Characterization of Microplastics and 6-PPD Quinone in a Suburban Lake–Tributary System Impacted by Highway Runoff

Characterizing and enumerating microplastics (MPs) have received considerable attention in recent years. Most attention has focused on major waterways and especially in the water column. However, less is known about MPs in smaller lakes and tributaries, as well as in sediments, under both baseflow and storm flow conditions. This study examines the role of an unnamed tributary in transporting microplastics and 6PPD-quinone from a major state highway into a small, salt-impacted lake in Grand Rapids, Michigan. Water and sediment samples were collected over a one-year period to characterize microplastic types, abundances, and seasonal variability, while environmental conditions were monitored to assess storm event impacts. Results confirmed widespread microplastic presence in both habitats, with abundances ranging from 61 to 16,390 MPs/L. Many microplastics were black fragments, often unidentifiable but suspected to be rubber-derived. Storm events significantly increased microplastic transport, particularly in the tributary, where concentrations peaked at 16,390/L during fall stormflow. Concentrations of 6-PPD-quinone were very low overall (<1 ng/L), with spikes during storm events in the tributary (up to 201 ng/L). Sediment analysis revealed higher microplastic abundances in the tributary compared to the lake, with black fragments dominating in both habitats. These findings underscore the critical role of storm-driven runoff in microplastic and chemical transport, highlighting the need for improved road infrastructure design to mitigate environmental impacts on downstream water bodies.