Trace metals are non-degradable pollutants in the aquatic environment, and in high concentration they can threaten organisms through the food chain and cause serious ecological problems(Cravo and Bebianno, 2005; Rainbow, 2007). In the last 20 years with rapid development of coastal zone economies in China, increased amounts of trace metals have been transported to the coastal environment through river and urban runoff, sewage discharge, atmospheric deposition, and other pathways(Xu et al., 2013; Gao et al., 2014). Trace metals in receiving waters can be absorbed directly by phytoplankton and transferred through the food chain. Sediments are an important repository of trace metals and other chemicals. Mostly, trace metals in receiving waters are absorbed by suspended particulate matter and accumulated by sediment through a series of processes. Once environmental conditions change, the balance between sediment and overlying water is destroyed, and trace metal contaminants in sediment reenter the water body to cause secondary pollution.

Laizhou Bay is in southern Bohai Bay, and is semienclosed with a shallow and poor water exchange capacity. Inflow from more than 10 rivers, such as the Huanghe, Xiaoqing and Weihe Rivers, transport rich nutrients to Laizhou Bay. Rich food organisms, suitable temperature and salinity conditions make the bay a natural feeding and spawning ground and habitat for economically valuable animals. However, with rapid development of the economy and population, terrigenous emissions of pollutants including trace metals have increased greatly. Trace metals became one of the major pollutants in seawater and the sedimentary environment. One of the greatest problems of trace metal pollutants is the potential threat of bioaccumulation and biomagnification, generating greater exposure for certain organisms than in the present environment. Thus, the spatial distribution of trace metals in seawater and marine sediments have been frequently monitored to provide basic information for determination of environmental health risks(Fernandes, 1997; Gao and Chen, 2012; Dou et al., 2013). In this study, distributions and pollution assessment in seawater and sediments of four trace metals(Cu, Pd, Cd, and Hg) and arsenic in Laizhou Bay are reported and discussed, based on investigation during August 2010. 2 MATERIAL AND METHOD 2.1 Sample collection

The investigation was proceeded via a cruise in Laizhou Bay, traversing 119.6°–120.1°E and 37.2°–37.7°N from July 29 through August 8, 2010. Twenty stations were used in this area for seawater and surface sediment sampling(Fig. 1). Seawater samples were taken by a Niskin collector in the surface layer and bottom of the water column. The samples were mixed and filtered immediately through acid-treated Millipore filters(0.45-μm mesh)into pre-cleaned high-density polyethylene containers. The filtered samples were acidified to pH<2 and then stored in a dark ice chest filled with crushed ice until laboratory analysis. Sediment was sampled by a grab borrow device at each station. The surface sediment samples were preserved in polyethylene bags that had been pre-washed with nitric acid and distilled water, and placed in a cooled box onsite.

Fig. 1 Study area and sampling stations in Laizhou Bay

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After transport back to the laboratory, 0.3–0.4 g of dried sediment sample was analyzed for content of arsenic and the trace metals Cu, Pb, Cd, and Hg, using the digestion procedure and measurement conditions recommended by the State Environmental Protection Administration of China(GB 17378.5-2007)(CSBTS, 2008). Digestion for Hg and As was done using a mixture of concentrated HCl+HNO₃. Digestion for Cu, Pb and Cd was accomplished using a mixture of concentrated HCl+HNO₃+HClO₄. Concentrations of trace metals Cu, Pb, and Cd were analyzed by nonflame atomic absorption spectrophotometry(PEAA600, USA). Concentrations of As and Hg were determined by atomic fluorescence spectrometry(JITIAN-SA-10, China).

St and ard samples were used to ensure measurement accuracy and precision. St and ard deviation was within 10%. Each sample was detected three times. Statistical data analysis included average, st and ard deviation, maximum and minimum. 2.3 Assessment methods 2.3.1 Pollution index

The single factor pollution index(C_fi) and complex pollution index(C_d)are widely used to evaluate water quality and sediment contaminants in an oceanic environment(Hakanson, 1980).

Table 1 Background concentration and toxicity coefficient of trace metals and arsenic

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The C_fi reflects the pollution level of a single trace metal in seawater or sediment, and C_d reflects the pollution level of overall trace metals in those media. Relationships between C_fi, C_d and pollution level are shown in Table 2.

Table 2 Relationship between C_fi, C_d and pollution level

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The Hakanson ecological risks index(RI)was used to evaluate potential risk associated with the accumulation of trace metals in surface sediments, as proposed by Hakanson(1980).

RI was used to evaluate the potential risk of one metal or a combination. Calculated RI values were categorized into five classes of potential ecological risk(Table 3).

Table 3 Relationships betweenE_ri, RI and potential ecological risk level

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and arsenic in seawater Dissolved trace metals and arsenic in seawater at 20 sites in Laizhou Bay are summarized in Table 4. The contents of different trace metals and arsenic at every sampling site were plotted by Surfer 8.0 software, which is displayed in Fig. 2 to precisely reveal their spatial distribution.

Fig. 2 Trace metals and arsenic distributions of Laizhou Bay seawater(μg/L)

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High levels of each trace metal occurred mostly near estuaries. The greatest concentration of As and Pb was at site 12 in the Xiaoqing River estuary. Concentrations were greater than 2 μg/L for both elements. The highest concentration of Cd was at site 6, near the Jiehe River estuary. The distribution of Cd showed a strong concentration gradient from east to west. The maximum concentration of Hg was at site 20, near the Weihe River estuary. High concentration areas of Cu were at the estuaries of the Weihe and Jiehe Rivers. Concentrations of Hg varied from 0.014 to 0.094 μg/L, with average 0.058 μg/L. High Hg concentrations were in the northwest(Dongying area), southeast(Weifang area), and middle of the bay. Areas between the estuaries of the Xiaoqing and Laomi Rivers showed low Hg concentration.

High concentrations were mainly at river estuaries, indicating that rivers were the main trace metal pollution sources to Laizhou Bay. The Xiaoqing River was once a heavily polluted river that received industrial effluent and domestic sewage from the cities of Jinan, Zibo, Dongying, and Weifang. Its watershed encompasses numerous densely industrialized zones with papermaking, electronics, printing and dyeing, along with petroleum and chemical industries(Sun, 2009). Peak levels of As and Pb at the Xiaoqing River estuary may be ascribed to the influx of river water containing effluents from a variety of industrial plants scattered along the riverbank. The Jiehe River is on the border of Zhaoyuan County, which is known in China for its abundant gold reserves. A significant increase in environmental toxic metal concentrations can occur during gold mining(Edinger et al., 2008), because of the association of such metals with gold in ores and gravels. The maximum concentrations of Cd and high levels of Cu at the Jiehe River estuary can be largely attributed to the influx of Jiehe River water(Xu et al., 2013). The coastal current and inner current of Laizhou Bay affect the deposition of trace metals and arsenic. The coastal current flows counterclockwise, from north to south, southeast, and then east. In contrast, the inner current has a clockwise rotation, from east to west, north, northeast, and finally east. The combined effects of river flow, coastal current and inner current cause the variable distributions of the trace metals and arsenic. For example, high As and Pb concentrations extending southeastward were influenced by the coastal current.

A statistical summary of the trace metal contents in seawater is given in Table 4. Based on mean contents, these contents were in the following descending order: Cu > As > Pb > Cd > Hg. For comparison, contents in other regions and the Chinese national st and ard for seawater quality are listed in the table. Among all coastal areas chosen for comparison, average contents of Cu and Hg in the present study were higher than those of other study regions. Average concentrations of the three other elements(Pb, Cd and As)are comparable with those of our study area. The marine seawater quality st and ard(CSBTS, 1997)contains three st and ard criteria for seawater. The primary seawater st and ard criterion is applied to protect habitats for marine life including natural, rare and endangered species, as well as to areas for human recreation and sports. The second criterion is applied to regulate general industrial use, aquacultural activity, and coastal tourism. The third criterion is applied to port activity. Referring to marine seawater quality st and ard GB 3097-1997, overall average concentrations of trace metals in Laizhou Bay seawater were less than or close to the primary st and ard criteria(Grade I), except for Cu(Table 4). The average Cu concentration met the third st and ard criteria(Grade III). However, the other three metals and arsenic concentrations at several sampling sites exceeded Grade I. Among the 20 sites, 55% exceeded Grade I for Hg and 15% for Pb.

Table 4 Concentration ranges and averages of trace metals and arsenic in seawater (μg/L) and surface sediment (mg/kg) in Laizhou Bay and other study areas

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C_fi(Fig. 3a) and C_d(Fig. 3c)are also applied to assess the pollution level of trace metals in seawater. C_fiof As and Cd at all sites were less than 0.12 and 0.38, respectively. Values for these two elements of <1 show that As and Cd pollution in Laizhou Bay was weak. C_fivalues for Hg and Pb were <1 at some stations, but between 1 and 3 at others. Of the entire study area, 60% and 15% had medium pollution levels of Hg and Pb, respectively. Pollution of the trace metal Cu was substantial. Its C_fiwas larger than 2.54 at all stations. Twenty percent of the study area had a medium pollution level and 80% a high level. Cu was the major pollutant in Laizhou Bay seawater, followed by Hg, Pb, Cd and As. Most stations had seawaterC_dlarger than 5(Fig. 3c), meaning that seawater in the bay had a mediumC_dpollution level. In general, the trace metal pollution results show that Laizhou Bay seawater has been polluted by such metals, especially Cu, but the pollution was not serious.

Fig. 3 Single factor pollution index(Cfi) and complex index(Cd)of seawater and sediments in Laizhou Bay

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Sediments are important in identifying the pollution patterns of aquatic systems. Sediment pollution by trace metals is a worldwide problem(Wang et al., 2007; Fernandes et al., 2008; Fujita et al., 2014) and is considered a serious threat to aquatic ecosystems because of their toxicity, non-biodegradability, and ability to bioaccumulate in the food chain. Trace metal distributions in sediment reflect the history of pollution and provide records of inputs to aquatic ecosystems(Pan and Wang, 2012; Xu et al., 2013). Concentration distributions of four trace metals and arsenic in surface sediments of Laizhou Bay were plotted by Surfer 8.0 and are presented in Fig. 4.

Fig. 4 Trace metal and arsenic distributions of Laizhou Bay surface sediments(mg/kg)

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The high concentrations of As, Hg, Cu and Pb were in the middle through north of Laizhou Bay. Distribution trends for these elements were largely affected by the coastal current and inner current in the bay. These currents transported sediment particles with trace metals from rivers to the aforementioned high concentration areas. The highest Cd concentration was at site 15, near the Jiehe River estuary, and there were also high concentrations around the Xiaoqing River estuary. Cd is relatively mobile in soils(Babic et al., 1998) and can thus migrate more readily than other elements. The distribution trend of Cd was mainly impacted by river inputs and reflects the brief history of settlement. Xu et al.(2013)reported that among all sampled river sediments, Cd concentrations in Jiehe River sediments were the highest; Xiaoqing River was also a collector of sewage that contained high levels of particulate Cd.

Table 4 furnishes a statistical summary of the trace metal contents in sediments. Based on mean contents, the metals had the following decreasing order: Pb > Cu > As > Cd > Hg. Contents of other bays and the national st and ard of China for sediment quality are listed in the table 4 for comparison. Among all the compared bays, the average concentration of Hg in the present study was slightly lower than the others. Average concentrations of the other four elements(Pb, Cd, Cu and As)were comparable with those of our study area. The marine sediment quality st and ard(CSBTS, 2002)contains three st and ard criteria that are the same to the marine seawater quality st and ards cited on page 7. Referring to marine sediment quality st and ard GB 18668-2002, overall average concentrations of the trace metals in Laizhou Bay sediments were less than the primary st and ard criteria(Grade I)(Table 4). However, Cd concentration at several sampling sites(15% of the 20 total)exceeded Grade I.

C_fi and C_dare also applied to assess the pollution of trace metals in sediment(Fig. 3b and c). C_fiof Pb, Cu, Hg and As at all sites were <1, indicating low pollution levels for these trace metals and arsenic in Laizhou Bay. C_fiof Cd at some stations was between 1 and 3. For Cd, 15% of the entire study area had a medium pollution level, and this was an important trace metal pollutant in sediment.C_dat each study site was <5(Fig. 3c), meaning that trace metal pollution in bay sediment had a lowC_dpollution level. In general, the trace metal pollutant results suggest that overall sediment in the bay has not been significantly polluted by these metals.

RI of trace metals in surface sediments were used to evaluate the potential ecological risk, with results shown in Fig. 5. Mean E_riof all trace metals were smaller than 40, portraying a low single potential ecological risk of bay sediment. The descending order of mean risk was Cd, Hg, As, Cu, and Pb. Cd was the main trace metal potential ecological risk factor. The maximum RI of 20 stations was 53.36 at site 15, and the minimum was 21.99 at site 14. According to the evaluation st and ard, all sites had low potential ecological risk levels, with RI <150.

Fig. 5 Single and comprehensive potential ecological risk indices(RI)of trace metals in surface sediments of Laizhou Bay

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Average concentrations of the trace metals(Pb, Cd, Cu and Hg) and arsenic were respectively 0.88, 0.28, 15.88, 0.056 and 1.40 μg/L in seawater, and 16.9, 0.33, 13.6, 0.031 and 9.42 mg/kg in surface sediment. Cu was the predominant trace metal pollutant in seawater, followed by Hg, Pb, Cd and As.C_d in Laizhou Bay seawater was at a medium level. Seawater has been polluted by trace metals, especially Cu, but not seriously so. High concentrations of the four trace metals and arsenic were mainly near river estuaries, indicating that river discharge was the main pollution source. The most important trace metal pollutant in sediment was Cd, followed by As, Cu, Pb, and Hg. Although contents of individual trace metals were elevated at several sites, the mean C_fiin sediment was less than 1, indicating a low pollution level. The potential ecological risk level was also low in bay surface sediment. 5 ACKNOWLEDGEMENT

We thank the editors and reviewers for their valuable comments and suggestions. We also thank Prof. Wang and Mr. Butcher for their writing suggestions.