Polycyclic Aromatic Hydrocarbons (PAHs) are a complex group of organic compounds, each composed of two or more fused benzene rings arranged in various structural configurations.^{1, 2} These compounds are ubiquitous in the environment and have garnered significant scientific and regulatory attention due to their persistence, potential for bioaccumulation, and, most notably, their toxicological properties.^{1, 2, 3, 4, 5, 6, 7, 8, 9} Many PAHs are recognized as carcinogenic (cancer-causing), mutagenic (causing genetic mutations), and teratogenic (causing developmental abnormalities).^{2, 4, 7, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22}

PAHs are introduced into the marine environment through a wide array of pathways, which can be broadly classified into natural and anthropogenic (human-derived) sources. Natural sources include geological processes such as natural oil seeps, volcanic eruptions, and the products of natural combustion events like forest and grassland fires.^{1, 2, 3, 4, 7, 8, 10, 16, 23, 24, 25, 26, 27, 28, 29, 30} Anthropogenic activities, however, are typically the dominant contributors to PAH loading in coastal and marine systems, especially in industrialized and urbanized regions. These activities include accidental oil spills during extraction and transportation, operational discharges from shipping, incomplete combustion of fossil fuels (coal, oil, and gas) by vehicles, power plants, and industrial processes, emissions from waste incineration, and runoff from paved surfaces and industrial sites.^{1, 2, 3, 4, 6, 7, 8, 9, 10, 13, 16, 17, 23, 24, 25, 26, 27, 28, 29, 30, 31a/31b, 32, 33} Atmospheric deposition of PAHs adsorbed to particulate matter also serves as a significant diffuse source to marine waters.^{1, 2, 3, 4, 26}

The dual origin of PAHs, being both naturally occurring substances and significant anthropogenic pollutants, presents a challenge for environmental management. While PAHs are natural components of fossil fuels like coal and oil ^{1, 2, 3, 4, 16}, human activities have drastically increased their concentrations and altered their distribution patterns in the environment. Regulatory frameworks and environmental assessments must therefore differentiate between natural background levels and anthropogenic contributions that pose an unacceptable risk to ecosystem health and human well-being. Understanding the intrinsic properties of individual PAH compounds, such as their toxicity, persistence, bioaccumulation potential, and bioavailability, is paramount for prioritizing regulatory actions and remediation efforts, focusing on those PAHs that are either predominantly anthropogenic in their elevated environmental concentrations or pose the most significant ecological or health risks.

PAHs are characterized by their hydrophobicity (low solubility in water) and lipophilicity (high affinity for lipids and organic matter).^{1, 2, 4, 7, 8, 10, 18, 27, 34} Consequently, upon entering the aquatic environment, they tend to rapidly adsorb onto suspended particulate matter. These PAH-laden particles eventually settle, leading to the accumulation of PAHs in bottom sediments.^{1, 2, 3, 4, 6, 7, 8, 10, 13, 23, 25, 27, 31a/31b, 32, 34, 35} Marine sediments thus function as significant long-term reservoirs or sinks for these persistent organic pollutants, reflecting both current and historical inputs.^{1, 2, 4, 6, 7, 32, 36}

The analysis of PAH concentrations in sediments is a cornerstone of marine environmental monitoring and assessment programs worldwide, including OSPAR's Coordinated Environmental Monitoring Programme (CEMP).^{1, 2, 4, 5, 6, 37a/37b, 38, 39} Such analyses are indispensable for evaluating the health of marine ecosystems, managing natural resources effectively, assessing the potential ecological risks posed by contaminants, and determining compliance with established environmental quality standards.^{1, 2, 3, 4, 5, 6, 8, 10, 11, 13, 19, 25, 26, 27, 31a/31b, 32, 36, 37a/37b, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49} Because PAHs can persist in sediments for extended periods, these matrices provide an integrated measure of contamination over time, offering insights into both recent pollution events and long-term contamination trends.^{36, 48}

The accumulation of PAHs in sediments creates a direct exposure pathway for benthic organisms, which live in or on the sediment layer. These organisms can take up PAHs through direct contact, ingestion of sediment particles, or from porewater.^{4, 31a/31b, 32} This exposure can lead to adverse biological effects in benthic fauna and facilitate the transfer of PAHs into the broader marine food web, potentially impacting higher trophic levels, including fish, marine mammals, and humans who consume seafood. Therefore, sediment PAH concentrations are a critical indicator of the overall health of the marine environment and the potential risks to its inhabitants.

In the United Kingdom, the Marine Management Organisation (MMO) plays a key role in regulating activities in marine waters. As part of its responsibilities, particularly concerning marine licensing for activities such as the disposal of dredged material, the MMO mandates the chemical characterization of sediments. This includes the analysis of a standard suite of 22 specific Polycyclic Aromatic Hydrocarbons.⁴¹ This list is comprehensive, encompassing not only parent (non-alkylated) PAHs but also several groups of their alkylated homologues, namely C1-Naphthalenes, C2-Naphthalenes, C3-Naphthalenes, and C1-Phenanthrenes.⁴¹

The selection of these 22 determinands is designed to provide a robust assessment of PAH contamination, enabling the identification of potential sources and the evaluation of environmental risk. The inclusion of alkylated PAHs is particularly significant from a diagnostic and risk assessment perspective. Alkylated PAHs, such as methylnaphthalenes, dimethylnaphthalenes, and methylphenanthrenes, are often more prevalent in crude oils and other petrogenic sources than their unsubstituted parent compounds.⁹ Furthermore, some alkylated PAHs can exhibit greater persistence in the environment and, in certain cases, enhanced toxicity compared to their parent PAHs.^{9, 17, 28, 50, 51} The relative concentrations of alkylated PAHs to their parent compounds, and the distribution patterns of different alkyl homologue series, serve as valuable chemical fingerprints. These fingerprints help distinguish between petrogenic contamination (e.g., from oil spills or uncombusted fuel) and pyrogenic contamination (resulting from the incomplete combustion of organic materials).^{1, 2, 4, 23, 24, 26, 30} By mandating the analysis of this specific suite of 22 PAHs, the MMO ensures a more thorough understanding of the nature and origin of PAH contamination, which is crucial for informed decision-making regarding the management of marine sediments.

The primary objective of this report is to conduct an expert-level analysis of the 22 PAHs specified by the Marine Management Organisation for sediment analysis. This analysis will focus on five key environmental properties that dictate their fate, behaviour, and potential impact in the marine environment:

1. Toxicity: The inherent capacity of each PAH to cause adverse biological effects.
2. Persistence: The ability of each PAH to resist degradation in marine sediments, indicating how long it will remain in the environment.
3. Bioaccumulation Potential: The propensity of each PAH to be taken up and concentrated in the tissues of marine organisms.
4. Bioavailability: The fraction of each PAH in marine sediments that is accessible for uptake by marine organisms.
5. Natural Abundance: The extent to which each PAH occurs from natural environmental processes versus anthropogenic inputs.

For each of these five properties, this report will:

• Provide a clear definition and discuss the relevant metrics used for assessment (e.g., Toxic Equivalency Factors for toxicity, half-lives for persistence, octanol-water partition coefficients (Log K_ow) and Bioconcentration/Bioaccumulation Factors (BCF/BAF) for bioaccumulation).
• Present an ordered list of the 22 MMO PAHs, ranked in increasing order of the specific property.
• Offer detailed justifications for these rankings, drawing upon scientific literature and established environmental chemistry principles, primarily using the research material provided.
• Discuss the implications of these rankings for environmental risk assessment and sediment management.

This comprehensive analysis aims to provide a scientifically robust resource for environmental regulators, marine scientists, and consultants involved in the assessment and management of PAH-contaminated marine sediments.

The Marine Management Organisation (MMO) of the United Kingdom stipulates a standard set of 22 Polycyclic Aromatic Hydrocarbons (PAHs) that are potentially required for the chemical characterisation of sediment, particularly in the context of marine licensing applications such as those for dredged material disposal.⁴¹ The analysis of these specific compounds allows for a comprehensive assessment of PAH contamination. The definitive list, along with key structural information, is presented in Table 2.1.

Table 2.1: The Marine Management Organisation's 22 Target PAHs for Sediment Analysis.⁴¹

Notes: ^a Molecular weights are for the parent (unsubstituted) PAH or representative isomer for alkylated groups (e.g., methylnaphthalene for C1-Naphthalenes). ^b Number of aromatic rings refers to true fused benzene rings unless otherwise specified. ^c These LMW PAHs contain a five-membered ring fused to a naphthalene or biphenyl system, contributing to their three-ring classification in some contexts.^{19 d} Fluoranthene and Indeno[1,2,3-cd]pyrene contain a five-membered ring in addition to their fused benzene rings.

This list is fundamental for the subsequent analysis, as it defines the specific chemical entities whose properties will be ranked. The inclusion of molecular weight and ring number provides immediate structural context, which is crucial for understanding the general behavior and classification of these PAHs.

PAHs are commonly categorized into two main groups based on their molecular structure and weight: Low Molecular Weight (LMW) PAHs and High Molecular Weight (HMW) PAHs.^{19, 40} This classification is pivotal as it often correlates with significant differences in their physical-chemical properties, environmental fate, and toxicological profiles.

• Low Molecular Weight (LMW) PAHs: These compounds typically consist of two or three fused aromatic rings.^{19, 40} Examples from the MMO list include Naphthalene, C1-Naphthalenes, C2-Naphthalenes, C3-Naphthalenes, Acenaphthylene, Acenaphthene, Fluorene, Phenanthrene, C1-Phenanthrenes, and Anthracene. LMW PAHs are generally more water-soluble, more volatile, and more susceptible to biodegradation compared to HMW PAHs.^{1, 2, 27, 34} They are often associated with acute toxicity to aquatic organisms ^{1, 2, 13, 19, 20, 27, 34, 52} and are predominant in petrogenic (oil-derived) sources.^{1, 2, 4, 23, 24, 29, 30, 40}

• High Molecular Weight (HMW) PAHs: These compounds contain four or more fused aromatic rings.^{19, 40} From the MMO list, HMW PAHs include Fluoranthene, Pyrene, Benz[a]anthracene, Chrysene, Benzo[b]fluoranthene, Benzo[k]fluoranthene, Benzo[e]pyrene, Benzo[a]pyrene, Perylene, Indeno[1,2,3-cd]pyrene, Dibenz[a,h]anthracene, and Benzo[g,h,i]perylene. HMW PAHs are characterized by lower water solubility, lower volatility, and greater persistence in the environment.^{1, 2, 7, 8, 15, 27, 34} They tend to adsorb more strongly to sediment particles and organic matter. Many HMW PAHs are of greater concern for their carcinogenic potential.^{2, 13, 15, 16, 27, 34} Pyrogenic sources, such as the combustion of fossil fuels and biomass, are major contributors of HMW PAHs to the environment.^{1, 2, 4, 23, 24, 29, 30, 33}

This LMW versus HMW classification provides a foundational framework for anticipating trends in the environmental properties discussed in the following sections. For instance, one would generally expect HMW PAHs to be more persistent and have higher bioaccumulation potential due to their increased hydrophobicity, while LMW PAHs might exhibit greater bioavailability from the dissolved phase and higher acute toxicity.

This section presents the ordered lists of the 22 MMO PAHs based on increasing toxicity, persistence in marine sediment, bioaccumulation potential, bioavailability from marine sediment, and natural abundance. Each property is defined, the metrics for ranking are explained, and the ranking is justified using data and principles derived from the provided research material.

PAH toxicity encompasses a spectrum of adverse biological effects, including carcinogenicity (cancer-causing), mutagenicity (causing genetic mutations), teratogenicity (causing developmental malformations), and acute toxicity to aquatic organisms.^{7, 10, 11, 14, 15, 16, 17} The primary metric used for comparing the carcinogenic potency of different PAHs is the Toxic Equivalency Factor (TEF). TEFs express the toxicity of an individual PAH compound relative to that of Benzo[a]pyrene (BaP), which is a well-studied and potent carcinogen assigned a TEF of 1.0.^{14, 21, 35, 41, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67} The concentration of a PAH multiplied by its TEF yields its Benzo[a]pyrene equivalent concentration (BaP-TEQ). Different TEF schemes exist (e.g., developed by Nisbet and LaGoy, 1992; U.S. EPA), which may assign slightly different values to the same PAH. For consistency in this report, TEF values primarily based on the Nisbet and LaGoy (1992) scheme will be used where available for the MMO PAHs, as it is frequently cited for a broad range of compounds. EPA TEFs will be used supplementally.

While TEFs are invaluable for assessing carcinogenic risk, it is important to note that LMW PAHs, which often have low or zero TEFs for carcinogenicity, can exhibit significant acute toxicity to marine life.^{1, 2, 13, 19, 20, 27, 34, 52} This report's toxicity ranking will primarily focus on carcinogenic potency via TEFs due to the availability of comparative data, but the acute toxicity of LMW PAHs will be acknowledged.

Sediment Quality Guidelines (SQGs) such as Effects Range Low (ERL) and Effects Range Median (ERM) values are also used to assess the potential ecological risks of PAHs in sediments.^{2, 4, 6, 11, 31a/31b, 38, 40, 49} ERLs represent concentrations below which adverse effects are rarely observed, while ERMs indicate concentrations above which adverse effects are frequently observed. While these are crucial for sediment risk assessment, they are concentration thresholds in sediment rather than intrinsic toxicity values of the PAHs themselves, and thus are not the primary basis for ranking the PAHs against each other for inherent toxicity in this section.

TEF values for the parent PAHs on the MMO list have been compiled from various sources within the provided research material.^{14, 21, 35, 53, 54, 56, 57, 60, 61, 62, 63, 64, 65, 66, 67} For the alkylated PAH groups (C1-Naphthalenes, C2-Naphthalenes, C3-Naphthalenes, C1-Phenanthrenes), specific TEFs for the entire group are generally not defined. Following the approach suggested in ¹⁴, where the TEF of 2-methylnaphthalene (0.001) is applied to 1-methylnaphthalene and also to dimethyl- and trimethylnaphthalenes, a TEF of 0.001 will be assigned to C1-Naps, C2-Naps, and C3-Naps. Similarly, C1-Phenanthrenes will be assigned a TEF of 0.001, consistent with the TEF for the parent Phenanthrene in many schemes. Perylene is generally considered non-carcinogenic or to have very low carcinogenic potential, often assigned a TEF of 0.001 or is not listed in carcinogenic assessments.^{68, 69} Benzo[e]pyrene is also typically assigned a TEF of 0.001 or 0.01 in some schemes, reflecting its lower carcinogenicity compared to Benzo[a]pyrene.^{20, 54, 62, 63, 70}

It is important to recognize that TEF values can vary between different assessment schemes. For example, Dibenz[a,h]anthracene is assigned a TEF of 1 by the EPA ⁵³ but a TEF of 5 by Nisbet and LaGoy (1992).⁵⁴ This report will primarily use the Nisbet and LaGoy (1992) values where available for consistency, as they are widely cited for a broader range of PAHs, and note significant discrepancies.

Table 3.1.1: MMO PAHs Ordered by Increasing Carcinogenic Potency (TEF values relative to Benzo[a]pyrene).

Notes on Table 3.1.1: ^a TEF values are relative to Benzo[a]pyrene (TEF=1.0). ^b Contains a five-membered ring in addition to two benzene rings. ^c Contains a five-membered ring in addition to fused benzene rings. ^d Some sources ⁵⁷ suggest TEF 0.0005 for Fluorene and Phenanthrene; 0.001 is also widely used. ^e EPA TEF for Chrysene is 0.001 ⁵³, while Nisbet & LaGoy (1992) also use 0.01.⁵⁴ The more conservative (lower) value of 0.001 is used for ranking. ^f Perylene is often considered non-carcinogenic or having very low carcinogenic potential; some sources assign a TEF of 0.001 or 0.01.^{68 g} EPA TEF for Benzo[k]fluoranthene is 0.01 ⁵³, while Nisbet & LaGoy (1992) use 0.1.⁵⁴ The higher value is used here reflecting some schemes. ^h EPA TEF for Dibenz[a,h]anthracene is 1 ⁵³, while Nisbet & LaGoy (1992) use 5.⁵⁴ Ranked based on EPA value, with Nisbet & LaGoy value noted as it would place it highest.

The ranking by TEFs highlights a wide range in carcinogenic potency among the MMO PAHs. Many LMW PAHs and some HMW PAHs like Fluoranthene, Pyrene, and Chrysene have very low TEFs (0.001), indicating significantly lower carcinogenic potential compared to BaP. Conversely, BaP itself and Dibenz[a,h]anthracene (especially with a TEF of 5) are the most potent carcinogens on the list. This differentiation is critical for risk assessment, as simple summation of PAH concentrations without applying TEFs can grossly misrepresent the actual carcinogenic risk of a PAH mixture.⁵³

The use of surrogate TEFs for alkylated PAH groups introduces an element of uncertainty. While pragmatic due to limited data on individual alkylated isomers, it is a simplification. Individual isomers within an alkylated group can exhibit varying toxicities, and future research may lead to more refined TEFs for these groups. The current approach, however, acknowledges their presence and assigns a conservative, low carcinogenic potential based on their parent structures or representative congeners like 2-methylnaphthalene.¹⁴

It is also crucial to remember that this ranking focuses on carcinogenicity. LMW PAHs such as Naphthalene, Acenaphthylene, Acenaphthene, Fluorene, Phenanthrene, and Anthracene, despite their low TEFs, are recognized for their acute toxicity to aquatic organisms.^{1, 2, 13, 19, 20, 27, 34, 52} Therefore, a low TEF value does not imply a complete absence of ecological risk, particularly for these LMW compounds which can cause different types of biological harm. A comprehensive risk assessment should consider both carcinogenic potential (primarily from HMW PAHs) and acute toxicity (often associated with LMW PAHs).

Persistence refers to the ability of a chemical to remain in an environmental compartment, such as marine sediment, over time without undergoing degradation or transformation.^{1, 2, 3, 4, 6, 7, 8, 9, 17, 72, 73} The primary metric for quantifying persistence is the half-life (t_1/2), which is the time required for 50% of the initial concentration of a substance to degrade or dissipate from the specified medium.^{15, 22, 42, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103} A longer half-life signifies greater persistence.

PAH persistence in marine sediments is influenced by several factors:

• Molecular Structure: Generally, persistence increases with increasing molecular weight and the number of aromatic rings. HMW PAHs are more resistant to degradation than LMW PAHs.^{7, 8, 15, 77} Alkylation can also increase persistence compared to the parent PAH.^{9, 17}
• Sediment Characteristics: The organic carbon content of the sediment is a critical factor, as PAHs adsorb to organic matter, which can protect them from degradation. The presence of black carbon (soot, char) can lead to even stronger sorption and reduced degradation rates.^{10, 31a/31b}
• Environmental Conditions: Oxygen availability is paramount; anaerobic (anoxic) conditions, common in deeper or highly organic sediments, significantly slow down PAH biodegradation compared to aerobic conditions.^{15, 74, 83, 104} Temperature and microbial community composition also play vital roles.^{43, 74, 84}
• Bioavailability: Processes that reduce bioavailability, such as strong sorption or aging, can indirectly increase persistence by making PAHs less accessible to degrading microorganisms.^{85, 105, 106}

Compiling definitive half-lives for all 22 MMO PAHs specifically in marine sediments is challenging due to variability in study conditions and data availability. This report uses available marine sediment data where possible, supplemented by soil sediment data or general principles of PAH degradation.

• LMW PAHs (2-3 rings): Generally less persistent.
  • Naphthalene: t_1/2 ~20-21 days (San Francisco Estuary model ⁷⁷); 2.3-130 days in GW/Soil (slowest inf (3851) days) ⁹⁴; 0.13-inf (88) days in Sed/GW.⁹⁴
  • Acenaphthene: Uptake t_1/2 onto microplastics ~10 hours ⁷⁸, not degradation in sediment. No specific degradation t_1/2 in marine sediment found in provided snippets, but generally considered less persistent than HMW PAHs. Howard Handbook data (via OEHHA ¹⁰⁷) gives an average soil half-life for PAHs as 570 days, but this is a very broad average.
  • Acenaphthylene: Similar to Acenaphthene, expected to be relatively less persistent.
  • Fluorene: t_1/2 ~32-60 days reported.⁸⁰
  • Phenanthrene: t_1/2 ~63 days (San Francisco Estuary model ⁷⁷); 16-126 days (soil/sediment ¹⁵); 4.5-137 days (diesel-contaminated sediment ¹⁰⁰); 1.5-2.8 weeks (landscaping material ⁸⁴).
  • Anthracene: t_1/2 57-210 days (unacclimatised sediments), 5-7 days (oil-treated sediments) ⁷⁶; experimental sediment t_1/2 “days to months”, calculated 80.2 years.⁸²
• HMW PAHs (4+ rings): Generally more persistent.
  • Fluoranthene: t_1/2 ~302 days (San Francisco Estuary model ⁷⁷); 2.4-39 weeks (landscaping material ⁸⁴).
  • Pyrene: t_1/2 ~9-16 years (Gulf sediments ⁷⁴); 2.7-52 weeks (landscaping material ⁸⁴).
  • Benz[a]anthracene: t_1/2 ~338 days (San Francisco Estuary model ⁷⁷).
  • Chrysene: t_1/2 372-993 days (soil ⁸³); 22-198 weeks (landscaping material ⁸⁴).
  • Benzo[b]fluoranthene: t_1/2 ~5.6 years (San Francisco Estuary model ^{77, 108}); 360 days - 1.67 years (aerobic soil ⁸⁵). Resistant to degradation in some studies.⁸⁴
  • Benzo[k]fluoranthene: No specific marine sediment t_1/2 found, but as an isomer of BbF, expected to be very persistent. EPA data indicates soil t_1/2 of 360 days - 1.67 years for Benzo(b)fluoranthene ⁸⁵, BkF likely similar.
  • Benzo[e]pyrene: No specific marine sediment t_1/2 found. Expected to be very persistent, similar to BaP.
  • Benzo[a]pyrene: t_1/2 229→1400 days (soil/sediment ¹⁵); EPA data: 57-522 days (aerobic soil ⁸⁷), 85-inf (2100) days (sediment ⁹⁴).
  • Perylene: Primarily diagenetic, so its “persistence” reflects formation and stability over geological timescales rather than degradation of recent inputs.^{89, 90, 98, 109, 110} Biodegradation t_1/2 of 3.4 days reported in a yeast consortium study, but this is under optimal lab conditions, not representative of sediment.⁸⁹
  • Indeno[1,2,3-cd]pyrene: Biodegradation t_1/2 ~330-331 days (EPA CompTox ⁹¹).
  • Dibenz[a,h]anthracene: t_1/2 ~5.7 years (San Francisco Estuary model ⁷⁷).
  • Benzo[g,h,i]perylene: t_1/2 ~5.7 years (San Francisco Estuary model ⁷⁷); EPA data: 590-650 days (aerobic soil ⁸⁵).
• Alkylated PAHs: Generally more persistent than their parent compounds.^{9, 17}
  • C1-C3 Naphthalenes: Expected to be more persistent than Naphthalene. ¹⁰⁰ reports rapid metabolism of naphthalenes in diesel-contaminated sediments, but this includes parent and alkylated. ⁵⁰ notes alkylated PAHs (including naphthalenes) are abundant and their movement understudied.
  • C1-Phenanthrenes: Expected to be more persistent than Phenanthrene. ¹¹¹ notes C1-Phenanthrene can be an evaluation index for oil pollution release. ¹⁰⁰ indicates rapid metabolism of phenanthrenes in diesel-contaminated sediments.

Table 3.2.1: MMO PAHs Ordered by Estimated Increasing Persistence (Half-life in Sediment).

Notes on Table 3.2.1: Half-life values are highly dependent on specific sediment conditions (aerobic/anaerobic, organic carbon, microbial activity, temperature). Ranges reflect this variability. ^b Contains a five-membered ring in addition to two benzene rings. ^c Contains a five-membered ring in addition to fused benzene rings.

The data clearly indicate that PAH persistence in marine sediments varies dramatically, spanning from days for some LMW compounds to many years, or even decades under certain conditions, for HMW compounds. This wide range has significant implications for long-term ecological risk and the selection of sediment management strategies.

A critical factor emerging from the data is the substantial variability in reported half-lives for the same PAH. This variability underscores that persistence is not an intrinsic property of the chemical alone but is heavily influenced by site-specific environmental conditions such as sediment organic matter content, the presence of black carbon, oxygen levels (aerobic vs. anaerobic), microbial community structure, and temperature.^{74, 76, 82, 84, 93, 94} For example, anaerobic conditions, common in buried or highly organic sediments, drastically reduce degradation rates for most PAHs.^{15, 74, 83, 104} Therefore, a single half-life value for a PAH may not be universally applicable.

Despite this variability, a consistent trend is the markedly greater persistence of HMW PAHs (those with four or more rings) compared to LMW PAHs (two to three rings).^{7, 8, 15} This is attributed to their lower water solubility, stronger sorption to sediment particles, and greater resistance to microbial degradation. This principle is fundamental in predicting the relative persistence of PAHs when specific half-life data are scarce.

The inclusion of alkylated PAHs in the MMO list is particularly relevant to persistence. Alkylated PAHs are generally found to be more resistant to degradation than their parent (unsubstituted) compounds.^{9, 17} This increased persistence, combined with their often high abundance in petrogenic sources, means that alkylated PAHs can contribute significantly to the long-term PAH burden and associated risks in contaminated sediments. The ranking reflects this by placing alkylated PAHs as more persistent than their respective parent compounds.

Perylene's persistence is a special case. Its high concentrations in some sediments, particularly deeper layers, are often attributed to its in situ diagenetic formation from natural organic precursors rather than direct anthropogenic input and subsequent degradation.^{34, 89, 90, 98, 109, 110} Thus, its “persistence” reflects its inherent stability once formed and the continuous nature of its formation process over long timescales, distinguishing it from the environmental persistence of externally introduced PAHs.

Bioaccumulation refers to the process by which organisms accumulate chemical substances in their tissues to concentrations higher than those in the surrounding environment (water, sediment, or food).^{1, 2, 3, 4, 5, 6, 8, 15, 99, 108, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141} For PAHs, which are lipophilic (fat-loving), accumulation primarily occurs in the lipid-rich tissues of organisms.

Key metrics for assessing bioaccumulation potential include:

• Octanol-Water Partition Coefficient (Log K_ow): This is a measure of a chemical's hydrophobicity. A higher Log K_ow value indicates a greater tendency for the chemical to partition from water into octanol (a surrogate for lipids), and thus a higher potential for bioaccumulation in fatty tissues.^{99, 112, 113, 114, 115, 116, 118, 124, 125, 126, 142, 143}
• Bioconcentration Factor (BCF): This is the ratio of the concentration of a chemical in an organism to its concentration in the surrounding water, specifically considering uptake from water across respiratory surfaces (e.g., gills).^{99, 108, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143}
• Bioaccumulation Factor (BAF): This is similar to BCF but accounts for uptake from all exposure routes, including water, diet (ingestion of contaminated food), and direct contact with contaminated sediment.^{112, 113, 114, 116, 117, 118, 122, 123, 125, 126, 127, 142, 143}

For regulatory purposes, such as under REACH, BCF values are often used to categorize substances: Not Bioaccumulative (nB, BCF < 2000 L/kg), Bioaccumulative (B, BCF 2000-5000 L/kg), and Very Bioaccumulative (vB, BCF > 5000 L/kg).¹¹⁴ This report will focus on marine invertebrates (e.g., mussels, crustaceans) for BCF/BAF data, as they generally exhibit higher PAH accumulation than fish due to their limited metabolic capacity to break down PAHs.^{2, 5, 32, 52, 114, 118, 119, 122, 124, 125, 137, 139}

Log K_ow values were systematically extracted from PubChem entries (experimental if available, otherwise computed XLogP3 values). BCF data for marine invertebrates were primarily sourced from the RIVM report ¹¹⁴ and supplemented by other provided snippets.

• Naphthalene (Nap): XLogP3 = 3.3.^{144, 145} BCF in crustaceans 131-736 L/kg.¹¹⁴ Category: nB.
• C1-Naphthalenes (C1-Naps): (e.g., 1-Methylnaphthalene, 2-Methylnaphthalene) XLogP3 = 3.9.^{146, 147, 148, 149, 150, 151} Expected BCF < 2000 L/kg. Category: nB.
• C2-Naphthalenes (C2-Naps): (e.g., Dimethylnaphthalenes) XLogP3 = 4.3.^{152, 153, 154, 155, 156} Expected BCF < 2000 L/kg. Category: nB.
• C3-Naphthalenes (C3-Naps): (e.g., Trimethylnaphthalenes) XLogP3 = 4.7.^{157, 158} Expected BCF potentially approaching 2000 L/kg. Category: nB to B.
• Acenaphthylene (Acy): Experimental Log K_ow = 3.93 ¹⁰⁸; XLogP3 = 3.7.^{108, 159, 160, 161, 162} BCF in carp 560 L/kg (lipid norm.).¹³⁷ Category: nB.
• Acenaphthene (Ace): Experimental Log K_ow = 3.92.^{163, 164} BCF (invertebrates) < 2000 L/kg (e.g., Nereis BDL ¹³²). Category: nB.¹¹⁴
• Fluorene (Flu): XLogP3 = 4.2.^{165, 166, 167} BCF in Daphnia magna 506 L/kg.¹¹⁴ Category: nB.
• Phenanthrene (Phe): XLogP3 = 4.5.^{140, 168, 169} BCF in some crustaceans (Diporeia, Pontoporeia) > 5000 L/kg; molluscs < 2000 L/kg.¹¹⁴ Category: vB in some invertebrates.
• C1-Phenanthrenes (C1-Phe/Ant): XLogP3 = 5.1.^{170, 171, 172} Expected BCF > Phenanthrene, likely vB in sensitive invertebrates.
• Anthracene (Ant): XLogP3 = 4.4.^{173, 174, 175, 176, 177} BCF in molluscs (Perna) 19000 L/kg; crustaceans (Pontoporeia) 16800-40000 L/kg.¹¹⁴ Category: vB.
• Fluoranthene (Flt): XLogP3 = 5.2.^{178, 179, 180, 181, 182} BCF in molluscs (Mytilus) 5920 L/kg, (Perna) 12250 L/kg; crustaceans (Diporeia) 15136-58884 L/kg.¹¹⁴ Category: vB.
• Pyrene (Pyr): XLogP3 = 4.9.^{134, 183, 184} BCF in molluscs (Dreissena) 13000-77000 L/kg, (Perna) 44550 L/kg; crustaceans (Pontoporeia) 166000 L/kg.¹¹⁴ Category: vB.
• Benz[a]anthracene (BaA): XLogP3 = 5.8.^{185, 186, 187, 188} BCF in crustaceans (Daphnia, Pontoporeia) 10109-63000 L/kg.¹¹⁴ Category: vB.
• Chrysene (Chr): Experimental Log K_ow ~5.6-5.86; XLogP3 (for dione) = 3.9 ¹⁸⁹, (for perhydro) = 7.4.¹⁹⁰ For Chrysene itself, XLogP3 is typically ~5.8. BCF in Daphnia magna 6088 L/kg.¹¹⁴ Category: vB.
• Benzo[b]fluoranthene (BbF): Experimental Log K_ow = 5.78-6.12 ^{179, 191, 192, 193}; XLogP3 = 6.4. No reliable BCF data for categorization in ¹¹⁴, but high Log K_ow suggests vB potential.
• Benzo[k]fluoranthene (BkF): Experimental Log K_ow = 6.0-6.12 ¹⁹⁴; XLogP3 = 6.4.^{86, 194, 195, 196, 197, 198, 199} BCF in Daphnia magna 13225 L/kg.¹¹⁴ Category: vB.
• Benzo[e]pyrene (BeP): Experimental Log K_ow = 6.25-6.57 ^{134, 135, 200}; XLogP3 = 6.4. BCF for marine invertebrates not specified in ¹¹⁴, but high Log K_ow suggests vB potential.
• Benzo[a]pyrene (BaP): Experimental Log K_ow = 5.97-6.13 ^{75, 87, 88, 135}; XLogP3 = 6.0.^{71, 135, 201, 202, 203} BCF in molluscs (Dreissena) 24000-273000 L/kg, (Perna) 8500 L/kg; crustaceans (Daphnia, Pontoporeia) 8496-73000 L/kg.¹¹⁴ Category: vB.
• Perylene (Per): Experimental Log K_ow = 6.25-6.30 ^{98, 204}; XLogP3 = 5.8. BCF for marine invertebrates not specified in ¹¹⁴, but high Log K_ow suggests vB potential.
• Indeno[1,2,3-cd]pyrene (IP): Experimental Log K_ow = 6.50-7.66 ^{91, 92, 205, 206, 207}; XLogP3 = 7.0. No reliable BCF data for categorization in ¹¹⁴, but high Log K_ow suggests vB potential.
• Dibenz[a,h]anthracene (DBA): Experimental Log K_ow = 6.75-7.10 ^{208, 209, 210}; XLogP3 = 6.5.²¹¹ BCF in Daphnia magna 50119 L/kg.¹¹⁴ Category: vB.
• Benzo[g,h,i]perylene (BghiP): Experimental Log K_ow = 6.50-7.23 ^{212, 213, 214, 215, 216, 217}; XLogP3 = 6.6. BCF in Daphnia magna 28288 L/kg.¹¹⁴ Category: vB.

Table 3.3.1: MMO PAHs Ordered by Increasing Bioaccumulation Potential (Log K_ow and BCF Category in Marine Invertebrates).

Notes on Table 3.3.1: ^a BCF Categories based on RIVM ¹¹⁴ for invertebrates where available: nB (BCF < 2000 L/kg), B (BCF 2000-5000 L/kg), vB (BCF > 5000 L/kg). “Estimated” indicates categorization based on Log K_ow trends where specific BCF data for invertebrates was lacking in the primary review document. ^b Contains a five-membered ring in addition to two benzene rings. ^c Contains a five-membered ring in addition to fused benzene rings.

The Log K_ow value serves as a strong primary indicator for the bioaccumulation potential of PAHs, with higher values generally correlating with increased partitioning into organism lipids.^{99, 112, 113, 114, 115, 116, 118, 124, 125, 126, 142, 143} However, this relationship is not always linear across the entire range of PAHs. Factors such as very large molecular size can hinder membrane permeation, and the metabolic capacity of the organism can significantly reduce the actual accumulated concentration, particularly in vertebrates like fish which possess efficient enzyme systems (e.g., cytochrome P450) for metabolizing PAHs.^{2, 5, 32, 52, 114, 118, 119, 122, 124, 125, 137, 139}

This is why marine invertebrates, such as bivalve molluscs (e.g., mussels) and crustaceans, are often better indicators of PAH bioaccumulation in the environment. Their generally lower metabolic capacity for PAHs allows these compounds to accumulate to higher concentrations in their tissues, reflecting environmental exposure levels more directly.^{2, 5, 32, 52, 114, 118, 119, 122, 124, 125, 137, 139} The RIVM report ¹¹⁴ confirms that many HMW PAHs are categorized as “very bioaccumulative” in invertebrates, even if they are less so in fish.

Alkylation tends to increase the Log K_ow of PAHs. For example, C1-Naphthalenes (Log K_ow ~3.9) are more hydrophobic than Naphthalene (Log K_ow ~3.3), and this trend continues with C2- and C3-Naphthalenes (Log K_ow ~4.3 and ~4.7, respectively). This suggests an increased intrinsic potential for bioaccumulation with increasing alkylation, assuming metabolic processes do not counteract this effect. The data for C1-Phenanthrenes (Log K_ow ~5.1) compared to Phenanthrene (Log K_ow ~4.5) also supports this. The ranking reflects this by generally placing alkylated PAHs higher in bioaccumulation potential than their parent compounds within the same ring-size category.

Bioavailability, in the context of sediment contaminants, refers to the fraction of the total amount of a PAH present in the sediment that is accessible for uptake by, and can cause an effect in, an organism.^{7, 9, 17, 20, 22, 31a/31b, 32, 49, 51, 85, 99, 105, 106, 118, 124, 218, 219} It is a critical concept because not all of the PAH mass measured in a bulk sediment sample is necessarily available to cause harm.

Key factors influencing PAH bioavailability from sediment include:

• PAH Physicochemical Properties:
  • Water Solubility & Log K_ow: LMW PAHs, with their relatively higher water solubility and lower Log K_ow values, are generally more readily available from the sediment porewater (the water filling spaces between sediment particles).^{1, 2, 27, 34} HMW PAHs, being more hydrophobic (higher Log K_ow), have very low concentrations in porewater and their bioavailability is often more linked to the ingestion of sediment particles by benthic organisms.^{1, 2, 27, 32, 34, 118}
• Sediment Characteristics:
  • Organic Carbon (OC) Content: PAHs strongly adsorb to the organic carbon fraction of sediments. Higher OC content generally leads to stronger binding and lower concentrations of freely dissolved (and thus readily bioavailable via aqueous uptake) PAHs in porewater.^{4, 10, 31a/31b}
  • Black Carbon (BC) Content: Black carbon, which includes materials like soot, char, and coal particles, has an exceptionally strong affinity for PAHs, binding them much more tightly than natural amorphous organic carbon. The presence of BC significantly reduces PAH bioavailability.^{31a/31b, 106, 218, 219} PAHs sorbed to coal-derived particles are noted to have minimal biodegradation and slow release rates.^{218, 219}
  • Grain Size: Finer sediments (silt and clay) have a larger surface area per unit mass and often higher organic carbon content, which can lead to higher bulk PAH concentrations. However, PAHs associated with the clay/silt fraction might be more mobile or available under certain conditions compared to those strongly bound to coarser, black carbon-rich particles.^{10, 31a/31b, 218}
• Aging and Weathering: Over time (aging), PAHs can become more strongly sequestered within the sediment matrix, diffusing into micropores or becoming more intimately associated with organic matter. This process generally reduces their extractability and bioavailability.^{51, 105} Weathering processes also preferentially remove more soluble and degradable PAHs, leaving behind a more recalcitrant mixture.
• Organism-Specific Factors: The feeding strategy (e.g., deposit feeder, filter feeder), habitat, and physiology of benthic organisms also influence their actual exposure and uptake of sediment-bound PAHs.

A direct quantitative ranking of all 22 MMO PAHs for bioavailability is complex and highly context-dependent. The ranking below is qualitative, based on general principles of solubility, sorption (inferred from Log K_ow), and molecular size.

No direct comparative bioavailability data for all 22 PAHs from marine sediment was found in the provided snippets. The ranking relies on established principles:

• LMW PAHs (2-3 rings): Higher water solubility and lower Log K_ow suggest greater potential for partitioning into porewater, making them more readily bioavailable via aqueous uptake to organisms living in or irrigating sediment burrows. However, their higher volatility and degradability can reduce their persistence in the bioavailable fraction.
• HMW PAHs (4+ rings): Lower water solubility and higher Log K_ow mean they are strongly bound to sediment particles. Bioavailability is primarily through ingestion of these particles by deposit-feeding organisms. The strength of sorption to organic carbon and especially black carbon significantly reduces the freely dissolved fraction.
• Alkylated PAHs: Increased alkylation generally decreases water solubility and increases Log K_ow, which would tend to decrease bioavailability from the dissolved phase but could increase uptake via ingestion if associated with digestible organic matter.

Table 3.4.1: MMO PAHs Ordered by Estimated Increasing Bioavailability from Marine Sediment (Qualitative Ranking).

Notes on Table 3.4.1: This ranking is qualitative and represents general tendencies. Actual bioavailability is highly site- and organism-specific. ^b Contains a five-membered ring in addition to two benzene rings. ^c Contains a five-membered ring in addition to fused benzene rings.

The bioavailability of PAHs from sediment is a highly complex and dynamic property, influenced by an interplay of chemical, sediment, and biological factors. It is not an intrinsic property of the PAH alone but rather a result of these interactions.^{10, 31a/31b, 105, 106, 218, 219} This complexity makes a universal, quantitative ranking of bioavailability challenging. The provided qualitative ranking is based on general principles of solubility, sorption behavior (inferred from Log K_ow and molecular structure), and typical exposure pathways for benthic organisms.

A critical factor is the nature of the sediment itself. The amount and type of organic carbon significantly control PAH partitioning. While higher total organic carbon (TOC) generally reduces the freely dissolved PAH concentration in porewater, the presence of black carbon (e.g., soot, coal particles from industrial or combustion sources) has a disproportionately strong effect, binding PAHs much more tenaciously than amorphous organic carbon and thereby drastically reducing their bioavailability.^{31a/31b, 106, 218, 219} Studies have shown that PAHs sorbed to such carbonaceous materials exhibit minimal biodegradation and slow release rates, effectively locking them away from biological uptake.^{218, 219} Conversely, PAHs associated with finer sediment fractions like clay and silt, which may have higher amorphous organic carbon, might be relatively more mobile and bioavailable under certain conditions.²¹⁸

The concept of “aging” is also crucial. Over time, PAHs in sediment can become more strongly sequestered, diffusing into micropores within sediment particles or becoming more intimately incorporated into the organic matrix.¹⁰⁵ This aging process generally leads to a decrease in their extractability by mild solvents and, consequently, a reduction in their bioavailability to organisms. This implies that recently deposited PAHs may pose a greater immediate bioavailable risk than older, historically contaminated sediments, even if bulk concentrations are similar.

While LMW PAHs are generally more water-soluble and thus potentially more bioavailable from porewater, their higher volatility and faster degradation rates (as discussed in Section 3.2) can reduce their long-term presence in the bioavailable fraction. HMW PAHs, although less soluble and more strongly sorbed, are also more persistent. Their bioavailability is often linked to the direct ingestion of contaminated sediment particles by deposit-feeding benthic organisms. Therefore, the “most bioavailable” PAH depends on the specific exposure route and organism being considered. For organisms primarily exposed via porewater, LMW PAHs might dominate uptake, whereas for sediment ingestors, persistent HMW PAHs on particles can be a significant source of exposure.

PAHs in the marine environment originate from both natural processes and human activities. “Natural abundance” in this context refers to the contribution of natural sources to the overall presence of a specific PAH, relative to its anthropogenic inputs. A PAH with high natural abundance would be one where natural formation or release mechanisms are significant contributors to its environmental levels, even in relatively pristine environments. Conversely, PAHs with low natural abundance are those whose presence in notable concentrations is overwhelmingly indicative of anthropogenic contamination.

Natural Sources of PAHs:

• Diagenesis: The slow, low-temperature transformation of sedimentary organic matter by geological and microbial processes over long periods. This is a primary natural source for Perylene.^{34, 89, 90, 98, 109, 110}
• Natural Pyrolysis: Incomplete combustion of organic matter from natural events like forest fires and volcanic eruptions.^{1, 2, 3, 4, 7, 9, 10, 16, 17, 23, 24, 25, 26, 27, 28, 29, 30} These processes generate a range of PAHs, often dominated by unsubstituted HMW PAHs.
• Petroleum Seeps: Natural seepage of crude oil and gas from underwater geological formations into the marine environment. These seeps release a mixture of PAHs, typically rich in LMW and alkylated PAHs.^{1, 2, 4, 8, 9, 10, 24, 26, 28, 31a/31b}

Anthropogenic Sources of PAHs:

• Pyrogenic Sources: These involve high-temperature processes, primarily the incomplete combustion of organic materials. Major sources include the burning of fossil fuels (coal, oil, and gas for power generation, industry, and transportation – including shipping emissions), wood burning (domestic heating, biomass burning), and waste incineration.^{1, 2, 3, 4, 7, 9, 10, 13, 16, 17, 23, 24, 25, 26, 27, 28, 29, 30, 33} These sources typically produce mixtures dominated by unsubstituted HMW PAHs.
• Petrogenic Sources: These relate to uncombusted petroleum and its products. Sources include crude oil spills, operational discharges from oil tankers and platforms, runoff contaminated with lubricating oils and fuels, and releases from asphalt and creosote-treated wood.^{1, 2, 4, 7, 8, 9, 10, 12, 16, 17, 23, 24, 25, 26, 27, 28, 29, 30, 31a/31b, 33} Petrogenic mixtures are characterized by a higher proportion of LMW PAHs and, importantly, abundant alkylated PAH homologues.^{1, 2, 4, 9, 17, 23, 24, 28, 30}

The ranking for natural abundance will order PAHs from those whose presence is most strongly indicative of anthropogenic activity (low natural background relative to typical pollution levels) to those with more significant or dominant natural sources.

• Perylene: Uniquely among the MMO PAHs, Perylene is widely recognized as having a predominantly diagenetic origin in many sedimentary environments. Its presence and concentration are often linked to the in-situ transformation of terrestrial or aquatic organic matter under specific (often anoxic) conditions, rather than direct input from combustion or petroleum sources.^{30, 34, 89, 90, 98, 109, 110}
• Alkylated PAHs (C1-C3 Naphthalenes, C1-Phenanthrenes): These groups are characteristic of petrogenic sources. While crude oil is a natural substance and natural seeps contribute these PAHs to the marine environment ^{8, 9}, the widespread and high concentrations often encountered in contaminated areas are typically due to anthropogenic petroleum spills, fuel combustion, or industrial releases of petroleum products. The ratio of alkylated PAHs to their parent compounds is a key indicator of petrogenic versus pyrogenic sources, with petrogenic sources showing a dominance of alkylated forms.^{1, 2, 4, 24, 26, 30}
• LMW Parent PAHs (Naphthalene, Acenaphthylene, Acenaphthene, Fluorene, Phenanthrene, Anthracene): These PAHs can originate from both petrogenic (crude oil, natural seeps) and pyrogenic sources (natural fires, anthropogenic combustion). Their natural background levels can be variable, but significant elevations are often linked to anthropogenic inputs.^{1, 2, 4, 19, 23, 24, 27, 29, 30, 33, 34, 40}
• HMW Parent PAHs (Fluoranthene, Pyrene, BaA, Chrysene, BbF, BkF, BeP, BaP, IP, DBA, BghiP): These are primarily associated with pyrogenic processes. Natural sources like forest fires and volcanic activity do produce these HMW PAHs. However, the extensive and elevated concentrations found in many marine sediments, particularly in urban and industrial areas, are overwhelmingly attributed to anthropogenic combustion of fossil fuels and biomass.^{1, 2, 7, 13, 16, 23, 24, 26, 27, 29, 30, 33, 34} Some of these, like Benzo[a]pyrene, are often used as markers for anthropogenic combustion.

Table 3.5.1: MMO PAHs Ordered by Estimated Increasing Natural Abundance in Marine Sediments.

The concept of “natural abundance” for PAHs is nuanced because many of these compounds, while having natural origins, are also produced and released in much larger quantities by human activities. The ranking aims to reflect the relative significance of natural formation or release pathways compared to anthropogenic inputs for each PAH typically found in marine sediments. OSPAR's strategy, for instance, aims to achieve concentrations in the marine environment near natural background values for naturally occurring substances.^{1, 2, 4, 6} Understanding which PAHs have a substantial natural background is key to interpreting monitoring data and setting realistic environmental targets.

Perylene is a standout compound due to its predominantly diagenetic origin in many aquatic sediments.^{34, 89, 90, 98, 109, 110} Its presence and concentration profile in sediment cores can provide insights into past environmental conditions and organic matter sources rather than solely indicating direct anthropogenic pollution like other PAHs. This unique characteristic means that high perylene concentrations do not necessarily equate to high anthropogenic impact in the same way they might for, say, Benzo[a]pyrene.

Alkylated PAHs (C1-C3 Naphthalenes, C1-Phenanthrenes) are strongly indicative of petrogenic sources.^{1, 2, 4, 9, 17, 23, 24, 28, 30} Crude oil is a natural substance, and natural petroleum seeps are a significant natural source of these alkylated compounds to the marine environment.⁸ However, the widespread, high concentrations of alkylated PAHs often found in contaminated coastal and marine areas are typically the result of anthropogenic activities such as oil spills or chronic releases of uncombusted fuels and lubricants. The relative abundance of different alkylated homologues (e.g., C1-, C2-, C3-naphthalenes) versus their parent PAH (Naphthalene) is a critical diagnostic tool for distinguishing petrogenic sources from pyrogenic sources, with petrogenic sources generally showing a higher proportion of alkylated compounds.^{1, 2, 4, 24, 26, 30}

Most HMW parent PAHs (e.g., Benzo[a]pyrene, Indeno[1,2,3-cd]pyrene, Dibenz[a,h]anthracene) are primarily formed during combustion processes. While natural fires (forest fires, volcanic activity) contribute to their presence in the environment, the levels found in industrialized and urbanized coastal zones are usually dominated by anthropogenic combustion of fossil fuels and biomass.^{1, 2, 7, 13, 16, 23, 24, 26, 27, 29, 30, 33, 34} For these compounds, the natural background is often significantly lower than concentrations observed in impacted areas, making them strong indicators of anthropogenic pollution.

The analysis of the 22 MMO PAHs across toxicity, persistence, bioaccumulation potential, bioavailability, and natural abundance reveals distinct profiles for different compounds and groups. Generally, HMW PAHs (≥4 rings) tend to exhibit higher carcinogenic toxicity (higher TEFs), greater persistence in sediments, and higher bioaccumulation potential (higher Log K_ow and BCF values in invertebrates) compared to LMW PAHs (2-3 rings). However, these HMW PAHs often have lower water solubility, which can reduce their bioavailability from the dissolved phase, making sediment ingestion a more critical exposure route for benthic organisms. Conversely, LMW PAHs, while often less carcinogenic and less persistent, can be more acutely toxic to aquatic life and may be more readily bioavailable from porewater due to higher solubility, though their persistence in the bioavailable fraction might be shorter due to faster degradation and volatilization.

PAHs that consistently rank high across multiple adverse properties (e.g., high toxicity, high persistence, AND high bioaccumulation potential) represent compounds of particular concern for long-term ecological health and sediment management. For instance, Dibenz[a,h]anthracene and Benzo[a]pyrene score high in toxicity, persistence, and bioaccumulation, marking them as priority pollutants. Other HMW PAHs like Benzo[b]fluoranthene, Benzo[k]fluoranthene, Indeno[1,2,3-cd]pyrene, and Benzo[g,h,i]perylene also exhibit combinations of high persistence, bioaccumulation, and moderate to high toxicity.

The molecular structure of a PAH, particularly the number of aromatic rings and the presence and nature of alkyl substituents, profoundly influences its environmental properties.

• Number of Aromatic Rings: As the number of fused aromatic rings increases (i.e., moving from LMW to HMW PAHs):
  • Toxicity (Carcinogenic): Generally increases, with many 4- to 6-ring PAHs having higher TEFs.^{13, 16}
  • Persistence: Significantly increases due to greater chemical stability and resistance to microbial degradation.^{7, 8, 15}
  • Bioaccumulation Potential (Log K_ow/BCF): Increases due to greater hydrophobicity and lipophilicity.^{112, 113, 114, 118}
  • Water Solubility: Decreases sharply, which in turn affects bioavailability from the dissolved phase.^{1, 2, 27, 34}
  • Volatility: Decreases, reducing atmospheric transport in the gas phase but increasing association with particulate matter.

• Alkylation: The addition of alkyl groups (e.g., methyl, ethyl) to a parent PAH structure typically:
  • Persistence: Often increases resistance to degradation compared to the parent compound.^{9, 17}
  • Bioaccumulation Potential (Log K_ow): Increases hydrophobicity and thus Log K_ow, suggesting higher intrinsic bioaccumulation potential.^{119, 124}
  • Toxicity: The effect of alkylation on toxicity is complex and can vary. Some alkylated PAHs may exhibit increased toxicity or different toxic mechanisms compared to their parent compounds.^{4, 17} For carcinogenic TEFs, alkylated PAHs are often conservatively assumed to have similar or slightly lower potency than their parent if specific data are lacking.¹⁴
  • Source Indication: The presence and relative abundance of alkylated PAHs are key indicators of petrogenic (petroleum-derived) contamination, as crude oils are rich in these compounds, whereas combustion sources tend to produce predominantly parent PAHs.^{1, 2, 4, 9, 17, 23, 24, 28, 30, 40}

The rankings and property data presented in this report have several important implications for the environmental risk assessment and management of PAH-contaminated marine sediments:

• Prioritization: PAHs that exhibit a combination of high toxicity, high persistence, and high bioaccumulation potential (e.g., Dibenz[a,h]anthracene, Benzo[a]pyrene, and other carcinogenic HMW PAHs) should be prioritized in monitoring programs and risk management strategies. Their long residence time in sediments and ability to accumulate in biota pose significant long-term threats.
• Source Apportionment: The inclusion of alkylated PAHs (C1-C3 Naphthalenes, C1-Phenanthrenes) in the MMO list is critical. Analyzing the full suite allows for more robust source apportionment, distinguishing between pyrogenic (combustion) and petrogenic (oil) inputs. This is vital for identifying pollution sources and implementing targeted control measures. For example, a high proportion of alkylated naphthalenes and phenanthrenes relative to their parent compounds would strongly suggest petroleum contamination.^{1, 2, 4, 24, 30}
• Sediment Quality Guidelines (SQGs): Understanding the varying toxicities (TEFs) and bioaccumulation potentials (BCFs) is essential when applying or developing SQGs. Generic SQGs based on total PAH concentrations can be misleading if the mixture composition and relative potencies of individual PAHs are not considered. The TEF approach provides a more refined method for assessing the carcinogenic risk of PAH mixtures.⁵³
• Bioavailability Considerations: Risk assessments must acknowledge that not all PAHs measured in bulk sediment are bioavailable.^{31a/31b, 105, 106, 218} Factors like organic carbon content, black carbon presence, and aging significantly modify bioavailability. Site-specific assessments may require more sophisticated approaches, such as equilibrium partitioning models or direct measurement of freely dissolved PAH concentrations in porewater, to better estimate actual exposure and risk.^31a/31b
• Management of Alkylated PAHs: Given their potential for increased persistence and abundance in petrogenic spills, specific attention should be paid to the alkylated PAHs on the MMO list. Their distinct environmental behavior warrants their individual consideration in risk assessments rather than solely relying on data for parent PAHs.

The multifaceted nature of PAH risk—encompassing acute toxicity, carcinogenicity, persistence, and food web transfer—necessitates a holistic assessment approach. A PAH that is highly toxic but rapidly degrades may pose an acute, localized risk, whereas a less acutely toxic but highly persistent and bioaccumulative PAH can lead to chronic, widespread impacts and food chain contamination. Effective management requires considering these diverse risk profiles.

This report synthesizes information from the provided research material, but several limitations and uncertainties should be acknowledged:

• Data Gaps: Comprehensive, directly comparable data for all 22 MMO PAHs across all five environmental properties (especially for individual isomers within the alkylated PAH groups) are not always available in the provided snippets. Rankings, particularly for bioavailability and natural abundance, have relied on established principles and data for representative compounds where specific information was lacking.
• Variability in Experimental Data: Reported values for properties like half-lives and BCFs often show considerable variation. This is due to differences in experimental conditions (e.g., sediment type, temperature, organism species, exposure duration) and analytical methodologies across studies. The report has attempted to use representative values or ranges where possible.
• Simplifications in Ranking:
  • For alkylated PAHs, rankings often rely on the properties of the parent compound or a representative alkylated isomer (e.g., using 2-methylnaphthalene data to infer properties for C1-C3 naphthalenes). This is a necessary simplification but introduces uncertainty as different isomers can have varying properties.
  • Log K_ow values are often based on computational estimates (e.g., XLogP3 from PubChem) when experimental data are unavailable. While useful, these are predictions.
  • Bioavailability ranking is largely qualitative due to its high dependence on site-specific factors.
• Mixture Effects: PAHs almost always occur in complex mixtures in the environment, not as single compounds. The TEF approach addresses additivity for carcinogenic effects, but other synergistic or antagonistic interactions between PAHs, or with other co-contaminants in sediments, are not fully captured by individual PAH rankings.
• Natural Abundance Complexity: Distinguishing true natural background from diffuse anthropogenic contributions for many PAHs is challenging, making the “natural abundance” ranking an estimation of relative contributions rather than absolute pristine levels.

Despite these limitations, the structured approach of ranking the MMO's 22 target PAHs based on these five key environmental properties provides a valuable framework for understanding their relative environmental behavior and potential risks.

This report has provided a detailed analysis of the 22 Polycyclic Aromatic Hydrocarbons (PAHs) mandated for sediment analysis by the Marine Management Organisation, ranking them according to increasing toxicity, persistence in marine sediments, bioaccumulation potential, bioavailability from marine sediments, and natural abundance. This multi-faceted approach offers a more nuanced understanding of the potential risks and environmental behavior of these compounds than considering any single property in isolation.

Key findings indicate that:

• Toxicity: Carcinogenic potency, assessed by Toxic Equivalency Factors (TEFs), varies significantly, with Dibenz[a,h]anthracene and Benzo[a]pyrene being the most potent. Many LMW PAHs and some HMW PAHs exhibit very low carcinogenic potential based on TEFs, though LMW PAHs can pose acute toxicity risks to marine organisms. Alkylated PAHs are generally assigned low TEFs based on their parent structures or representative congeners.
• Persistence: HMW PAHs are markedly more persistent in marine sediments than LMW PAHs, with half-lives potentially extending to years or decades for compounds like Benzo[a]pyrene, Dibenz[a,h]anthracene, and Benzo[g,h,i]perylene. Alkylated PAHs are generally considered more persistent than their parent compounds. Perylene's high persistence is often linked to its diagenetic origin.
• Bioaccumulation Potential: HMW PAHs, with their higher Log K_ow values, generally show a greater potential for bioaccumulation. Many are classified as “very bioaccumulative” (vB) in marine invertebrates, which lack efficient metabolic pathways for PAHs. Alkylation tends to increase Log K_ow, suggesting enhanced bioaccumulation potential for alkylated PAHs compared to their parents.
• Bioavailability: LMW PAHs tend to be more bioavailable from the dissolved phase in sediment porewater due to higher solubility. HMW PAHs are more strongly sorbed, with bioavailability often linked to sediment ingestion; their availability is significantly reduced by high organic carbon and especially black carbon content, as well as by aging processes in sediment.
• Natural Abundance: Perylene stands out as having a predominantly natural (diagenetic) origin. Alkylated PAHs are strongly associated with petrogenic sources (natural seeps and anthropogenic oil). Most HMW parent PAHs are primarily pyrogenic, with anthropogenic combustion often overshadowing natural fire contributions in impacted areas. LMW PAHs have mixed natural (seeps, fires) and anthropogenic sources.

The PAHs consistently ranking high in multiple adverse categories, such as Dibenz[a,h]anthracene, Benzo[a]pyrene, and other persistent, bioaccumulative, and toxic HMW PAHs, warrant the highest level of concern in sediment management. The inclusion of alkylated PAHs in the MMO's monitoring list is crucial for accurate source apportionment (distinguishing petrogenic from pyrogenic inputs) and for assessing the full spectrum of PAH-related risks, as these compounds can be more persistent and abundant than their parent PAHs in petroleum-contaminated sediments.

This comprehensive understanding of the distinct environmental profiles of the 22 MMO PAHs should inform regulatory decision-making, guide the development and application of sediment quality guidelines, aid in the prioritization of sites for remediation, and contribute to more effective strategies for the protection of the marine environment. Future research should continue to refine our understanding of the properties of individual alkylated PAH isomers and the complex interplay of factors governing PAH bioavailability in diverse marine sediment types.

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13. HELCOM Indicators (pahs-and-metabolites, version 1) HELCOM Indicators PAHs v1

14. PMC NCBI (PMC5634701, version 1) PMC5634701 v1 15. PMC NCBI (PMC111252, version 1) PMC111252 v1

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23. PMC NCBI (PMC3991632) PMC3991632

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31b. PMC NCBI (PMC6852300, version 2) PMC6852300 v2

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37a. OSPAR (843-en-1-0-1-h3, version 1) OSPAR v1

37b. OSPAR (843-en-1-0-1-h3, version 2) OSPAR v2

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