The water quality modeling capability is accessed from existing HEC-ResSim watersheds. This chapter describes how to activate and implement the new plug-in HEC-ResSim water quality modeling capability, and provides an introduction to water quality mapping capabilities. Note: This manual is supplemental to the HEC-ResSim User's Manual, and users should refer to the HEC-ResSim Version 4.0 for general HEC-ResSim software information.