in the northwest and center of the Bohai Bay suggest that the sedimentations in these regions are stable due to relatively weak hydrodynamic conditions in the centers of the vortexes. Under the weak hydrodynamic conditions, suspended sediments and associated trace metals in the overlying water can settle down to the seabed. With the sediment deposition in these regions, the trace metal concentrations in the seabed sediments increase. In addition, the relatively long residence time of the water in the centers of the vortexes also contributes to trace metal accumulation in the seabed sediments. Moreover, the weak hydrodynamic conditions restrain the sediment resuspension and mixing, which preserves relatively stable decay profiles of excess 210 Pb activities in sediments. Therefore, the high trace metal concentrations and normal exponential decay profiles of excess 210 Pb in sediments are the evidences to the weak hydrodynamic conditions in these regions.
The geochemical analysis shows slightly higher trace metal concentrations in the seabed sediments in the northwest of the Bohai Bay than those in the center of the Bohai Bay. This can be attributed to several reasons. Firstly, the northwest of the Bohai Bay is closer to the riverine outlets and higher trace metal concentrations were found in the water in the northwest of the Bohai Bay (SOAC, 2009(SOAC, , 2010. Secondly, the excess 210 Pb profiles in sediments in the northwest of the Bohai Bay show relatively higher sediment deposition rates than those in the center of the bay. In fact, the center of the Bohai Bay has finer sediments (silty clay) than those (clayey slit) in the northwest of the Bay (Zhou et al., 2014;Chen et al., 2017;Duan and Li, 2017), which means that the sediments in the center of the Bohai Bay should be prone to absorb more trace metals. In a preliminary study of the submarine sediments in the Bohai Bay, Qin and Liao (1962) found that the clay-mud was predominant in the center, the fine silt was predominant in the south, and the fine sediments was predominant in the coastal region. Their results showed that there were two regions in the center and northwest of the Bohai Bay, respectively, where fine particles accumulated. However, this fact cannot explain the slightly lower concentrations of trace metals in the center of the Bohai Bay than those in the northwest of the Bay. It indicates that the current circulations, sedimentation rates and geographical locations have more significant influence on the trace metal distributions than those of the particle size difference in such a coastal region.
Owing to a lack of 210 Pb profiles in sediments, the sedimentation condition in this region is unknown. In addition, because its location is on the mouth of the bay, this region is influenced by the currents and contaminants from the Liaodong Bay as well. Therefore, there is no direct explanation of the mechanisms and processes that can be given in this study to discuss the cause of the high trace metal concentrations in this region.
Nevertheless, both sedimentation and trace metal concentrations are found to be correlated with the magnitude of tidal current speed. Fig. 7 shows a two-year average current speed (scale average) in the Bohai Bay. It is seen that a region in the central-east of the bay has a relatively high current speed with an average speed up to 0.40 m/s. However, the three regions with higher trace metal concentrations mentioned above are just located outside this region. The regions with high trace metal concentrations and steady-state sediment deposition correspond to those of lower tidal current speed. The existence of high average speed in this region was also found in a previous study (Zheng et al., 2015). All of these confirm the rationality of the water circulation pattern and the existence of regions with high trace metal concentrations found in this study. In addition, the average speed of tidal currents in the northwest of the Bohai Bay is smaller than that in other regions. This further confirms the weak hydrodynamic condition in the northwest of the Bohai Bay and explains the high trace metal concentrations there.
Although the particle-tracking model simulation shows that the particles discharged from the Yongdingxin River and the Jian River accumulate on the south of Caofeidian. However, high trace metal concentrations are not found in the seabed sediments in this region. The speed of residual current (Fig. 3) and average speed of tidal current (Fig. 7) in this region indicate a strong hydrodynamic environment that can cause sediment disturbance and resuspension. On the one hand, the deep water and high gradient slope of the seabed in this region [ Fig. 1b] may cause strong flows in the vertical direction that can also inhibit fine sediment depos-ition and make the steady-state sedimentation difficult in this region. On the other hand, it also explains the presence of the coarse sediments in this region. In previous studies, both Duan and Li (2017) and Zhou et al. (2014) found that sediments in this region were dominated by silty sand which was coarser than that on its southern area(i.e., the center of the Bohai Bay).
Additionally, the water circulation pattern in the Bohai Bay is a controversial topic. The results in this study indicate that high trace metal concentrations and steady-state sedimentation, as supported by excess 210 Pb decay profile in the sediments, are correlated with the presences of the vortexes in the northwest and center of the Bohai Bay. The close correlations serve as a valid evidence to support the existence of such a circulation pattern in the Bohai Bay and imply the credibility of the model used in this paper. This may provide a new idea to study this controversial problem. Jiang et al. (1997) considered that the residual currents in the Bohai Bay were weak and the inside circulation was clockwise according to their tracing of drifting buoy. Based on the observation of floating chambers to study the similar problem, however, Xu et al. (2006) suggested that there was a northeastward flow from the Yellow River estuary and the water from the central basin of the Bohai Sea entered the Bohai Bay along the north bank. Xu et al. (2006) also indicated that the water current along the north bank could not further flow into the bay, and it turned to the southeast and flowed out of the bay once entering it. Moreover, Wei et al. (2004) indicated that there was a clockwise circulation in the northwest of the Bohai Bay. Recently, Huang et al. (2017) have simulated the tidal currents in the Bohai Sea and found the existence of strong tidal residual currents near Tianjin and the weak residual currents in the offshore region. These findings and observations are similar to the results from the numerical models in this study. It means that results from the numerical models in this study not only agree with field observations and numerical simulation results by other researchers, but also can account for the mechanisms of the transport and distributions of trace metals in the Bohai Bay. These facts further confirm the credibility of the numerical simulation results and the analysis based on them.

Conclusions
This study combines the numerical models and geochemical analysis to study the transport and distributions of trace metals and its controlling mechanism in the Bohai Bay. Geochemical analysis of the trace metal (Cr, Cu, Pb and Zn) distributions show that these trace metals have similar distribution patterns in the Bohai Bay. Three regions with high trace metal concentrations in the seabed sediments in the Bohai Bay are identified. Two of them are located in the northwest and center of the Bohai Bay, and the other one is located at the mouth of the Bohai Bay. The results from the hydrodynamic model simulations show that there are three vortexes in the Bohai Bay. The vortex in the northwest of the Bohai Bay is clockwise and the vortexes in the center and the mouth of the bay are anticlockwise. Interestingly, the centers of the vortexes are just located in the regions where high trace metal concentrations are found in the seabed sediments. Moreover, case studies using the particle-tracking model show that the trace metals discharged from the wastewater outfalls around the Bohai Bay can be transported to these regions and hover there for a long time. Combined with the analysis of the data of radionuclide ( 210 Pb) in the sediments cores, it showed that the weak hydrodynamic condition in the centers of the vortexes is in favor of steady-state sedimentation and particle reactive contaminant accumulation (e.g., trace metals), which cause high trace metal concentrations in sediments.
Overall, the trace metal transport and sedimentation process can be well explained by the residual current circulations. Excess 210 Pb activity profiles in the sediment cores verify the steady-state sedimentation in the centers of the vortexes, where the trace metal concentrations are relatively high, and it indicates the weak hydrodynamic conditions in the centers of the vortexes. The results imply that the distributions of trace metals, sedimentation conditions and residual currents are closely correlated and the tides are the governing factor controlling the distributions of the trace metals in the bay. Moreover, these correlations indicate the credibility of the transport tend of water and contaminant found in this study.