One of the most common uses of water distribution system (WDS) water quality modeling is to simulate the chlorine residual in the network. Most standard software packages such as EPANET model the decay of chlorine using a bulk decay rate and a wall decay coefficient. In EPANET, the bulk decay rate is defined as a single constant for the entire distribution system or can be assigned as different fixed values for specific pipes. If there are multiple sources within a distribution system that produce finished water with different chlorine decay characteristics, it is not possible within EPANET to accurately represent the decay in areas of the system where the water from multiple sources is blended. The Multiple Species eXtension (MSX) to EPANET allows the user to create a simple text file (MSX file) that more accurately represents the bulk decay when there are multiple sources with different decay rates. An MSX file was developed that (1) calculated the percentage of water coming from each source to each pipe over time, and (2) used the percentages to calculate a bulk decay rate for each pipe for each time step as a weighted combination of the bulk decay rate for each of the multiple sources. This MSX representation was tested and demonstrated using the standard EPANET NET3 distribution system example and is currently being applied to a water distribution system at an Army Base. Similar MSX representations can be written to simulate other parameters such as disinfection byproducts.