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Microplastic transport and fate in an Urban Estuary
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Scientists studied how tiny pieces of plastic pollution move through and get trapped in an estuary (where rivers meet the ocean) in Southern California. They found that these microplastics build up in muddy areas and marshes, especially during storms when more plastic washes in from rivers. This matters because estuaries are important sources of seafood, and understanding where plastic pollution accumulates helps scientists predict contamination risks in fish and shellfish that people eat.
Plastic transport and storage dynamics in estuaries have important implications for environmental risk in these systems and also modulate the transfer from terrestrial to oceanic spheres. Here we synthesize fieldwork, theory, and experiments with numerical modeling to elucidate the source, transport, and fate of microplastics in an urban estuary in southern California. Riverine concentration-discharge models based on streamflow sampling are used to estimate microplastic flux at the two major riverine inputs to the estuary. Utilizing the Delft-3D hydrodynamic model coupled with particle tracking, microplastic transport is simulated for a dry and wet Water Year (October - September). Subtidal sediments collected from the estuary support modeled results of microplastic accumulation rates in bed layer sediments. Intertidal sediment cores collected within high and low areas of the saltmarsh and dated using fallout radionuclide analysis revealed the present and historical influence of stormflow event-driven suspended sediment (and microplastic) mobilization as well as the effect of dredging-based sediment management on microplastic accretion. Additionally, we investigate the importance of tides (i.e., bi-directional flow, phase, and range) and stormflow peak discharge on determining particle transport distance, flushing, and areas of peak accumulation. Local hydrodynamics and particle characteristics are also examined to contextualize observations of spatial partitioning of microplastic types. Finally results of this study are considered to inform future microplastic and sediment management in watersheds and estuaries.
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Estuaries — where rivers meet the ocean — act like traps for microplastics, with tides, sediment, and shoreline interactions causing much of this plastic pollution to get stuck rather than flushed out to sea. This matters because these are often the same waters used for fishing and shellfish harvesting, meaning trapped microplastics may build up in local food sources over time rather than dispersing and diluting in the open ocean.
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