Concentration of polycyclic aromatic hydrocarbons (PAHs) in sediments from the Tampamachoco lagoon, Tuxpan River mouth, Gulf of Mexico

Polycyclic aromatic hydrocarbons (PAHs) are organic chemicals that consist of carcinogenic and mutagenic properties, which are considered as high-priority persistent organic pollutants (POPs). Their source is due to an incomplete burning of fossil fuels and organic materials. In the present study, the distribution and concentrations of 16 priority PAHs in sediments of the Tampamachoco lagoon and Tuxpan River estuary at the Gulf of Mexico were analyzed. The surface sediment samples were collected during the dry (March–June) and rainy (August–November) seasons. PAHs were analyzed by means of a gas capillary chromatography-FID. In dry season, the average concentration of PAHs in sediments was 860 ± 390 ng/g, and in rainy season, it was 1140 ± 450 ng/g. During both seasons, the total concentration of chrysene was high and a domain of the compounds with 4 benzene rings was observed. The source of PAHs was investigated in 78.6% of the lagoon sites. 45.5% revealed indicators of a mixed origin between pyrolytic and petrogenic sources, and 36.4% was petrogenic, and the remaining 18.2% was pyrolytic PAHs.


Introduction
Polycyclic aromatic hydrocarbons (PAHs) are considered as a major concern regarding public health and environmental impact, because of their mutagenic and carcinogenic properties (Guinan et al. 2001;Ramos et al. 1989).They are present in air, land, and water, through natural and anthropogenic sources.However, the contribution from natural sources is minimal compared to emissions caused by human activities (Abdel-Shafy and Mansour 2016; Pichler et al. 2021).Its origin is mainly attributed to the incomplete combustion processes of oil spills, organic matter, and fossil fuels.It is also recognized that most of the anthropogenic activities are the important sources for the release of PAHs into the coastal environment, i.e., municipal sewages, industrial discharges, and vehicular emissions (Mastandrea et al. 2005;Nadal et al. 2011;Amador-Muñoz et al. 2020;Ambade et al. 2022;Ghorbanzadeh et al. 2022).The compounds originated by these anthropogenic activities are introduced into the marine and coastal environments through atmospheric transport and by rivers run-off (Gambaro et al. 2009;Patel et al. 2020).Most of the pollutants have high potential bioaccumulation, high persistence, and carcinogenicity properties (Ponce-Vélez et al. 2006).In addition, physicochemical properties such as the octanol-water partition coefficient (log K ow naphthalene 3.3-benzo[g,h,i]perylene 6.6) means that PAHs have an intermediate to high environmental persistence; hence, they are considered as persistent organic pollutants (Zhang et al. 2015;Skic et al. 2023).
In the study area southern Gulf of Mexico, anthropogenic activities are common (Armstrong-Altrin et al. 2018, 2019, 2021a, b, 2022;Ramos-Vázquez and Armstrong-Altrin 2021;Ramos-Vázquez et al. 2018, 2022;Flores-Ocampo Responsible Editor: Domenico M. Doronzo 556 Page 2 of 11 and Armstrong-Altrin 2023).The Tampamachoco lagoon is an important area for fishing, largely occupied by the local habitats.On the other hand, Tuxpan city is located on the banks of the Tuxpan river, which consists of several petroleum industries (Flores-Cortés and Armstrong-Altrin 2022).The port lies on the northern bank of the river.The anthropogenic activities in Tuxpan liberate refuses and pollutants that impact the fisheries and the biodiversity of the lagoon (Chu et al. 2002;Arias et al. 2010;Botello et al. 2016;Ayala-Pérez et al. 2021).
The quantification of PAHs in the Tampamachoco lagoon sediments is analyzed in the dry and rainy seasons.Potential sources of these compounds are port activity, electricity generation, gasoline, and oil reception and storage, as well as the discharge of industrial and municipal wastes into the Tuxpan River.In addition, the natural processes that define the condition of dispersion and presence of PAHs in this region are linked to hydrological conditions, including the amount of water discharge by the Tuxpan River, wind direction, tidal cycle, coastal currents, and waves.This study aims to analyze the behavior of PAHs in sediments of the Tampamachoco lagoon and at the Tuxpan River estuary, Gulf of Mexico.

Study area
The estuary-lagoon system of Tuxpan-Tampamachoco is located in the northern part of the Veracruz State, northeast of Tuxpan town in the northern Gulf of Mexico (21°00′18.10″N and 97°21′17.65″W; Fig. 1).The estimated lagoon area is ~ 57.5 km 2 , with an approximate length of 10 km and 3 km in its widest part.The lagoon lacks notable bathymetric features, except for the channel that runs in a north-south direction and that continues until its communication with the Tuxpan estuary, with an average depth of 2.5 m.In the northern part of the lagoon, there are two channels, new and old, with approximately 4 m depth that link to the north with the Galindo barrier (Fig. 1).
The lagoon is surrounded by mangrove trees with a great contribution of organic materials into the lagoon that supports important fisheries.The hydrology regime is controlled by the climatic variations in the region.Generally, the study area presents two main seasons: dry from March to June and rainy from August to November (Botello et al. 2001(Botello et al. , 2016)).

Sampling and analytical methods
Samples were collected in the study area during March (dry season) and August 2018 (rainy season).The sediment samples were collected by means of a van Veen dredge.The samples were selected in the first 10 cm of the sedimentary column, and with the help of a spatula, a portion of the sample was taken and placed in a glass jar, previously washed and treated with pure acetone.The samples were kept frozen until the moment of analysis.Sampling stations M1-A1, M2-A2, M3-A3, and M4-A4 are located inside the lagoon, and in the lower course of the Tuxpan River, three stations are located.Samples M5 and A5 are collected at the mouth of the lagoon.Tumilco (M6 and A6) and dock areas (M7 and A7) are located near to the estuary shore (Fig. 1).Sample numbers M1, M2, M3, M4, M5, M6, and M7 represent March sampling, and A1, A2, A3, A4, A5, A6, and A7 represent August sampling.All sampling stations are subjected to various anthropogenic activities as listed in Table 1.

Organic matter (OM)
The organic matter (OM) contents in the lagoon sediments were determined by the oxidation-reduction technique with K 2 Cr 2 O 7 and Fe(NH 4 )(SO 4 ) 2 •6H 2 O (Loring and Rantala 1992;Ortíz et al. 1993).The precision and accuracy of the method were evaluated with replicates of C 6 H 8 O 6 resulting in an experimental value of 39.91%, which is lower than the expected value of 39.99% for the dextrose standard (CV 0.30%).

Particle size
Sand, silt, and clay ratios were analyzed by the Beckman Coulter model LS230 laser analyzer.The sediments were treated with hexametaphosphate as a solvent for homogenization.A variability of less than 1% was observed (Solleiro-Rebolledo et al. 2011).

PAHs
PAHs in sediments were analyzed by the USEPA ( 2007) methodology.The sediments were dried at 50 °C temperature, then homogenized in a porcelain mortar and sieved through a 250-μm mesh.Three grams of dried sediments were powdered and sieved, then placed on a Whatman No. 2 filter paper disc.Extraction was with 25 mL of hexane:acetone (50:50 v/v) mixture a microwaveassisted procedure (MAE).Centrifuged the samples for 5 min at 1500-2000 rpm to remove any sedimentary particles present in the extract.Concentrate the extract in a roto evaporator up to about 5-8 mL (the bath temperature should not exceed 30 °C, and the vacuum pump pressure was maintained at 40 psi).
The qualitative and quantitative analyses were carried out with an Agilent 5890 Series II gas chromatograph equipped with a flame ionization detector (FID).The external standard method was used with 8-level calibration curve of concentration (10,20,30,40,50,60,80,

Results and discussion
Table 2 lists the percentage of organic matter and type of sediments in the estuarine-lagoon system of the Tampamachoco-Tuxpan, Veracruz, Mexico.The textural characteristics of sediments are important, because they accumulate organic pollutants such as PAHs in the aquatic ecosystems.
The sediments are fine-grained, varying from silt-clayey to sandy-clayey with a homogeneity in the organic material content.
The average concentration of PAHs in sediments during the dry season (March) is 860 ± 390 ng/g (~ 50-3080 ng/g).However, the average concentration of PAHs during the rainy season (August) is higher than in the dry season (1140 ± 450 ng/g), which ranges from 160 to 3670 ng/g.Table 3 lists the results of the 16 PAHs analyzed in March and August, as well as the international sediment quality criteria values established for the coastal environment (Long et al. 1995;Buchman 2008).By comparing the average concentration of this study with TEL (Threshold Effects Level), the level of toxicity by PAH compounds in sediments is inferred.The reference concentrations most commonly used to approximate the probability of ecotoxicological risk in sediments are ERL (effect range low) and ERM (effect range median) (Buchman 2008).In general, it is observed that toxic effects in organisms occur rarely when < ERL, occasionally when ERL = ERM, and frequently if > ERM (Long et al. 1995;Buchman 2008).A total of 16 compounds are detected in sediments throughout the annual cycle, among them 9 are in the dry season and 14 in the rainy season.In dry season, the PAHs with the highest level are chrysene and benzo(a)anthracene, the rest of the compounds are detected in 3, and in few stations, they are below in detection limit (Table 3).The total concentration of PAHs (∑PAHs) in station M6 is 3080 ng/g, which is the highest among other stations.The hydrocarbon with highest level is chrysene (3050 ng/g), which is slightly higher than the compound benzo(a)anthracene and also exceeds the average ERL and ERM values (Long et al. 1995;Table 3;Fig. 2).
In rainy season, the level of PAH compounds increased significantly, as listed in Table 3.The highest ∑PAHs were recorded in station A7 (3670 ng/g), which exceeds the threshold concentration (1684 ng/g; Fig. 2), and chrysene is statistically different from the rest of the compounds, with a single extreme value of 3270 ng/g (Fig. 3).It is observed that in some stations, chrysene, benzo(a)anthracene, dibenzo(a,h) anthracene, phenanthrene, fluorene, acenaphthylene, and acenaphthene are superior to one or more of the established sediment quality criteria (Fig. 3; Table 3).Based on the number of benzene rings of the PAHs, it is inferred that during both seasons, the dominant compound aromatic rings reported are in the following order: 4 > 5 > 6 > 3 > 2. In both seasons, compounds of 4 aromatic rings are with variable concentrations and in greater quantity than the rest of the compounds (Fig. 4).
The source indices of PAHs recorded in the analyzed sediments (78.6%) are calculated.
Numerous studies reported that the PAH compounds formed by 4 and 5 aromatic rings in lagoon-estuarine sediments are similar to that of atmospheric particles,

Petroleum
which derived largely from the urban areas and reach the marine aquatic environment by wind, and subsequent dragging by surface water channels and industrial effluents (Wu et al. 2003;Gambaro et al. 2009;Amador-Muñoz et al. 2020;Patel et al. 2020).
Comparatively, the average concentration of PAHs quantified in the present study was lower than that registered by Botello and Calva (1998) for the Tampamachoco lagoon and Tuxpan river.These authors reported an average concentration of 4480 ± 1840 ng/g, with a concentration maximum of 9390 ng/g in the rainy season, as well as the diversity of compounds is lower than in the current study (Table 5).This difference is probably due to the dredging activities along the channel region, which might have caused the removal of pollutants in the outer portion of the lagoon sediments.The comparison of PAHs of this study with other studies reveals that the values of this study are higher than others (Table 5).
The results of this study reveal that the highest concentration of PAHs is recorded in sampling stations at Tuxpan River estuary, in both seasons (M5, M6, M7, A5, A6, and A7).In the Tampamachoco lagoon, the PAH value is homogenous throughout the year; however, it is higher during the rainy season.In general, the mixed origin is dominant (petrogenic and pyrolytic).However, during the rainy season, PAH values increased due to the combustion of fossil fuels mainly from aquatic vehicles or from emissions from industrial processes.
In most of the sampling stations, the total concentration of PAHs detected is below the threshold concentration, may cause adverse effects to the benthic community, except for the stations located in the Tuxpan riverbed.However, the values of various compounds such as chrysene, benzo(a)anthracene, and dibenzo(a,h) anthracene are higher than the international sedimentary quality criteria, which reveals an environmental risk for the benthos and for the species that consume sediments or absorb water contaminated with these compounds.

Conclusions
The results of this study reveal that in the Tampamachoco lagoon, most of the PAH compounds are high in the areas where the velocity of water is low.During rainy season, the mixing of water to the lagoon system is greater, which causes dragging and increase the velocity of mixing of pollutants in sediments.In the dry season, the volume of water and the flow velocity decrease; therefore, the concentration of compounds in sediments is lower and it is expected much lower concentrations of PAHs in the dissolved phase of the water.It is also observed that when current speed decreases, the pollutants dispersed in the water column will settle slowly, and therefore, the bioavailability of pollutants for fish and other aquatic species is lower in the dry season.
The concentrations of chrysene, benzo(a)anthracene, and dibenzo(a,h)anthracene are higher than the international sedimentary quality criteria, which could represent an environmental risk for the benthos and for the species that consume sediments and water contaminated with these compounds.Hence, it is necessary to establish a continuous surveillance of PAHs in sediments to prevent the benthic organisms that inhabit in the aquatic system.However, the level of PAHs determined in this study is lower than the other aquatic systems of Mexico.

Fig. 2 Fig. 3
Fig.2∑PAHs in surface sediments collected in March and August 2018.Sediment quality criteria's like TEL (threshold effect level), ERL (effect range low), and ERM (effect range media) are also included for comparison(Buchman 2008)

Fig. 4
Fig. 4 Average concentrations of aromatic rings PAHs in surface sediments collected in March and August 2018

Fig
Fig. 5 PAH ratios to identify dominant origin in sediments of the Tampamachoco lagoon and Tuxpan River estuary, Veracruz, Gulf of Mexico

Table 2
Mexico, collected in March and August 2018.Sediment quality criteria TEL, ERL, and ERM values are ng/g dry weight ND = < 10 ng/g.Adverse biological effects: TEL = threshold effect low; ERL = effect range low; ERM = effect range median *Not reported Values marked in bold letters are higher than in the average TEL and ERL values No.Of rings Table 4 lists the result of the seven calculated indicators and are compared with suitable compounds reported by various authors (Sicre

Table 5
Concentrations of PAHs reported in sediments of aquatic ecosystems in Mexico and other regions of the world