New Study Shows Rain Gardens Filter Chemicals Out of Stormwater

A new study found that Bay Area local rain gardens are highly effective at filtering harmful
contaminants out of stormwater before it reaches the Bay.

The study, published earlier this month by the San Francisco Estuary Institute, examined rain
samples from four San Francisco rain gardens across 15 rain events between 2022 and 2024.
Twenty-one different contaminants — primarily vehicle-derived chemicals from materials like
tires, pesticides, preservatives, pharmaceuticals, and microplastics — were measured in
stormwater runoff as it both entered and exited the rain gardens.

In many cases, the gardens successfully removed more than 90% of the chemicals.

“We didn’t have a number expectation going into the study,” says principal investigator Melissa
Foley. “So to see, for some of those contaminants, a 90% reduction between stormwater coming
in and stormwater coming out, that was really exciting for the team.”

According to Foley, many modern rain gardens are often specifically engineered to filter out
highly-toxic “legacy contaminants” like heavy metals and synthetic pollutants. But SFEI wanted
to know how rain gardens were performing against “emerging contaminants” — found in
products like cosmetics, paint, pesticides, and vehicle parts — which are suspected of being
pollutants but are as yet unregulated.

The study took a close look at 6PPD-quinone, an especially toxic emerging contaminant that
vehicle manufacturers use to prevent tire degradation. It’s estimated that trillions of tire particles
enter the San Francisco Bay every year. The chemical poses a threat to coho salmon, which are
extinct in the Bay, and steelhead trout.

On average, the four rain gardens studied filtered out more than 95% of the stormwater’s
6PPD-quinone, and were overall even better at removing emerging contaminants than legacy contaminants. However, the remaining amount of 6PPD-quinone was still above the
Environmental Protection Agency’s safe threshold to protect marine life.

The state is working with tire makers to find a safer replacement for 6PPD-quinone, and the California legislature has preemptively passed a law banning the chemical in new vehicles once a substitute is found. 

Still, we know very little about emerging contaminants and the threats they pose to the Bay ecosystem, according to SFEI researchers. 

“To my knowledge, the contaminants that we’re looking at, for the information that we have, are at levels that are OK,” says Foley. “The thing that we don’t know is, what about this urban cocktail of contaminants that’s coming into the Bay? That’s something we don’t generally do a good job of testing.” 

For now, though, Foley and other SFEI researchers stress that rain gardens are a vital piece of a larger urban greenery puzzle. 

Rebecca Sutton, a managing senior scientist at SFEI studying emerging contaminants, describes rain gardens as a “targeted intervention” that can be highly effective when deployed near places like industrial sites or farms that use a lot of pesticides. 

“There’s just so much stormwater in our cities … we can’t make enough rain gardens to handle all that flow,” says Sutton. “But if you have key habitats you’re really focusing on, that might be a great place to establish more rain gardens.” 

Currently, the San Francisco Public Utilities Commission has built or is in the process of building 575 urban rain gardens that will filter 333 million gallons of stormwater annually, with a goal to reach a billion gallons by 2050. Generally, those projects call for replacing the soil every 25 to 30 years, which is in line with the level of contaminants trapped by soil samples analyzed by SFEI, according to Foley and Sutton. There’s not much information available about potential harmful effects on plants in rain gardens, though. 

Foley also notes that, in addition to formal rain gardens, backyard gardens or small patches of greenery maintained by neighborhood volunteers can make a difference in stormwater filtration. The problem, though, is that “greenery is very unequally distributed around the city,” she says, with the south end of San Francisco suffering from much more concrete and runoff than other neighborhoods. So certain parts of the Bay Area could benefit more from the introduction of new rain gardens. 

“I know this study is really about water quality, but I want to make it clear that these rain gardens are so much more,” Foley says. “If properly designed and maintained, they’re also really important in urban spaces for biodiversity support and heat mitigation, and just green space in cities. It’s a multi-benefit solution in cities. They can be a powerhouse.”