Intrusion of contaminants into a water distribution system (WDS), deliberately or accidentally, is a potential risk facing water authorities and utilities. Deployment of methodologies for water quality sensor placements in water distribution systems is an approach for mitigation of such risks. This study incorporates a methodology for optimal placement of water quality sensors, through including contaminants detailed chemistry reactions. This is performed through using multispecies water quality model (EPANET-MSX) coupled with a statistical dose-response model. At the first stage, a series of contamination events are simulated, and the number of incident consumers' are evaluated. For each contamination event three decision variables are selected: injection location, contaminant type, and injection time. At the second stage, a genetic algorithm (GA) is invoked for selecting the optimal sensor placements through minimizing the average expected incidents. The method is demonstrated on an example application and two methods for events generation are compared and discussed.