TERRESTRIAL TRASH TAPHONOMY: CHARACTERIZING ROADSIDE LITTER AS POTENTIAL SOURCES OF PLASTIC IN WATERWAYS

The accumulation of plastic in bodies of water is well documented, but the role of terrestrial litter sources, transport pathways into waterways, and deterioration of terrestrial litter prior to and during transport are less well known. We collected and characterized roadside litter from 300 m transects along 4 streets in Holland, Michigan that included stream crossings or storm drain inlets. Opposite sides of 3 roads were collected as separate sampling sites, and 4 of these were resampled after ~30 days, yielding 11 different collections.

Litter was unevenly distributed within single transects, with some concentrations of litter (e.g., car fragments, styrofoam packing material, cardboard box fragments) apparently resulting from single, localized releases of material. Despite such idiosyncratic occurrences, factor analysis of collected items placed associated sites (opposite sides of roads and resampled sites) in non-overlapping domains.

Of 11,223 items analyzed, 83.2% were varieties of plastic (38.5% undifferentiated, 37.5% cigarette butts, 7.1% styrofoam), 12.1% were paper, 3.2% metal, and 1.5% glass. In all, 88.2% of items floated in freshwater, suggesting that the great majority of roadside litter is susceptible to rapid transport into and through waterways.

Samples from a residential street grate and storm drain catch basin during scheduled cleaning yielded little litter (1 and 10 pieces, respectively), suggesting that once roadside litter enters the storm drain system it is rapidly flushed to the outfall.

Experiments with a car and lawnmower examined how plastic and styrofoam break when run over. Mowing resulted in extensive, rapid fragmentation, yielding small pieces of styrofoam, thin, pointed fragments of plastic cups, and miniscule, shattered fragments of plastic utensils and drinking straws. In contrast, running over items 3 times with a car traveling at 40 km/hr yielded larger, quadrilateral fragments of styrofoam and plastic cups, while plastic utensils showed little damage and straws showed lengthwise splits but remained essentially whole. These differences may help identify the origins of plastic in water bodies as roadside versus lawn debris, and suggest that removing litter near streams and storm drains before mowing may reduce plastic input into watersheds.