Contributing to the assessment of the impact of urban activities on microplastic transport through air and runoff infiltration

Plastics have permeated society and are used in all sectors of industry, from packaging to construction to the automotive industry. One consequence of this extensive, unregulated use is the now ubiquitous presence of plastic debris of all sizes in ecosystems. Plastic pollution, and in particular the presence and effects of microplastics in the environment, has been studied since 1972 and has been the focus of increasing scientific interest over the last two decades. Microplastics have been identified in virtually all environments, from marine sediments to mountaintops, While it is considered to be a dominant source of microplastics, the impact of local urban activities on the fate of microplastics in the environment remains poorly understood. This work aims to provide new insights into the relationships between anthropic activities, specifically road traffic, atmospheric deposition of microplastics, and infiltration of microplastics into roadside soils.Firstly, the methodological choices of this PhD are presented with an insight into the current challenges of microplastic quantification in continental environments in the literature. After collection, samples undergo a treatment consisting of density-based separations with NaI and oxidative treatments. The microplastic content of each sample is characterized and quantified using a micro-FTIR imaging analysis, followed by a post-treatment using the open software SiMPle.The impact of urban activities on microplastics in the atmospheric compartment was then addressed. Microplastic bulk atmospheric deposition was measured over 5 campaigns in 4 different sites of interest, with variable levels of urban pressure, including rural and urban sites. Key results were observed, including significantly lower deposition rates in an urban site during a Covid-related national lockdown (median 5.3 MP m-2 d-1) than in a period of normal activity (median 29.2 MP m-2 d-1). When comparing different sites in similar campaigns, higher deposition rates were observed in urban areas than in rural, agricultural sites, suggesting an effect of local activity on depositions. The characteristics of the deposited microplastics also varied with the level and type of activity. Different dominant polymers, and differences in size distribution, were observed.Thirdly, the infiltration of microplastics and tyre and road wear particles into the soil of a biofiltration swale located on the side of a high-traffic highway was quantified. After manually coring soil samples from the biofiltration swale, both microplastics and tyre and road wear particles were quantified using two specialized analytical methodologies. Significantly higher concentrations were measured on the surface than in the deeper samples, with a clear decrease in number of particles and concentration. This suggests the majority of particles were filtered by the soil and remained close to the surface. Despite consisting in different particles with different sources, similar vertical profiles were for tyre wear particles and microplastics, albeit tyre wear particles had significantly higher concentrations than other microplastics (median 2.32 mg g-1 for SBR, against a median of 0.05 mg g-1 for other microplastics in the surface samples).This PhD contributed to the overall understanding of microplastic transfer mechanics and the role of less-studies microplastic transport vectors and reservoirs in urban environments. In particular, it helped highlighting an immediate impact of traffic on microplastic contamination and accumulation.