Marine Debris in River Margins: Wet and Dry Weathering Effects on the Fragmentation and Degradation of Discarded Plastic

Abstract Land-based debris fragmentation has been recognized as a major source of marine microplastics. We evaluated the effects of extended wet and dry weathering of everyday land-based debris items reported in riparian zone trash surveys: cigarette filters, blankets, tents, medical masks, single-use plastic bags, water bottles, and food containers composed of Styrofoam, hard plastic, and “bioplastic.” Newly purchased items were subjected to outdoor wet and dry weathering microcosms over a 271-day period (October-July). To simulate additional fragmentation during riverine transport, tumbling experiments were conducted with pristine and weathered items. Photographic evidence and scanning electron microscopy analyses after weathering and tumbling revealed that dry weathered plastic had greater surface alteration, characterized by pitting, cracking, and grooves, and had greater fragmentation than wet weathered plastic. The base polymer and surface area-to-mass ratios controlled an item’s susceptibility to weathering. First-order fragmentation rates for the dry weathered medical mask and thin single-use bag were calculated as ~ 2.00 × 10 12 mask fragments/m 2 and ~ 1,900 bag fragments/m 2 , respectively, after one year of weathering. Under wet conditions, the accumulation of biofilm on material surfaces likely inhibited items from degradation and fragmentation. Most wet weathered items also increased in mass and density and lost buoyancy, affecting their potential for riverine transport. Tumbling resulted in the greatest degree of fragmentation for dry weathered materials, which further suggests that dry weather has the highest risk of microplastic formation in agitated and dry environments. These findings can inform the design and placement of trash capture devices, guide waste collection planning, and contribute to policies that prevent debris from reaching oceans and coastal areas.