Page 5: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Tetrachloroethylene
Part II. Science and Technical Considerations
4.0 Identity, use and sources in the environment
Tetrachloroethylene (C2Cl4; molecular mass 165.85 g/mol; Chemical Abstracts Service No. 127-18-4), also known as tetrachloroethene, perchloroethylene, PERC, PER, PCE and ethylene tetrachloride, is a clear, colourless, non-flammable liquid with an ether-like odour (ECETOC, 1999; WHO, 2000). The odour threshold for tetrachloroethylene in water has been reported as 0.3 mg/L by other agencies (ATSDR, 1997;WHO, 2003), although no primary reference has been found. The odour threshold for tetrachloroethylene in air is ≥0.77 ppm Footnote 1 (Leonardos et al., 1969; Nagata, 2003). At room temperature, tetrachloroethylene is a volatile liquid with a high vapour pressure (1.9 kPa at 20°C). Tetrachloroethylene has a melting point of −22°C and a boiling point of 121°C, with a density of 1.623 g/cm3 (European Commission, 2005). Tetrachloroethylene is relatively insoluble in water (i.e., 150 mg/L at 25°C) (WHO, 2003) and has a low log octanol/water partition coefficient (2.53) (European Commission, 2005). With bioconcentration factors reported to range from 39 to 49, tetrachloroethylene is not expected to bioconcentrate in organisms or to biomagnify within food chains (U.S. EPA, 2012a).
In Canada, the predominant uses of tetrachloroethylene are as a solvent in the dry cleaning industry and as an intermediate in chemical synthesis (e.g., of fluorocarbons). Other uses for this solvent include processing and finishing in the textile industry, as an extraction solvent, as an anthelmintic, as a heat exchange fluid and in grain fumigation. In addition, tetrachloroethylene is used as an insulating fluid and cooling gas in electrical transformers and in paint removers, printing inks, adhesive formulations, paper coatings and aerosol formulations such as water repellents (Government of Canada, 1993; CPI, 2004; HSDB, 2007). Regulations concerning the use of tetrachloroethylene in Canadian dry cleaning operations and establishing reporting requirements on the importation, recycling, sale and use of tetrachloroethylene were adopted in 2003 and amended in 2011 (Environment Canada, 2011).
The manufacture of tetrachloroethylene in Canada ceased in 1992 (CPI, 2004). Import levels in 2013 were 10.4 kilotons (9.5 kilotonnes), and were primarily (97%) from the United States. Imports in 2009-2012 ranged from 7.7 to 12.3 kilotons (7.0 to 11.2 kilotonnes; International Trade Centre, 2014).
Tetrachloroethylene's entry into the environment is primarily from anthropogenic sources (Government of Canada, 1993),but there has been one report of marine algae that can naturally produce tetrachloroethylene (Abrahamsson et al., 1995). The majority (>97%) of anthropogenic tetrachloroethylene releases in Canada are to air, with <0.1% of releases in water. Releases to water have decreased over time, with the highest levels (annual average of 0.076 tonnes) observed between 1994 and 1996, decreasing to an annual average of 0.032 tonnes in 1997-2005, to an even lower annual average of 0.003 tonnes in 2006-2012. Air releases similarly decreased from an average annual release of 204 tonnes in 1994-2001 to 40.8 tonnes in 2002-2011. However, air releases increased between 2012 and 2014, averaging 101 tonnes per year (Environment Canada, 2015).
Tetrachloroethylene is distributed between environmental compartments by volatilization, precipitation and adsorption. The behaviour of tetrachloroethylene in the environment is affected by a number of processes, such as atmospheric photooxidation, volatilization and biotransformation. Once released in soil or water, tetrachloroethylene can accumulate in groundwater if not removed by degradation or evaporative processes (Government of Canada, 1993).
Tetrachloroethylene has the potential to volatilize from water and moist soil, with a Henry's Law constant of 1.8 × 10-2 atm-m3/mol at 25°C (or a unitless value of 7.23 × 10-1) (Gossett, 1987). Volatilization into the atmosphere is the dominant fate process for tetrachloroethylene in aquatic systems; 99.45% of tetrachloroethylene in water can be released into the atmosphere (Callahan et al., 1979; Schwarzenbach et al., 1979; Wakeham et al., 1983; Kaiser and Comba, 1986; ECETOC, 1999). Evaporation half-lives of less than 1 hour were observed in laboratory studies, but those from field measurements and theoretical considerations were higher, ranging from 2 to 10 days in rivers and from 10 to 30 days in lakes and ponds. Volatilization rates from groundwater are expected to be low, since both volatilization and biodegradation are greatly reduced (IPCS, 1984; ECETOC, 1999; U.S. EPA, 2012a).
Tetrachloroethylene is expected to evaporate rapidly from soil surfaces, as it has a high vapour pressure and moderately low adsorption to soil (Wilson et al., 1981). Soil adsorption of tetrachloroethylene depends on the partition coefficient, the organic carbon content of the soil, the type of release and the concentration of tetrachloroethylene in the liquid phase (Seip et al., 1986; Poulsen and Kueper, 1992). Based on experimental and estimated soil sorption coefficients of 209-1685, tetrachloroethylene has low to moderate mobility through soil (U.S. EPA, 2012a). Seip et al. (1986) determined that in sandy soil, tetrachloroethylene moves at almost the same rate as in water, but retention can occur in soils with higher organic carbon and clay contents. Migration of the chemical through soil is determined by the permeability and porosity of the soil as well as the amount of tetrachloroethylene released. It is assumed that tetrachloroethylene will be mobile in most soils and able to penetrate to depths where groundwater can be contaminated (Schwille, 1988; Poulsen and Kueper, 1992).
Microbial degradation of tetrachloroethylene has been shown to occur under anaerobic conditions, through microcosm and pilot-scale laboratory studies, but degradation under aerobic conditions has not been observed (Bouwer et al., 1981; Barrio-Lage et al., 1986; Fogel et al., 1986; Freedman and Gossett, 1989). Anaerobic degradation of tetrachloroethylene proceeds by reductive dechlorination to trichloroethylene, dichloroethylene and vinyl chloride, with microbial oxidation and dehalogenation leading to carbon dioxide and ethylene as end-products, respectively (Freedman and Gossett, 1989; Bradley, 2000).
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