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 documentation and scanning electron microscopy analyses after weathering and tumbling revealed that dry weathered plastic experienced greater surface alteration, characterized by pitting, cracking, and grooves, and had greater fragmentation than wet weathered plastic. An item's susceptibility to weathering was primarily controlled by its base polymer type and its surface area-to-mass ratio. Applying a first-order fragment generation model to our specific Mediterranean riparian environment, we estimated that after one year of exposure, a medical mask would release ~2.00 × 10 9 fragments/m 2 and a single-use plastic bag would release ~480 fragments/m 2 . Under wet conditions, the accretion of biofilm on material surfaces likely inhibited degradation and fragmentation. Most wet weathered items also increased in mass and density and lost buoyancy, affecting their potential for riverine transport. Tumbling experiments with sand, cobbles, and water produced the greatest degree of fragmentation in dry weathered materials, which further suggests that macroplastics discarded in open, dry, and sunlight exposed areas may generate far more microplastics during subsequent riverine transport than unweathered macroplastics or macroplastics deposited in wet, stagnant environments. These findings can inform optimal placement and design of trash capture devices, guide waste collection planning, and contribute to policies that prevent debris from reaching oceans and coastal areas.