Pathways of pollutants in stormwater systems

Abstract: The choice of stormwater procedures will affect the pathways of the water and its pollutants, since different stormwater facilities reduce different types of substances and fractions to varying degrees. Traditional stormwater handling has mainly been emphasised to transport the water from the cities. However, during the last year, the interest in environmental aspects has increased. Different ecological solutions, such as infiltration, wetlands, and swales will affect the path of water and its substances. The main objective of this thesis is to characterize and study the pathways of pollutants (heavy metal and organic substances) in stormwater systems. The main focus has been on gully pot water and sediment after it has been removed from the grit chamber. In order to study the pathways of pollutants, a substance flow analysis was done in Hammarby Sjöstad. Since no measurements were possible in the area, theoretical calculations were performed. The results show that, in a separate system with treatment, most of the pollutants will end up in the sediment or soil, depending on which BMP is used. The largest amount of pollutants will be discharged to receiving waters with a separate system without treatment. The samples of gully pot water and sediment were taken during the autumn 2004 in Luleå, Sweden. Three different sites were chosen due to different types of area and traffic loads: a residential area with 500 v/d and two sites in the city with different traffic intensities, 13,800 v/d and 25,500 v/d, respectively. The water samples were analysed for particle-size distribution, suspended solids (SS), heavy metals, PAH, phthalate, and nonylphenols, while the sediment samples were analysed for particle-size distribution and heavy metals. The metals in the water phase were analysed for six fractions: truly dissolved (<1kD), colloids, <0.2 µm, <0.45 µm, and two different extracted fractions. The results showed that the gully pot mixture (water and sediment) contained a wide range of substances. For the different fractions in the water phase, the highest concentration of metals varied between the three sampling sites. All analysed metals were found in the mixture and, out of 29 organic substances, 18 were detected. Most of the metals and PAH were attached to particles in the water. The concentration of PAH was highest in the city with 13,800 v/d. The only phthalates substance detected in all three areas was di-(2-etylhexyl) phthalate and the highest total concentration was found in the residential area. The concentration of total 4-nonylphenol was detected only in the residential area. The sediment's smallest fraction (<63 µm) had the highest concentration of metals. The leaching test indicates that the metals in the sediment are harder bound in the city. However, in the water, the metals are harder bound in the residential area. The water exceeds the guidelines for freshwater for most of the substances, while the sediment only exceeds the guidelines for soil for a few substances. This indicates that the gully pot mixture must undergo treatment before it can be discharged to the environment.