Traffic-related pollutants in urban snow : Concentrations, size fractionation, and release with snowmelt

Abstract: In urban areas with seasonal snow, traffic-related pollutants such as solid particles, metals, chlorides, organic pollutants in vehicular emissions, and microplastics (MPs) can be temporarily stored in snowbanks along roads and streets. The concentrations of these pollutants in snow typically dominate the pollution stored in snow and are mostly influenced by meteorological conditions, traffic intensity and composition, and winter road maintenance, including snow removal by ploughing or hauling to disposal sites, and applications of chemical de-icers and traction materials. Thus, urban snow on ground acts as a reservoir storing many pollutants and when it melts, the pollutants accumulated in snow may be released. The resulting snowmelt with diverse pollutants may infiltrate into the ground, or enter storm sewers and eventually be discharged into, and impact on, the receiving waters. To mitigate the associated environmental impacts, it is essential to improve the contemporary management of snow in urban areas by mitigating the following knowledge gaps addressed in this thesis: (i) occurrence, distribution, and size fractionation of conventional as well as emerging pollutants (the former group includes solids (TSS), Cu, Zn, Pb, chlorides and Polycyclic Aromatic Hydrocarbons (PAHs), and the latter includes Tire and Road Wear Particles (T&RWPs), Platinum Group Elements (PGEs), tungsten (W) and antimony (Sb), in urban snow, (ii) Spatial and temporal changes in urban snow quality, and (iii) behaviour of pollutants during snow melting. Thus, these objectives form the core focus of this doctoral thesis. The work presented in the thesis includes a literature review of metal pollution in urban snow, field sampling of urban roadside snowbanks and snow storage piles, and laboratory-scale snow melting experiments. The field sampling included snow sampling surveys at three sites in Sweden – Frihamnen (Stockholm), Umeå and Luleå (municipalities in Northern Sweden) to understand the potential variation in snow quality in terms of solids, metals, chlorides, PAHs and MPs. These field samples of snow were also used for making snow piles serving for laboratory-scale snow melting experiments to understand the fate of various snow pollutants during the snow melting process.The literature survey identified Zn, Cu, Pb, Cd and Ni as the metals most frequently studied in urban snow. However, even though Sb, W and PGEs in urban snow are of environmental concern, they were seldom studied. Consequently, this group of metals was designated here as ‘emerging pollutants’ in urban snow. The average concentrations with standard deviations of various pollutants in urban snow samples collected at three study areas on various occasions were: TSS – 1500±1900 mg/l, Zn – 530±860 µg/L, Cu- 180±280 µg/L, Cd- 0.45±0.56 µg/L, Cr -97±150 µg/L, Pb- 45±76 µg/L and sum of 16 PAHs – 3.5±4.8 µg/L. The emerging pollutants also showed large variations among the sites, with average concentrations determined as T&RWPs - 20000±48000 n/L, TWP - 1300±2600 n/L, RWP - 18000±46000 n/L, other MPs - 24±16 n/L, W- 98±200 µg/L, Sb – 8.4±13 µg/L, Pd – 0.13±0.08 µg/L, Rh – 0.03±0.008 µg/L. In the laboratory snow melting experiment, it was observed that only 10% of total metals, 12% of PAHs, and 20% of T&RWPs were carried away by the meltwater, while the remainder stayed in the (immobilised) sediment residue. The dissolved (<0.45µm) and truly dissolved (<3000 MWCO) metals and chloride exhibited a preferential elution during melting, whereas TSS and PAHs displayed a delayed release. Such research findings underscore the potential for effective pollutant control by implementing appropriate snow management strategies, focusing on the collection of snowmelt residue left on the catchment surface.A comparative analysis of snow quality across six locations in Luleå, sampled over three winter periods within 27 years, revealed significant temporal and spatial variations. These variations were attributed to such factors as differences and variations in meteorological conditions (particularly temperature fluctuations; and precipitation patterns, including distributions of snowfalls and rainfalls), traffic intensity, snow management practices (snow removal and applications of road grit and salt), and evolving pollutant control regulations. The findings underscore the importance of conducting regular urban snow monitoring to assess changes in pollutant levels, detect the emergence of new pollutants, and consequently develop site-specific and up-to-date snow management plans.Investigation of estimation accuracies for pollutant loads in temporary snow storage piles highlighted the critical role of sampling design. Single-column samples were prone to underestimating or overestimating the pollutant loads in snow piles, with variations of up to 400%, observed in the samples collected at Frihamnen. This underscores the importance of collecting and analysing multiple samples for reliable pollutant load assessments.In summary, the thesis contributes to developing a comprehensive understanding of urban snow pollution dynamics and underscores the significance of and need for effective snow management for mitigating urban snow environmental impacts.