Estuaries are commonly characterised by high productivity due to frequent if not continuous inputs of nutrients from domestic and industrial discharge effluents and from freshwater and marine sources. These forms of nutrient enrichment can lead to eutrophication problems within an estuarine area. Although eutrophication of water bodies is a natural process, it can be greatly accelerated by artificial nutrient enrichment, i.e. nutrient enrichment caused by man's actions. These unnatural increases in a water body's nutrient content can lead to the development of eutrophic conditions with increased plant growth and the occurrence of planktonic algal blooms. However, by using effective management tools, artificial nutrient enrichment may be controlled in such a way as to minimise its environmental impacts on a waterbody.Cork Harbour is an estuary located on the southwest coast of Ireland. At present a number of industrial and domestic outfalls discharge into the harbour, resulting in nutrient enrichment and increased productivity. One area in particular in the northwest of the harbour, the Lee Estuary/Lough Mahon area, has been classed as eutrophic by the Irish Environmental Protection Agency (EPA). It is proposed to construct a new secondary treatment facility in the Lough Mahon area which will collect a number of existing outfalls, treat their wastes and discharge into Lough Mahon. This paper presents a selection of the findings of a numerical model study carried out to assess the changes, if any, in the trophic status of the Lee Estuary/Lough Mahon area as a result of the proposed treatment facility.