Water may undergo a number of changes in the distribution system, making the quality of the water at the customer's tap different from the quality of the water that leaves the treatment plant. Such changes in quality may be caused by chemical or biological variations or by a loss of system integrity. A computer-visualization technique, coupled with a dynamic water quality model for analyzing spatial and temporal water quality variations, was studied at two systems. The two case studies of a contaminant-propagation model application are detailed here. A form of animation was chosen in which the water quality (measured by fluoride tracer) at each sampling station was shown in a "display" circle, using a color to represent a concentration range. Includes 28 references.