Water quality routing in a combined sewer system model was used for flow source identification. Pseudo-pollutants at specific concentrations were employed as tracers to identify source components in combined sewer overflow. Source components include base wastewater flow and wet weather flow contribution from areas served by separated and combined sewer conveyance systems. One application for this approach is performance evaluation of green stormwater infrastructure. This methodology may also be used to evaluate any project that requires tracking various flow sources.