The US Environmental Protection Agency (EPA) instituted the Water Security Initiative to design, deploy, and evaluate a model contamination warning system (CWS) for drinking water utilities. Through a cooperative agreement with the Greater Cincinnati Water Works (GCWW), the first pilot system has been designed and implemented in Cincinnati, Ohio. Deployment of this CWS relied on in-depth understanding of the hydraulic and water quality parameters that exist in the GCWW's distribution network. As a result, a tracer test field study was designed as part of the pilot to assess the accuracy of the utility's distribution system model with particular emphasis placed on evaluating transport-related aspects. This field study is considered to be the largest tracer test of a distribution network completed in the United States. Its purpose was unique in that the study was designed to test how well the model would predict conditions in the actual distribution system, rather than being designed to calibrate the model. The field study provided an unparalleled opportunity to improve our understanding of the predictive ability of a network model for representing transport within a very large, complex network. Our experiences are shared to provide any utility interested in improving its knowledge of, and confidence in, its distribution system model valuable insights into the necessary procedures, equipment, effort, and costs.