Monday May 18, 2026
San Francisco Estuary Institute —
The chemical, called 6PPDQ, forms in the environment from an ingredient in tires and was first identified in urban stormwater runoff and streams in 2020 by a team of scientists that included SFEI.
“To my knowledge, this is one of the first accounts of this compound in an estuary,” says SFEI senior scientist Ezra Miller. “So we are on the forefront of this as we have been since the beginning.” Estuaries are areas where freshwater and saltwater meet, and the Bay is part of one of the nation’s largest estuaries.
6PPDQ is highly toxic to coho salmon, which are extinct in the Bay but are returning to streams as a threatened species just outside the Bay. Steelhead trout, a threatened Bay species, are also sensitive to the chemical. The data, which were collected by the Regional Monitoring Program for Water Quality in San Francisco Bay (RMP) by SFEI scientists from 2021 to 2024, show the highest 6PPDQ levels close to shore after winter storms—the same time of year that steelhead return from the ocean to their birth streams to spawn. Previous RMP data found even higher levels of 6PPDQ in Bay Area stormwater runoff and urban streams.