Water utilities are increasingly concerned with a distribution system's effect on water quality. A water distribution system has come to be viewed as a large reactor vessel that degrades water as it travels through the system. These reactions occur within the bulk phase of the water and also occur at the interface between the water and the pipe wall. To understand and predict these reactions, water quality modeling has been used extensively to study disinfection decay. Current models typically divide this decay into two distinct phases that include: decay occurring in the bulk phase of water; and, decay attributed to a demand exerted by the pipe wall. Transport between these two phases has been described with a variety of advective mass transfer processes, which utilize dimensionless flow parameters such as the Reynolds, Schmidt and Sherwood numbers (1,2,3). Older distribution systems frequently have a predominance of unlined cast iron pipe. These pipes can experience higher rates of disinfectant decay than other classes of pipe such as polyvinyl chloride (PVC) and ductile iron. This is particularly evident in dead end lines or areas of distribution systems with historically low flow patterns. Using the Norfolk Naval Base as a test site, field data was collected from low flow areas of the water distribution system from 1999 through 2002. For unlined cast iron distribution grids, field data exhibited significantly higher disinfectant decay rates than could be explained with conventional modeling. Based on this data, a new disinfectant decay model was developed with additional decay sinks/sources applied to low flow and stagnant pipe sections. This effort identified three other decay sinks that can be incorporated in a decay model. These sinks were identified as: diffusion; iron release from cast iron pipe walls; and, microbial detachment events from pipe wall biofilms. These microbial detachment events (wall shed) were found to be the most dominant decay sink for cast iron pipe in stagnant flow conditions. Microbial detachment events were indirectly measured by collecting and analyzing heterotrophic plate count samples using the R2A agar method. Includes 15 references, tables, figures.