1 INTRODUCTION

Trace elements are one of the main pollutants in the offshore environment; not only do they harm aquatic organisms, they can also affect human health through transmission along the food chain. The distributions of trace elements are influenced by the natural environment and human activities. In the study of trace element pollution, many parameters, such as the enrichment factor (EF), contamination factor (CF), and pollution load index (PLI), are frequently used (Müller, 1979; Håkanson, 1980; Xu et al., 2015, 2018b; El-Sorogy et al., 2018). In the calculation of these parameters, the selection of environmental background values is involved. Previously, the mean concentrations of elements in the upper continental crust (UCC) (Hu et al., 2013a), shale (Turekian and Wedepohl, 1961), preindustrial sediments (CobeloGarcía and Prego, 2003), and/or a large region (Xu et al., 2018c) were adopted as environmental background values. However, the environmental background values have strong regional characteristics. By neglecting the geochemical characteristics of the elements in a specificresearch area, the results of an environmental quality assessment will produce a large deviation (Romano et al., 2015; Xu et al., 2018b). Thus, the selection and study of environmental background values is the key to the assessment of trace element pollution.

Jiaozhou Bay, located on the southern coast of the Shandong Peninsula, is a typical semiclosed bay with more than 10 small rivers entering the bay (Fig. 1). With rapid economic development, pollution and other environmental problems in the Jiaozhou Bay catchment have intensified. The construction of the blue economic zone on the Shandong Peninsula, with Qingdao as the leading city, became one of China's national strategies. Thus, the environmental protection of Jiaozhou Bay has been promoted to an important strategic position. In the study of trace element pollution, the concentrations of trace elements in preindustrial sediments in Jiaozhou Bay (Liu et al., 2010; Xu et al., 2016), Mesozoic granites (Dong et al., 2007), and Chinese soil (Li et al., 2011) have been adopted as environmental background values. Inevitably, there will be a large difference in the assessment result depending on the selected background reference. Therefore, it is necessary to determine the environmental background values of the Jiaozhou Bay catchment.

Recently, we analyzed the environmental background values of the bulk sediments in the Jiaozhou Bay catchment (Xu et al., 2017b). A possible problem is that the large difference in the grain size of the bulk sediments inevitably has an important influence on the elemental concentrations (Zhang et al., 2002). There are also several methods to estimate the environmental background values of trace elements, such as (1) using the elemental concentrations of sediments in deep layers unaffected by pollution (Wan et al., 2015; Xu et al., 2018a); (2) using the raw elemental concentrations from surface sediments by eliminating the contamination values and calculating the statistical mean and standard deviation (Chen et al., 2001; Xu et al., 2017b); and (3) using the elemental concentrations in the finegrained sediments (< 63 μm) (de Paula Filho et al., 2015). In this paper, the trace elements (Co, Ni, Cu, Pb, Zn, Cd, Cr, and Sc) in the fine-grained (< 63 μm) sediments from the Jiaozhou Bay catchment were analyzed to evaluate their environmental background values and the current pollution situation.

2 MATERIAL AND METHOD 2.1 Sampling and analytical method

Surface sediment samples (top 2 cm) were scraped carefully from 72 sites in urban river and intertidal areas in the Jiaozhou Bay catchment in 2015 (Fig. 1). During sample collection, a hand-held global positioning system (GPS) was used to locate the sites. Plastic material was utilized to avoid metal pollution in the samples. After sampling, the sediment samples were sealed in clean polyethylene bags, transported back to the laboratory within a few hours, and kept frozen until further analysis. Areas with serious impacts from human activities and pollution sources were avoided during the sampling process. Seventytwo sediment samples were first passed through 63-μm wet sieves to remove the coarse-grained fraction. The fine-grained components (< 63 μm) were treated with 30% H₂O₂ and 1-mol/L HCl to remove organic matter and calcareous debris. The residues were oven-dried at 60 ℃ and then ground into powders. The trace elements (Sc, Co, Ni, Cu, Pb, Zn, Cd, and Cr) were analyzed by inductively coupled plasma mass spectrometry (ICP-MS, Thermo X series). The analytical accuracy was monitored by analyzing Chinese stream sediment reference materials (GSD4, GSD9, and GBW07345).

2.2 Determination of environmental background values

The calculation method for environmental background values is as follows. To avoid any individual station pollution, the Grubbs test was performed on the raw data to eliminate the abnormal values (Zhang and Du, 2005; Qiao et al., 2009; Xu et al., 2017b). The raw data were arranged from low to high values, and then the maximum and minimum are most likely to be abnormal values x_d. The values of x_mean, S, and G were calculated as follows:

x_mean is the arithmetic average, and S is the arithmetic standard deviation.

If the calculated G value is greater than the critical value level of the Grubbs test method, this x_d value is defined as an abnormal value. This abnormal value is eliminated, and the same test is then carried out until there is no abnormal value. Then, a normality test was performed on the raw and the logarithmically transformed data using the Shapiro-Wilk and skewness-kurtosis test method (Liu et al., 2014; Xu et al., 2017b). The statistical parameters of the average and standard deviation of element concentrations were calculated. For element concentrations characterized by normally distributed patterns, the environmental background values were adopted as x_mean, and the range was defined as x_mean±S. For element concentrations characterized by logarithmically distributed patterns, the environmental background values were adopted as the geometric mean (x′), and the range was defined as x′ multiplied/divided by (S′)2 (where S′ is the geometric standard deviation). If the element concentrations displayed skewed distribution patterns, the median values were adopted as the environmental background values, and the corresponding data interval of 5% to 95% was defined as the range values (Chen et al., 2001, 2014; Xu et al., 2017b). The values of x′ and S′ were calculated as follows:

2.3 Assessment of sediment pollution

The calculation method of EF normalizes the concentrations of the selected trace elements with conservative elements, such as Al, Fe, and Sc (Hu et al., 2013b; Jahan and Strezov, 2018). Major elements such as Al and Fe were not measured in this study; thus, the element Sc was adopted as the normalized element. The calculation formula is:

where X represents the concentrations of trace elements.

The CF and PLI are also commonly used to evaluate the status of trace element pollution in sediments. The calculation formula of CF is:

where CF₁, CF₂, CF₃, ∙∙∙, CF_n are the contamination factors of different elements, and n is the number of elements.

The PLI of trace elements in a region is expressed as PLI_zone. The calculation formula of PLI_zone is:

where PLI₁, PLI₂, PLI₃, ∙∙∙, PLI_m are PLI of different stations, and m is the number of stations.

3 TRACE ELEMENT CONCENTRATION

The mean values and ranges of trace elements in sediments from the whole, eastern, and western Jiaozhou Bay catchment are shown in Table 1. The trace element concentrations of Cu, Pb, Zn, Cd, and Cr are higher in the eastern part of the Jiaozhou Bay catchment than in the western part. The trace element concentrations of Sc, Co, and Ni in the eastern part are very close to those in the western part. In particular, the mean values (12.4 and 13.1 mg/kg, respectively) and ranges (7.8–16.4 and 9.3–19.8 mg/kg, respectively) of the conservative element Sc, which is used to normalize the other elements, are approximately coincident. This indicates that the environmental background values of the eastern and western parts of the Jiaozhou Bay catchment are basically the same. As shown in Table 1, the concentrations of almost all the trace elements in fine-grained sediments were higher than those in bulk sediments of the Jiaozhou Bay catchment. This is consistent with the grain size effect on multielement concentrations, namely, the concentrations of most elements increased when the sediment grain size became finer. This is because most of the elements are enriched in fine-grained samples, where clay minerals may be an important constituent (Zhang et al., 2002).

When compared to the China Marine Sediment Quality criteria (MSQ, General Administration of Quality Supervision, Inspection and Quarantine of the People's Republic of China, 2004, Table 1), the mean values of elements Cu, Pb, Zn, and Cr in the eastern Jiaozhou Bay catchment are higher than MSQ-1 but meet MSQ-2. The mean values of elements in the western Jiaozhou Bay catchment roughly meet MSQ- 1, while the values of elements Cu and Cr are slightly higher than MSQ-1. The mean values of all the trace elements meet MSQ-2. In general, the concentrations of the trace elements in this study suggest that the sediment quality in the Jiaozhou Bay catchment has been partly impacted by trace element contamination. The most substantial contamination occurred in the eastern part of the Jiaozhou Bay catchment, while the western part was less influenced by human activities. Thus, the element concentrations of sediments in the western part of the Jiaozhou Bay catchment could be selected to calculate the environmental background values following the assumption that the sampling points located in the western part of Jiaozhou Bay are less influenced by human activities.

4 ENVIRONMENTAL BACKGROUND VALUES OF TRACE ELEMENTS

It is worth noting that the environmental background values need to establish that there is no major difference in the underlying geology of the catchments that could contribute to differences in trace element concentrations. There are large granite complexes in the east and southwest of Jiaozhou Bay, i.e., Laoshan, Xiaozhushan, and Dazhushan, which are part of the huge Mesozoic granite belt in the Rizhao-Jiaonan-Qingdao area (Han et al., 2012). Thus, there is no significant difference in the underlying geology or lithology of the Jiaozhou Bay catchment. It should also be noted that the whole area is inhabited and shows agricultural activities (probably for a very extensive period), all of which will have had some effects on the trace element levels. However, the study does provide a useful set of 2015 reference values (environmental background values) for trace elements in the Jiaozhou Bay catchment that can be used to assess future increases in pollution both in the catchment and in the bay.

To eliminate the possibility that high concentrations of pollution occur at specific stations, the Grubbs test was first conducted on the element concentrations in 37 sediment samples from the western part of the Jiaozhou Bay catchment. The remaining numbers of samples for each element were 37, 37, 36, 36, 36, 36, 36, and 36, respectively. The Shapiro-Wilk and skewnesskurtosis test were then conducted at sampling sites that met the test criteria. Frequency distribution plots showed that Sc, Co, and Ni were normally distributed; Cu, Pb, Zn, and Cd were logarithmically distributed, and Cr was not normally distributed or logarithmically distributed (Fig. 2). According to the determination methods of environmental background values, the environmental background values and their ranges for the sediments in the western part of the Jiaozhou Bay catchment were estimated (Table 2). The environmental background values (ranges) of the elements Sc, Co, and Ni were 13.1 (10.8–15.4), 12.4 (8.6–16.2), and 32.0 (22.9–41.2), respectively. The environmental background values (ranges) of Cu, Pb, Zn, and Cd were 29.6 (13.5–64.9), 24.1 (13.0–44.6), 77.6 (38.5–156.5), and 0.07 (0.02–0.20) mg/kg, respectively. The environmental background value (range) of Cr was 82.5 (66.5–104.0) mg/kg.

We compared the reconstructed environmental background values with those in previously published values from bulk sediments in the Jiaozhou Bay catchment (Xu et al., 2017b), Chinese soil (Wang and Wei, 1995), UCC (Taylor and McLennan, 1995), and global shale (Turekian and Wedepohl, 1961, Table 3). The environmental background values in the finegrained sediments are higher than those in the bulk sediments of the study area (Xu et al., 2017b). The reconstructed environmental background values are similar to those of Chinese soil (Wang and Wei, 1995); however, there is no obvious relationship between them. These values are generally higher than those of the UCC (Taylor and McLennan, 1995) but lower than those in global shale (Turekian and Wedepohl, 1961). This shows that the environmental background values of the studied area have regional characteristics mainly because of the relatively simple granite components compared with the mixed components of large areas, e.g., Chinese soil, UCC, and global shale.

5 ENVIRONMENTAL ASSESSMENT OF FINE-GRAINED SEDIMENTS

Using the reconstructed environmental background values from the western Jiaozhou Bay catchment, the EFs of sediments in the whole Jiaozhou Bay catchment were 0.68 to 1.70 (mean of 1.02) for Co, 0.66 to 2.51 (mean of 1.10) for Ni, 0.55 to 29.97 (mean of 2.07) for Cu, 0.66 to 26.59 (mean of 2.36) for Pb, 0.70 to 11.97 (mean of 2.00) for Zn, 0.30 to 22.70 (mean of 3.26) for Cd, and 0.62 to 6.42 (mean of 1.29) for Cr, exhibiting the following order: Cd>Pb>Cu>Zn>Cr>Ni>Co (Fig. 3a). The mean CF values of sediments were 1.00 for Co, 1.07 for Ni, 2.03 for Cu, 2.23 for Pb, 1.95 for Zn, 3.11 for Cd, and 1.24 for Cr, decreasing in the same order as the EFs: Cd>Pb>Cu>Zn>Cr>Ni>Co (Fig. 3b). The EFs and CFs of Cd, Pb, Cu, and Zn were much higher than 1.5 and 1.0, respectively, suggesting that these elements have been influenced by a certain degree of human activity. In general, the EFs and CFs in the eastern part of the Jiaozhou Bay catchment are higher (Figs. 4–5), indicating that the pollution is heavier there than in the western part. Compared with our previous studies (Xu et al., 2017a), this result is much clearer. This further indicates that the reconstructed environmental background values are more reasonable than the previous values.

The PLI results showed that 36 stations are moderately polluted, and 11 stations are strongly to very strongly polluted, with the polluted stations mainly distributed in the eastern part of the Jiaozhou Bay catchment (Fig. 6). The PLI_zone value of the eastern part of the Jiaozhou Bay catchment is 1.60, which is much higher than that of the western part (1.03). In the eastern part of the Jiaozhou Bay catchment, there are many factories. The consumption of energy, materials processing, manufacturing, and other industrial production processes inevitably produces waste substances. These substances cause trace element pollution through atmospheric sedimentation, river transportation, and even anthropogenic dumping. In addition, the water and land transport in the eastern part of the Jiaozhou Bay catchment developed rapidly. The role of transportation in residents' lives and urban construction is becoming increasingly important. The trace elements produced by traffic activities may make the pollution more prominent in the eastern part of the Jiaozhou Bay catchment. Comparatively, the economic development of the western part of the Jiaozhou Bay catchment is weak. Combined with EF, CF, and PLI analysis, trace element pollution in the eastern part of the Jiaozhou Bay catchment is heavier than that in the western part.

6 CONCLUSION

In this study, the concentrations of trace elements (Sc, Co, Ni, Cu, Pb, Zn, Cd, and Cr) in the finegrained sediments of rivers and intertidal zones in the Jiaozhou Bay catchment were analyzed. The environmental background values of trace elements in fine-grained sediments are established, and the pollution situation of the Jiaozhou Bay catchment is evaluated by using EF, CF, and PLI to provide basic data for the planning and management of Jiaozhou Bay and ecological environment protection. We draw the following conclusions:

1. The environmental background values (ranges) of Sc, Co, Ni, Cu, Pb, Zn, Cd, and Cr were 13.1 (10.8–15.4), 12.4 (8.6–16.2), 32.0 (22.9–41.2), 29.6 (13.5–64.9), 24.1 (13.0–44.6), 77.6 (38.5–156.5), 0.07 (0.02–0.20), and 82.5 (66.5–104.0) mg/kg, respectively. These values are higher than those of the UCC and lower than those of global shale, indicating that the environmental background values have regional characteristics.

2. The EF, CF, and PLI showed that the pollution in the eastern part of the Jiaozhou Bay catchment was much higher than that in the western part. With increased economic development, more attention should be given to the eastern part of the Jiaozhou Bay catchment.

7 DATA AVAILABILITY STATEMENT

We have shared the research data in the manuscript, as well as the Supplementary data.

8 ACKNOWLEDGMENT

The authors would like to deeply thank the editor Drs. Yang CHEN and Jiao LIU, and three anonymous reviewers for their constructive suggestions, and this work has been greatly improved.

Electronic supplementary material

Supplementary material (Supplementary data) is available in the online version of this article at https://doi.org/10.1007/s00343-022-2099-9.