Early warning systems are often used for detecting contamination accidents. Traditional event detection methods suffer from high false negative and false positive errors. This paper proposes a detection method using multiple conventional water quality sensors and introduces a method to determine the values of parameters, which was configured as a multiple optimization problem and solved using a non-dominated sorting genetic algorithm (NSGA-II). The capability of the proposed method to detect contamination events caused by cadmium nitrate is demonstrated in this paper. The performance of the proposed method to detect events caused by different concentrations was also investigated. Results show that, after calibration, the proposed method can detect a contamination event 1 min after addition of cadmium nitrate at the concentration of 0.008 mg/l and has low false negative and positive rates.