California wildfires leave behind toxic ash

As the last flames from the deadly California wildfires are extinguished, the state faces another hurdle in the form of ash — possibly toxic — left behind, WIRED's Adam Rogers reports.

Why it matters: The environmental impacts of large volumes of ash range from contaminated soil to algal blooms. "We're anywhere from five years to 100 years in terms of the longevity of effects," Oregon State University researcher Kevin Bladon told WIRED. "That really depends on the severity of the fire and our ability to get some vegetation re-established on site."

Ash created in low temperature fires, under 840 degrees Fahrenheit, is grey or black and mostly organic carbon whereas ash created in hotter fires is white because most of the carbon has burned away, leaving calcium and magnesium. Depending on the chemical composition of the ash, it will be hydrophobic or hydrophilic. Rainfall will mix with more hydrophilic ash and could enter streams. If the ash is hydrophobic, it will repel the water and may then run off quickly, washing soil away in the process.

• If the ash runs off into streams, it could eventually reach the San Francisco Bay and stimulate algal blooms. With the volume of ash created in the recent fires, these potentially large blooms could "eat all the dissolved oxygen out of a waterway, making it unlivable for everything else," per WIRED.
• If it stays on land, a mixture of calcium oxide in certain types of ash and rain could create a layer of cement-like limestone on forest floors. "Basically, the ash can pave a forest," Rogers writes.

The big question: Scientists have experience with ash from burned vegetation but the recent California fires burned both forests and cities. So the resulting ash will "be full of heavy metals and toxins — no one knows exactly how much, and it depends on what burned and at what temperature."

The bottom line from Rogers: "All of which means the real trick in Northern California will be debris cleanup."