Road runoff quality in cold climates
Sammanfattning: The main objective of the doctoral thesis was to investigate the complex processes and influencing factors affecting snowmelt-induced runoff and snowmelt quality in a cold climate under wintry conditions compared to non- winter conditions in areas with a warmer climate. In order to improve the understanding and knowledge of road-runoff quantity, quality and pollutant transport, snowmelt and rainfall runoff were studied and characterized in the laboratory and in the field, respectively. Measurements were carried out at a field site during 2000, 2001 and 2004. The field site was a small urban runoff plot (540 m2) in the central part of Luleå, in northern Sweden. The runoff plot consisted of a road with a traffic intensity of 7400 vehicles/day and a strip of grassed land on the road boulevard used to store ploughed snow. For the laboratory experiments during 2006, snow was collected from the same field site and melted under defined conditions in climate rooms at Luleå University of Technology. Runoff samples from snowmelt and rainfall were collected and analysed for concentrations of total suspended solids (TSS), total and dissolved heavy metals (Cd, Cu, Ni, Pb, and Zn), chloride, and particles (4-120 µm) as well as for pH and conductivity. The results from the field site investigation showed higher concentrations and loads of TSS, particles, and total heavy metals for the snowmelt runoff compared to the rainfall runoff, and the highest concentrations were found during the rain-on-snow events. On the contrary, dissolved heavy metal loads were higher during the rain period. The metal elements investigated during the snowmelt runoff were more particulate- bound compared to the rain period, which was characterised by a higher percentage of the dissolved fraction. During the snowmelt period, investigated particle sizes and TSS were highly correlated with total concentrations of Cd, Cu, Ni, Pb, and Zn. During the rain period, the correlations between total metal concentrations and the different particle sizes were not as significant. The transport of H+-ions, the load of TSS and the total and dissolved heavy metal loads from the runoff plot to the gully pot showed no tendencies of a first flush, but, instead, showed a uniform transport. On the contrary, the transport of chloride showed tendencies of a slight first flush. Lysimeter setups in a laboratory setting showed that the addition of road salt, the surface slope, and the temperature had a significant influence on the transported TSS load; between 1 to 11 % of the TSS load was transported with the snowmelt compared to the TSS load in the initial snow. The lysimeter with added NaCl had the largest transported load of TSS. The study of a small, medium, and large lysimeter showed that the transported load of TSS in the snowmelt from the initial snow was 3, 3.7, and 4.8 %, respectively. The analysis of TSS underestimates the actual particle load contained in snow and snowmelt runoff, making total particle mass balances difficult to perform. A model of the runoff plot showed that the equations in the model used for describing snowmelt and the build-up and wash-off of particles during wintry conditions did not perform satisfactorily. In regions with cold climate, road runoff processes become more complex, compared to those in temperate regions. The effects that runoff has on receiving waters seem to be particularly severe during snowmelt and rain-on-snow in cold climates. Therefore, the special requirements of cold-climate conditions should be considered in planning processes related to the applicability, operation and need to specially design best management practices, snow-handling strategies, and environmental management practices as well as improving models.
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