An insightful overview of the distribution pattern of polycyclic aromatic hydrocarbon in the marine sediments of the Red Sea

1 Introduction

Humans have attempted to halt or alter the dynamic coastal zone throughout history. The creation or stabilization of inlets, beach nourishment and sediment bypassing, creation of dunes for property protection, dredging of waterways for shipping and commerce, and the introduction of hard structures, such as jetties, groins, and seawalls, are examples of anthropogenic (human-influenced) changes to coastal environments. These changes alter coastal characteristics and have far-reaching consequences for coastal processes and ecosystems. For the protection and preservation of coastal areas, it is critical to understand how human changes affect shoreline habitats. Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental pollutants with moderate-to-poor water solubility, which promotes sorption to particles and subsequent sediment buildup [1]. Particulate matter deposition in the atmosphere, runoff from contaminated ground sources, and contamination of rivers and lakes by industrial effluents, municipal wastewater discharge, and oil spills are the primary sources of PAHs in water bodies [2,3]. Furthermore, PAHs are produced during the combustion of coal and oil, which can contribute to sediment contamination, particularly in the vicinity of manufactured gas plant sites. Marine sediments are a valuable source of information on human activities in coastal areas as well as the long-term destiny of xenobiotics. Monitoring of contaminants in sediments, even if no applicable legislation exist, can provide information for analyzing the potential adverse effects of these compounds as well as supporting management decision-making [4]. Because of the large number of toxic substances transported from human activities, marine pollution in coastal areas is a global concern [5,6]. Traditional coastal industrialization and urbanization have resulted in the development of large metropolises, harbors, and industrial and shing bases, resulting in the deterioration of neighboring marine environments. PAHs are a major environmental concern for marine ecosystems because of their negative health effects on organisms, including endocrine-disrupting activity [7,8,9]. PAHs enter the marine environment through processes, such as organic matter combustion (pyrolytic origin), the slow geothermal transformation of organic matter (petroleum hydrocarbons), and degradation of biogenic material (diagenesis). The naturally formed PAHs are biosynthesis products or oil upwelling products that occur in marine sediments at very low levels ranging from 0.01 to 1 ng g⁻¹ dry weight (background concentrations). Anthropogenic activities, on the other hand, are sources of a number of PAHs in the aqueous environment, with the highest levels found in estuaries and coastal areas, as well as areas with heavy vessel transport and oil treatment. Physical transport and mechanical considerations have a major role in the dispersion of PAHs in sediments once they are introduced into the marine environment. Furthermore, in situ factors, such as PAH fractionation, amongst sorbed and aqueous phases, infaunal remodeling (bioturbation), and specific microbial degradation may affect both the observed PAH composition and absolute PAH levels [10,11].

The Kingdom of Saudi Arabia produces 12,402,761 barrels of oil per day (as of 2016), placing it second in the world. Saudi Arabia produces an amount equivalent to 1.7% of its total proven reserves [12,13] each year (as of 2016). The most obvious and visible source of oil pollution in the marine environment is accidental or deliberate discharges and spills of oil from ships, particularly tankers, offshore platforms, and pipelines. The Red Sea coast of Saudi Arabia stretches for 2,000 km. A steep western escarpment rises to 1,500–3,000 m at the edge of a plateau that gradually declines eastward, flattening to a broad coastal plain. In July, the Red Sea’s surface temperatures range from 31°C in the south to 27°C in the north, and between 24 and 26°C in January, with temperatures dropping to 20°C in the Gulf of Aqaba. The salinity is high (37–40 ppt), and inflow from the Indian Ocean via the Bab el Mandeb compensates for about 2 m per year of evaporation from the sea surface. The warm saline water, which is clear due to the lack of river inflow, allows coral to grow anywhere in the Red Sea, and coral reefs are common, with fringing reefs along much of the Saudi coast, usually with shallow lagoons behind them [14]. The present review study discussed the distribution pattern of PAHs along the Red Sea coast of Saudi Arabia, its general characteristics, and the comparison with the other locations around the world. The study will reflect a baseline idea about the distribution pattern along these regions including the local habitats like estuaries.

2 General characteristics of PAHs in the sediment

PAHs derived from combustion sources bind tightly to soot particles in natural sediments in general. They primarily accumulate in organism lipids due to their high lipophilicity. Some PAHs and petroleum hydrocarbons in seawater and soil have been reported to damage aquatic life even in low concentrations [15]. PAHs are a family of compounds that may have an influence on animals and people via the food chain due to their lipophilicity, poor water solubility, and adsorption to marine particles and sediments [16]. PAHs are deposited in the sedimentary environment via processes similar to those that govern surface soil deposition. PAHs sorb to atmospheric particles in rural areas and can settle on the surface of lakes, streams, and oceans via dry or wet deposition. Particulates are transported in suspension and on the surface of surface water when they fall out. PAH that has been adsorbed eventually ends up in fresh water or marine sediments. PAH is strongly bound to these sediments, which could serve as a pollution reservoir for PAH release under certain conditions. As a result, organisms that live in sediments and filter water are the most vulnerable to contamination [17]. PAHs are somewhat immobile once they have been incorporated into sediments because their non-polar structures prevent them from dissolving in water. Nonetheless, PAHs, particularly lower molecular weight PAHs, are not completely insoluble. As a result, small amounts of PAHs dissolve and become part of the pore water, where they are bioavailable [17].

Because PAHs will sorbet onto these organic colloids, the presence of pore water organic colloids can raise PAH concentrations above their aqueous solubility. These, in turn, are easily transported through the sediment’s pore spaces. As a result, sorption to colloids can increase PAH mobility and bioavailability in sediments [18]. PAHs in aquatic sediments decay slowly in the absence of penetrating radiation and oxygen and may survive permanently in oxygen-depleted basins or anoxic deposits. In aquatic habitats, PAH degradation is slower than in the atmosphere, and PAH cycling in aquatic environments, like other biological systems, is poorly understood [19]. PAHs are organic contaminants found in aquatic sediment that form when virtually all organic matter is pyrolyzed. Because of their acute toxicity and sublethal impacts on aquatic creatures, as well as their mutagenic and genotoxic potential through the food chain, researchers are interested in learning more about the composition and distribution of PAHs in the environment [20,21,22]. PAHs formed from pyrolytic activities are more firmly connected with sediments and withstand microbial degradation significantly better than PAHs derived from petrogenic mechanisms [23].

3 The transport of PAHs into the marine environment

Anthropogenic PAHs reach the ocean through a variety of routes, including air deposition, river runoff, household and industrial emissions, and oil spills. Natural input sources are often less-expensive than anthropogenic sources [24]. PAH concentrations and distribution within a country’s marine environment must be investigated to evaluate likely consequences on related ecosystems due to their environmental durability and possible ecotoxicological effects [25,26]. PAH bioavailability and type are determined by the origin of PAH chemicals detected in marine sediments [27], with PAHs classed as either petrogenic or pyrogenic [28]. Oil spills, urban runoff, and atmospheric deposition are the principal components of PAHs in coastal marine habitats [29]. Organic matter content and particle size influence PAH levels in sediments, affecting the sorption of these pollutants onto the solid phase [30,31,32]. According to ref. [33], a sediment organic carbon concentration of more than 1/10th percent is satisfactory to considerably enrich PAH sorption onto marine sediments. The kind of organic materials present in the sediment may also impact PAH concentrations [25]. Seasonal climate factors, such as wind direction, as well as anthropogenic factors with seasonal variability hot seasons and the intensity of seawater and shipping activities can affect PAH content and composition in the coastal sediment environment [34]. Two major physiochemical properties of PAHs determine their fate in marine sediments: solubility and vapor pressure [35]. Temperature, UV radiation, organic carbon content, and sediment dynamics are some of the geochemical and environmental elements that determine PAHs’ final destiny in sediments and near-bottom water (e.g., resuspension, grain size, and bioturbation). Temperature and salinity are two of the most essential elements that influence PAH dissolution, adsorption, and transport in coastal waters.

4 Distribution pattern of PAHs along the red sea sediment of Saudi Arabia

The Red Sea city of Jeddah is Saudi Arabia’s most industrialized city [36], and much of the PAHs research has taken conducted near the Jeddah shoreline and in the neighboring areas. In this location, oil contamination is a major cause of pollution. Approximately 10.8 million barrels of crude oil were purposely spilled into the Arabian Gulf during the 1991 Gulf War [37]. In the Jeddah coastal region sediments, a little quantity of information regarding the various PAHs has been documented. The current review study will be extremely important and beneficial as a desk review as a result of the growing human population and rapid development in the area. Al-Mur [36], conducted a study along the Jeddah coast and found a total level of 1,170–3,003 ng g⁻¹ dry wt for 16 PAHs. The study by [38] conducted a similar study in the same location and found a lower concentration of 40–1,730 ng g⁻¹ dry weight. Furthermore, their concentrations are four to six times higher than those found in the Gulf of Suez, the western side (Egyptian) of the Red Sea coast (0.75–457 ng g⁻¹ dry weight) [38], and the Gulf of Aden was 2.5–605 ng g⁻¹ dry wt [21,35].

There are few studies were done along the Jeddah coastal lagoon as well. The levels of numerous contaminants in the lagoon water and sediments of the Al-Arbaeen and Al-Shabab lagoons in southern Jeddah have increased [40,41]. PAHs were found in very high concentrations in the sediments of these lagoons (5.4–5,372 and 60–7,927 ng g⁻¹, respectively) [38], while moderate values (97.1–507.3 and 97.3–314 ng L⁻¹, respectively) were found in the water column [42,43]. Low to extremely polluted levels of pollution have been found in both Jeddah coast lagoons. When considering total PAH levels between the two lagoons, Al-Arbaeen is somewhat higher than Al-Shabab for both 38 and 16 PAHs; carcinogenic PAHs (cPAHs) were estimated to be two orders of magnitude higher in Al-Arbaeen than in Al-Shabab. The difference in wastewater input and the geographical structure of the Al-Arbeen lagoon, which hampered water circulation inside the lagoon, might explain the comparatively high PAH levels in Al-Arbeen. The study was conducted by Ruiz-Compean et al., [44], along the Red Sea coastal sediments (Duba, Rabigh, Thuwal, Jeddah, Farasan Island, Jazan Economic City, Jazan city, and Farasan Islands) showing that 1-methyl naphthalene, 2-methyl naphthalene, acenaphthene, acenaphthylene, anthracene, and benzo[a]anthracene were not detected in any of the samples collected. In general, all the concentrations were very low. None of the analytes presented concentrations above 10 ng g⁻¹ suggesting minor contamination for PAH in the study areas. Most of the studies along the Red Sea coast of Saudi Arabia show the pyrogenic and petrogenic sources of PAHs and it is shown in the other parts of the world as well. Industrial activities, municipal waste run off, petroleum spilling, and sewage run off were also highly impacted on the distribution of PAHs along the coastal estuaries of Red Sea [38]. Industrial and sewage wastewater from Jeddah have an impact on PAH concentrations, either directly or indirectly. Furthermore, for most stations, industrial and sewage wastewater discharge, as well as urban emissions from the surrounding area, could be a major source of PAHs.

5 The comparative study of the PAHs between Red Sea sediment and worldwide

Few investigations on Red Sea sediments have been conducted, and sedimentology and geochemical studies along the Red Sea coast have problems. To our knowledge, only a modest quantity of data on specific PAHs has been documented in Saudi Arabia’s Red Sea coastal region sediments. The inclusive PAH values in the Red Sea coastal sediment of Saudi Arabia were lower than the PAH levels in Guanabara Bay, Brazil (78–7,750 ng g⁻¹ dry wt) [45]; in Vhembe District, South Africa (27,101–55,931 ng g⁻¹ dry wt) [46]; in the Mediterranean Sea (13.6–22,601 ng g⁻¹ dry wt) [47]; and in Boston Harbor, USA (7,301–358,001 ng g⁻¹ dry wt) [48]. The total PAHs’ concentration in sediments from Qatar’s exclusive economic zone in the Arabian Gulf ranged from 2.5 to 1,026 ng g⁻¹ [49]. The level of PAHs along the Gulf countries is shown in Table 1.

The current study’s PAH contents are similar to those discovered in Iranian and Qatari coastal sediments but lower than those observed in Bahrain and Kuwait [49,58]. PAH concentrations in Gulf sediments were usually elevated before and soon after the Gulf war (1991), reaching 1,405 μg g⁻¹ dw in sediments off the coast of Saudi Arabia, but have fallen dramatically in the subsequent 20 years period of time. The values in the current research are much lower than those documented in other marginal seas, such as the Mediterranean Sea (1,115–17,005 ng⁻¹ dw) [59]. The carcinogenic PAHs (PAH_CARC) concentrations along the Egyptian Red Sea coast were found to be between 0.065 and 3.523 µg g⁻¹, according to the study (dry weight). The samples from Ras Suder (5.185 µg g⁻¹), El-Tour (4.055 g g⁻¹), and Sharm (3.368 µg g⁻¹) had the highest PAHs’ content [60]. The total concentration of PAHs in sediment along the Red Sea coast of Yemen samples ranged from 33.8 µg g⁻¹ at Rass-Issa to 5.25 µg g⁻¹ dry wt at Al-Mukha [61]. A comparative study of PAHs along the estuarine coastal system other than Red Sea and Arabian Gulf is shown in Table 2.

In previous studies, high PAH concentrations, largely from human activities, were discovered in Brazilian estuary environments [74,75]. However, a lack of rigorous examination of PAH distribution and behavior in various compartments of subtropical estuarine systems, as well as an integrated interpretation of PAH content and water column physico-chemical properties, limits our current understanding of these contaminants. The PAH concentrations ranged from low to high, with the lowest values being commensurate with other human-affected settings, such as various Chinese locales [76,77]. Luo et al. [76] discovered the greatest PAH concentrations in the SPM in the Pearl River (China), Dauner et al. [78] in Guaratuba Bay (Brazil), and Maioli et al. [79] in the Manguaba and Munda lagoons (NE Brazil) and the Paraba do Sul River (Brazil). Guo et al. [77] reported lower quantities in the Daliao River (China) while Qin et al. [80] discovered lower values in Chaohu Lake (China). The amount of PAHs recorded in the Middle East area is greater than in eastern Asia and the Americas.

6 Conclusion

The principal PAH pollutants differ in different functional sectors, according to an examination of the features of PAH pollution in distinct coastal locations. In locations with a lot of anthropogenic activity, pollution is frequent, and PAH contamination is the most common. Fuel leaks and other causes in oil and gas development sites and commercial ports contribute to a high concentration of PAHs. The PAH mixture formed by pyrogenic origin is categorized by unsubstituted and high molecular weight congeners, whereas the PAH mixture produced by petrogenic origin is characterized by alkyl-substituted low molecular weight congeners. Differentiating between pyrolytic and petrogenic sources of PAH mixture in marine sediments is particularly difficult due to the complexity of controlling PAH dispersion in marine sediments. The majority of research found pyrogenic and petrogenic sources of PAHs along Saudi Arabia’s Red Sea coast, as well as elsewhere in the world. The quality guidelines reflect the lesser impacts of PAHs in the sediment compared to the Arabian Gulf region. However, the bioaccumulation and the persistence in the sediment are significantly higher than the few of the European and American countries.