Microplastics (MPs), defined as between 1μm to 5 mm in diameter, are abundant within freshwater ecosystems and deposit and accumulate within stream transient storage areas, such as streambed sediments. Pathogenic bacteria use microplastics as a substrate, and therefore MPs can be used as a vector of disease transmission in streams. MPs can both impair the ecological quality of aquatic systems and pose a public health risk. Monitoring programs are often combined with mathematical models to assess risk for a wide range of flow conditions. A hydrodynamic model provides a powerful tool to identify high risk zones of MPs and pathogens in streams, such as hot spots of accumulation within sediments, and to predict the response to dynamic flow conditions. The overall goal of this proposal is to pioneer the development and field validation of a microplastic fate and transport model for predicting the persistence of microplastics and pathogens in streams worldwide, particularly lowland streams prevalent in the UK and Europe. The field study site is the Tame river, a headwater stream in Birmingham greatly impacted by urban influence. The project will assess three main objectives: 1) to accurately predict the fate and persistence of MPs in lowland streams by applying a hydrodynamic model that appropriately characterise their transport and varied residence time based on size, 2) to measure the spatial heterogeneity of MPs and pathogenic bacteria accumulation (separated by size fractions) in streambed sediments and important hydraulic drivers, and 3) improve predictions and fate of both MPs and pathogens by incorporating size-dependent immobilization and remobilization rates into the hydrodynamic model. The proposed project will advance a critical step for ongoing MP research by providing an advanced hydrodynamic model as a tool to improve predictions of MP and pathogen persistence in streams, and a synthesis study to advance knowledge on the fate of MPs in urban streams.