Coupled watershed and estuary modeling of microplastic transport and retention in the Delaware river basin

Abstract Microplastics (MPs) are an emerging environmental concern owing to their widespread presence and ecological impacts. We develop a coupled watershed–estuary modeling framework to simulate MP transport from terrestrial sources to estuarine systems. The Distributed Hydrology Soil Vegetation Model (DHSVM) is enhanced to represent point and nonpoint MP sources and is coupled with the Finite Volume Community Ocean Model (FVCOM) to capture estuarine transport. Applied to the Delaware River Basin, the framework simulates MP build-up and wash-off, including airborne deposition and wastewater treatment plant (WWTP) discharges. Coupling the estuary model enables a more accurate estimation of MP discharge to the ocean, highlighting retention within Delaware Bay from salinity-induced circulation. MP retention on land is strongly influenced by watershed topography, urban distribution, and source configuration, with the “Lower Delaware” subwatershed exhibiting minimal residence time. These results demonstrate the importance of watershed–estuary coupling for accurate MP export estimates and provide a framework for evaluating MP fate in aquatic environments.