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The occurrence of the most prevalent chlorinated disinfection by-products (DBPs) was investigated in three Quebec City distribution systems that deliver drinking water subjected to different treatment strategies. Trihalomethanes (THMs), haloacetic acids (HAAs), and other complementary parameters were monitored in several locations representing variable water residence times over 16 months. The occurrence of chlorinated DBPs levels was considerably lower in the system where raw water is subjected to pre-ozonation instead of pre-chlorination (as pre-treatment) or direct chlorination. Seasonal analysis of the data showed that according to the system and to the group of chlorinated DBPs, the average DBP concentrations in winter were two to four times lower than the average DBP levels of the entire period under study. Considerable variations of both DBP groups were observed according to the water residence time in the distribution system, but the behavior was not the same for THMs as for HAAs. In fact, correlations between THM and HAA levels were good only for cold water and low residence time conditions. Seasonal and spatial variations of DBPs documented in this study have important implications on regulatory issues and from an epidemiological point of view. [ABSTRACT FROM AUTHOR]