Sources, contamination, and fate of microplastics and other anthropogenic microdebris in an urban bay: A case study in San Francisco Bay

, Zhu Xia, Munno Keenan, Grbic Jelena, Werbowski Larissa Meghan, Bikker Jacqueline, Ho Annissa, Guo Edie, Sedlak Meg, Sutton Rebecca, Box Carolynn, Lin Diana, Gilbreath Alicia, R.c. Holleman, Fortin Marie-Josee, Rochman Chelsea.

Plastic particles less than 5 mm in diameter, also known as microplastics, have an enhanced ability to interact intimately with a greater diversity of organisms due to their smaller size. To mitigate risk, there is a need to trace microplastics found in nature back to their original sources to inform prevention. The physical and chemical properties of microplastics and their environmental distributions provide clues as to their sources and inform their fate. Here, we present a case study of local monitoring in San Francisco Bay, USA. Surface water, fish, sediment, stormwater runoff, and treated wastewater were sampled across the Bay and adjacent National Marine Sanctuaries (NMS). Appropriate clean-up and quantification procedures were applied to samples, and Raman and FTIR spectroscopically confirmed material types. We found microplastics and other anthropogenic microdebris in all sample types. Concentrations were greater in the Bay than in the NMS, and, within the Bay, greater during the wet season than the dry season. Different morphologies dominated different sample matrices, indicating that the fate of microdebris varied depending on their morphologies and densities: fibers were common in fish, black rubbery fragments in sediment, and buoyant fragments and fibers in surface waters. Stormwater contained predominantly black rubbery fragments and was a significant pathway for microplastics and other anthropogenic microdebris, with concentrations 140 times higher than wastewater, which was dominated by fibers. Thus, potential sources of microplastics and other anthropogenic microdebris to the Bay include rubber from car tires and fibers from textiles. Overall, we demonstrate the value of multi-matrix regional studies to evaluate sources and fate of microplastics, which can inform effective mitigation. We propose strategies to help mitigate microdebris in the Bay including the installation of rain gardens and stormwater catch basin filters as well as behavioural campaigns to reduce the littering of plastic items.

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