Reverse-osmosis seawater desalination processes produce a hypersaline effluent waste (brine) which is usually discharged back into the sea via an underwater outfall. Brine discharges can cause environmental problems due to their high salt concentration, but the effects can be minimised by installing a diffuser structure at the end of the pipeline. In May 2010, a mono-port diffuser was installed at the Nuevo Canal de Cartagena desalination plant pipeline (located in Murcia, Spain) in order to increase effluent dispersion. The aim of this paper was to characterise the brine-plume dispersion throughout a series of oceanographic surveys that were carried out before and after the diffuser installation (2006-2011), and to quantify the effects of the diffuser on the behaviour and dilution rate of the effluent. The results of six years of operational monitoring were also compared with simulations predicted by a numerical mixing model simulation (CORMIX 1), obtaining high concordance with field data. After the diffuser installation, both field data results and model simulations showed a strong increase in the dilution rate of the brine plume, and a significant reduction in the size of the affected area.