Lake Vanern is the largest lake of Sweden and the fourth largest lake of Europe. It is situated in a region with Archaean rocks, but the deposits in the drainage area mainly consist of moraines poor in nutrients. A rift zone of north-south extension divides the lake into two major basins, Varmlandssjon and Dalbosjon. The numerous small islands form a considerable archipelago in relation to the open water areas. The good water exchange between the archipelago zone and the open water is, however, decisive for the generally extensive spreading of pollutants.

The main problems associated with water quality in the lake are due to outlets from pulp mills mainly localized on the northern shore of Varmlandssjon. Effluents from these industries contain high concentrations of organic material with lignin components and contribute to the brownish colour of the water. Pollution of another kind is discharged from a chlor-alkaline plant situated in one of the northern bays of Varmlandssjon. This factory caused serious mercury contamination of water, sediments and fish.

Lake Vanern is of special interest since it is large enough for geostrophic currents to occur, and this phenomenon has been measured in the basin of Varmlandssjon (Kvarnas, 1975). The circulation is anticlockwise and lasts throughout the summer. The vertical transports in the lake are of little importance. Twice a year, however, in spring and autumn, the water masses are totally mixed, but in the summer a thermocline is established. In the spring the heating of water is fastest in the shallow areas and therefore a zone with warmer water appears around the margins of the lake. This thermal bar temporarily retards the water exchange between the coastal zone and the open lake, resulting in an obvious quality difference between these two areas.

There are more than one hundred tributaries, the main one flowing into the northern part. Lake Vanern drains into the Kattegat (the Atlantic) via the Gota River and the drainage area of these two bodies cover 10% of the total area of Sweden.