Tracing the horizontal transport of microplastics on rough surfaces

Abstract Occurrence and distribution of microplastics in different ecosystems have recently become subjects of numerous studies. However, to date the research has focused mainly on marine and freshwater ecosystems and widely neglected terrestrial environments. Only recently, first studies investigated the microplastics contamination of soils. Therefore, we know little about the transport mechanisms of microplastics in soils and sediments and virtually nothing about their surface transport. In this study we investigate surface transport mechanisms by tracking fluorescent, irregularly shaped polymethyl methacrylate (PMMA) particles in real time in a laboratory setup. In 108 experimental runs, we vary the irrigation rates, inclinations and surface roughnesses. Additionally, we simulate the small-scale flow patterns to resolve the role of the roughness-induced microrelief. Our results suggest that microplastics are transported along preferential pathways resulting from the micro- and macrorelief, which can be correlated to the flow pattern observed in the computer simulation. Our model study facilitates a deeper insight into microplastic transport on different soil surfaces and serves as a pilot for investigating mechanisms of horizontal microplastic transport. However, microplastics are a diverse group of contaminants with varying shapes, densities and sizes. Therefore, for a full understanding of transport of microplastics in terrestrial environments, it is important to address these properties as well as more variable surfaces for horizontal migration and to include vertical transport mechanisms in future research.