This research used the tracer study method to determine more accurate disinfectant contact times in the City of Carbondale Water Treatment Plant. In this investigation, three clarifiers were selected to be the experimental segments. Water samples coming from each clarifier and the confluence of the three clarifiers were analyzed to obtain the disinfectant contact times. Based on a continuous source of infinite duration model and the tool "Solver" in Microsoft Excel, this study optimized the experimental data. The experimental data and the optimized data were then analyzed to determine the disinfectant contact times using the graphical method and the numerical method. The disinfectant contact times obtained between using the experimental data and the optimized data, and between using the graphical method and the numerical method were then compared. The results from the numerical method on the optimized data were most accurate and reliable according to these comparisons. The results suggested that the disinfectant contact times were 192.7 min for the first clarifier when its flow rate was 5.72 ML/d; 161.46 min for the second clarifier when its flow rate was 5.99 ML/d; 156.06 min for the third clarifier when its flow rate was 6.26 ML/d; and 168.48 min for the confluence of all clarifiers when its flow rate was 17.5 ML/d, respectively. The feed flows increase down the distribution manifold changing the disinfectant contact times for the clarifiers.