FOLLOW THE FLOATING FOAM: STREAM TRANSPORT OF PLASTIC WASTE IN A LAKE MICHIGAN WATERSHED

Plastic and microplastic pollution in the world’s oceans is acknowledged as a global problem. Comparing roadside litter on land to floating litter captured in streams can give insight into the origin, transportation, and alteration of mismanaged plastic waste that ultimately becomes ocean plastic. We manually collected roadside litter at three 300-m stretches of roadside in Holland, Michigan, consisting of two residential streets and a 4-lane road in a commercial area, with the commercial site having more litter overall. Both aquatic and terrestrial litter were dried and processed by photographing, sorting and counting materials, massing various categories such as foam, bottles, and plastic fragments, and conducting a brand analysis when possible.

We used a combination of industrial and homemade litter booms and a floating cage trap to sample floating litter at three field sites in June and July, 2022: a stream draining extensive proximal wetlands (velocity = 0.11 m/s, discharge = 0.32 m³/s, days sampled = 17), a small urban stream integrated into the local storm drain network (0.12, 0.24, 15), and a smaller stream in a residential area (0.15, 0.18, 9). Most sampling occurred during normal flow conditions. In release and capture experiments homemade booms, made of pool noodles strung on rope and sewn into a sleeve at the top of a 30 cm wide strip of burlap, retained 100% of floating test debris but in field use failed under high flow when damaged by woody debris.

Whereas expanded plastic foam accounted for 10.0% of all items of roadside litter, it made up 88.9% of floating litter (range 63–100%) captured in streams. The stream with extensive storm drain input was intermediate in velocity and discharge but had the greatest variety of litter composition. More floating plastic was captured following rain events than under normal flow conditions. Our results suggest that foam is the most mobile kind of plastic waste under normal flow. Other plastic materials may move more episodically during rain events or spring snow melt. Furthermore, cheap homemade litter booms may be useful for monitoring many sites simultaneously during normal flow conditions but more robust and expensive commercial devices may be needed to capture litter movement during high flows, which possibly account for the majority of litter movement.