The levels and spatial distribution of Hg are presented in Fig. 2. The results were compared with the predicted values for dredging materials established by Brazilian legislation (CONAMA 454/2012) because the region is subjected to periodical dredging of its navigation channels. This regulation, as well as other sediment quality guidelines (e.g., Long et al., 1995; Macdonald et al., 1996), designates a level 1, below which there is a low probability of negative effects on organism and a level 2, above which there is a high probability of negative effects on the organisms. The comparison shows that 23 samples, that is, 48.9% of the total samples, presented concentrations between the two levels (0.3–1.0 mg kg− 1) and only two samples showed values above level 2 (1.0 mg kg− 1).
The mean ± standard deviation (SD), median and range of geochemical tributes, such as mud content (%mud), CaCO3 content (% CaCO3) and TOC, are presented in Table 1.
Table 1
Statistical Descriptive values for geochemical parameters
| % mud | % CaCO3 | TOC |
Mean ± SD | 63.5 ± 32.3 | 17.5 ± 12.4 | 3.15 ± 2.3 |
Median | 69.4 | 15.7 | 2.6 |
Min | 1.7 | 3.1 | 0.3 |
Max | 99.3 | 66.8 | 10.8 |
The results of the hydrodynamic model (Fig. 3) show the distribution of currents within the estuary, with the most dynamic areas being the main estuarine channels and weaker currents are observed in the upper estuary. Such a pattern seems to control the Hg distribution in which areas of higher current velocities present lower Hg concentrations. Samples with higher Hg values are found in areas with weaker currents, leading to a depositional environment that favors sedimentation and organic matter concentration.
The high correlation between Hg and TOC is widely reported due to the strong affinity with TOC, which is also associated with the fine-grained fraction (e.g., Chakraborty et al. 2014; Gao et al. 2016). Moreover, the concentration of Hg in the sediment is strongly influenced by the solubility of the sedimentary organic material, especially fulvic acids (Bergkvist, 2001). This acid is released when the organic material is decomposed by microorganisms or by fluctuations in the amount of water surrounding the environment, which produces low molecular weight organic acids (Johansson and Tyler, 2001).
Once regions with higher Hg concentrations have weaker hydrodynamics, becoming essentially depositional environments, the sediment has a high degree of compaction, resulting in a lower oxygen level and less interstitial water. These environmental settings prevent decomposition, resulting in organic material of higher molecular weight (Johansson and Tyler, 2001).
Two positive significant correlations (p < 0.05) were observed between Hg and TOC by removing outlier samples (#36 with [Hg] = 7.69 mg kg− 1 and #45 with [Hg] = 1.45 mg kg− 1) from the data set (Fig. 4). Nonetheless, when the prediction interval (Kim et al., 2017) was applied, the correlations were statistically equal (α = 0.05). In addition to these correlations, seven samples (triangles) presented low Hg levels and high TOC contents. These samples are located inside the Bertioga channel, a pristine region surrounded by well-developed mangrove forests. Sample #42, which located at the innermost part of the SSVES, did not fit any correlation once it was in a confined area with a punctual anthropogenic source.
Several studies apply the enrichment factor (e.g., Duodu et al., 2016; Kim et al., 2017; Zhang et al., 2016) to evaluate the concentration of a particular metal in sediment. This procedure involves normalization with reference elements to reduce the variability caused by grain size. Taking the sample located inside the Bertioga channel (#73, [Hg] = 0.06 mg kg− 1) as a reference, due to the pristine conditions of this region, the enrichment factor (EF) was calculated for all samples (Fig. 5), and the results ranged from 0.91 to 115.0. The latter corresponds to sample #36 and is not shown in the graphic.
The EF results indicated strongly enriched samples, but they were not all in agreement with the comparison to the CONAMA 454/2012 thresholds. These high EF values may be attributed to the background Hg value, which is a natural contribution that is almost absent compared to contaminated samples.
Values of Al, Cu, Fe, Pb, Sc, V, and Zn were taken from (Kim et al., 2019) and a principal component analysis was performed to understand the relationship between Hg and the other analyzed parameters and infer possible sources and pathways, from the interelement contribution of each variable (Table 2). Taking into consideration that the SSVES is an estuarine environment, multiple forces coexist, and strong marine and lithogenic contributions may hinder other possible influences, so PCA can increase the sensitivity of the analysis.
As several authors reported (e.g., Kim et al., 2020; Siepak and Sojka, 2017; Zhao et al., 2016), the lithogenic contribution, represented here by dimension 1 (Table 2), shows the strong contribution of Al, Fe, Sc and V and corresponds to the weathering and leaching of the rocks. Dimension 2 could be considered a marine component because most of the calcium carbonate has a biogenic origin and plays an important role as a dilutant material of the trace elements. Dimensions 3 and 4 showed higher contributions of Cu, Pb and Zn, indicating an anthropogenic influence, as already recorded in previous studies (Kim et al., 2019). Higher contributions of Hg were observed in the fifth component, demonstrating that dimensiona 3 to 5 grouped elements were linked to an anthropogenic origin. Dimension 5 contributed 6.38% of the variance, but the Hg contribution accounted for 69.02, followed by TOC (12.11) and carbonate (4.68), which was not negligible.
Table 2
The variance percent and metal contribution of each dimension of the Principal Component Analysis.
| Dim.1 (49.04%) | Dim.2 (16.01%) | Dim.3 (9.81%) | Dim.4 (7.32%) | Dim.5 (6.38%) |
Al | 13.51 | 0.06 | 4.69 | 0.48 | 3.62 |
Cu | 11.39 | 4.75 | 5.55 | 1.20 | 0.74 |
Fe | 14.46 | 0.00 | 0.85 | 0.00 | 0.88 |
Pb | 7.49 | 6.59 | 16.75 | 4.96 | 0.62 |
Sc | 14.73 | 0.03 | 2.83 | 1.03 | 1.89 |
V | 13.86 | 0.80 | 3.81 | 0.30 | 0.97 |
Zn | 5.90 | 6.25 | 27.09 | 0.04 | 3.04 |
Hg | 0.99 | 5.01 | 23.11 | 0.19 | 69.02 |
%mud | 11.42 | 4.66 | 3.84 | 0.09 | 1.70 |
% CaCO3 | 0.26 | 26.43 | 1.20 | 44.09 | 4.68 |
TOC | 0.93 | 14.24 | 9.98 | 47.61 | 12.11 |
TN | 5.05 | 31.15 | 0.28 | 0.00 | 0.70 |
Dimension 5 showed strong contributions of Hg, TOC and carbonate, a comparison with the marine dimension (dimension 2) is given in Fig. 6. Sample #36, as described above, presented high and positive values for dimension 5 but negative values for dimension 2, suggesting that Hg is linked to high molecular weight organic matter. Normally, elements linked to carbonate and bicarbonate have a more labile character (Chakraborty et al., 2014) and are easily available in the environment. Samples #5, #6 and #9, which were collected in Bertioga channel, a pristine region, presented high values for dimension 2 and positive values for dimension 5. This element in these samples may be in its organic form of low molecular weight, and due to its higher mobility, we hypothesize that this element is bioavailable.
Due to its characteristics, such as high mobility, affinity with organic matter and stability, Hg is generally not degraded in the environment, especially in the methylated form (Methyl-Hg), which is easily assimilated by marine biota (Bearhop et al., 2000; Seixas et al., 2009). Moreover, methyl-Hg has proven its biomagnification capacity and is the most toxic form of Hg (Bearhop et al., 2000; Chouvelon et al., 2017; Seixas et al., 2009). Almost all mercury that is accumulated in fish is methylated and, consequently predatory biota, at the top of the aquatic food chain, such as marine mammals, tunas and sharks, are exposed to higher Hg concentrations (Teffer et al. 2014; Chouvelon et al., 2017). Even with low Hg concentrations, biomagnification has been observed in marine trophic webs, which is a cause of great concern, since the trophic pathway is the main Hg absorption path for top predators and humans (Hall et al., 1997; Lavoie et al., 2013; Le et al., 2017; Zhang et al., 2012).
By comparison with other studies in Brazil and around the world, without outliers, this study showed the highest Hg levels (Table 3), except for the study by Gao et al. (2016) in China, that includes results of Hg in the Yellow River, which is in the process of recovering from past pollution activities, and the study by Scanu et al. (2016), with in sediments from the northern coastal region of Lazio, Italy, that is known for anomalously high Hg concentrations due to decades of mining and industrial activities. These results show that the SSVES can also show anomalous concentrations of Hg resulting from the history of anthropogenic activities in the region.
Table 3
Comparison of the levels of Hg (mg kg− 1) in different coastal/marine areas
Location | Range (mg kg− 1) | Reference |
Mucuripe harbor - Brazil | < 0.03–0.04 | Buruaem et al. (2012) |
Pecém Harbour - Brazil | < 0.03–0.04 | Buruaem et al. (2012) |
Yellow River - China | 0.005–2.663 | Gao et al. (2016) |
Campos Basin - Brazil | 0.002–0.052 | Araujo et al. (2017) |
Tyrrhenian sea - Italy | 0.03–2.2 | Scanu et al. (2016) |
Western North America | < 0.01–0.030 | Fleck et al. (2016) |
SSVES | 0.02–0.88 | This study |