Page 5: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Benzene

Part II. Science and Technical Considerations

4.0 Identity, use, and sources in the environment

Benzene, the simplest homologue of the aromatic hydrocarbons, is a planar, cyclic molecule with six carbon atoms arranged in a regular hexagon. The molecular formula for benzene is C6H6. It is a volatile, colourless liquid with a characteristic odour; the odour and taste thresholds for benzene are 4.68 ppmFootnote 1 and 0.5-4.5 mg/L, respectively (HSDB, 2005). Benzene has a relatively high vapour pressure (10.1-13.2 kPa at 25°C), a high water solubility (820- 2167 mg/L at 25°C), and a low log octanol/water partition coefficient (1.56-2.69) (Mackay et al., 1992).

Benzene is produced commercially from petroleum, natural gas, or coal. From 1988 to 2002, benzene production in Canada rose from 827 to 1142 kt per year; imports dropped from 29 to 2 kt per year, and exports rose from 92 to 210 kt per year (CPI, 2003). In Canada, benzene is produced in Ontario, Alberta, and Quebec. Benzene is used in industry as a volatile solvent and as an intermediate in the production of many chemicals, including ethylbenzene/styrene (used in plastics), cumene, linear alkyl benzene, and maleic anhydride (Jaques, 1990; CPI, 2003). The majority of benzene produced or imported into Canada is used in the production of ethylbenzene/styrene, with benzene usage for this purpose increasing from 582 kt in 1988 to 737 kt in 2002 (CPI, 2003). Benzene is also present in gasoline as an octane enhancer and anti-knock agent; since July 1999, however, levels of benzene in gasoline have been reduced to below 1% by volume.

Benzene is found naturally in the environment in very low concentrations; concentrations in the Atlantic and Pacific oceans have been reported to range between 100 and 200 ppt (Singh and Zimmerman, 1992). Natural sources of benzene include volcanoes, crude oil, forest fires, and plant volatiles (Graedel, 1978; IARC, 1982). Benzene may enter water and soil through petroleum seepage and weathering of exposed coal-containing rock. It can enter groundwater from petroliferous rocks and can enter air from volcanoes, forest fires, and releases of volatile chemicals from plants (Graedel, 1978; Westberg et al., 1981; Whelan et al., 1982; Fishbein, 1984; Slaine and Barker, 1990). Natural sources are believed to be generally low in comparison with anthropogenic sources (Rasmussen and Khalif, 1983; Rudolph et al., 1984). Anthropogenic releases of benzene may occur at any stage of the production, storage, use, and transport of isolated benzene and crude oil and gasoline, including emissions resulting from fuel combustion. Vehicular emissions constitute the main source of benzene in the environment.

The atmosphere and surface waters are the major environmental sinks for benzene owing to its relatively high vapour pressure, moderate water solubility, and low octanol/water partition coefficient. Virtually all (99.9%) of the benzene released into the environment eventually distributes itself into the air (Wallace, 1989a). Volatilization and biodegradation are the major processes involved in the removal of benzene from water. The half-life of benzene in water 1 m deep is estimated to be 4.8 hours as a result of volatilization (ATSDR, 2007); ice cover in the winter may impair benzene volatilization from surface waters. Reported half-lives of benzene have ranged from 33 to 384 hours for aerobic biodegradation in surface waters; for anaerobic biodegradation in deeper waters or in groundwater, half-lives ranged from 28 days to 720 days (Vaishnav and Babeu, 1987; Howard et al., 1991).

It is estimated that every year in Canada, 34 kt of benzene are released into the atmosphere (Jaques, 1990); major sources include combustion of gasoline and diesel fuels, emissions during benzene production, primary iron and steel production, solvent uses, residential fuel combustion, and gasoline marketing. Benzene is generally introduced into water from industrial effluents and atmospheric pollution.

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