Microplastic accumulation and transport in the subsurface under weathering cycles
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Microplastic accumulation and transport in the subsurface under weathering cycles


Microplastics are continuously released into the terrestrial environment from sources near where they are used and produced. These microplastics are conveyed via wind and water to the surrounding areas, where they accumulate in soil, sediment, and freshwater environments. The concentration gradient between terrestrial inland and boundary regions, the factors that influence the concentration, and the fundamental transport processes that could dynamically affect the microplastic transport in subsurface soil are unclear. In urban areas, stormwater control measures can accumulate microplastics, where they can break down into smaller fragments and be transported into the subsurface environment to groundwater. Subsurface soil experiences weathering cycles such as freeze-thaw cycles and dry-wet cycles, which could increase the downward transport potential of the accumulated microplastics. Yet, it is unknown whether or how the fluctuation in the water phase and content during these transient weather conditions could affect microplastic transport in the subsurface. The overall objective of the dissertation is to improve the understanding of microplastic transport and accumulation pathways in terrestrial environments with an emphasis on urban areas and stormwater. The dissertation consists of six research chapters. Chapter 2 estimates the global distribution and abundance of microplastics in terrestrial environments and shows that urban areas and glaciers are the hotspots of microplastic pollution. Chapter 3 calculates the mass balance of microplastics in wastewater treatment plants based on the global data and reveals that wastewater sludge could contain 25 times more microplastics than the reported concentration. Chapter 4 analyzes microplastic sources and types inside urban playgrounds in Los Angeles and shows that playgrounds could contain a higher concentration of microplastics than the surrounding due to extensive release of the microplastics from plastic structures inside the playground. Chapter 5 analyzes microplastic contamination across stormwater treatment systems in Los Angeles and shows atmospheric deposition, not just stormwater, could be the major conveyor of microplastics to the stormwater treatment systems, but the accumulated microplastic concentration decreased exponentially with an increase in soil depth. Chapter 6 shows that freeze-thaw cycles increase the vertical penetration of microplastics in the subsurface, but the effect is more prominent if the subsurface media contain more sand. Chapter 7 shows that microplastics of higher density can be preferentially transported deeper into subsurface during the freeze-thaw cycles. Overall, the results help understand the transport and accumulation of microplastics in urban areas and identify research needs to assess microplastic exposure in urban areas.

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