Polycyclic aromatic hydrocarbons (PAHs) are a diverse group of aromatic compounds containing two or more fused arenes structures. Commonly occurring PAHs include:

They are formed by the incomplete/inefficient combustion of organic material, diagenesis and biosynthesis. PAHs are usually coloured crystalline solids with high melting and boiling points, low vapour pressures and low water solubilities. In general, it can be concluded that low molecular weight PAHs are more soluble and volatile and have less affinity for surfaces than do high molecular weight PAHs.

PAHs are ubiquitous in the environment, with natural background levels resulting from forest fires, volcanoes and possibly production by some plants. However, a significant fraction of PAHs resulting in the environment are due to anthropogenic sources (e.g. burning of fuel, internal combustion engines etc.). Their widespread occurrence results largely from formation and release during the incomplete combustion of coal, oil, petrol and wood, but they are also components of petroleum and its products. PAHs reach the marine environment via sewage discharges, surface run-off, industrial discharges, oil spillages and deposition from the atmosphere (CCME 1992).

PAHs are ubiquitous environmental contaminants. Although they can be formed naturally (e.g. forest fires), their predominant source is anthropogenic emissions, and the highest concentrations of PAH are generally found around urban centres.

Concentrations of PAHs in the aquatic environment are generally highest in sediment, intermediate in biota and lowest in the water column (CCME 1992).

Monitoring data for water and sediments from the National Rivers Authority and the National Monitoring Programme Survey of the Quality of UK Coastal Waters are presented in Appendix D.

The available data suggest that significant concentrations of PAH can be found in some major estuaries. However, PAH concentrations at offshore sites were generally low or undetectable.

For sediments, while PAH concentrations are generally low or undetectable at most intermediate and offshore sites, further work should be concentrated on fine sediments and depositional areas. Significant concentrations of PAHs were found in a number of estuaries. However, the bioavailability of sediment associated PAHs and potential uptake still need to be further evaluated.

PAHs have a low water solubility and hydrophobic nature and so they will tend to be associated with inorganic and organic material in suspended solids and sediments. In general, most PAHs (with the exception of some low-molecular weight compounds, such as naphthalene) will be strongly sorbed by particulate matter and biota in the aquatic environment (CCME 1992).

An exhaustive literature review on the toxicity of PAHs to marine organisms has not been carried out for the purposes of this profile. The information provided in this section is taken from existing review documents (CCME 1992). The most sensitive groups of organisms have been identified.

The lower molecular weight PAHs can be acutely toxic to aquatic organisms, but the major concern is that some PAHs form carcinogenically-active metabolites (benzo[a]pyrene is the prime example) and PAH concentrations in sediments have been linked with liver neoplasms and other abnormalities in bottom-dwelling fish (Malins et al., 1988, Vethaak and ap Rheinallt, 1992). Elevated PAH concentrations may therefore pose a threat to aquatic organisms and potentially also to human consumers of fish and shellfish (MPMMG 1998).

Environment Canada has issued interim marine sediment quality guidelines which include data for a number of individual PAHs (see table below).

¹ ISQGs and PELs presented here have been calculated using a modification of the NSTP approach.

These guidelines have been derived for use in Canada and should only be used as an indication of the concentrations of individual PAHs that may affect sediment-dwelling organisms. MPMMG (1998) reported highest recorded concentrations of individual PAHs and National Monitoring Programme sites two orders of magnitude greater than the Canadian guidelines for these substances. If such concentrations were reported from a European marine site, they should be a serious cause for concern.

Aquatic organisms may accumulate PAHs from water, sediments and food. In general, PAHs dissolved in pore water are accumulated from sediment and digestion of sediment may play an important role in the uptake of PAH by some species. The relative importance of the uptake routes from food and sediment is not known.

The bioconcentration factors of PAH in different species vary greatly (WHO 1998). Species that do not metabolise PAH at all or to only a limited extent, such as algae, oligochaetes and molluscs, and the more primitive invertebrates (protozoans, porifers and cnidaria) accumulate high concentrations of PAHs, as would be expected from their high log Kow values. However, organisms that metabolise PAHs such as fish and higher invertebrates, such as arthropods, echinoderms and annelids accumulate little or no PAHs.

Biomagnification of PAHs through the food chain has been shown to occur to some degree (from annelids to fish for example) but the greater capacity of higher organisms to metabolise PAHs reduces the efficiency of the transfer.

Potential effects include:

• toxicity of low molecular weight PAHs to aquatic organisms in the water column;
• accumulation in sediments and potential hazard to sediment-dwelling organisms at concentrations between 6 and 150 µg kg^-1 (dry weight), depending on the individual PAH;
• bioaccumulation of PAHs, especially in algae and lower invertebrates and some biomagnification to higher trophic levels.

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