Wastewater outfalls provide a natural laboratory for the study of nutrient dynamics in coastal seas, however if properly designed and operated their impact can be difficult to detect. A model was developed and applied to investigate the effect of variation in hydrodynamic conditions on the transport and dilution of a treated wastewater plume in Belfast Lough, Northern Ireland. To validate these predictions we measured the physiochemical properties of the waters surrounding the outfall with a specific focus on inherent plume tracers likely to be relevant to the study of macroalgae (salinity, nitrogen and phosphorus). The model performed well and our data show high dilution of the plume, even under neap-tide conditions. This provides a spatially and temporally defined predictive framework for future studies investigating the compliance of Northern Ireland’s coastal waters with European Water Framework Directive objectives and for feasibility studies investigating macroalgal aquaculture near wastewater outfalls.