Biogeochemical Effects of Particle Filters in Forest Drainge Ditches
Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
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 packed and inTärendö the particle filters were too small for working satisfactorily.The influence of the acid sulphate soil was significant in the Alvik ditch. The pH waslowest during May (mean 4.5), at the same time sulphur and metal concentrations were high(S mean 17 mg/l, Al mean 2464 μg/l and Zn mean 41 μg/l). During summer (June-September)the pH increased and sulphur and metal concentrations decreased (pH mean 6.2, S mean 10mg/l, Al mean 950 μg/l and Zn mean 15 μg/l). In November the trend reversed again, pHdecreased and sulphur and metal concentrations increased again to the highest values for someelements (pH mean 4.8, S mean 24 mg/l, Al mean 2880 μg/l and Zn mean 47 μg/l). pH wassignificantly negatively correlated with S and with several metals (Al, Cd, Co, Mg, Mn, Niand Zn). Sulphur and the metals were significantly correlated to each other. These resultssuggest that sulphate oxidation occurred in the soil which affected the geochemistry in theditch water. After drainage ditching the groundwater level sinks and the sulphate soils areoxidised. After oxidation, hydrogen ions and metal ions leach from the soils when the soilsagain are saturated by melt water. Because of this, the pH will decrease and further metal ionsare leaching from the soils. The leaching and transport of metals from the ditch in Alvik wassignificant. The transport of S in November was estimated to be ca. 11 kg/day. The annualaverage transport of S was ca. 2200 kg/year from the ditch in Alvik. The transport of metals inNovember in Alvik was for Al ca. 1 kg/day, for Zn 22 g/day and for Cd 64 mg/day. Theresults suggest that drainage ditching in areas containing acid sulphate soils should beavoided.
Place, publisher, year, edition, pages
2011. , 46 p.
Life Earth Science
Bio- och geovetenskaper
IdentifiersURN: urn:nbn:se:ltu:diva-53920Local ID: ae71f9c7-5d92-4016-8580-42abc79ce635OAI: oai:DiVA.org:ltu-53920DiVA: diva2:1027297
Subject / course
Student thesis, at least 30 credits
Validerat; 20110630 (anonymous)2016-10-042016-10-04Bibliographically approved