Working for the County of Hawaii, Brown and Caldwell conducted the Hilo Bay outfall mixing zone field study on May 25, 2005. The primary objective was to quantify the dilution of the Hilo Waste Water Treatment Plant (WWTP) wastewater plume in relationship to a 1,000-foot radius around the outfall diffuser using a tracer dye. Treated wastewater from the Hilo WWTP travels approximately 2.6-miles through a pipeline to the shoreline at Puhi Bay near the Pua Sewage Pumping Station. From here, the offshore section extends 4,200-feet offshore. At the end of the outfall pipe is a 225-foot long, 48-inch-diameter diffuser. The diffuser is fitted with 15 five-inch diameter ports spaced on 15-foot centers. The ports extend vertically on three foot tall risers capped with a 90-degree elbow and sit in approximately 50-feet of water. Figure 1 shows underwater pictures of the diffuser ports with ports oriented 45 degrees from the diffuser centerline. Results of the field studies were used to map the horizontal and vertical distribution of the plume and calculate the dilution within the mixing zone. The results were compared with the mathematical plume dilution software models PLUMES, which includes UM and RSB, and CORMIX to identify the model that best describes conditions observed in the field.