Levels and sources of hydrocarbons in the Patos Lagoon estuary and Cassino Beach mud bank (South Atlantic, Brazil): evidence of transference between environments

This study assessed the concentrations and sources of natural and anthropogenic aliphatic (AHs) and polycyclic aromatic hydrocarbons (PAHs) in superficial sediments collected along the Patos Lagoon estuary and in sediment cores obtained from the Cassino Beach mud bank. Levels and distribution of n-alkanes indicate terrestrial sources, overlapping with a low amount of petrogenic hydrocarbons (heavy oils). Unresolved complex mixture (UCM) was observed in all samples. On the other hand, the distribution of PAHs in the sediments showed a predominance of pyrolytic over petrogenic sources. In general, hydrocarbons (HCs) contamination in the Patos Lagoon estuary and its adjacent coastal area can be considered low, except for sites near urban or industrial effluents, where moderate to high levels of contamination were found. Concentrations of hydrocarbons were homogeneous throughout the sediment cores, suggesting that mixing processes may have occurred along the layers or that HCs inputs to the mud banks were uniform during the studied deposition period. In addition, the levels and profile of HCs in the coastal sediments were similar to those observed in the estuary. Moreover, the frequent remobilization of sediments from the mud bank towards Cassino beach does not seem to pose any threats to the local biota or beach users since the levels of contamination were relatively low and below the threshold limits of sediment quality guidelines.


Introduction
Coastal zones are the receptors of many substances from natural processes and human activities (Naidu et al., 2021), ranging from organic matter to complex mixtures of synthetic organic and inorganic contaminants. As population density and economic activity in coastal regions increase, these environments are vulnerable to anthropogenic inputs that may lead to concerning levels of environmental contamination (Cabral et al., 2019). Among the most relevant and ubiquitous contaminant groups, hydrocarbons (HCs) are chemical markers of land-based organic inputs (Lu et al., 2022;Volkman et al., 1992). Due to their partition coefficients, HCs preferentially accumulate in the particulate phases (Santos et al., 2023), making sediments the most suitable matrix for spatial and temporal studies.
In aquatic environments, HCs are commonly found as complex mixtures derived from numerous overlapping sources and are potentially deleterious to the environment (Goswami et al., 2016;Ramzi et al., 2017). The relative concentrations of aliphatic (AHs) and polycyclic aromatic (PAHs) hydrocarbons in sediments have been used to distinguish among different hydrocarbon sources (petrogenic, pyrogenic, biogenic, and diagenetic). AHs are a major component of petroleum and its derivatives, although they can also be synthesized by algae and higher plants (Kumar et al., 2016;Meyers, 1997;Volkman et al., 1992). PAHs are mostly derived from anthropogenic sources, specifically the incomplete combustion of organic matter (fossil fuel and wood) and the spillage of petroleum and its byproducts (Chen & Chen, 2011;Readman et al., 2002;Zhang et al., 2015).
Estuaries are important transition zones between fluvial and marine systems that can act as temporary or permanent traps for anthropogenic contaminants. The sediment balance between inputs of terrestrial or adjacent coastal zones and outputs relies on the local hydrodynamics of tides, waves, and river flows (Zou et al., 2016). Since many contaminants may adversely affect estuarine and coastal ecosystems, it is important to increase knowledge of their distribution, concentration, transference processes, and impacts (Wu et al., 2023).
The Patos Lagoon (PL), located on the southern Brazilian coast (30-32°S), is the world's largest choked coastal lagoon (Bueno et al., 2021) (Fig. 1). The estuarine region, which covers approximately 10% of the total area, has great economic and ecological importance due to its high productivity (Seeliger, 2001). This area, however, has been subject to intense anthropogenic pressure related to maritime (Rio Grande port complex is the second largest in Brazil), industrial (oil refinery, fertilizers plants, food industries, etc.), and urban activities (the cities of Rio Grande and São José do Norte used to discard untreated sewage into the estuarine waters) (Wallner-Kersanach et al., 2016). Its riverine tributaries, running off organic matter from extensive natural grasslands (Boldrini & Eggers, 1996), provide a large amount of fine sediments to the PL system. According to the hydrodynamics, these fine materials can be trapped in the estuary or carried out to the adjacent coastal areas (Burchard et al., 2018). As a result, a large mud deposit (mud bank) has formed south of Patos Lagoon output (offshore Cassino Beach), mostly between the 6 and 20-m isobaths   (Fig. 1). During severe storm events, mud bank sediments can be remobilized and transported to the shoreface and/or beach (Calliari et al., 2001), which could potentially be linked to dredging operations in Rio Grande port. However, this is still a controversial topic . As fine sediments can adsorb and accumulate contaminants, local biota and beach users at Cassino Beach might be under threat.
Previous studies have shown moderate to high concentrations of hydrocarbons in estuarine sediments near the main sources of contamination (domestic/industrial sewages and harbor activities) Medeiros et al., 2005). However, neither the processes of transference of hydrocarbons from the estuarine system to the adjacent coast nor their levels in the adjacent coastal depositional areas have been previously appraised. Thus, the present study assessed the levels and sources of hydrocarbons to address evidence of processes of transport and degradation of these compounds from the estuary to the adjacent coastal mud bank. Levels and sources of HCs were discussed in the face of the estuarine morphology and depositional aspects of the mud bank.

Sampling
Superficial sediment samples (top 2 cm) were collected inside the Patos Lagoon estuary using a stainless-steel grab and stored in precleaned aluminum containers (−15 °C) until analysis. Sediment samples from the shallow areas and/or nearby potential anthropic sources (sites 1 to 17) were collected once in September 2003 (N = 1), while samples from the main navigation channel (sites 18 to 27) were collected five times (January and July 2006 and April, June, and October 2007; N = 5)) ( Fig. 1). In addition, four sediment cores were collected in November 2005 by scuba diving in the mud bank located in front of Cassino beach ( Fig. 1)  . The locations and descriptions of the sampling sites are available in Table S1 of the Supporting Information. In the laboratory, cores were sliced into 2-cm layers to obtain interposed samples for grain size analysis,  Holland et al., 2009) and sampling sites: ■surface samples inside the Patos Lagoon estuary and -sediment cores at the Cassino Beach mud bank total organic carbon (TOC), age dating, and hydrocarbon determinations. Samples were placed in precleaned aluminum containers and stored at −15 °C until analyses.

Bulk parameters
Sedimentation rates and sediment radiochronology were determined based on the relative concentrations of 210 Pb and/or 137 Cs, as described by Reed et al. (2009). TOC was determined using a CHNS Perkin-Elmer 2400 Serie II, according to Yang et al. (1998). Grain size and sediment density analyses were performed as described by Gray and Elliott (2014).

Analysis of hydrocarbons
All samples were freeze-dried, grounded, and analyzed as described by Niencheski and Fillmann (2006) within 1 year of sampling. Briefly, 15 g of dried sediments were spiked with surrogate standards (1-hexadecene and 1-eicosene, and p-Terphenyl-D 14 ) and Soxhletextracted for 12 h with n-hexane/dichloromethane (1:1) after stabilization (2 h). Activated copper was added to remove sulfur. Extracts were concentrated (~1 mL) and cleaned up and fractionated using a column of silica gel (6 g, deactivated with 5% water) and neutral aluminum oxide (8 g, deactivated with 5% water). Elution was performed using 25 mL of hexane to yield the first fraction (which contains the AHs-F1), followed by 30 mL of n-hexane/dichloromethane (90:10) and 25 mL of nhexane/dichloromethane (50:50) (which combined contain the PAHs-F2). Fractions F1 and F2 were concentrated, transferred to vials, and fortified with internal standards for AHs (1-tetradecene) and PAHs (naphthalene-d8, acenaphthene-d10, phenanthrene-d10, chrysene-d12, perylene-d12), respectively. The final volume was adjusted to exactly 1 mL using N 2 , and an aliquot of 1 µL of each extract was analyzed by GC-FID (F1) and GC-MS (F2). AHs concentrations were not determined for sites 18 to 27, since these sites were originally used exclusively to assess the PAHs levels associated with activities at Rio Grande port.
AHs (F1) were analyzed on a Perkin Elmer Clarus 500 gas chromatograph equipped with a flame ionization detector (GC-FID), an autoinjector, and an Elite-1 capillary column (100% dimethylpolysiloxane; 30 m × 0.25 mm × 0.25 µm film thickness). Helium was used as the carrier gas (1.5 mL min −1 ). The injector temperature was maintained at 280 °C in splitless mode, while the GC temperature was programmed from 40 °C to 290 °C at 5 °C min −1 , maintained at 290 °C for 10 min, increased to 300 °C at 10 °C min −1 , and then held at 300 °C for 10 min.
PAHs analyses were carried out using a gas chromatograph equipped with a mass spectrometer detector (Perkin Elmer Clarus 500-GC-MS) and an Elite-5MS silica capillary column (5% phenyl-95% methylpolysiloxane; 30 m × 0.25 mm, 0.25 µm film thickness). The injector was kept at 280 °C in spitless mode. The GC temperature was programmed from 40 to 60 °C at 10 °C min −1 , from 60 to 290 °C at 5 °C min −1 , maintained at 290 °C for 5 min, increased to 300 °C at 10 °C min −1 , and then held at 300 °C for 10 min. Helium was used as the carrier gas (1.5 mL min −1 ). Ion source and transfer line temperatures were set at 290 °C, and 70 eV was used for ionization. Selected ion monitoring mode was used for quantification. Data acquisition was performed in SIFI (Selected Ion and Full Ion Scanning).
Quality assurance and quality control were based on regular analyses of blanks, spiked matrices, and certified reference material (IAEA-417). Recoveries for surrogate standards varied between 40 and 100%, while for CRM-IAEA-417, they were between 70 and 112% (n = 10) ( Table S2). Limits of quantitation (LOQ) were the lowest quantifiable concentration of the calibration curve (0.3 ng g −1 for PAHs and 0.03 μg g −1 for AHs). During the period of these analyses (2005 -2008), our Laboratory (CONECO-FURG; https:// coneco. furg. br) was certified by the Canadian Association for Laboratory Accreditation (www. cala. ca) for PAH analysis in sediment samples.

Hydrocarbons data analysis
The sources of n-alkanes were assessed by applying selected diagnostic ratios. The CPI (carbon preference index, n-C 24 -n-C 35 ) was calculated to further verify the predominant source of n-alkanes. According to Bi et al. (2005), CPI values above 2.3 suggest the predominance of higher plants, while CPI values close to 1 usually imply the predominance of anthropogenic n-alkanes derived from oil contamination (Keshavarzifard et al., 2020). However, some studies suggest that a CPI below 1.5 can also indicate planktonic inputs as the main source of n-alkanes (Bakhtiari et al., 2011 (Vaezzadeh et al., 2015). The proxy P aq (aquatic macrophytes/emergent and terrestrial species) uses the relative proportion of mid-chain to long-chain homologs (Ankit et al., 2017). P aq < 0.1 corresponds to terrestrial plants, 0.1 -0.4 to mixed sources, and > 0.4 -1 to submerged/floating macrophytes.

Results and discussion
AHs and PAHs results for the Patos Lagoon estuary are shown in Table 1, while the Cassino Beach mud bank samples are shown in Table 2.

Aliphatic hydrocarbons
Concentrations of total aliphatic hydrocarbons (total AHs)-comprehending the resolved aliphatic fraction plus the unresolved complex mixture (UCM)-in superficial sediments varied from 0.68 to 3383 µg g −1 dry weight (d.w.) inside the estuary and 20.3 to 36.0 µg g −1 d.w. in the mud bank (Tables 1 and 2, and Fig. 2). The highest values were found nearby the possible anthropogenic sources, while the lowest were in the main navigation channel and areas further away from the possible direct sources (Fig. S1). According to Volkman et al. (1992), total AHs concentrations above 100 µg g −1 are generally associated with oil inputs to the environment, and values above 500 µg g −1 are indicative of chronic hydrocarbon contamination. Considering these thresholds, sites 2 (marina), 3 (city market), and 4 (dry docking) were likely contaminated by oil inputs, while sites 6 (Sewage), 7 (petroleum distributor), and 8 (petroleum refinery) were chronically contaminated. The total AHs concentrations were below 10 µg g −1 at sites 1 (Pombas Island), 11, 12, 13 (Marinheiros Island), 14, 16, and 17 (Mangueira Cove). Such values (and even higher in areas with significant biogenic inputs) are similar to concentrations reported for areas considered to be free of contamination Readman et al., 2002;Volkman et al., 1992).    No correlation was observed between total AHs concentrations and grain size (p > 0.05) or between total AHs and TOC (p > 0.05).
Individual n-alkane distributions revealed the predominance of odd long-chain n-alkanes (n-C 29 and n-C 31 ) at most stations, suggesting a biogenic contribution from terrigenous sources (higher plant waxes) (Medeiros & Bícego, 2004;Volkman et al., 1992). This is confirmed by the predominance of n-alkanes associated with terrestrial sources (alk terr ) at most sites (Table 1). n-C 31 showed the highest concentrations at most sites, indicating inputs from C4-type grasses (Bush & McInerney, 2015;Schefuβ et al., 2003) from both salt marshes of the Patos Lagoon estuary (e.g., Spartina alterniflora, Spartina densiflora, etc.) (Marangoni & Costa, 2009) and the runoff of the extensive natural grasslands along the Patos Lagoon drainage basin (Boldrini & Eggers, 1996).
In the present study, most sediments presented CPI ratios above 2.3, suggesting inputs from higher plants (Table 1). CPI ratios, however, were close to 1 at sites 1 (Pombas Island), 11, 12, and 13 (Marinheiros Island). These sites also presented low levels of total AHs and no UCM, suggesting that n-alkanes from planktonic sources were more likely than petrogenic inputs. n-C 17 was also observed at most sites, suggesting that planktonic inputs from algal blooms (Chevalier et al., 2015) were another probable source of n-alkanes inside the Lagoon.
Although the terrigenous/aquatic ratio (TAR) ratios varied between 0.19 and 7.23, most sites showed values indicating terrigenous sources, as seen for CPI ratios. Ratios for sites 1 (Pombas Island), 11, 12, and 13 (Marinheiros Island), as also seen for CPI ratios, indicated aquatic inputs. The ratios for sites 3 and 16 also indicated aquatic sources. P aq ratios varied between 0.11 and 0.28, remaining on the mixed sources range and, therefore, showing little differentiation along the sampling area.
The average chain length (ACL) reflects the average number of carbon atoms for n-alkanes (Vaezzadeh et al., 2015). Alkanes n-C23 and n-C25 are associated with Sphagnum mosses, n-C27 and n-C29 are related to woody plants, and n-C31 is mostly found in graminoids (grasses). However, the signals of the last two groups may be mixed in the sediments. In the present study, the ACL ranged from 27.8 to 30.0, indicating a mixed source of both woody plants and graminoids.
Perylene is a major diagenetic PAH commonly found in marine sedimentary environments (Silliman et al., 1998). Although its diagenetic precursors are associated Fig. 4 Cross plot of fluoranthene/(fluoranthene + pyrene) and anthracene/(anthracene + phenanthrene) ratios for surface sediments of Patos Lagoon estuary (■) and Cassino beach mud bank sediment cores (*) with terrestrial organic matter (Varnosfaderany et al., 2014), pyrogenic processes may yield perylene as well, and such an origin may be dominant in areas receiving high anthropogenic inputs. Its occurrence in levels > 10% of the total penta-aromatic PAHs isomers is attributed to diagenetic sources (Oyo-Ita et al., 2013. In the present study, perylene levels ranged from < LOQ to 130.3 ng g −1 (d.w.) (estuary) and 0.85 to 62.91 ng g −1 (d.w.) (mud bank) and only 2 out of 27 sites (7-petroleum distributor and 16-Mangueira cove 4) showed %PER/5-ring PAH rations < 10%, suggesting that diagenetic sources prevailed in most of the sampling area.
Sediment quality guidelines are broadly used to assess the contamination of aquatic environments. According to Buchman (2008), TEL (Threshold Effect Level) levels are values below which effects on organisms are rarely expected, while PEL (Probable Effect Level) indicates values above which effects on organisms are frequently expected. All 27 sites showed concentrations of total PAHs below PEL (ΣPAH < 16,770 ng g −1 ), while one site (4-dry docking) presented levels above TEL (ΣPAH > 1,684 ng g −1 ). In addition, according to the Brazilian guideline (Brasil, 2012), the sum of PAHs should not exceed 4,000 ng g −1 for saline/brackish sediment; therefore, none of the sites surpassed this threshold level.

Sediment cores of the Cassino Beach mud bank
The fine sediments originating inside the Lagoon are transported southward by coastal drift and settle offshore, forming the Cassino beach mud bank . The mud bank sediments are periodically remobilized and transported to the beach during high-energy events (storms) caused by cold fronts . In these situations, the beach can be covered in mud, affecting local tourism and the biota.
Currents and waves modify the mud deposit by transporting and suspending the sediments, influencing the mud density, and classifying the mud by grain size. As wave and current shear forces progressively decrease, the mud finally settles and consolidates (Reed et al., 2009). These processes yield two endmember types of mud: one type that is considered a viscous fluid and another one that is consolidated and can be considered an elastoplastic material (Foda et al., 1993). Usually, the mud type is defined by the particulates per volume of fluid (g L −1 ): viscous muds have a density from 10 to 480 g L −1 , which corresponds to a density of 1.05 to 1.30 g cm −3 . In contrast, consolidated mud exceeds this upper limit (Reed et al., 2009).
According to Reed et al. (2009), in cores 2 and 3, the density ranged between 0.73 and 1.53 g cm −3 and 1.10 to 1.29 g cm −3 , respectively, while it ranged from 1.33 to 2.03 g cm −3 in core 4 (Table S3). Core 1 was not analyzed. Therefore, cores 2 and 3 consist of fluid mud, while the density is higher in core 4, suggesting consolidated layers. However, the 210 Pb analyses indicate that all cores presented an initial mixed layer of approximately 40 cm (Reed et al., 2009). The lack of stable layering prevents any temporal analyses of hydrocarbon contamination in the area, and the reconstruction of a depositional history was not possible. Moreover, storm surges may create gaps in the historical registry through sediment remobilization (Figueiredo & Calliari, 2006).
Sediments in the Cassino beach mud bank cores were predominantly fine (silt + clay), in agreement with the results reported for the area , ranging from 60.0 to 73.5% in core 1, 18.5 to 87.2% in core 2, 90.2 to 100% in core 3, and 65.0 to 93.7% in core 4 (Table 2). Total organic carbon varied between 0.09% and 1.56%, with the highest TOC values found in core 3 (Table 2).
UCM was present in all analyzed samples and comprised more than 70% of the total AHs in 90% of the samples (Fig. 5). The high abundance of UCM, an indicator of oil contamination (Yunker et al., 2002), suggests a chronic petrogenic input in the area. However, levels were not very high, and the ratio between UCM and resolved aliphatic hydrocarbons (UCM/RA) was > 4 in only 5 samples: core 2 (10-13 cm); core 3 (0 -2 cm); core 3 (6-8 cm); core 4 (0 -2 cm); and core 4 (6-10 cm), being < 4 in the majority of samples ( Table 2). The CPI results were above 1 along all cores and did not suggest petroleum inputs. The Σ n-C 12 -nC 36 concentrations were relatively low, ranging from < LOQ to 2.77 µg g −1 . There was no evident trend in n-alkane deposition along the cores (Fig. 5). Similar to total AHs, the n-alkane concentrations were higher in core 3, with a significant correlation between Σ n-C 12 -n-C 36 vs TOC (p < 0.05) and Σ nC 12 -nC 36 vs fine sediments (p < 0.05) in all cores. Individual n-alkane distributions revealed the predominance of long-chain odd n-alkanes (C > 23), indicating input from terrigenous sources. The alk ter ratio results also suggest prominent contributions of higher plants in all cores (Table 2). Similar to the samples collected inside the Lagoon, the n-C 31 alkane showed the highest concentrations throughout all sediment cores, indicating the predominance of inputs derived from C4-type grasses (Schefuβ et al., 2003). Regarding the other proxies for n-alkanes, ACL was similar to the values observed for the estuarine samples, remaining in the range of mixtures of woody and graminoid plants (27.9 -30.0). The proxy P aq was also like the estuarine area (0.11 -0.28), pointing out a mixed source range. Additionally, these proxies presented a homogenous distribution along the cores.
The TAR values could not be calculated since shortchain n-alkanes (n-C15, n-C17, and n-C19) were not detected in any core sediments.
Total AHs and UCM concentrations were relatively constant throughout all four sediment cores, suggesting that either the inputs were constant over time, or the sediment layers were mixed due to physical processes, which is more plausible considering the fluid state of the cores, as explained in the section above. This is also corroborated by the lack of shortchain n-alkanes and the high percentage of UCM in the cores. The resuspension and mixing of the sediments make the more labile short-chain n-alkanes vulnerable to decomposition, while UCM is more resistant to degradation and is preserved. Therefore, physical processes may influence the record of hydrocarbons stored in mud bank sediments.
Perylene was the most abundant PAH in all layers of all sediment cores, with concentrations varying from < LOQ to 20.4 ng g −1 (d.w.) in cores 1, 2, and 4, and from 26.7 to 62.9 ng g −1 (d.w.) in core 3. All samples showed a prevalence of perylene over the Σ5-ring PAHs (more than 69% of perylene), also indicating a predominance of diagenetic sources for the Cassino mud bank (Table 2).

Hydrocarbons distribution
Overall, AHs and PAHs concentrations inside the Patos Lagoon estuary were higher at those sites under the influence of potential sources of hydrocarbons (e.g., sites 2 -8). These sites are subject to oceanographic conditions (e.g., flocculation), which favor the deposition of hydrocarbons generated in the vicinity of their sources. No significant correlation was observed between the hydrocarbons found in the sites inside the Lagoon and either grain size or TOC, which suggests that source proximity is probably the main cause of hydrocarbon concentration. Medeiros et al. (2005) have previously assessed hydrocarbon concentrations and sources in 10 out of the 27 sites examined in the present study (sites 1-8, 10 and 26). Compared to the results obtained by Medeiros et al. (2005), the concentrations of PAHs found in the present study were lower at sites 7 and 8 and slightly lower at the other sites, indicating that the study area can still be considered moderate to highly contaminated by hydrocarbons. The contamination levels at these 10 specific sites were similar to those found in other areas considered moderate to highly contaminated with hydrocarbons, such as the Pearl River estuary in China (Zhang et al., 2015), the Iko River estuary mangrove system in Nigeria (Essien et al., 2011) and the Cochin estuary in India (Ramzi et al., 2017).
Intermediate levels of PAHs were observed in sites located along the main navigation channel, directly influenced by maritime activities (sites 18 -27). These sites, despite being close to expected sources of PAHs, are subjected to stronger currents (António et al., 2020;Martelo et al., 2019), which apparently can dilute the contamination and/or prevent sediment and hydrocarbon deposition (Janeiro et al., 2008), resulting in the intermediate concentrations observed. Lower AHs and PAHs concentrations were observed in sites located in areas far from the expected sources (sites 1, 11 -13) or with low contents of fine sediments, such as Mangueira cove (sites 14 -17). Hydrocarbon concentrations at these specific sites were comparable to those found in areas considered uncontaminated, such as Admiralty Bay in Antarctica (Martins et al., 2010) and Laranjeiras Bay (marine protected area) in Brazil (Martins et al., 2012).
Overall, AHs and PAHs concentrations in the Cassino beach mud bank sediment cores were low, much lower than concentrations found near the main hydrocarbon sources at Patos Lagoon estuary (sites 2-10) and similar to concentrations found in sites located in areas far from the expected sources (sites 1, 11 -13) or with low content of fine sediments, such as Mangueira cove (sites 14 -17). The exception was PAHs in core 3, where concentrations were similar to those seen in sites located in the main navigation channel (19-27). The higher contents of fine sediments and TOC may explain the higher levels of PAHs in core 3. However, the lack of LMW PAHs could also suggest that physical processes of mixing sediment, caused by mud fluidity, promote remobilization or degradation of LMW PAHs, which are more susceptible to this process than HMW PAHs. This may explain why there are low concentrations of LMW PAHs and short-chain n-alkanes.
Hydrocarbons inside the Lagoon and in the mud bank seem to have similar sources, showing both biogenic and anthropogenic signatures. Aliphatic hydrocarbons indicated the predominance of terrigenous n-alkanes derived from higher plant waxes and chronic inputs of petrogenic sources. Regarding PAHs composition and diagnostic ratios, pyrogenic PAHs were evidenced inside the Patos Lagoon estuary as well as in the adjacent coastal area.
The Patos Lagoon estuary discharge transports high amounts of suspended material to the adjacent coast, being the main source of sediments to the Cassino beach mud bank ). Thus, considering the similarity between the levels and profiles of individual hydrocarbons in sediments from the estuary and mud bank, the Patos Lagoon is also the main source of hydrocarbons to the adjacent coastal area. However, all sediment cores showed PAH concentrations below PEL and TEL (Buchman, 2008) and threshold limits set by the Brazilian sediment quality guidelines (Brazil, 2012).

Conclusions
The hydrocarbon individual distribution, as well as the diagnostic ratios, reveals a mixture of natural and anthropogenic inputs inside the Patos Lagoon estuary and in the Cassino Beach mud bank (outside the estuary). Source proximity seems to be the main cause of the hydrocarbon concentrations observed around the study area, with higher hydrocarbon concentrations observed closer to the main potential sources. According to Brazilian and international sediment quality guidelines, 5 sites inside the Lagoon were shown to be moderate to highly contaminated. Nevertheless, the mud transported by storm surges to Cassino Beach shoreface poses minimal risk to local biota and humans, since sediment cores collected in the mud bank showed hydrocarbon concentrations below the threshold limits of sediment quality guidelines. Although depositional history is not registered in the cores due to physical mixing, it is possible to observe that biogenic n-alkanes derived from terrestrial plants, n-alkanes derived from chronic petrogenic inputs, and pyrolytic PAHs contributions are the predominant sources of hydrocarbons to the samples both inside the estuary and in the mud bank. Since the Patos Lagoon estuary is the main source of suspended sediments to the mud bank and the hydrocarbon signatures were similar in both sampling areas, it can be stated that hydrocarbons (and sediments) originated inside the Lagoon are being transported and deposited on the Cassino beach mud bank.