Ash from wildfires in Southern California's residential neighborhoods poses a serious threat to people and ecosystems because it is extremely caustic and contains high levels of arsenic, lead and other toxic metals, according to a study by federal geologists released Tuesday.
U.S. Geological Survey scientists warned that rainstorms, which are forecast for the region beginning Friday, are likely to wash the dangerous substances into waterways, polluting streams and threatening wildlife.
Some ash collected in residential areas after the October fires registered a pH of 12.7, a level more caustic than ammonia and nearly as caustic as lye.
Metals, particularly arsenic, were found in such high concentrations in the ash that they would violate federal standards for cleaning up hazardous waste sites. Metals could have come from treated wood in decks, old lead-based paint, plumbing solder and other household substances.
Hazardous runoff flowing from the burned areas "is a very substantial concern" for the environment and public health, said geochemist Geoffrey Plumlee, who led the research team at a USGS laboratory in Denver.
The scientists in their report called for concerted efforts to clean the sites before winter rains, and to monitor them afterward.
Local officials in the burn areas are racing to beat the approaching rain but said they will not be able to remove debris by Friday. Some runoff may have occurred during previous storms, the USGS said, but this week's system is expected to bring heavier rainfall.
"No way can we get all the cleanup done by then," said Nick Vent, San Diego County's supervising environmental health specialist. "We could use two dry weeks, but we're not going to get them."
The study is the first major attempt to test ash and soils after California wildfires. The scientists last month collected 28 samples from residential areas burned by the Grass Valley fire in the Lake Arrowhead area and the Harris fire near the U.S.-Mexico border.
"We knew that the ash was going to generate alkalinity because that's a well-known feature," Plumlee said. "But the pH levels were quite high, higher than what we'd seen before, in other burned areas."
Donna Turbyfill, deputy director of the San Diego County Public Works Department, said officials had removed some hazardous materials, including drums of oil, propane tanks and paint cans. But piles of debris and ash remain; the county is in the process of planning dates and locations for people to dispose of it.
