Estimating Riverine Microplastic Flux by Accounting for Transport Dynamics

Sampling microplastics at the surface of a stream in order to derive average concentrations and estimate microplastic loading without recognition of hydrologic processes is misguided. These methods were adapted from oceanic sampling and are employed for ease. Surface sampling assumes equal concentration of microplastics within the water column. Using a one-dimensional Rouse profile approach we show that such surface sampling methods can introduce up to eight orders of magnitude uncertainty in any assessment of average concentrations. This uncertainty is primarily due to a wide range of plastic polymer types with diverse shape factors and densities that strongly vary in their concentration-depth profiles. For example, many polystyrene products have densities lower than water with near-surface concentration maxima, so estimations using near-surface discharge as opposed to full discharge are more accurate. These results indicate that in order to effectively monitor fluvial microplastic transport we need to assess the hydrologic mechanisms that affect our measurements, correct for them in our reporting of abundance, and develop new methods for assessing the concentration depth profile of microplastics in streams.