| Summary: | In Sweden and in Finland large forest areas have been ditched to increase forestproductivity. In Sweden more than 1.5 million ha forest have been ditched with a total lengthof about 220 000 km ditches for the last 100 years. Forest drainage ditching results in theerosion of minerals and humus from the ditch slopes and further transport of the erodedmaterial with the ditch water. This probably affects the biological and ecological conditions inthe ditches and brooks due to increased turbidity and sedimentation. Such adverse effects canpotentially be prevented by particle filters. In 2005-2007, the Swedish Forestry Board built ca80 particle filters in Northern Sweden. The effects of these particle filters on water quality arehowever unknown. If ditching is done in acid sulphate soils in areas of for example isostaticland uplift there is also a risk of acidification, which potentially can lead to leakage of metals.The aim of the study was to evaluate if particle filters in forest drainage ditches improve waterquality.The study was performed in a ditch in Alvik, Luleå municipality, during May toNovember 2007 and in three ditches in Tärendö, Pajala municipality in October 2007. TheAlvik catchment is characterized by sulphide soils. In Alvik, only suspended materialdecreased significantly after the second particle filter during the study period. All otherinvestigated elements were unaffected by the particle filters. There were no differences in thewater levels before and after the particle filters, which indicates that the particle filters weretoo loosely packed. The effect of the ditches in Tärendö was difficult to assess. In one ditchboth of the particle filters were flooded and in another ditch, two of three particle filters wereflooded. In the third ditch there was almost no flow. On the basis of this study it is hard to saywhich benefit the particle filters constitute. The construction of the particle filters seams to becritical for their function. In Alvik, the particle filters probably were too loosely ...
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