3.1. Temporal-Spatial Variation of Total Metal(loid) Concentrations
Seasonal and spatial variations of heavy metals and As were shown in
Figure 2, which were in the surface sediments of the intertidal zones. In surface sediments, only the concentrations of As, Cd and Pb were significantly different between two seasons (
p < 0.01). As and Pb presented higher concentrations in April than them in September. Contrarily, the higher concentration of Cd was observed in September instead of in April. The effect of hydrodynamics was the important factor for the seasonal differences of heavy metals or As. And the water level was higher in September than in April. As and Pb showed the conservative behavior and their forms were more stable when the water level was low in the Yellow River estuary [
27,
28]. Cd was controlled by the transformation mechanisms between the dissolved and particulate state [
29]. Moreover, it was attributed to the characteristics of Cd which could be affected easily by some factors such as the concentration of suspended solids and the organic matter content [
30]. The average concentration of organic matter in April was lower than that in September (
Table S2) and the organic matter could promote the Cd adsorption or complexation [
31,
32]. No significant seasonal variations were observed for Cr, Cu, Mn, Ni and Zn in surface sediments (
p > 0.05). The result indicated that most of the heavy metals concentrations were not influenced violently by the changes of short-term environmental conditions between seasons.
All the heavy metals and As exhibited great variations among different sampling sites in both seasons (
Figure 2). In order to identify the severity of heavy metal or As contamination in the sediments of the sampling sites, background concentrations of heavy metals and As in the sediments of intertidal zones were compared with them. Based on this, the sources of heavy metals and As could be further pinpointed. In April, the S2, S3, S5, S8 and S12 sediments had relatively higher total As concentrations at 9.30, 10.70, 10.94, 9.71 and 11.64 mg·kg
−1, respectively, which were 1.00–1.25 times of the background total As concentration. Sewage discharge into the sea may be the reason for the higher concentrations of As in these samples. Total As concentrations in other sites were lower than the background concentration. Total Cd concentrations in the sediments at the sites of S3, S4, S5, S6 and S9 were much higher than the background concentration of 0.084 mg·kg
−1, which were about 2.00–4.00 times of it. The results were mainly related to the anthropogenic activities such as fish culture, which could lead to the pollution of sediments in these sample sites. A significant positive correlation was found between total Cd concentrations and organic matter contents (
p < 0.05) (
Table S3 Supporting Information), which indicated that the organic matter was an important carrier for Cd. At the sites of S6, S7 and S12, the total Pb concentrations were separately 67.00, 68.77 and 147.34 mg·kg
−1 which were 2.60, 2.67, 5.71 times of the background concentration. Total Zn concentrations in the sediments at the sites of S3, S5, S7 and S8 were slightly higher than the background concentration. In the sediments of S3 and S5, higher total Cu concentrations were observed at 24.20 and 32.21 mg·kg
−1, respectively. Accordingly, the both sites also had higher total Ni concentrations of 29.52 and 29.33 mg·kg
−1, respectively, implying that Cu and Ni were likely released from the same sources. They both had significant positive correlations with the organic matter contents (
Table S3). Total Cr and Mn concentrations at the sampling sites were all lower than their background concentrations.
In September, total Cd concentrations at all the sites were 2.50–5.00 times higher than its background concentration. Conversely, total As, Cr and Mn concentrations at the sites were all lower than their background concentrations. Total Cu and Zn concentrations had the highest value at the site of S5 and total Pb concentrations had the highest value at S11. The three heavy metals concentrations at other sites were all lower than the background concentrations. Total Ni concentrations in the sediments at the sites of S3, S5 and S9 were slightly higher than the background concentration. Therefore, the concentration characteristics of heavy metals and As indicated a minimal contamination by external sources except Cd in September.
In addition to the effect of hydrodynamics for the seasonal differences, the sediment grain size (
Table S2) was also the important factor for the distribution of heavy metals. The heavy metal contents can gradually increase along with the change of the sediment particle-size from coarse to fine. It was attributed to the greater surface areas and adsorption exchange capacities for the fine particles in sediments [
33,
34]. What’s more, seasonal variation can also cause the change of the biological activity including plants, animals, or microorganisms, which play a very important role in the accumulation and migration of heavy metals and As [
35,
36].
The PCA (
Figure 3 and
Table 1) showed that Cr, Cu, Mn, Ni and Zn were clustered to one component (PC 1) accounting for 45.59% of the variance and As, Pb and Cd to the second component (PC 2) accounting for 28.00% of the variance. To sum up, the two components explained 73.59% of the variance.
Different heavy metals and As were grouped into PC 1 or 2, indicating that they might be derived from common sources. Cr and Ni belong to the siderophile elements, which are the primary rock-forming elements [
37]. It is easy for them to integrate with iron magnesium silicate minerals due to their similar ionic radius [
38]. They are derived from terrigenous detritus material transported by surface runoff [
39], indicating a natural input. Therefore, the heavy metals (grouped into PC 1) might be mainly from natural sources. In addition, As, Pb and Cd were grouped into PC 2, indicating that they might originate from another common source. The pollution in the coastal water and the oil exploitation in this region might be important sources for them. Moreover, Tang et al. (2010) observed that higher As and Cd contents in the seawater of the Yellow River Estuary were primarily affected by inputs from the Yellow River [
40].
3.2. Temporal-Spatial Variation of the Metal(loid) Speciation
The percentages of four heavy metal fractions in sediments (i.e., F1–F4) as previously described are shown in
Figure 4. The exchangeable and carbonate-associated (F1), reducible (bound to Fe/Mn oxides, F2) and oxidizable (bound to OM, F3) fractions are extractable and have the biological effectiveness [
41]. There exist some potential hazards for the three forms. Furthermore, the anthropogenic pollution has a great impact on these forms [
42]. Nevertheless, the residual fraction (F4) with minimal toxicity is very stable, which is difficult to migrate or transform under general conditions [
43,
44].
In April, the F4 fraction was predominant and the exchangeable and the F1 fraction was relatively lower for As, Cr, Cu and Ni in the sediments. The F4 fraction was also dominant for Zn in most sampling sites. Because Zn mainly came from the natural weathering of rock minerals, thus making the proportion of the F4 fraction relatively high. The ranges of the F1 and F4 fractions for the five metal(loid)s were separately 0.52–7.68%, 62.12–89.06% for all the sampling sites. Because the F4 fraction is imbedded in the silicate crystalline structures of sediments, this fraction of heavy metals is very stable and unlikely to be released to pore water [
5]. The results were also supported by the Pearson’s correlation analysis (
Table 2), in which a very significant positive correlation was found between the five metal(loid)s and the F4 fraction (
p < 0.01). Therefore, these results indicated that the study area had a lower environmental risk of As, Cr, Cu, Ni and Zn. In the sediments of most sampling sites, the F3 fraction was the main form of Pb averaging 48.76%, whereas the corresponding F1 fractions were also very low with a mean value of 5.97%. The results indicated the organic matter had a stronger geochemical affinity to Pb, resulting in higher F3 fraction of Pb. The fraction of Pb can be released to pore water under potential environmental risks [
5,
23].
However, the F1 fraction of Cd was dominant in the sediments for most sampling sites averaging 49.14%. What’s more, the sediments of total Cd concentrations such as S2, S8, S10, S11 and S12 had relatively higher F1 percentage at 67.41%, 66.84%, 51.17%, 65.89% and 69.62%, respectively, implying that external sources more likely contributed to the more bioavailable and mobile forms of Cd. The F1 and F4 fractions were the dominant forms for Mn averaging 42.83% and 47.07%, respectively. Meanwhile, total Mn concentration and its F1 fraction presented a significant positive correlation (p < 0.01). The results indicated that Mn in the sediments had the higher biological effectiveness. And it is easy to migrate or transform.
In September, the F4 fraction was predominant and the F1 fraction was very low for most heavy metals and As except Cd and Mn in the sediments. The significant positive correlations were also found between total As, Cr, Cu, Ni, Pb or Zn concentration and the corresponding F4 fraction (
Table 2). In general, the results indicated that the study area had a lower environmental risk of As, Cr, Cu, Ni, Pb and Zn in September. The dominant forms for Mn were the F1 and F4 fractions averaging 42.46% and 48.85%, respectively. And the significant positive correlations between total Mn concentration and its F1 fraction (
p < 0.01) and a negative correlation between total Mn concentration and its F4 fraction were observed (
Table 2). But with respect to Cd, the main form was the F2 fraction averaging 36.10%, followed by the F3 fraction and the F1 fraction with the mean value of 27.00% and 25.44%, respectively. The extractable fractions take up a large proportion for Cd, indicating a high environmental risk.
3.3. Environmental Risk Assessment
In April, the
Igeo values of As, Cr, Cu, Mn, Ni and Zn at all sites, Cd at the site 2, 7, 11, 12 and Pb at the site 1, 2, 5, 8, 10, 11 were less than or at zero (
Table 3), suggesting that these sites were not polluted. The values of
Igeo for Cd at the site 1, 4, 6, 8, 9, 10 and Pb at the site 3, 4, 6, 7, 9 were from 0 to 1 in the sediments which usually had “unpolluted to moderately polluted” class, while the
Igeo values of Cd at the site 3, 5 and Pb at the site 12 were both from 1 to 2, suggesting that the
Igeo class in the sediments of these sites was “moderately contaminated”. The results showed that the accumulation of Cd and Pb in the season was relatively serious in surface sediments of some sites.
Sediments of some sampling sites were heavily polluted by Cd, of which the higher environmental risk was found due to higher percentages of the bioavailable/mobile fraction (i.e., the F1 fraction) (
Figure 4,
Table 4). And sediments of most sampling sites presented high environmental risks induced by Mn (
Table 4). Although S12 showed heavy Pb pollution according to the
Igeo values, no environmental risk was induced by Pb in this site (
Table 4). On the contrary, Zn showing relatively lower pollution levels had a medium environmental risk for S1, S2 and S12 (
Table 3 and
Table 4). As, Cr, Cu and Ni had no or low environmental risks for the studied sediments.
The results for metal(loid) concentration in the surface sediment according to the potential ecological risk index (PERI) were shown in
Table 5. In April, the
values of As, Cr, Cu, Mn, Ni, Pb and Zn were all less than 40, which showed low ecological risk. Also, the values of
RI at all sites were less than 150, which indicated low ecological risk. But the
value of Cd was from 40 to 80, indicating moderate ecological risk.
The concentration of heavy metals and As in the sediment samples were contrasted with the consensus-based ERL, ERM, TEL and PEL values (
Table 6). In April, the results show that As, Ni and Pb were between ERL and ERM for 66.67%, 50% and 25% and were between TEL and PEL for 83.33%, 91.67%, 66.67% of the samples, respectively, indicating that the concentration of As, Ni or Pb was likely to occasionally exhibit adverse effects on the ecosystem. However, the concentrations of Cd were all below the ERL and TEL guidelines showing that the biological effects were rarely observed and negative ecological effects were not expected to occur.
In September, the
Igeo values of As, Cr, Cu, Mn, Ni, Pb and Zn at all sites were less than zero (
Table 3), suggesting that these sites were not polluted. The value of
Igeo for Cd at the site 12 were from 0 to 1 which had “unpolluted to moderately polluted” class but the
Igeo values of Cd at all other sites were from 1 to 2, suggesting that the
Igeo class was “moderately contaminated.” The results showed that the accumulation of Cd in the season was relatively serious in surface sediments.
Some sediments were also heavily polluted by Cd in September, which also had the higher environmental risk because of the higher percentages of the F1 fraction (
Figure 4,
Table 4). And sediments of most sampling sites also presented high environmental risks induced by Mn (
Table 4). As had a medium environmental risk for the sediments of S5, S9, S10 and S12, which were different from the results in April. Only S12 presented a medium environmental risk induced by Zn and other sediments showed lower pollution levels. Cr, Cu, Ni and Pb had no or low environmental risks for the studied sediments.
The values of As, Cr, Cu, Mn, Ni, Pb and Zn were all less than 40, which showed low ecological risk. The value of Cd was from 80 to 160, indicating appreciable ecological risk in September. The values of RI at the sites 8, 9 and 10 were from 150 to 300, which indicated moderate ecological risk. Site 8 was located at the entrance of the Yellow River which flowed through many densely populated areas and received a large amount of wastewater. Besides, there were some irrigated or aquiculture regions around which could lead to the serious pollution in the sites 9 and 10.
The results show that As and Ni were between ERL and ERM for 25%, 66.67% and between TEL and PEL for 41.67% and 91.67% of the samples (
Table 6), respectively, indicating that the concentration of As or Ni was likely to occasionally exhibit adverse effects on the ecosystem in September. However, the concentrations of Cd were also all below the ERL and TEL guidelines showing that adverse effects on the ecosystem were rarely observed.