Microplastics in stormwater runoff : measuring tire wear particles (TWP) concentration in the road environment and effect of treatment systems

Abstract

Tire wear particles (TWP) are a major source of microplastics that are mainly transported by stormwater from roads to the environment. Their risk has not yet been sufficiently evaluated, mainly because of the lack of suitable analytical methods for identifying and measuring their environmental concentrations. However, the ecotoxicological effects of TWP or chemical substances present in, or on, the particles are widely reported. For example, recent findings attributed the mass mortality of coho salmon (Oncorhynchus kisutch) in the Pacific Northwest region of the USA to the acute toxicity of a by-product of a tire manufacturing additive. Acute toxicity to other fish species such as brook trout (Salvelinus fontinalis) and rainbow trout (Oncorhynchus mykiss) is also reported. Moreover, TWP are persistent in the environment while their generation is increasing, which calls for action to limit their environmental spread. Conversely, stormwater management solutions are becoming a growing fixture in the road environment for their multipurpose role in controlling peak runoff and reducing pollution. However, knowledge of the effect of stormwater management solutions in removing TWP is limited. The overall goal of this Ph.D. study was to introduce a suitable analytical method for detecting and quantifying TWP in the environment and measuring the actual concentrations of TWP in sediments of stormwater management solutions associated with roads. Three study sites and laboratory experiments were used as data sources for the studies included in this thesis (Papers I–IV). Simultaneous thermal analysis (STA) and Fourier transform infrared spectroscopy (FTIR) were used to analyze the samples, and parallel factor analysis (PARAFAC) was used for data analysis. Analysis of variance (ANOVA) and t-tests were used for statistical analysis. In the first part, the laboratory study introduced a method that detected the presence of rubber materials (RM) in formulated sediment spiked with tire granules (TGIS, Paper I). The combination of STA, FTIR, and PARAFAC enhanced the identification of TWP in formulated sediment. The method appeared to aggregate all likely RM (butadiene rubber (BR), styrenebutadiene rubber (SBR), ethylene propylene diene monomer (EPDM), and natural rubber (NR)) present in the spiked tire granules, which is a good attribute given that TWP in road environments originate from different types of tires, whose rubber compositions are varied. The method estimated unknown concentrations of RM by including a minimum of one sample with known RM concentration (reference sample) in the analysis setup (Paper I). However, it is uncertain if the results from the small data size used in the linearity analysis could be applied to environmental samples. It is also uncertain if using a fixed ratio of RM to TWP in the reference sample estimates unknown concentrations accurately since a wide range of ratios has been reported in the literature. Therefore, the method was further improved to suit the estimation of RM in environmental samples with a method detection limit (MDL) of 0.7 mg/g (Paper II). The improved method used the proportionality relationship between component score from PARAFAC model and sample mass loss during heating in the STA following Beer–Lambert’s law. In the second part, we measured TWP concentrations in stormwater treatment solutions; gully pots in Paper II, and bioretention cells in Paper III. The study demonstrated that stormwater management solutions retain TWP. Concentrations of TWP ranging between 0.1 and 15% were measured in gully pots’ sediments (Paper II). The study revealed the association between TWP concentrations, traffic density, and braking/acceleration intensity. The study demonstrated the potential of gully pots to act as temporary sinks. The study further demonstrated that another stormwater management solution, bioretention cells constructed with engineered soil, retain TWP (Paper III). TWP were detected in all size fractions of the bioretention soil. It appears particle sizes and locations of the inlet (entry point of road runoff to the bioretention cell) influenced TWP concentrations in the bioretention cell. In the third part, we tested the removal efficiency of engineered soil in laboratory column experiments and found very high TWP retention of TWP >25 μm (Paper III). The observed high removal efficiency in the laboratory test and measured TWP concentrations in functional bioretention cells showed the potential of bioretention cells in removing >25 μm TWP from stormwater runoff and limiting their spread in the environment. In the fourth part, we assessed TWP concentrations in soil along the racetrack at Rudskogen motorsport center (Paper IV). The study demonstrated the presence of large pieces of tires (4–9 cm) and two different microplastics, namely RM, and tire reinforcement microplastics (TRMP). The distance from the edge of the racetrack and locations along the racetrack has shown to influence RM and TRMP concentrations. Finally, the thesis recommends future research in applying high-resolution STA to further improve the quantification method. Establishing a linear range between scores and concentrations is also recommended to reduce uncertainty in estimates. Studying TWP retention in gully pots in settings where gully pot operations and environmental conditions are controlled is recommended to conclusively determine the influence of traffic conditions. It is also recommended to study the vertical distribution of TWP in the bioretention soil and conduct a long-term assessment of TWP concentrations in the influent and effluent. The thesis further recommends studies on the mass transport in the environment, degradation impacts on TWP concentration over time, the removal efficiency of sweeping, and analysis of finer particles <25 μm to fill the knowledge gaps on the fate of TWP in the environment. Notwithstanding these future research needs, this thesis contributes to the improvement of measuring environmental concentrations of TWP and filling the knowledge gaps with respect to stormwater management solutions’ effect, which has implications for controlling and monitoring TWP in the environment and designing stormwater management solutions.

Dekkslitasjepartikler (DSP) er en viktig kilde til mikroplast, som hovedsakelig transporteres med overvann til miljøet. Likevel er risikoen ved DSP ikke tilstrekkelig evaluert, hovedsakelig på grunn av utilstrekkelige analytiske metoder for å identifisere og måle konsentrasjoner i miljøet. Toksiske effekter er imidlertid ofte rapportert ved at DSP selv eller de kjemiske stoffene i eller på partiklene har negative miljøpåvirkninger. Nylige studier tilskrev massedødeligheten av coholaks (Oncorhynchus kisutch) i Stillehavet, Norvest i USA til den akutte toksisiteten av et biprodukt lekket ut fra DSP. Akutt toksisitet for andre fiskearter som bekkerøye (Salvelinus fontinalis) og regnbueørret (Oncorhynchus mykiss) er også rapportert. DSP-dannelsen øker og DSP er et bestandig materiale, noe som krever tiltak for å begrense spredningen i miljøet. På den annen side er lokal overvannsdisponering i ferd med å bli et vanlig element i veimiljøet, og kan både kontrollere spissavrenning og redusere forurensning. Kunnskapen om virkningen av løsninger for overvannshåndtering i å fjerne DSP er imidlertid begrenset. Det overordnede målet for denne Ph. D. studien var å introdusere en adekvat analytisk metode for å identifisere og kvantifisere DSP i miljøet og måle konsentrasjoner i sedimenter av lokal overvannsdisponering og i jord assosiert med veier. Tre studiesteder og laboratorieeksperimenter ble brukt som datakilder for studiene inkludert i denne oppgaven (Artikler I–IV). Simultan termisk analysator (STA) og Fourier transform infrarød spektroskopi (FTIR) ble brukt til å analysere prøver, og parallellfaktoranalyse (PARAFAC) ble brukt for dataanalyse. Tilsvarende ble variansanalyse (ANOVA) og t-tester brukt for statistisk analyse.