Wildfires Leave Behind More Than Ashes

But what people may not know is that the wildfires are also causing damage on an "elemental" level — that is, in increased amounts of elements such as iron, aluminum and mercury accumulating in watershed systems after a fire. Enhanced concentrations of such elements in stream water adversely affect the quality of downstream water supplies and the rate of vegetation regrowth.

After the Topanga Fire of 2005 — which scorched areas in western Los Angeles County, including Malibu Canyon, Agoura Hills and Calabasas — Assistant Professor Terri Hogue of the Department of Civil and Environmental Engineering collected and studied soil and water samples taken from the watershed area of Malibu Creek. She found that the mercury levels in the creek had risen significantly, a discovery that caused some concern.

"All natural systems contain metals; for example, even rocks in some geologic areas contain arsenic," said Hogue. "But we are concerned about mercury because it is toxic once it bioaccumulates and gets into food sources that humans consume."

Mercury levels increase as they move up the food chain, beginning with the tiniest animals as they are consumed by fish; those fish, in turn, get eaten by larger fish, and so on. That fact becomes significant when you consider events such as the Day Fire of September 2006, which consumed more than 160,000 acres of northern Los Angeles and Ventura counties. The Day Fire was upstream of the Pyramid Lake area, which serves as a reservoir for drinking, fishing and other recreational uses.

Recently, Hogue, a physical hydrologist, teamed with Assistant Professor Jenny Jay, an environmental contaminant specialist, to investigate mercury cycling in post-fire watershed systems. After studying the soil samples from the 2005 Topanga Fire, Hogue and Jay saw that there was a dramatic loss of mercury at the soil surface in the burn areas.

They hypothesized that this was due to the vaporization of surface mercury into the atmosphere, along with the erosion and transport of mercury into streams during post-fire rain. Water samples taken from Upper Malibu Creek after the Topanga Fire revealed elevated mercury levels — 30 times higher than samples taken from a neighboring control stream.

Based on these findings, Hogue and Jay received an $80,000 Small Grant for Exploratory Research from the National Science Foundation in January to investigate the impact of the 2006 Day Fire. They and their team — Ph.D. student Megan Burke, master's student Marcia Ferreira and undergraduate students Carolina Mendez and Bridget Navarro — are also investigating the post-fire conditions that may contribute to the transformation of elemental mercury into methylmercury, which Jay says is an especially toxic form of mercury.

"It's mostly mediated by bacteria that live in anaerobic conditions — those with no oxygen, like mud at the bottom of lakes," she said.

Already, the Hogue-Jay team is putting its research to practical use: They are collaborating with the National Weather Service and the U.S. Geological Survey's Debris Flow Task Force to improve post-fire debris flow warnings issued to the public through the NWS offices in Oxnard and San Diego.

"The big picture is trying to understand the processes that control mercury transport in post-fire stream systems," Hogue said. "If we can understand how metals are transported — and how they are converted into toxic forms — and tie the two together, we'll understand how we might have toxic forms of mercury in our stream and reservoir systems when fires do occur."

For more information about Hogue and Jay's wildfire research, visit their research Web site: http://www.seas.ucla.edu/~thogue/FireProject.html