Gdansk water supply system belongs among the oldest in Continental Europe. In 1992 one of the first joint-venture water companies was established in the city. Under a contract concluded between the firm and the municipality, the company was obliged to secure quick and considerable improvement of drinking water quality. At the same time a considerable water consumption decrease was observed. The drop entails new environmental, technical and economic problems. The biggest threat to the supplies of safe and good quality water is the phenomenon of secondary pollution of water resulting from the overdimensioning of the water supply network. Positive aspects of water consumption decrease are related to the opportunity of more rational and sustainable water resources management. The solutions adopted in Gdansk can serve as a starting point for working out the best model for water quality management in the coastal cities.