The most critical water quality conditions are generally found at a water distribution network's extremities, given their high residence times. As chlorine injected at the water treatment plant (WTP) or at rechlorination sites has more time to react, residual chlorine concentrations at the extremities may not be enough to prevent microbial regrowth. This study focuses on the relationships between residence times, and residual chlorine and trihalomethane (THM) concentrations in a sector supplied with water coming directly from the WTP and from a large reservoir within the network. A hydraulic model was calibrated based on residence times obtained from a tracer study and a water quality characterization campaign at 47 sampling sites. Results showed that chlorine decay in water from the reservoir is faster than for water directly from the WTP. THM concentrations differ, with those in water from the reservoir being far higher than those from the WTP. A slight increase in THM concentrations is seen with residence time in both cases. By using a hydraulic model, we can evaluate – at any time and in any place – the impact of a hydraulic change on the network in terms of hydraulics, as well as in terms of the vulnerability associated with low residual chlorine and high THM concentrations.