Tire chemical found in Humboldt stormwater at salmon-threatening levels

The April 14, 2026 Humboldt Waterkeeper and Wiyot Tribe report found a tire chemical in local stormwater at levels that exceeded federal screening values in most of the samples it collected from Arcata, Eureka and other urban runoff sites. Twelve of 19 samples, or 63 percent, were above the U.S. Environmental Protection Agency’s 11 ng/L screening level for 6PPD-quinone, the compound tied to coho salmon kills in storm drains and creeks.

The sampling ran from October 2024 through May 2025 and covered seven small urban streams and four large parking lots draining into the Humboldt Bay, Lower Eel River and Mad River watersheds. Eight of nine samples from Jolly Giant Creek in Arcata exceeded the EPA threshold, and all four parking lot samples were above the LC50 for juvenile coho salmon. It was the first study to examine 6PPD-q concentrations in Humboldt County.

6PPD-q forms when 6PPD, an additive used to keep tires from cracking, reacts with ozone as tires wear down on roads. Rain then washes the particles from streets and parking lots into storm drains, creeks and estuaries. The EPA’s screening values for 6PPD and 6PPD-q, 8,900 ng/L and 11 ng/L respectively, are non-regulatory and non-binding, but are meant to help states, tribes and local governments monitor waterways and protect aquatic life.

Juvenile coho rely on small tributaries to Wigi, Humboldt Bay, for overwintering habitat. Urban drainage in Arcata, Eureka, Fortuna and Blue Lake feeds the same basin.

Urban runoff mortality syndrome was first observed in Puget Sound monitoring between 1999 and 2001 and was linked to 6PPD-q in 2021. Washington State Department of Ecology traced more than two decades of research before scientists pinpointed 6PPD-q as the coho killer in 2020.

The Humboldt Waterkeeper and Wiyot Tribe report called for more sampling, better road maintenance, street sweeping, stormwater capture and treatment such as bioswales, and steps to reduce tire wear.