Quantifying microplastic fluvial flux from a coastal watershed—A microplastic rating curve approach

As primary conveyers of dissolved and solid materials from the terrestrial to the marine sphere, rivers are dominant transport pathways for both natural and anthropogenic particles, including plastics. Given rising concern over the ubiquity, persistence, and harmful effects of plastics, it follows that researchers are modeling global microplastic fluvial fluxes in an effort to understand the source, transport, and fate of plastic in the environment. However, there are significant data gaps contributing to uncertainty in flux estimates, including selective particle bias introduced through collection methods, failure to account for flow-field variation, and poor characterization of concentrations across the discharge regime. Furthermore, these types of models do not translate well to the local watershed scale, where the potential utility of accurate flux data is the highest. To address this, we modeled the microplastic concentration-discharge behavior near the outlets of two tributaries to Upper Newport Bay, California, USA, using samples collected as part of a flow-integrated fluvial monitoring campaign. Representative microplastics concentrations collected from a range of flows were paired with streamflow discharges and fit with rating curves. By integrating over the 2021 Water Year discharge record, we calculated a total flux to the Bay of 23-41 billion microplastics and found that despite the infrequency of rainfall in the region, the majority of the total microplastic load was transported during stormflows at each site.