The Costa do Estoril sewerage system is located on the west side of the Lisbon region, off the mouth of the Tagus river. This system was planned in order to free the pleasant beaches of the region from any wastewater discharge and is prepared to serve, at the final phase, more than one million inhabitants.

In the Costa do Estoril a long sea outfall with two diffusers is used for final wastewater disposal. This system has been operating since 1994. In this paper nearfield dilution corresponding to present working conditions are presented. For simulation of nearfield dilution CORMIX 1 and CORMIX 2 (Cornel Mixing Zone Expert System) were applied. The basic data for modelling was selected according to field studies obtained under an ongoing environmental monitoring program.