Collaborators: Natalie F. Hernandez, M.S., and Elizabeth A. Hasenmueller, Ph.D.

Microplastics, 1 μm – 5 mm plastic particles, are ubiquitous global contaminants. The debris is concerning to human and ecological health due to ingestion by a range of species and presence in commonly consumed human foods and beverages. To regulate microplastics effectively, we must understand their sources, storage, and transport. Small urban streams connect city plastic sources to rivers and the global ocean. These systems experience extreme changes in flow, exhibiting floods that may dramatically enhance sediment and contaminant transport, followed by periods of low discharge when debris may be stored. Erratic flow in urban streams suggests dynamic partitioning of microplastics across sediment and water, influencing stream storage and transport. Our research characterizes the role of an urban stream (Deer Creek near St. Louis, Missouri) in microplastic storage and transport across time and space. Samples were collected March to August 2021 from a single site weekly at low flow and at high frequency during a flood. Microplastics and other geochemical parameters were measured and compared with discharge to assess potential plastic sources including particle suspension within the catchment (e.g., using total suspended solids (TSS)) and wastewater inputs (e.g., using fluoride and optical brighteners (whiteners in laundry detergent)). Microplastics were found in most samples (93%; n = 27) with the highest concentration (35.9 particles/L) found near peak flood flow. For all plastics, the dominant shape was fiber (88%) and common colors were clear (34%) and blue (33%). Total microplastics in flow lower than or near the stream’s average of 0.36 m3/s exhibited no correlations with other geochemical parameters. However, preliminary flood data indicate enhanced microplastic concentrations during high discharge (flow up to 4.87 m3/s) when TSS was elevated. Microplastics in floods are likely sourced from a combination of surface runoff inputs and resuspension of bed sediments. Ongoing analyses of temporal and spatial water and sediment samples will identify sources, storage, and transport of plastic debris across the basin, seasons, and floods. Our study results will clarify how microplastics partition between sediment and water in variable hydrologic conditions, with implications for export downstream.