Microplastic characterization and transport mode -A flow-integrated approach to sampling urban waterways.

Concentrations of microplastics are both temporally and spatially variable in streamflow. Yet, an overwhelming number of published field studies do not target a range of flow conditions and fail to adequately capture particle transport within the full flow field. Since microplastic flux models rely on the representativeness of available data, current predictions of riverine exports contain substantial error. In an effort to capture more representative concentrations in streamflow, we implemented a flow-integrated fluvial monitoring program for microplastics within two channels draining an urbanized watershed in Southern California, USA. Concentrations ranged up to 5 orders of magnitude between low flow and stormflow conditions. Tire road wear particle concentrations were significantly higher for the stormwater control channel draining a smaller but more densely developed watershed. Additionally, we leveraged each site's hydrologic conditions and the physical characteristics of particles extracted from depth-integrated samples, in order to compare microplastic Rouse number distributions relative to transport mode. Results indicated that full wash load transport capable of encompassing all types of microplastics was not achieved at either site. Additionally, for the channel with the larger drainage area and less uniform cross-section, the proportion of microplastics transported as wash load was approximately 10-30% lower near the bank compared to the thalweg. The results of this study indicate the importance of considering flow field transport variability when designing fluvial microplastics studies to improve the accuracy of both monitoring and modeling efforts.