7 Trillion Microplastic Particles Pollute the San Francisco Bay Each Year

San Francisco Bay, like Monterey Bay to its south, is a rare success story in ocean conservation. In the 1960s, three grassroots activists—Sylvia McLaughlin, Kay Kerr, and Esther Gulick—launched Save the Bay, which beat back developers trying to fill in parts of the iconic body of water.

But also like Monterey Bay, San Francisco Bay has all the while been poisoned by an invisible menace—microplastic. Yesterday at a summit in Berkeley on the east shore of the Bay, the San Francisco Estuary Institute and the 5 Gyres Institute presented findings from a three-year survey of microplastics in Silicon Valley’s massive watershed.

The news is grim: an estimated 7 trillion (yes, with a “t”) pieces of microplastic are entering the Bay via stormwater each year, much of the debris likely coming from car tires, with treated wastewater contributing another 17 billion (this time with a “b”) particles, largely from synthetic fibers in clothing. They found particles everywhere—in surface waters, in sediments, in fish. The less grim news: The research inadvertently discovered a way to help stop microplastics from reaching aquatic habitats, and it’s bringing some rigor to the still-nascent field of microplastics research.

An important definitional distinction here. The researchers found microplastics and microparticles, both of which are bits of debris less than 5 millimeters long. The former they were able to test in the lab and confirm as plastic, while the latter they eyeballed and determined to potentially be plastic. Take, for instance, a fiber of dyed blue cotton versus a fiber of blue polyester—they look similar, but the cotton is more or less organic, while the polyester is industrial. Given the number of particles the team collected, it wasn’t feasible to test every microparticle to confirm it is indeed a microplastic. So while they’ve determined that stormwater flushes 7 trillion microplastic particles into the Bay each year, the figure is more like 11 trillion for microparticles in general.

In their surface water samples, the researchers found 74 percent of particles to be fibers (for example the bits that come off clothing), more than half of which they identified as plastic. In the samples of sediment they studied, the researchers found mostly fibers, 20 percent of which they confirmed to be plastic.

Interestingly, in sediment the researchers also found high concentrations of black, rubbery fragments, much more so than they found in surface waters. They concluded these are likely bits of rubber flying off tires, washing into the Bay, and sinking down to the muck. Because they are less buoyant, these chunkier pieces are more likely to sink to the bottom.

By examining where the particles came from—stormwater versus wastewater—they could edge closer to understanding how and when plastic starts disintegrating into smaller pieces. “We can look at these types of fragments and try to tie them to the breakdown of these plastic items,” said Carolynn Box, of the 5 Gyres Institute, during the summit. “We can look at these polyester fibers and try to link them upstream to clothing, bedding, carpets, and so on.” Then we might put in place better controls in wastewater treatment plants to filter out more microparticles.

It’s these kind of upstream interventions that scientists are increasingly urging as a way to stem plastic pollution, instead of focusing all our time and money on reactive efforts like the Ocean Cleanup campaign and its giant U-shaped catcher in the Great Pacific Garbage patch. That device broke on its first deployment, after failing to actually catch plastic. Just yesterday the campaign says its retrofitted catcher has begun ensnaring plastic in the Patch. Even so, floating catchers are no match for the ocean plastic problem, with much of the gunk swirling thousands of feet deep.

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