Page 6: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Trihalomethanes
4.0 Identity, use and sources in the environment
Trihalomethanes (THMs) are halogen-substituted single-carbon compounds with the general formula CHX3, where X represents a halogen, which may be chlorine, bromine, fluorine, or iodine, or combinations thereof. The THMs most commonly present in drinking water are chloroform (CHCl3), bromodichloromethane or dichlorobromomethane (CHBrCl2) (BDCM), dibromochloromethane or chlorodibromomethane (CHClBr2) (DBCM), and bromoform (CHBr3); consideration of information relevant to the derivation of drinking water guidelines for THMs is restricted to these compounds. THM measurement assesses these four common THMs, with chloroform usually constituting the largest proportion. As well as being the most common THM, chloroform is also the principal DBPs in chlorinated drinking water (LeBel and Williams, 1995).
These compounds are formed in drinking water primarily as a result of chlorination of organic matter present naturally in raw water supplies, and they are released into the environment from industrial sources as well as through indirect production in the chlorination of drinking water and municipal sewage. The rate and degree of THM formation increase as a function of the chlorine and humic acid concentration, temperature, pH, and bromide ion concentration (Stevens et al., 1976; Amy et al., 1987). In the presence of bromides, brominated THMs are formed preferentially and chloroform concentrations decrease proportionally (Aizawa et al., 1989).
The four compounds considered here are liquids at room temperature. They are relatively to extremely volatile, with vapour pressures at 25°C ranging from 0.80 kPa for bromoform to 23.33 kPa for chloroform. The THMs are only slightly soluble in water, with solubilities less than 1 mg/mL at 25°C. Their log octanol-water partition coefficients range from 1.97 (chloroform) to 2.38 (bromoform). Chloroform decomposes via photochemical oxidation to dichlorocarbonyl (phosgene) and hydrogen chloride (Environment Canada and Health Canada, 2001).
Chloroform has not been manufactured in Canada since 1978, and its use as an anaesthetic has been largely discontinued. The presence of chloroform in dentifrices, liniments, and antitussives has contributed to the exposure of Canadians in the past, but the use of chloroform in these products has now been banned under the Food and Drugs Act. Manufacturers are not permitted to import or sell a drug that contains chloroform for human use in Canada (Environment Canada and Health Canada, 2001). Canadian imports of chloroform were 402 tonnes in 1993, 69 tonnes in 1995, and 118 tonnes in 1996, with imports declining in recent years. Chloroform is used as a solvent and in the production of other chemicals (Environment Canada and Health Canada, 2001). BDCM is used in the synthesis of other chemicals and as a solvent, whereas DBCM is an intermediate in the manufacture of refrigerants, pesticides, propellents, and other organic chemicals (Keith and Walters, 1985). Bromoform is used in the synthesis of pharmaceuticals, as a solvent, and in the aircraft and shipbuilding industries as an ingredient in fire-resistant chemicals and gauge fluid.
The formation of CDBPs is a complex process that occurs when chlorine reacts with naturally present organic matter. The process is a function of naturally occurring organic precursor concentration, chlorine dose, contact time, water pH and temperature, and bromide ion concentration. An important parameter in CDBP formation is pH: THM formation increases at high pH and decreases at low pH, whereas the formation of haloacetic acids (HAAs) (the second most common group of disinfection by-products) decreases at high pH and increases at low pH. Therefore, some remedial measures applied to minimize THM formation could potentially maximize the formation of other CDBPs.
Results from Health Canada studies (Williams et al., 1995, 1997; LeBel et al., 1996, 1997), including a national survey of CDBPs in Canadian drinking water (53 systems) and a 1-year monthly survey of three systems using different disinfection processes, indicated that THMs and HAAs were the major CDBPs found in all facilities for all treatment processes including chlorine disinfectant, and that HAA levels often equalled or exceeded THM concentrations. The CDBPs levels and variation were also dependent on the DBP group, temperature (seasonal variation), water sampling location within the distribution system (contact time, spatial variation), and type of disinfection(chlorination, chloramination, ozonation). Also, within a CDBP group, the bromo-chloro speciation was dependent on the bromide ion level in water.
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