Understanding Hg Distribution in Sediments From the Santos and São Vicente Estuarine System, Southeastern Brazil

The Santos and São Vicente Estuarine System (SSVES) is a region with great economic importance in southeastern Brazil. It contains petrochemical and metallurgical industrial complexes and the largest commercial harbor in South America, which have made the region vulnerable to degradation by chemical pollutants, such as mercury (Hg). This study is the rst to evaluate the environmental parameters that control the spatial distribution of Hg in the surface sediments of the SSVES which is inuencied mainly by hydrodynamics. The innermost part of the estuarine system, where weaker currents lead to an environment with more sediment deposition, has anomalous Hg concentrations resulting from historical anthropogenic activities. Althought there is a low probability of negative effects of Hg in organisms, this element demonstrates greater mobility and bioavailability in the environment, indicating the need for monitoring Hg for the conservation of this region.


Introduction
Mercury (Hg) is a highly toxic, persistent and, nonbiodegradable contaminant. It is recognized worldwide due to its absence of biological function and ability to be biomagni ed in the food chain (e.g., Shi et al. 2005; Syversen and Kaur 2012; Gao et al. 2016). It can be released into the environment by natural and man-made sources and, atmospheric deposition in one of the main routes, leadind to aquatic ecosystems (Le et al., 2017;Seixas et al., 2012;USEPA, 1997). The atmospheric emission from industrial waste are a major concern and, the awareness of Hg poisoning started in the early 1950s with Minamata disease. This disease was discovered by to the consumption of sh and shell sh contaminated with methyl mercury compounds, discharged into the ecosystem as factory wastes. Among the most important effects were neurological and biochemical symptoms (Hachiya, 2006;Minamata Disease Municipal Museum, 2007). Due to strict regulations, the use of Hg has been reduced over time; nonetheless, high concentrations are still found in the environment, even in remote locations (Le et  In aquatic systems, trace elements, such as Hg, have a high a nity for the solid phase and are adsorbed onto suspended particles. Moreover, occulation, coagulation and coprecipitation may remove them from the water column and deposit them in the sediment (Kim et (Birch, 2017;Hakanson, 1980). Despite being considered ultimate sinks of pollutants, sediments can also act as a source of contamination once environmental characteristics change in the water column (Schintu et al., 2015). This can also change the speciation, in which is dependent on environmental parameters, converting inorganic species to others that are far more labile and toxic, such as methylmercury (Chakraborty et al., 2016;Deng et al., 2013).
Due to the proximity to potential sources of Hg, coastal and estuarine regions have increasingly been degraded by chemical inputs (Gao et al., 2021;Kehrig et al., 2006). There are many studies assessing heavy metal contamination in sediments, but the evaluation and relationship of Hg with surrounding factors are uncommon, specially in highly urbanized regions. In Brazil, some studies were conduted in others estuarine/coastal environments, such as Patos Lagoon (Quintana et al., 2020), Guanabara Bay (Wasserman et al., 2000), Todos os Santos Bay (Fostier et al., 2016) and Tapajós River (Lino et al., 2019), but at Baixada Santista, despite its economic signi cance, studies assessing Hg contamination are still scarce.
The Baixada Santista, which is located in the Santos and São Vicente Estuarine System (SSVES), southeastern Brazil is a region with great economic importance due to the presence of a important industrial area (Cubatão Industrial Complex) and the largest commercial harbor of South America Besides the economic importance, more than 40% of the SSVES territory is protected by conservation units since it is part of Atlantic rainforest biome (São Paulo, 2013).
Facing the challenges proposed by the 14th goal of 17 Sustainable Development Goals (SDGs), aiming conservation and sustainability of the oceans, increasing scienti c knowledge, the determination of Hg levels is crucial for the assessment of pollution status and conservation of SSVES, considering that human pressure into the environment and economic activities must coexist. Thus, this study aims to evaluate the environmental parameters that control the spatial distribution of Hg levels in the surface sediments of the SSVES.

Materials And Methods
In the summer of 2014, a total of 47 super cial sediment samples were collected along the SSVES using a stainless steel Van Veen grab ( Figure. Grain size analyses (< 0.063 mm) were conducted through wet sieving using a 0.063 mm mesh sieve.
Total organic carbon (TOC) was determined in subsamples, after acidi cation, in an elemental analyzer (EA) coupled with an isotopic ratio mass spectrometer (IRMS).
Total Hg (organic and inorganic) was determined by the cold vapor technique SW 846 US EPA 7471 A (USEPA, 1998). Hg was reduced to the elemental state and aerated from solution in a closed system and analyzed by cold vapor generation coupled with inductively coupled plasma optical emission spectrometry (VGA-ICP-OES). Quality control was assessed by subjecting a certi ed reference material ERM (European Reference Materials) -Estuarine Sediment CC580 n° 0369 (n = 7), to the same analytical procedure. The method had accuracy and precision, with a relative standard deviation of 7% and recovery of 97%. In addition, the detection limit of the method (DL) was obtained (DL = 0.006 mg kg − 1 ; m = 0.6 g; v = 45 mL), and all samples were above this value.
To better understand estuarine hydrodynamics and relate them to local sediment dynamics, a numerical

Results And Discussion
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), CaCO 3 content (% CaCO 3 ) and TOC, are presented in Table 1. 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 a nity with TOC, which is also associated with the ne-grained fraction (e.g., Chakraborty et al. 2014;Gao et al. 2016). Moreover, the concentration of Hg in the sediment is strongly in uenced 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 uctuations 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).  (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 in uences, so PCA can increase the sensitivity of the analysis.
As several authors reported (e.g., Kim   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, a nity 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 ( 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. The mercury distribution in the SSVES is dependent on several factors. The accumulation and spatial distribution of Hg in surface sediments is strongly in uenced by hydrodynamics. The highest levels were found in the innermost part of the estuarine system, where weaker currents lead to an environment with more deposition. The lowest levels were found in the Bertioga channel, which is considered pristine, where well developed mangrove forests are found.
Although most samples have Hg levels below which there is a low probability of negative effects for organisms, this element is related to total organic carbon and carbonate, demonstrating greater mobility and bioavailability in the environment, especially in the Bertioga Channel. This fact highlights the need for Hg monitoring in this estuary, which has anomalous Hg concentrations resulting from a history of anthropogenic activities. Authors' contribution BSMK analyzed and interpreted data and wrote major part of the manuscript. THV and JLFA reviewed and contributed in writing. ES performed hydrodynamic analysis and interpretation. MMM and RCLF were responsible for funding acquisition, project administration and supervision. All authors read and approves the nal manuscript mass spectrometer (IRMS). Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors. Figure 2 (a) Levels and; (b) spatial distribution of Hg (mg kg-1) in the study area Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.

Figure 3
Model averaged current velocity magnitude in the SSVES over a neap-spring tide cycle. Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.

Figure 4
Correlation between TOC and levels of Hg (mg kg-1) Figure 5 Enrichment factors of Hg at SSES without the sample #36.

Figure 6
Scatter plot of the dimension (Dim) 2 and 5 from the PCA of samples from SSVES