Introduction

Urban and industrial activities discharge significant amounts of pollutants (including heavy metals) into the marine environment, and these pollutants directly affect the coastal systems where they are often deposited. In recent decades, rapid industrialization and economic development in China, especially in coastal areas, have resulted in severe environmental pollution (Zheng et al. 2008; Zhang et al. 2009; Qiu and Yu 2011; Xia et al. 2011a, b; Pan and Wang 2012; Fu et al. 2013; Hu et al. 2013a; Li et al. 2013; Xu et al. 2015; Zhou et al. 2015). As a natural sink of environmental pollutants, marine sediments are vulnerable and sensitive to environmental pollution, and they can be used as an indicator of potential environmental contamination (Zhang et al. 2009; Qiu and Yu 2011; Xia et al. 2011a, b; Balzer et al. 2013; Fu et al. 2013; Hu et al. 2013a; Li et al. 2013; da Silva et al. 2015; Xu et al. 2015; Zhou et al. 2015).

The South China Sea (SCS) is the largest tropical marginal sea, and the northern portion of the SCS shelf is among the largest sub-tropical shelves in the world. The open SCS has been intensively studied over the past several decades (Wei et al. 2006; Liu et al. 2010; Tian et al. 2010; Wan et al. 2015) and has been the focus of several national and international studies, such as the Tropical Ocean/Global Atmosphere (TOGA) program, the South China Sea Monsoon Experiment (SCSMEX), and the Southeast Asian Time-series Study (SEATS) (Wong et al. 2007; Pan et al. 2015). However, the shelf seas around the rim of the SCS have received much less attention. Previous studies on the northern portion of the SCS shelf have focused on sub-regional phenomena, such as the influence of the Pearl River Estuary (Xia et al. 2004; Wei and Wu 2011; Yang et al. 2011), Pearl River outflow along the coast (Liu et al. 2009; Liu et al. 2014b), and upwelling off the Taiwan Bank (Lan et al. 2009) and Hainan Island (Jing et al. 2009). However, the quality of the sediments on the continental shelf has been rarely studied.

Over the previous decades, various indices have been developed to assess the heavy metal contamination of sediments and its ecological risk. These indices, such as the enrichment factor (EF) and geoaccumulation index (I geo), estimate the impact of human activities on sediment quality and the sediment quality guidelines (SQGs), which are used to evaluate the ecological risk that is posed by heavy metals in sediments (Müller 1979; Müller 1981; Idris 2008; Hu et al. 2013a; Zhao et al. 2015). Furthermore, appropriate statistical approaches have increasingly been applied for environmental studies (Idris 2008; Hu et al. 2013a; Zhao et al. 2015). This study addresses the identified research gaps that would provide valuable information regarding the spatial distribution of the selected heavy metals in the northern portion of the SCS shelf. The aims of this paper are to (1) determine the concentrations of heavy metals (Cr, Ni, Cu, Zn, As, Cd, and Pb) in the surface sediments of the northern portion of the SCS shelf, (2) evaluate the potential for adverse biological impacts by comparing our results to the SQGs, (3) assess the heavy metal contamination by using the EF and I geo, and (4) identify the heavy metal sources by using a Pearson’s correlation analysis.

Regional setting

The northern portion of the SCS shelf is considered a passive continental margin (Yim et al. 2006) that stretches southwestward from a ridge system at approximately 23° N and 119° E, which delineates the southern end of the Taiwan Strait from the northeastern coasts of the Leizhou Peninsula and Hainan Island at approximately 20° N and 111° E. In addition, the northern portion of the SCS shelf extends from the southeastern coast of China to the shelf break near the 120-m isobath, which can be subdivided into three primary hydrographic sub-regimes, namely, the inner, middle, and outer shelf, which have water depths of <40 m, 40–90 m, and 90–120 m, respectively (Pan et al. 2015; Wong et al. 2015). The hydrographic characteristics of the inner shelf are heavily influenced by the input of terrestrial material, which results in lower salinity and higher suspended particle concentrations (Pan et al. 2015), whereas the hydrographic characteristics of the outer shelf are governed by mixing with the open oligotrophic SCS, which results in higher salinity and temperatures and lower suspended particle concentrations (Pan et al. 2015). The middle shelf contains a mixture of water from the inner and outer shelves. Superimposed on these shelf-wide patterns are secondary features that result from sub-regional processes, such as coastal upwelling (Jing et al. 2009; Su and Pohlmann 2009) and internal wave activities (Guo et al. 2012; Pan et al. 2015).

The highly urbanized Pearl River empties into the middle section of the northern portion of the SCS shelf with an annual sediment discharge of ∼54–80 × 106 t (Zhang et al. 2008; Zhang et al. 2012). The total sediment discharge from local rivers in eastern Hainan Island is ∼1 × 106 t year−1 (Liu et al. 2014a). Modern siliciclastic sediments that primarily originate as mud from the Pearl River are dominant from the coast to a depth of approximately 50 m on the middle shelf (Niino and Emery 1961; Yim et al. 2006; Liu et al. 2014b). Relic sediments have been identified at greater water depths (Niino and Emery 1961; Yim et al. 2006).

Monsoonal winds dominate over the northern portion of the SCS shelf, and they have a northeasterly direction in winter and a southwesterly direction in summer. The seasonal influence of monsoonal winds on the ocean circulation of the northern portion of the SCS shelf is clear, and wind-forced upper layer currents primarily flow along the shelf and move northeastward in summer and southwestward in winter (Fang et al. 2015). Coastal upwelling resulting from wind and/or topographic forcing has been reported in the northern portion of the SCS shelf during the summer off the Taiwan Bank (Lan et al. 2009) and the eastern coast of Hainan Island (Jing et al. 2009). Typhoons or tropical storms regularly affect the northern portion of the SCS shelf during the hot and wet summer monsoon season (Liu et al. 2011; Sun et al. 2011). The northern portion of the SCS shelf is dominated by irregular diurnal and mixed semi-diurnal microtides, mostly with <2 m tidal ranges (Krumme et al. 2012; Zhang et al. 2013). The maximum tidal current velocity is less than 0.5 m s−1 (Liu et al. 2002). The prevailing direction of both regular and strong waves is from the southeast. The average wave height is 1.06 m, and the maximum wave height is 4 m (Wang et al. 2013). Therefore, their effect on sediment erosion should be minor.

Materials and methods

Sampling

The sampling sites that were chosen for this study were located along the continental shelf south of Guangdong and east of Hainan Island at water depths from 30 to 130 m below the current sea level. A total of 33 surface sediment (0–5 cm) samples were collected by using a Van Veen grab sampler using a small vessel by R/V Shiyan 3 from the South China Sea Institute of Oceanology, Chinese Academy of Sciences between 2012 and 2014 (Fig. 1). All the samples were refrigerated at 4 °C until analysis.

Fig. 1
figure 1

Study area and locations of sampling sites in the northern portion of the SCS shelf. T Taiwan, G Guangdong, LP Leizhou Peninsula, and H Hainan Island

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Sedimentary properties

All the samples that were used for the grain-size analysis were pre-treated with excess 30 % H2O2 and 1 mol L−1 HCl in a water bath at 60 °C for 1 h to remove the organic matter and calcium carbonate, respectively. The suspension was centrifuged twice at 3500 rpm for 6 min with distilled water, and the supernatant was discarded each time. Subsequently, the samples were dispersed and homogenized by using ultrasound and then passed through a Mastersizer 3000 laser particle analyzer at the First Institute of Oceanography, State Oceanic Administration, China. This facility can measure grain sizes from 0.01 to 3500 μm with a measurement repeatability error of <1 %. The textures of the sediment samples were classified according to the Udden-Wentworth grade scale based on the relative percentages of clay (<4 μm), silt (4–63 μm), and sand (63–2000 μm) (Wentworth 1922).

Sediment geochemistry

The chemical analysis samples were also pre-treated with excess 30 % H2O2 and 1 mol L−1 HCl in a water bath at 60 °C for 1 h to remove the organic matter and calcium carbonate, respectively. The suspension was centrifuged twice at 3500 rpm for 6 min with distilled water, and the supernatant was discarded. The residues of the samples were heated until dry at 60 °C and then ground into a powder by using an agate mortar and pestle. Approximately 0.04 g of powdered sample was then digested with an HF + HNO3 + HClO4 acid mixture in Teflon vessels. The major and heavy element concentrations were determined at the Institute of Oceanology, Chinese Academy of Sciences (IOCAS) by using a Thermo Icap6300 ICP-AES and a Perkin-Elmer ELAN DRC II ICP-MS, respectively. The analytical accuracy was assessed by comparison with selected certified reference materials from the United States Geological Survey (USGS) and China (BCR-2, BHVO-2, GBW07315, and GBW07316). Overall, the measured values of the reference material were within the range of the certified values (Table 1), indicating satisfactory recovery. Because the concentrations and spatial distribution of heavy metals may be related to the grain size, Pearson correlation coefficients were calculated for these parameters by using the statistical software SPSS (Version 19).

Table 1 Results from an analysis of certified reference materials. The concentrations are in mg kg−1
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Results and discussion

Sediment types

The types of sediment were identified (Fig. 2) according to the ternary diagram of Folk’s classification (Folk et al. 1970) and are listed in Table 2.

Fig. 2
figure 2

Ternary classification diagram of the surface sediment samples based on Folk et al. (1970). Z silt, sZ sandy silt, zS silty sand, S sand

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Table 2 Aluminum (%) and heavy metal concentrations (mg kg−1) and other general characteristics of the sediments from the northern portion of the SCS shelf
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Four samples (S3, S5, S24, and S25) were classified as silt, with sand, silt, and clay contents of 5.4, 80.7, and 13.9 %, respectively. Nine samples (S8, S13, S20, S21, S23, S27, S28, S29, and S30) were classified as silty sand, with sand, silt, and clay contents of 64.3, 30.2, and 5.5 %, respectively. Three samples (S31, S32, and S33) were classified as sand, with sand, silt, and clay contents of 95.0, 4.3, and 0.7 %, respectively. The remaining samples were classified as sandy silt, with sand, silt, and clay contents of 25.9, 61.6, and 12.5 %, respectively. The mean grain size (Mz) of the sediments varied from 1.93 to 6.37 φ and averaged 4.78 ± 1.25 φ.

Sediment geochemistry

As shown in Table 2, the concentrations of the metals in the surface sediments from the study area were 4.9–17.2 % for Al, 12.4–72.5 mg kg−1 for Cr, 4.4–29.2 mg kg−1 for Ni, 7.1–38.1 mg kg−1 for Cu, 19.3–92.5 mg kg−1 for Zn, 1.3–12.1 mg kg−1 for As, 0.03–0.24 mg kg−1 for Cd, and 8.5–24.4 mg kg−1 for Pb. The mean values of the heavy metal concentrations in the northern portion of the SCS shelf were comparable to or lower than those in other regions, such as Liaodong Bay (Hu et al. 2013c), Bohai Bay (Gao and Chen 2012), the Yangtze River Estuary (Wang et al. 2015), and the Pearl River Estuary (Yu et al. 2010) in China, as listed in Table 3. In addition, the metal concentrations in the northern portion of the SCS shelf were significantly lower than those in other larger industrialized/urban ports and estuaries around the world, such as Masan Bay in Korea (Hyun et al. 2007), the Gironde Estuary in France (Larrose et al. 2010), and the Guadiana Estuary in Spain (Delgado et al. 2010).

Table 3 Mean values and ranges of heavy metals in the surface sediments from the northern portion of the SCS shelf. Metal concentrations from other coastal areas, the upper continental crust (UCC), and the Marine Sediment Quality standard criteria are also listed (unit, mg kg−1)
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Assessment of potential ecological risk

Marine Sediment Quality Standards (GB 18668–2002) were promulgated by the China State Bureau of Quality and Technical Supervision (CSBTS 2002, Table 3) to prevent and control marine sediment pollution, protect marine life and resources, encourage the sustainable use of marine resources, maintain marine ecological equilibrium, and protect human health. GB 18668–2002 contains three criteria for marine sediments: the primary criteria (MSQ–1) for metal toxicity to wildlife and humans; the secondary criteria (MSQ–2) for general industries and coastal tourism; and the tertiary criteria (MSQ–3) for harbors and ocean exploration. According to the Marine Sediment Quality Standards (GB 18668–2002), the average concentrations of most of the heavy metals in the northern portion of the SCS shelf’s surface sediments were below or close to the primary standard criteria. Among the 33 sampling sites, only two sites exceeded the primary criteria for Cu, whereas they met the secondary standard criteria. Because Ni standards were not available in the GB 18668–2002, we could not evaluate the sediment quality with respect to Ni. In general, the concentrations of the seven heavy metals in this study suggest that the overall sediment quality in the study area has not been significantly impacted by heavy metal pollution.

Threshold effect level (TEL)/probable effect level (PEL) SQGs were also applied to assess the degree to which the sediment-associated metals might adversely affect aquatic organisms, and these SQGs were designed to assist in the interpretation of sediment quality (Long et al. 1995; MacDonald et al. 2000). The TELs are intended to represent chemical concentrations below which adverse biological effects rarely occur, and the PELs are intended to represent chemical concentrations above which adverse biological effects frequently occur (Long et al. 1995; MacDonald et al. 2000). A comparison of the TEL and PEL SQGs indicates that the concentrations of Cr, Ni, Cu, Zn, As, Cd, and Pb were below the TEL for 52, 30, 33, 100, 79, 100, and 100 % of the sample sites, respectively. The remaining samples all fell in the range between the TEL and PEL (Table 4). Therefore, the Cr, Ni, Cu, and As in the northern portion of the SCS shelf are likely to cause occasional adverse biological effects on the local aquatic ecosystems.

Table 4 Comparison between the heavy metal concentrations (mg kg−1) in the northern portion of the SCS shelf and sediment quality guidelines (SQGs), with the percentage of samples in each guideline
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Anthropogenic enrichment of heavy metals

To better understand the current environmental status and assess the metal contamination in the northern portion of the SCS shelf, both the EF and I geo were utilized to differentiate the metal sources (anthropogenic source vs. natural origin) and evaluate the degree of anthropogenic impacts. In this study, the EF were calculated by using the following equation: EF = (Xsample/Alsample)/(Xbaseline/Albaseline), where Xsample, Xbaseline, Alsample, and Albaseline represent the heavy metal concentrations and aluminum contents of the samples and background references, respectively. The elemental abundance in the upper continental crust (UCC, Table 3, Rudnick and Gao 2003) was used as the reference baseline because of a lack of data on the background concentrations in the study area. In general, EF values of 0.5–1.5 are typical of heavy metal levels and reflect regional rock compositions, whereas EF values >1.5 indicate non-crustal contributions and/or non-natural weathering processes (e.g., anthropogenic influences) (Zhang and Liu 2002).

The EF of the surface sediments in the study area are shown in Fig. 3, and they are in the order Cd > Pb > Zn > As > Cu > Cr > Ni. The average EF of Cu (1.2 ± 0.2), Cr (0.8 ± 0.1), and Ni (0.6 ± 0.2) were less than 1.5 in the northern portion of the SCS shelf, which suggests that these metals are not a major concern, although several sites showed minor enrichment in these metals. However, moderate enrichment in Pb, Zn, and As was also found in certain areas, with 18 sites enriched in Pb (1.6 to 2.5, mean of 2.0), 16 sites enriched in Zn (1.5 to 2.0, mean of 1.7), and 11 sites enriched in As (1.6 to 4.5, mean of 2.4). These EF values that exceed 1.5 indicate minor to moderate Pb, Zn, and As contamination in the study area and suggest that the metals may originate from both natural and anthropogenic sources. In contrast, the average EF of Cd (2.5 ± 1.2) was greater than 1.5, which suggests that Cd contamination occurs in the northern portion of the SCS shelf.

Fig. 3
figure 3

Box-and-whisker plots for the a EF and b I geo of heavy metals (Cr, Ni, Cu, Zn, As, Cd, and Pb) in the surface sediments from the northern portion of the SCS shelf. Both the EF and I geo suggest moderate Cd pollution in the study area

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Similar to EF, the I geo can also be used to estimate the degree of metal pollution. The I geo values were calculated with the following equation: I geo = log2(C n/1.5B n), where C n is the concentration of a metal and B n is the geochemical background concentration of that metal. The factor of 1.5 represents a background matrix correction factor that includes possible variations in the background values because of lithogenic effects (Müller 1979). The elemental abundance of the UCC (Rudnick and Gao 2003) was also adopted as the geochemical background concentration for the metals. The calculated I geo values in the northern portion of the SCS shelf sediments were −3.47 to −0.93 for Cr, −4.01 to −1.27 for Ni, −2.56 to −0.14 for Cu, −2.38 to −0.12 for Zn, −2.51 to 0.75 for As, −2.14 to 0.85 for Cd, and −1.58 to −0.06 for Pb. Based on the Müller scale (Müller 1981), the average I geo values below zero that were observed for Cr (−1.59 ± 0.57), Ni (−1.95 ± 0.66), Cu (−1.00 ± 0.61), Zn (−0.77 ± 0.60), and Pb (−0.78 ± 0.39) suggest a lack of pollution by these metals in the region. However, the I geo values for Cd (−0.07 ± 0.68) suggest moderate Cd pollution in this area. Although the I geo values for As (−1.00 ± 0.94) were less than zero, several sites had positive values, indicating minor to moderate metal pollution.

Several sites had positive I geo values for Cd and As, demonstrating minor to moderate metal pollution. Most of these sites occurred to the east of Hainan Island (Fig. 4a, b), where the sediments are mostly sandy silt and silt (Fig. 4c). This result suggests that the sediment granulometry can affect heavy metal concentrations in marine environments (Cai et al. 2011; Hu et al. 2013b).

Fig. 4
figure 4

Spatial distribution of the I geo values for a Cd and b As, and c the mean grain size (Mz) of the surface sediments in the northern portion of the SCS shelf. The yellow shaded area indicates the heavy Cd and As contamination in the study area

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Pearson’s correlation coefficients were calculated to explore the relationship among the major element (Al), heavy metals (Cr, Ni, Cu, Zn, As, Cd, and Pb), and Mz in the surface sediments (Table 5). Strong positive correlations were observed among Cr, Ni, Cu, and Zn and between these metals and Al and Mz. Al is a structural element of terrigenous aluminosilicates and a primary lithogenic component, and the correlations between Al and Cr, Ni, Cu, and Zn suggest that these metals are likely associated with fine-grained terrigenous sediments because of their greater specific surface area (Williams et al. 1994; Hu et al. 2013b). Thus, Cr, Ni, Cu, and Zn most likely have a natural source that is related to the erosion of parent rocks and weathering crusts. Cd presented moderate positive correlations with other metals and Mz; however, the EF and I geo results suggest that Cd is influenced by anthropogenic inputs. Previous studies have shown that the Cd distributions were closely related to the intensive usage of phosphate fertilizers (Jones and Johnston 1989; Zhang and Shan 2008; Xia et al. 2011a, b). The continuous application of these fertilizers and other soil amendments potentially exacerbates the accumulation of heavy metals in agricultural soils (Huang et al. 2007; Lambert et al. 2007). In addition, Cd might originate from the alloying, electroplating, and dyeing industries (Li et al. 2009; Xu et al. 2015). More recently, Zhao et al. (2015) found that the Cd in the surface sediments of rivers from eastern Hainan Island was predominantly sourced from anthropogenic sources. Therefore, Cd is primarily sourced from anthropogenic activities and partially from lithogenic components.

Table 5 Pearson’s correlation matrix for heavy metals (Cr, Ni, Cu, Zn, As, Cd, and Pb), Al, and mean grain-size (Mz)
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Significant correlations were not observed between As and Pb and other metals or between As or Pb and Mz, which suggest that the distribution of As and Pb in the sediments of the northern portion of the SCS shelf could be affected by factors such as varying input sources or pathways (e.g., coal burning activities and aerosol precipitation) (Li et al. 2000; Gao and Chen 2012; Hu et al. 2013b). Similar situations were also found in the Pearl River Estuary (Li et al. 2000) and the Changhua River Estuary of western Hainan Island (Hu et al. 2013b).

As discussed above, the average concentrations of most of the heavy metals in the northern portion of the SCS shelf surface sediments were below or close to the China Marine Sediment Quality Standards (MSQ–1). However, a comparison of the TEL and PEL SQGs indicates that several heavy metals in the study area are likely to cause occasional adverse biological effects on the local aquatic ecosystems. As mentioned above, EF values that are higher than 1.5 and positive I geo values are indicators of anthropogenic sources and heavy metal pollution. In this study, the EF and I geo values of Cd (2.5 ± 1.2 and −0.07 ± 0.68, respectively) clearly show that elevated concentrations of Cd occur in the region. Therefore, both EF and I geo are more suitable tools for the assessment of heavy metal pollution.

As shown in Fig. 4, the most apparent feature of the metal distribution is that the contamination in eastern Hainan Island was generally higher than that in the other areas. Over the last 50–60 years, Hainan’s coastal zone and its hinterland underwent considerable changes because of continuously increasing human activities; this phenomenon has been especially characteristic since the 1980s, when China initiated its economic innovation and later established Hainan Province in 1988 (Zhang et al. 2013). The population of Hainan Island was 2.59 million in 1952. By 2013, the population had increased to 8.95 million. The gross domestic product (GDP) of Hainan increased from 1.64 billion Yuan RMB in 1987 to 314.65 billion Yuan RMB in 2013, i.e., more than 190 times greater. The cultivated land of Hainan Island was 344.7 × 103 ha during the early 1950s and increased to approximately 418.2 × 103 ha by 2013. The surface area for marine aquaculture increased from 0.2 × 103 ha in 1957 to 14.5 × 103 ha in 2010 (Zhang et al. 2013). The amount of chemical fertilizers and pesticides that have been applied has also increased. For example, the application of chemical fertilizers in 2013 amounted to 124.05 × 104 t year−1, the overall consumption of pesticides was 4.35 × 104 t year−1, and the total waste water discharge was 36,214.7 × 104 t year−1 (Hainan Municipal Statistics Bureau 2014). Therefore, large amounts of heavy metals (especially Cd and As) from industries, sewage wastewater, and marine aquaculture were discharged directly into the eastern Hainan Island continental shelf because of the rapid economic and social developments in this region and caused the deterioration of the local aquatic environment.

Conclusions

The concentration and spatial distribution of heavy metals (Cr, Ni, Cu, Zn, As, Cd, and Pb) and sediment grain sizes were analyzed for the surface sediments of the northern portion of the SCS shelf. The heavy metal concentrations in these sediments generally met the criteria of the China Marine Sediment Quality Standards (GB18668–2002). Based on the effect-range classification with TEL and PEL SQGs, however, the concentrations of Cr, Ni, Cu, and As are likely to occasionally cause adverse biological effects on the local aquatic ecosystems. Pearson’s correlation analysis indicated that fine-grained particles are an important carrier of heavy metals. Cr, Ni, Cu, and Zn are predominantly sourced from lithogenic components in the surrounding area, while Cd is primarily sourced from anthropogenic activities (e.g., phosphate fertilizers that are used in nearby catchments) and partially from lithogenic components. The As and Pb concentrations could be influenced by different sources or pathways (e.g., coal burning activities and aerosol precipitation). The EF and I geo values clearly showed that Cd contamination (2.5 ± 1.2 and −0.07 ± 0.68, respectively) occurs in the study area. Heavy metal contamination (especially Cd and As) mostly occurs to the east of Hainan Island because of the rapid economic and social developments in the region. This work represents the current state of the sediment quality in the northern portion of the SCS shelf. Greater attention should be paid to anthropogenic sources of heavy metals, i.e., Hainan Island, because of further industrialization and economic development in the northern portion of the SCS shelf region.