Field studies were conducted in August 1988 to verify the performance of the new deep sea outfall for the Greater Vancouver Regional District (GVRD) Iona Island Sewerage Treatment Plant (STP). On two days, Rhodamine WT fluorescent tracer dye was injected into the Iona STP effluent under different tidal conditions for about 3 hours. The wastewater plume was then tracked offshore using two vessels equipped with insitu fluorometric and water quality instrumentation and accurate positioning equipment to monitor the plume’s 3-dimensional location and movement. The study determined that the effluent plume was about 40 m thick and remained trapped at least 30 m below the surface. Minimum dilutions within the mixing zone 200 m from the diffuser were generally greater than 370:1. The USEPA initial dilution model ULINE was found to predict dilutions matching field measurements reasonably; however, model-predicted trapping depths were greater than observed trapping depths. The study results demonstrated the effluent discharge system is successfully meeting performance expectations. The validity of available USEPA mathematical initial dilution models was also tested.