Vattenkvalitén i Fredstorpsbäcken - dikad bäck på fastigheten Rämningstorp i Skara kommun

Abstract: The estate of Rämningstorp in the region of Västergötland in southern Sweden is administered by the holding trust company Skogssällskapet i Skövde. In 1999 measures were taken by Skogssällskapet to drain the stream Fredstorpsbäcken as a means to increase the forest productivity of the surrounding lands, which are situated on the estate of Rämningstorp. Almost immediately after the drainage, the streamwater turned turbid and a landowner downstream reported the issue to the county administrative board of the region. The aim of this thesis was firstly to analyse the suspended matter that appeared during summer/autumn and secondly to explain why it came to be. Initial hypotheses for the explanation to the greyish turbid water were that it was either 1) a chemical/biochemical precipitate, or 2) an bacterial/algal blooming. The results from soil- and water samples from the area indicated that the peat surrounding the river and the lake Fursjön, has intermediate levels of nitrogen, but high level of ash constituents and relatively high pH values. The water in the stream and the lake also showed high pH and alkalinity, but no signs of an ongoing leaching of mineral nitrogen or phosphorus from the surrounding peat. High levels of iron were found, especially in small secondary streams from the peaty lands and also further downstream in Fredstorpsbäcken. Microscope studies of water and sediment material from Fredstorpsbäcken and element analyses of this material, indicated an ongoing process of biochemical precipitation of iron oxides conducted by iron oxidizing bacteria in the lake and the main stream. The observed phenomenon is probably connected to natural presence of iron in the specific area and to a temporary change in the groundwater hydrology close to the stream, as a result of the ditching in 1999 and clear cutting of forests surrounding the lake Fursjön. This, combined with the fact of dense vegetation in and surrounding the shallow lake, creates a very favourable environment for iron oxidizing bacteria. The turbid waters of the stream Fredstorpsbäcken thus seems to be a result of the biochemical precipitation of iron oxides and the bacteria conducting the oxidation.