Snow quality in urban areas
Abstract: The studies reported on this thesis aim to provide information and increase knowledge about the quality of urban snow and the path of pollutants. The results develop a basis for decisions regarding the prevention of environmental damage. A number of projects have been carried out to provide more detailed knowledge about: snow quality in cities, sediments that had accumulated during the winter, and that were left at the surface when the snow had melted, and the paths of pollutants at urban snow deposit. For these purposes studies have been performed in laboratory, pilot and full scale. The snow quality was studied in the city of Luleå with regard to variation with time and area. The aim was to investigate urban snow in its actual environment, throughout an entire winter season. Snow samples were taken in a housing area and in the city centre, at sites with different traffic load. It was found that traffic or activities related to traffic was a major source of heavy metals and phosphorus in urban snow, both in the city centre and the housing area. Also the type of area and the design of the street were important for the snow quality. A clear relation between the pH and the quantity of suspended solids in the snow was found. Higher pH values were found in snow from sites with higher traffic loads, compared to sites with small quantities of particles which were the no-traffic sites. The dissolved fractions showed more complicated results. The concentration of the studied substances in the snow was affected by the precipitation at the site with no traffic. At the street where the snow was left beside the street, the concentration of substances increased with time, while the precipitation was of less importance. The mass loads of substances increased with time for both the streets, where the snow stayed in the vicinity of the street, and for the non-traffic site. For the site with traffic the snow handling operations evidently affected the mass loads. This was especially the case for particle-connected substances. As a result of the increased interest in reuse of friction material the sediment which remained along the streets after snow melt was studied with regard to physical and chemical characteristics. The side walk affected the amount of substances on the street more than the traffic load. For the streets without sidewalk an increased traffic load increased the amounts of sediment. The particle size distribution of the sediments swept from the street with a sidewalk differed from the distribution at the streets without a sidewalk. With two exceptions the smallest size fraction (>75 µm) had the largest concentration of heavy metals. For composite samples the concentration increased with traffic load. The pathways of pollutants in urban snow deposits have been studied in three different scales; laboratory, pilot, and full-scale. The initial snow, melt water and sediment were analysed with regard to chemical content. It was found that the dissolved substances to a great extent folowed with the melt water while for the particle bounded substances as much as 90-99% stayed in the sediment below the deposit. The dissolved substances left the snow in the beginning of the melt period, while the substances connected to particles showed a uniform release. The laboratory study showed that melt- freeze cycles delayed the release of substances. Almost all substances were attached to particles in the snow, while in the melt water a large part was in solution. It should be noted that a so-called acid shock was not found during these studies.
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