This study introduces a methodology for assessing the residence times of drinking water in distribution systems using a tracer. The injection of a tracer followed by an intensive sampling campaign was used to evaluate the residence times of water in Quebec City's main system, which is supplied by the St Charles River. Samples were also analysed to determine the hardness and conductivity of the water in order to identify interconnections with a neighbouring system supplied by the St Lawrence River, a source with different properties. To validate the assumptions of interconnectivity, a complementary conductivity campaign was carried out. The tracer campaign allowed us to obtain the mean residence times (MRTs) within the study area and to target areas with low and high MRTs between 6 and 33 h. The mixing zones between water from the various sources and with longer MRTs following a stay in a reservoir were also identified. Results of this study were used to develop strategies to minimise MRT in order to improve water quality. These strategies are presented in the ‘companion paper’ (Part II) in this issue.

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