WATER-QUALITY MANAGEMENT IN THE RIVER GUDENAA, A DANISH LAKE STREAM ESTUARY SYSTEM

Waste water purification has been established to some extent throughout the 3,260 kM2 catchment area of the River Gudenaa water system during the last 20 years. Biological treatment and phosphorus removal are now undertaken in almost all towns, and organic matter and phosphorus loading from fishponds and farms has been reduced. The pollution abatement strategy employed has been to coordinate and differentiate waste water treatment so as to maximise the environmental benefit/cost ratio. The time lag between implementing an abatement measure and observing an improvement in water quality varies considerably depending on the nature and cultural history of the water bodies involved and the processes occurring during the passage of water between them. Nutrient transformation and retention within water bodies of the Gudenaa system is of great importance for nutrient loading of the downstream lakes and of the estuary (Randers Fjord), and even for the Danish Belt sea. When loading is reduced retention changes, but to a different extent in different water bodies.