Investigation of N-Alkanes and Polyaromatic Hydrocarbons in Estuarine Sediments of Patos Lagoon (Brazil) – Anthropogenic Impacts

Research article

Investigation of N-Alkanes and Polyaromatic Hydrocarbons in Estuarine Sediments
of Patos Lagoon (Brazil) – Anthropogenic Impacts

Corresponding author: Larissa P. Costa Federal University of Rio Grande, Oceanography Institute, Av. Itália km 08 Campus Carreiros, 96201-900, Rio Grande, RS, Brazil. Email: larissap.costa@hotmail.com

Abstract

This paper presents a study of the historical record from organic contamination in two areas (Mangueira Bay (SM) and Arraial Bay (SA)) under the impact of various anthropogenic sources (e.g. pyrolytic and petrogenic) in Patos Lagoon Estuary. Aliphatic hydrocarbons (AH) and polyaromatic hydrocarbons (PAH) were determined by gas chromatography with flame ionization detection (GC/FID) and gas chromatography with mass detection (GC/MS), respectively. Geochemical indexes, pre-established in the literature, were used to diagnose the human impact. Overall, in Mangueira Bay, individual distribution and AH indexes suggested a preferential contribution of petrogenic source of hydrocarbons for the last 60 years, AH little or no prevailing in the odd/even ratio also pointed to an introduction of oil. However, it was also found high concentrations of PAH with 4-6 rings, suggesting the input of pyrolytic sources. The concentration levels reached values of 284.63μg.g-1 and 780.05ng.g-1, for AH and PAH respectively, telling high contamination. In the drill core from Arraial Bay, total aliphatic concentrations were below 69.98μg.g-1, reaching values less than 10μg.g-1, suggesting a place free of anthropogenic contamination and with large biogenic contribution. PAH concentrations for Arraial Bay were also low, reaching values of 55.09ng.g-1 or less, with greater contribution of pyrolytics sources. We conclude that in the estuarine region of Patos Lagoon coexist urban, port and industrial activities. Therefore, it was observed the presence of anthropogenic contributions, directly associated with the proximity of potential sources (urban pollution, petrogenic and pyrolytic), considered to have higher concentrations in Mangueira Bay region.

Keywords: Anthropogenic source; geochemical indexes; Hydrocarbons; Historical record.

Introduction

The continuous increase in urbanization in estuarine environments has led to the increase and accumulation of organic compounds at levels harmful to the biota and environmental quality [1]. The anthropogenic contributions most commonly found in the aquatic environment refer to dredged material dumps, urban and industrial effluents, leaching of rural areas and atmospheric inputs.[1 2] In a global perspective, the classes of contaminants considered critical to health of aquatic environment are petroleum hydrocarbons, halogenated hydrocarbons, heavy metals and solid waste.[3]

Hydrocarbons are a class of compounds that have good chemical stability in water and sediments, so they have been widely used as biomarkers and as indicators of oil pollution. The use of molecules or groups of molecules that can be unmistakably related to specific origins introduces the concept of molecular markers.[4-6] Sedimentary aliphatic hydrocarbons (AHs) have both natural (including biogenic and petrogenic) and anthropogenic sources[7] AH natural inputs are comprised mostly of vascular plants, algae, microorganisms and early diagenesis of natural products[8] and present an AH distribution with predominance of odd carbons.[9] Anthropogenic AHs in sediment originate mainly from petroleum residues and generally exhibit an unresolved complex mixture (UCM) of branched and cyclic hydrocarbons.[10]

Polycyclic aromatic hydrocarbons (PAH) are also widely used in environmental research due to their persistence, ability to bioaccumulate and high toxicity (e.g., mutagenic and carcinogenic effects) to many organism.[8,11] Anthropogenic PAH, petrogenic or pyrolytic sources, are generally differentiated by their composition. Mono, bi, triaromatic compounds are generally constituents of petroleum derivatives.[12,13] The presence of alkylated homologs and compounds containing heteroatoms such as oxygen and sulfur in their structure are common in petrogenetic sources.[14] However, PAH originated from the burning of fossil fuels or from wood, present compounds with four or more aromatic rings and non-alkylated groups.[14,15]

In this paper, we present the first survey of AH and PAH in drill core sediments of the Patos Lagoon Estuarine. Since the few studies carried out at this site are focused on bottom sediments, and most of them are focus on inorganic compounds [8,16-18] this study aims to assess AH and PAH sources and concentrations, related with their contribution along the time, to serve as a baseline for future monitoring programs and to improve environmental sustainability. To achieve this goal, the separation, identification and quantification of n-alkanes and PAH were carried out. Additionally, several organic geochemical indexes reported in the specialized literature [19-23] were calculated in the attempt to distinguish between the diverse aliphatic and polycyclic aromatic hydrocarbons inputs and evaluate the historical pollution level.

Material and Methods

Study Area and Characteristics of Sampled Sites

The Patos Lagoon estuary is the dominant feature of the southern Brazilian coastal plain. At its southern reach (31°41’ and 32°’12 S, 51°49’ and 52°15’ W) the lagoon connects to the Atlantic Ocean and therefore, brackish waters and fringing marshes comprise an estuarine ecosystem of approximately 1,000 km [2, 24] Most part of the estuary is shallow (1 to 5 m), and fluxes are mainly controlled by rainfall and wind. [25]

The estuary presents high biological productivity and provides significant ecosystem services including artisanal and industrial fisheries. Because of the presence of the second major Brazil port, Rio Grande Port, this region is also related industrial and shipping activities.[17] The study area of this research (Figure 1)

comprises regions close to the effluent disposal site (domestic, industrial and petrochemical) and chosen by the greater environmental vulnerability of the aquatic system.

The Mangueira Bay is a shallow bay (average depth of 1.5 m), connected to the Patos Lagoon estuary by a bottleneck of 240 m [26]. In this place are realized several economic activities, such as support to the industrial park to Rio Grande city, fishing and leisure. This area also receives a discharge of insufficiently treated, official and clandestine effluents from the city of Rio Grande and of its Industrial District [27]. The waters of this bay were classified as eutrophic, anoxic conditions [28] in the water column and sediment are not stable due to its hydrodynamics [29]

The Arraial Bay stands out as the largest bay of the estuary, presenting a high diversity of habitats, being considered an area of breeding, feeding and protection of various fish and invertebrates species of ecological and economic importance [28, 30] The sediment is mainly characterized by fine sand, indicating moderate dynamics[29]

Material and Analyzed Parameters

In order to evaluate hydrocarbons inputs from natural and anthropogenic sources to Patos Lagoon estuary, two drill cores, with 70 (Mangueira Bay – SM) and 105 (Arraial Bay -SA) cm of depth, were collected in June, 2014. The cores were sliced in layers of three centimeters wide, and sub samples were removed with a stainless steel spatula and packed in aluminum pots (previously calcined in muffle at 450 °C for 24 hours). Collected sediments were homogenized and separated in aliquots for the determination of organic matter, grain size distribution and hydrocarbons. Granulometric analyses were determined by the standard sieve method [31] pH and Eh (redox potential) in the sediment samples were performed at the time of the sediment sampling. The pH measurement was carried out in pH meter from Oakton (model pH6/00702-75), calibrated with standards of pH 4 and 7, using sword glass type electrode for sediment measurements. Measurements of the redox potential (Eh) were conducted using a Platinum electrode, calibrated with 470 ± 5 mV standard.

Organic Matter Analysis

About 100 mg of the homogenized dried sediment was powdered using an agate mortar and pestle, weighing about 30 mg of the sample for analysis and performed in a carbon-measuring device, model TOC – VCPH, coupled to the solid sample module, model SSM – 5000A, SHIMADZU with combustion detector. Total carbon (TC) was determined by oxidation of the organic and inorganic carbon of the sample when converted to CO2. The inorganic carbon (IC) determination was performed by acidification, with 0.5 ml of phosphoric acid, leading to the release of CO2 from inorganic carbon – carbonates and hydrogenated carbonates. By difference of total carbon and inorganic carbon, the total organic carbon value (TOC) of the samples is calculated.

The results were presented in percentage of carbon of the dried sediment, being all the analysis carried out in triplicate, with standard deviation smaller than 5 %. Five standard measurements were used to establish calibration curves for the measurement of TC, with glucose (C6H12O6), and of IC, with sodium carbonate (Na2CO3), both from Merck®.

Geochronological Analysis

Dating of the sediment samples from the drill cores has been described in detail elsewhere [32].The age of the sediments is calculated based on the unsupported Pb via CIC (Concentration Initial Constant). 210Pb activity was obtained by gamma spectrometry and its photopeak (47 keV), model Canberra Eclipse 5S. Constant initial 210Pb concentration model was adopted, obtaining average sedimentation rates of 0.60 and 0.34 cm.yr-1 for cores SM and SA, respectively.

Analysis of Hydrocarbons

In order to study the sources and distribution of AH and PAH in the Patos Lagoon estuarine system, 20 g of sediment were used in the analysis of them. Two sets of surrogates were added to quantify the overall recovery of aliphatic and aromatic fractions. The surrogates were 1-hexadecene and 1-eicoseno for aliphatic hydrocarbon fraction (recovery of over 40%) and ρ-terphenyl-d14 for aromatic fraction (recovery of over 40%).

The samples were then Soxhlet extracted with 200 mL of a mixture of dichloromethane (DCM) and hexane (50:50) for 24 h at a rate of 5 cycles per hour. The raw extracts were concentrated to 1mL in rotary evaporator and nitrogen flow (N2). The cleanup and fractionation of concentrated extract was achieved using silica gel chromatographic technique as described in reference.33 The sediment extract was fractionated into two fractions: the first fraction containing aliphatic hydrocarbons (F1) eluted with 20 mL of n-hexane, and the second fraction with polycyclic aromatic hydrocarbons (F2) eluted with 30 mL of hexane:dichloromethane (90:10) and more 30 mL of hexane:dichloromethane (50:50). The fractions were concentrated under a gentle flow of high-purity nitrogen to appropriate volumes, spiked with appropriate internal standards (1- tetradecene for the aliphatic and a mix of naphthalene-d8, acenaphthen-d10, phenanthrene-d10, chrysene-d10 and perylene-d10 for the aromatic fraction), and then adjusted to accurate pre-injection volumes (1.00 mL) for instrumental analysis.

The determination of the aliphatic hydrocarbons followed the USEPA 8015C methodology (USEPA, 2007) being performed in gas chromatograph with flame ionization detector (CG/DIC), model Perkin Elmer Clarus 500. For PAH identification, USEPA protocol 8270D34 was adopted and analysis were performed in a gas chromatograph equipped with a masses spectrometer (CG/MS), also model Perkin Elmer Clarus 500.

The hydrocarbons were identified by comparison of their mass spectra and relative retention times. Quantitative data were obtained by comparing the peak areas of internal standard with the interest compounds (in total ion current chromatogram). The results were based on dried weight sediments and were not corrected by the recovery number.

Results and Discussion

The pH values of sediment samples presented a mean of 7.21 (SM) and 6.87 (SA), with maximum reaching 7.44 (SM) and 7.36 (SA), what is expected in the samples location according to the literature [35] Surface sediments were slightly more acidic in the first 12 cm, becoming slightly alkaline towards the base of the cores. This behavior is expected due to early diagnosis processes that occur preferentially in the deep portion and lead to a geochemical zonation in sediments, being the deeper sediments more alkaline.

For both locations, Eh exhibited values in reducing range (mean of -65 mV and -242 mV for SM and SA, respectively), which is a sediment characteristic of the Patos Lagoon estuary [35] Sediments with finer grain size tended to be more reducing due to lower oxygen supply conditions at burial levels. This aspect can be verified by the most reductive values found in the SA sample, which is richer in fine sedimentary fraction. Reducing conditions favor the stability of the hydrocarbons in the sediments, reducing the rate of organic degradation and increasing the residence time of these compounds in the sediments

The granulometric analysis (Figure 2)

showed predominance in the content of sand fraction for SM drill core, 83.42% in average. Values are approximately constant throughout the core. These results indicate a stability of hydrosedimentary processes over time, a function of the morphological conditions from Mangueira Bay, which is a semi-closed aquatic system.

In SA, a higher percentage of fine sediments is observed, which can become majorities in grain size as occurs at depths of 9 to 12 cm, where fine content reaches 83.61%. The percentage of sand size sediments increases as it approaches the base of the drill core (greater depths), reaching values of 94.48%. The greater variations in granulometric sizes suggest instability of hydrosedimentary processes over time, since the bay dynamics is relatively more dependent on the deviations of estuary level, local effect of the wind and fluvial discharge intensity [25, 36]

The content of total organic carbon (TOC) (Figure 2) in the SM sample ranged from 0.13% to 0.37%, whereas for the SA, values from 0.01% to 1.09% were found. Previous results17 indicated TOC results equivalent to 1.43% in 1997, and 1.27% in 2003 for surface sediment sampling near the SA location. The same study showed lower TOC concentrations for SM area, with values of 0.71 % in 1997 and 1.07 % in 2003. Pederzolli37 points to TOC levels lower than 0.4% near SM area, which are values similar to those found in this study.

It should be noted that mentioned works used surface sediments, so this comparison should therefore be allowed within the limits of the temporal representativeness for present approach. The reduction of TOC contents with sediment depth is also attributed mainly to the degradation of organic matter over time, and secondarily to variations of the contributions. We also observed a direct correlation between the increases of fine fraction with TOC contents, as fine fraction has a greater capacity to fix the organic matter. The correlation index between granulometry and TOC was 0.71 for SM drill core and 0.87 for SA drill core, being statistically significant for a probability of 0.05.

For AH, the hydrocarbon species that presented the highest concentration was the triacontane (C30) for SM location and the heptacosan (C27) for SA location. Eicosane (C20) was the aliphatic hydrocarbon with the lowest mean concentration values for both cores. In general, hydrocarbons with up to 24 carbons were the compounds that showed the lowest concentrations. Total aliphatic hydrocarbons (sum of solved aliphatic hydrocarbons and UCM) ranged from 67.52 to 284.28 μg.g-1 for SM, and from 24.52 to 69.98 μg.g-1 for SA (Figure 3).

Total aliphatic concentrations of less than 10μg.g-1 in estuarine sediments are considered to be contamination-free and may reach 2 to 3 times higher values when there is significant contribution from higher plants [11] Sediments rich in organic matter may have total aliphatic hydrocarbon values close to 100μg.g-1. Concentrations above this value may indicate contamination of anthropogenic origin (usually related to petrogenic inputs), while concentrations higher than 500μg.g-1 are indications of sites with chronic oil contamination [38]

SM drill core showed concentrations greater than 100μg.g-1 from the present to 1950s, indicating anthropogenic contamination. However, older sediments represented values lower than 100μg.g-1, but higher than 50μg.g-1, indicating, therefore, sediments rich in organic matter and free of anthropogenic contamination. The concentrations obtained in this study are lower than those found by Portz [39] in surface sediments from SM area, with total aliphatic hydrocarbon concentrations reaching 1,453.44μg.g-1.

Nevertheless, this difference is justified by the relative small percentage of fine sediments in these samples, which favors the degradation and loss, mainly, of low molecular weight compounds. Brazilian coastal regions, which presented values of aliphatic concentration at levels similar to the present study, are considered as contaminated sites with strong anthropic influence, such as Santos and São Vicente Estuarine System[40] – 17 to 2,508 μg.g-1 and Guanabara Bay[41]- 25.6 to 9,184μg.g-1. On the other hand, sediments from SA were considered free of contamination, since they do not exceed 100 μg.g-1 total aliphatic hydrocarbons concentrations. Values found in the present study are similar to those of other coastal regions around the world considered free of contamination (e.g Black sea) [13, 42, 43].

By making use of geochemical indices, we could note that the first levels (0 – 30 cm) of SM and SA drill cores presented little or no odd/even predominance, with values close to 1, indicating the contribution of petrogenetic sources; the same results were found in previous studies[17, 39]. Sediments older than 60 years – 1950s or before that – presented predominance of odd-numbered carbon chains occurs, mainly the hydrocarbons C27, C29 and C31. The presence of these long chain homologs is characteristic of vascular terrestrial plants [38] The Resolved/MCNR index estimates relative degradation of resolved compounds, as these are more easily degraded than MCNR [19] The values found for this ratio, in the present study, were all lower than 1, indicating that the hydrocarbons were submit to active degradation for both (SM and SA).

The distribution of individual compounds from SM sediments showed the predominance of PAH of 5-6 aromatic rings: indene (1, 2, 3-cd) pyrene, dibenzo (a,h) anthracene and benzo (g,h,i) perylene. In the case of benzo (g,h,i) perylene, a concentration greater than 500 ng.g-1 was observed, which may be related to the lower solubility in water of this compound in relation to the others. The sediment samples showed a sum of PAHs ranging from 29.44 to 780.05ng.g-1 (Figure 4).

The highest concentrations were found in the first three levels (9 cm deep), indicating a significant influence of industrial activities from the past 20 years, in SM area. According to Seyffert43 sediments with PAH concentrations greater than 500ng.g-1 are considered contaminated, values between 250 and 500 ng.g-1 indicate moderate contamination. Some studies carried out [37, 42, 44] before showed that in superficial sediments indicated PAH concentrations around 960 ng.g-1, comparable to those of the present study, considering that degradation processes should be noticed.

For SA drill core, the highest values were found for phenanthrene, anthracene, fluoranthene and pyrene, with emphasis on fluoranthene, which presents values approximately twice as high as the others. The distribution of individual compounds showed the predominance of original PAH of 4 – 6 rings. Medeiros et al. [17] noticed this same predominance in their studies for the superficial sediments from Arraial Bay in 1997. For the first 20 cm the values found were similar, with an average around 50 ng.g-1 decreasing with deep. Nevertheless, total PAHs concentrations of 250 ng.g-1or less indicate sites free of contamination [44], as was observed for all levels of the SA drill core.

The little concentration of low molecular weight PAH (2-3 rings) is observed throughout both sample locations, probably because of the degradation processes that these compounds are subjected to along with the greater instability of these compounds. Light aromatic compounds when introduced into the aquatic environment begin to undergo various degradation processes, significantly decreasing their concentrations to reach the sediment [20]. The study of different PAH species is an important tool for distinguishing sources of hydrocarbons. Most of the samples presented predominance of high molecular weight PAH, characterizing a typical profile of pyrolytic origin [38], being commonly found in the particulate material of industrialized regions. Its entry into the aquatic ecosystem occurs preferentially by atmospheric dry deposition. The diagnostic reasons used (Fenantrene/Anthracene, Fluorantene/Pyrene, Indene (1,2,3-cd) pyrene /(Indene (1,2,3-cd) pyrene + Benzo (g,h,i) perylene), Fluorantene / Pyrene) showed a mixture of petrogenic and pyrolytic sources for SM drill core, and a predominance of pyrolytic sources for all sediments from SA drill core.

Conclusion

The presence of a refinery and oil transport terminals on the banks of Mangueira Bay, besides vehicle traffic and industrial inputs, corroborate the high contamination of SM sediments in the last 60 years. The values obtained for the sediments from Arraial Bay pointed to this site as being free of anthropogenic contamination. The presence of some PAH of higher molecular weight evidenced the contribution of pyrolytic sources (e.g.: burning fossil fuels from cars and trucks) in the last 20 years, due to the industrial and urban activities developed in the estuarine region of Patos Lagoon.

It should also be noted that due to degradation processes, organic compounds concentrations definitely decreases with depth, and decreasing profile of hydrocarbon concentrations is due not only to lower anthropogenic or biogenic contribution, but also to physicochemical processes (e.g.: organic matter degradation, diagnostic process) occurring in the sediments over time. Increases in hydrocarbon contents in deep areas of drill cores should be regarded as a major source of these compounds at respective age.

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