Combating highly complex matrices and exploring the human influence on microplastic distributions in Minnesota’s lakes

Abstract

Microplastics have become a major focus of research in environmental chemistry over the past decade due to their potential health risks to humans and their persistence in the environment. Under typical environmental conditions, petroleum-based plastics can take decades to centuries to fully degrade, leading to their accumulation in ecosystems, including surface waters such as lakes and rivers, which are often sources of drinking water. The fragmentation of plastics varies across different climates, influenced by biological, chemical, physical, and compositional factors, making it challenging to model the full degradation process. Over the past decade, microplastic sampling has shifted from marine and large lake settings to areas previously untouched by microplastic research, such as wastewater treatment plants and inland lakes and rivers, which present various environmental sampling challenges.In collaboration with the Minnesota Pollution Control Agency (MPCA), this study develops a new methodological approach to address the complex matrices found in a series of inland lakes in Minnesota. In addition to developing this new method, this study investigates the potential sources of microplastic particles in these lakes by integrating geospatial analysis of high-resolution land use data, census-derived socioeconomic factors, and in situ measurements of microplastic abundance and characteristics. The selected lakes span a range of watershed types, from urban areas in Minneapolis-St. Paul to agricultural regions and remote forested watersheds near the Canadian Shield. This new method incorporates the use of ethanol and an additional 50 µm filtering step following Fenton oxidation to address these matrices. This method was validated through a series of particle recovery tests and method blanks, which highlighted minimal to no particle loss and small amounts of fragmentation/coloring. Further testing confirmed that the method exhibits low to negligible contamination levels. When applied to samples from a series of inland lakes, the method found no statistically significant differences in microplastic accumulation across the study lakes, regardless of surrounding land use. However, size distribution analysis of microplastics in each lake indicates an exponential model, rather than the power law model commonly observed in other aquatic systems, and appears unaffected by land use. These findings suggest that the studied Minnesota lakes are more affected by direct microplastic inputs than by extensive in-lake fragmentation of plastic materials. This aligns with previous research on Lake Superior and the Duluth, MN-Superior, WI Harbor-Estuary, where size distributions in the open lake were better modeled by a power law, while harbor samples showed peaks in larger particle abundances, suggesting a more localized plastic source and less environmental reworking. This study further supports the idea that water residence time is a key factor in understanding microplastic fragmentation.