After the Napa fires, toxic ash threatens soil, streams, and the San Francisco Bay

Adam Rogers, WIRED
By any measure, the fires that tore through Northern California were a major disaster. Forty-two people are dead, and 100,000 are displaced. More than 8,400 homes and other buildings were destroyed, more than 160,000 acres burned—and the fires aren’t all out yet.

That devastation leaves behind another potential disaster: ash. No one knows how much. It’ll be full of heavy metals and toxins—no one knows exactly how much, and it depends on what burned and at what temperature. The ash will infiltrate soils, but no one’s really sure how or whether that’ll be a problem. And eventually some of it—maybe a lot—will flow into the regional aquatic ecosystem and ultimately the San Francisco Bay.

That’s the bomb. Here’s the timer: An old, grim joke about the California says that the state only has three seasons: summer, fire, and mudslides. Those mudslides happen because of rain; the Santa Ana (or Diablo, if you’d prefer) wind-driven wildfires of autumn give way to a monsoon season that lasts through winter and into spring. The rains of 2016-2017 ended a longstanding drought and broke all kinds of records.

Scientists and environmental health agencies know, mostly, what to expect from ash that comes from burned vegetation. But these fires included something a little new. They burned through the wildland-urban interface and into cities. “For how many structures that were burned in fairly small areas in these fires, I think that’s a first-of-its-kind event,” says Geoffrey Plumlee, associate director of environmental health for the US Geological Survey. “The concern is, can they get it cleaned up before the heavy rains come?”