SOURCING, TIMING, AND EXTENT OF MICROPLASTIC MOBILIZATION BY URBAN FLOODING

Flood events are mobilizers of microplastics stored in urban watersheds, transporting higher levels and more physiochemically diverse suites of these pollutants to downstream environments. Our research examines the sourcing, timing, and extent of flood-mobilized microplastics at Deer Creek, an urban stream near St. Louis, Missouri. We collected high-frequency water samples during an October 2022 flood, which produced four discharge peaks in response to 24 h of fluctuating rainfall pulses, with flow ranging from 0.04 m³/s at baseflow to 13.03 m³/s at the highest flood peak. Pre- and post-flood streambed sediment and stormwater runoff samples were taken to assess microplastic sourcing. Flood water samples were analyzed for suspected microplastics (using visual identification) and total suspended solids (TSS) to quantify sediment transport. All water samples (n = 35) contained microplastics, from 1 counts/L at baseflow to 17 counts/L near the largest flow peak. The first several microplastic peaks (>10 counts/L) occurred within 1 h of discharge and TSS peaks. In contrast, the final microplastic peak lagged the last discharge peak by 5 h and had no clear relationship with TSS. Nevertheless, throughout the flood, microplastics, discharge, and TSS were all positively correlated (r > 0.53, p < 0.01), despite their timing differences later in the event. Most microplastics were blue (46%) and clear (33%) fibers, but floodwaters introduced new and varied colors. Preliminary results suggest that sediment resuspension is largely responsible for microplastic abundance and physiochemical variations during urban floods. However, the unique timing of the final microplastic peak might indicate alternate sourcing or controls on microplastic transport in flood events once the sediment supply is exhausted. Ongoing work includes analysis of water samples for stable isotopes and major ions to separate baseflow and event flow contributions to the stream. Sediment and stormwater runoff samples will also be evaluated for microplastics and compared to flood samples to assess inputs from each source. Our study results will characterize sourcing, timing, and extent of microplastic mobilization during floods. These efforts can support management strategies to reduce the transport and impacts of flood-mobilized microplastics.