In the future, the Slovene coastal area will be in dire need of an additional source of drinking water. A possible solution is desalination of seawater with reverse osmosis (RO), currently being tested in Marina Koper. In the thesis we analysed the environmental impact of larger brine discharges from a RO desalination plant. Salinity of the discharge was used as the only indicator of pollution in this analysis, and the salinity threshold concentrations used in the Mediterranean area were taken into account. We analysed the impact of the discharge geometry on brine dilution and on the size of the mixing zone (near field) with significantly elevated salinity. Based on the water demand during various seasons, two operating regimes were chosen for the desalination plant: 240 l/s during the summer and 60 l/s in winter. The optimal discharge configuration – the smallest near field that still ensures adequate desalination – was determined. At the full operating capacity in the selected conditions, unfavourable for mixing, the affected area measures around 2100 m2; at the lower operating capacity this area is about eight times smaller. For a more precise evaluation of the dilution processes and the size of the near field, further measurements at the site of the potential desalination plant are necessary.