Pollution analysis of lake Mjøvann, Porsgrunn municipality

Summary

A pollution analysis of the drinking water source lake Mjøvann is performed, where possible pollution sources in the catchment area and their effect on the drinking water quality are considered. The water quality of lake Mjøvann is affected by natural inputs of organic matter and to a small extent by human activities. Analyses of phytoplankton and aquatic plants in the lake showed very good ecological status, and a simple phosphorus budget showed that current human activities in the catchment area, including wastewater from a small number of houses and cottages, recreational activities (hiking) and a few grazing animals, do not represent a risk for the eutrophication situation in lake Mjøvann. Such activities, however, can discharge potential pathogenic microorganisms to the drinking water source, and the current restrictions in the water source and catchment area are considered as important. We recommend to further investigate potential discharges of wastewater from houses and cottages to the tributary river Rekua, since the drinking water intake is sensitive for pollution from this river. The extent of future grazing activities in the catchment area should also be followed. The color of the raw water from lake Mjøvann has doubled over the last 20-30 years, and a further increase is expected in the coming decades. Increased inputs of organic matter and climate change can also affect the iron and manganese dynamics of the lake to a greater extent than previously. We recommend that this situation is followed by measuring the oxygen conditions and iron concentrations, as well as color and turbidity, in the bottom water and at the water intake.