A San Francisco Estuary Institute study found that rain gardens at four San Francisco sites cut levels of 21 toxic contaminants by 90 percent or more — including emerging pollutants like microplastics that the basins were never designed to intercept.

A new study by the San Francisco Estuary Institute found that rain gardens — the shallow planted basins built into SF streets to slow runoff — are substantially reducing levels of 21 different contaminants, with most cut by 90 percent or more, according to a report published Friday by ABC7 News. The list spans regulated substances like PCBs and mercury alongside a category researchers call "emerging contaminants": pollutants not yet covered by water-quality regulations, including microplastics and compounds shed by car tires.

Researchers sampled stormwater flowing both into and out of rain gardens at four locations around San Francisco. It's the performance on that second category — pollutants the gardens were never engineered for — that distinguished the results.

"We were looking at how do these rain gardens do with emerging contaminants that they weren't actually designed to intercept and to filter out?" Melissa Foley, Ph.D., who directs the Institute's Resilient Landscapes Program, told ABC7. "And what we found is that they actually do a pretty good job for most of them, which I think is really exciting and sort of a bonus add to these rain gardens."

Senior Scientist Rebecca Sutton, Ph.D., described the chemical inventory moving through the region's storm drains as broad and still expanding: "All the water flowing through our streets during our rainfall in the winter definitely has a lot of different chemicals in it. Some of these chemicals are regulated, and some of them are what we call emerging contaminants."

The Institute has documented rain garden performance at other Bay Area sites as well. Its El Cerrito Green Streets Pilot Project — 19 bioretention cells installed along the 10200 and 11000 blocks of San Pablo Avenue — found PCBs reduced by more than 90 percent and microplastics cut from 1.6 to 0.16 particles per liter over monitoring conducted between 2012 and 2017. That corridor drains to Baxter and Cerrito Creeks before reaching the Bay.

San Francisco now has nearly 600 rain gardens planned or built, with a city goal of capturing roughly one billion gallons of stormwater annually by 2050. SFEI researchers said they hope the findings prompt other Bay Area cities to treat the basins as a pollution-defense tool — not only a volume one. The data suggest what percolates down through gravel and root systems into the soil largely stays there.