Open access publications in the SLU publication database

Abstract

Log yard runoff can be harmful to receiving watercourses, mainly because of the high concentration of organic substances and phosphorus that can cause eutrophication and oxygen depletion. This thesis assesses the effectiveness of soil infiltration and constructed wetlands to purify runoff from two log yards in Sweden storing mainly Norway spruce (Picea abies (L.) Karst). The treatment efficiency of infiltration in a couch grass (Elytrigia repens (L.) Desv. ex Nevski) field was examined by comparing pollutant concentrations in the log yard runoff with resulting concentrations in groundwater pipes. The content of total organic carbon (TOC), total phosphorus (TP), total nitrogen (TN), suspended solids (SS) and distillable phenols was investigated during seven summer seasons. The results showed good purification capacity for TOC, TP and distillable phenols, but the infiltration had no effect on TN and SS. Intense irrigation led to increasing levels of TP in the groundwater after a few years. Reduced irrigation did not increase the purification efficiency for TOC but halted the increase of TP in the groundwater. Infiltration in soil systems planted with four different species (Alnus glutinosa (L.) Gärtner (common alder), Salix schwerinii x viminalis (willow variety ‘Gudrun’), Lolium perenne (L.) (rye grass) and Phalaris arundinacea (L.) (reed canary grass)) was assessed at a field-scale experimental site. No significant difference in treatment efficiency could be seen when comparing the different plants with each other. The infiltration was effective in reducing TOC and TP in the log yard runoff, but could not reduce SS. The self-organizing map (SOM) model was applied to the results to assess the relationships between different water quality variables, and the SOM model performed very well in predicting TN and TP concentrations. A small constructed treatment wetland showed no significant effect when treating log yard runoff, possibly because the hydraulic load on the wetland was too high and the oxygen content too low for the biochemical processes in the wetland to reduce the concentrations of TP, TOC and SS, or because the particles in the runoff do not settle easily. A constructed wetland should either be made larger or supplemented by further treatment solutions. The results in this thesis support the use of soil infiltration rather than wetlands to treat log yard runoff. Storage of aged windthrown logs on both log yards led to significantly higher content of TP in the runoff. Countries with a lot of forest should prepare for storing windthrown logs in a way that reduces the impact on receiving watercourses.