In order to control the risk of bacterial contamination in drinking water distribution systems, disinfectant is generally injected at the production point to ensure a sufficient residual throughout the distribution system. In doing this, disinfection must be kept at a minimum rate to limit the formation of disinfection byproducts. This trade-off is even more complex to deal with, when a network is supplied with mixed waters of various qualities. There is an increasing tendency to choose multi-source mixing within a distribution system for hydraulic, water-quality or economical reasons. The objective of this study was to evaluate the impact of mixing waters of various origins on the distributed water quality. To study this issue, the network of the City of Caen in Normandy (114,000 inhabitants), France was chosen because one of its distribution areas is supplied with two types of water of various origins and qualities: a treated surface water and a chlorinated groundwater. The hydraulic modelling software SynerGEE Water was used for this study to model the chlorine decay. This paper addresses the methodology and the results of winter and summer field tests which permitted the modelling of the chlorine residual. It shows how an innovative method has been developed and successfully applied to a complex distribution system to help the network operator optimize the disinfection strategy. Includes 7 references, tables, figures.