Microplastics measure less than five millimetres and are often invisible to the naked eye – but the term microparticles is used when the chemical composition is not clear.
Teams from the San Francisco Estuary Institute and the 5 Gyres Institute studied 12 small tributaries, the water flows from treatment plants, and the bay itself across three years.
Stormwater was the primary source of microparticles in the bay, the team found. Analysis found that almost half of these had a "rubbery texture" and could be synthetic or natural rubber, with vehicle tires one possible explanation.
Using modeling, the team estimated an annual discharge of 11 trillion microparticles into the bay.
"Approximately two thirds of these microparticles were estimated to be plastic, yielding an estimated annual discharge of seven trillion microplastics per year," the report said.
As for the rubber, it could be that rainwater is sweeping rubber stuck to roads into the tributaries and then on the ocean.
Water released from treatment plants meanwhile contained microplastics from textiles, like acrylics and polyester, probably coming from clothes washed in washing machines, and microbeads from hygiene and beauty products.
Polyethylene microplastics, probably from plastic bags and wrap, were also widely found.
The study is an important stepping stone, the authors wrote. "The field of microplastics pollution is in its infancy, and there are not yet widely accepted standards for sample collection, laboratory analysis, quality assurance/quality control (QA/QC), or reporting of microplastics in environmental samples," they concluded.
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