How gravity, wind, rain and surface runoff drive plastic transport on land

To accurately predict the transport routes of mismanaged plastic waste from land-based sources to their sinks, terrestrial plastic transport models require a robust empirical basis. The main driving forces behind the transport of macroplastics on land are assumed to be gravity, wind, rain and surface runoff. However, the underlying transport principles remain undescribed and unresolved. To determine the minimum wind velocities, rainfall, and surface runoff that are required to mobilize and transport macroplastic items, physical laboratory experiments on an artificial hillslope were performed. Four types of macroplastic waste items were used (bottles, cups, food packaging, and bags) while surface roughness (concrete versus grass) and slope angles (0°, 10°, 20°) were systematically varied. Here we present the identified wind, rain and surface runoff thresholds, as well as the relations between the wind velocity and the plastic transport velocity. These thresholds and relations can be implemented in terrestrial plastic transport models to forecast the transport and (re)distribution of macroplastic waste on land due to wind, rain and surface runoff. The overland pathways simulated by these models, reveal where macroplastic retention occurs on land, and where terrestrial macroplastics enter waterbodies. The locations of the terrestrial accumulation zones, and the main entry points into waterbodies are crucial input for the design of mitigation and prevention measures.