Benzene (C₆H₆) is a liquid at room temperature and pressure, is soluble in water (720-2,200 mg l^-1 variously reported, with probable typical values in freshwater of 720 mg l^-1 and in seawater of 1,224-1,550 mg l^-1), has a low to moderate log Kow value (2.13) and is highly volatile.

Production of benzene is associated primarily (>90%) with the petrochemical industry, with a smaller proportion produced as a by-product of coke production and small quantities derived from natural gas. Benzene is produced widely in the EC at high tonnage, including in the UK. Annual UK production was variously estimated around 690-1,025 Ktonnes during 1988-1990, with estimated annual capacity around 1,210 Ktonnes (Nielsen et al 1991) to 1,405 Ktonnes, at six sites (ChemInform 1992). The UK is a net exporter, with estimated usage in 1988-1990 of 694-879 Ktonnes (Nielsen et al 1991).

Most benzene is used as a constituent of BTX (benzene-toluene-xylene) in vehicle, aviation and other fuels, where benzene is used to increase the octane rating. This use has increased globally with the widespread introduction of unleaded petrol and the increasing use of vehicular transport in developing nations. However, in the EC and elsewhere, the benzene content of fuels is now coming under closer scrutiny and control. Benzene is limited to 2% by volume in leaded and unleaded fuel in the UK (i.e. below the 5% permitted by EU legislation). The other major area of use is as an intermediate in chemicals manufacture, particularly in the production of styrene, cyclohexane, ethylbenzene, nitrobenzenes and cumene. Benzene has also had widespread use as a solvent, although this is now declining and is probably insignificant in the UK; it is no longer regarded as a commercial solvent by the UK Hydrocarbon Solvent Association (Hedgecott and Lewis 1997).

During production and use, benzene is released primarily to the atmosphere, although it is quite mobile between environmental compartments. The principal anthropogenic releases of benzene include:

• accidental and deliberate release of crude oils and petrochemical products (including during refining of crude oils and distribution and use of products);
• production of benzene and derivative chemicals; exhaust emissions from fossil fuel combustion; and
• emissions from coal tar distillation plants and coal processing plants.

Entry into water may be direct or via atmospheric deposition, run-off and leaching. Low but significant levels may also arise from releases from natural sources, such as vegetation and fossil fuels.

Benzene has been reported in oceans, estuaries, precipitation, rivers, groundwaters, potable sources and drinking water, as well as aquatic sediments and biota (Jones et al 1990, Nielsen et al 1991). High usage and release, high mobility in the atmosphere, and natural sources all contribute to widespread occurrence in waters. Many of the data are for estuarine and coastal waters, with benzene apparently detected in relation to both industrial discharges and releases during oil extraction and transport. However, Hedgecott and Lewis (1997) concluded that none of the reported average concentrations in surface waters exceeded the proposed annual average EQSs of 30 µg l^-1 benzene in fresh and salt waters.

In the North-East region of the Environment Agency, 23% of 823 river and estuary samples over the period 1993-95 contained benzene, with a mean concentration of under 2 µg l^-1 and highest concentrations of 312 and 54 µg l^-1 (the only values that were higher than the proposed EQSs). Only one of 26 seawater samples contained detectable benzene (Hedgecott and Lewis 1997).

Monitoring data from the National Rivers Authority and the National Monitoring Programme Survey of the Quality of UK Coastal Waters are presented in Appendix D. However, benzene was not monitored in either survey.

The available data suggest that, while benzene has been detected in UK coastal and estuarine waters, in general, concentrations do not exceed relevant quality standards derived for the protection of saltwater life.

Hedgecott and Lewis (1997) reviewed data on the fate and behaviour of benzene. Benzene appears to be susceptible to microbial degradation by a variety of species. Mixed aerobic cultures have been reported to degrade benzene, with half-lives as low as 15-20 hours up to a few days or more depending on the source of the inoculum and the history of pre-exposure. Degradation in pure aerobic cultures of a variety of bacterial species has also been reported, with Pseudomonas spp. apparently being the major environmental degraders. Anaerobic degradation can potentially occur but is apparently subject to a long lag period (of the order of 20 weeks) and is nitrogen-limited. Without additional nitrogen, degradation has been reported to take six months to three years.

Only a low to moderate tendency to sorb to organic solids is suggested by the log Kow value of 2.13 and log Koc value of 1.91. In surface waters, only small amounts of benzene will sorb to aquatic sediments (Hedgecott and Lewis 1997).

Benzene is very volatile and is probably the major single removal process in most surface waters, particularly under conditions unfavourable to biodegradation. Theoretical volatilisation half-lives of 37 and 290 minutes have been calculated for a 1 m deep water column with completely mixed water and still water respectively. The importance of mixing in increasing the rate of volatilisation has also been demonstrated in experimental mesocosms, with half-lives of 13 days in winter (well mixed) but 23 days in spring (less well mixed) (Hedgecott and Lewis 1997).

An exhaustive literature review on the toxicity of benzene 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 (Hedgecott and Lewis 1997). The most sensitive groups of organisms have been identified.

Hedgecott and Lewis (1997) reviewed data on the aquatic toxicity of benzene. In a previous review (Jones et al 1990), the authors found that saltwater data were primarily derived from tests using static exposure conditions and nominal exposure concentrations, and there were some concerns that these would under-estimate benzene=s toxicity. Most of the information was derived from acute studies, and the lowest reliable data were acute LC50s of 4.9 mg l^-1 for sockeye salmon O. nerka and 5.5 mg l^-1 for Dolly Varden char Salvelinus malma. These results were based on analysed benzene exposures and were, therefore, considered to be reliable. However, Jones et al (1990) recommended that further research with echinoderms (which were found to be potentially sensitive) should be undertaken.

Additional data for bacteria (one species), invertebrates (five species) and fish (two species) were summarised by Hedgecott and Lewis (1997). All from acute studies, apart from one chronic study with an edible crab Scylla serrata . This appeared to be the most sensitive organism under acute exposure, with 72 hour LC50s of 3.3 and 4.6 mg l^-1, depending on reproductive status. In the chronic study, biochemical changes were seen when exposed for 3-30 days to 0.56 mg l^-1 or higher. However, their reliability was uncertain (Hedgecott and Lewis 1997). The lowest toxicity data based on analysis of benzene are a 96 hour LC50 of 9.03 mg l^-1 for the sole Solea solea. No data for sediment dwelling organisms could be located.

Data summarised in Jones et al (1990) for freshwater and saltwater studies were limited, but indicated that bioaccumulation of benzene was not significant, with maximum BCF values of 225 in fresh waters and 135 for salt waters. Few additional bioaccumulation data were available for Hedgecott and Lewis (1997), but they do not indicate higher bioaccumulation potential than that indicated previously.

Potential effects includes:

• Toxicity of benzene to invertebrates and fish at concentrations above the EQS of 30 µg l^-1 (annual average) in the water column.

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