Quantifying Sources, Sinks and Mitigation of Macroplastic and Other River Debris: A Trash Balance Model

Abstract Mismanaged consumer plastics and other waste that enters a river system, known as riverine debris, is a pervasive problem in urban rivers with consequences for ecosystem health and human livelihood. A better understanding of the loading pathways, fluxes, and fate of this debris is necessary for more effective mitigation efforts, and to reduce ocean emissions that become marine debris. This study presents a novel framework for quantifying riverine debris using a holistic mass balance modeling approach, applied in the San Diego River, California, a regionally important river with a large, urban watershed. This framework quantifies urban riverine debris sources, sinks and transport dynamics at the watershed scale. The model integrates a community science data set of floodplain debris with a simple stormwater runoff model to account for debris loading and applies probabilistic transport functions to estimate ocean emissions. Our key finding is that most riverine debris in the San Diego River is not washed in through storm drains but is directly deposited in the floodplain (79%–92% of total debris), with homeless encampments the largest contributing factor (62%–75% of total debris). Ongoing cleanup efforts substantially reduce the debris accumulation rate: without these efforts, debris stored on the floodplain increases by up to 48%. However, despite cleanups debris continues to accumulate over time with the potential for increased ocean emissions in subsequent years, especially during years with large flood events. Our approach is transferable to other urban rivers to understand the fate and flux of local debris, and therefore to inform effective mitigation initiatives.