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.
"We've gotten rid of the hazardous stuff," Vent said. "My big worry now is ash and silt. Hopefully, people have used their best-management practices to contain any runoff to keep it from storm drains."
In San Bernardino County, removal of toxic ash will not begin until next week. "You don't do 500 homes in a day; you can't," said Peter Wulfman, the county's solid-waste division manager, adding that the county had protections in place to prevent runoff.
San Bernardino County has offered free cleanup to homeowners. So far, 397 have signed up for the program, which is expected to cost $10 million to $20 million.
Arsenic in the ash tested as high as 140 parts per million in San Diego County's Harris fire area. The Environmental Protection Agency's cleanup level for arsenic in residential hazardous waste sites is less than 1 ppm. Those cleanup criteria are not applied to fire sites.
Arsenic occurs naturally in soil, but the Western United States' average is only a fraction of the amount found in the Harris Valley ash and soil.
Lead in ash from the Grass Valley fire was as high as 344 ppm, 20 times the average in Western soil, the USGS report says. Other metals found in high concentrations were antimony, chromium, iron and zinc. No asbestos was detected.
Breathing fine particles of the metallic substances might cause health problems, especially in people with respiratory or cardiovascular diseases. Arsenic is linked to cancer, and lead can damage children's brains.
The most immediate threat is to people handling the corrosive ash or debris. State and county health officials have warned people cleaning it up to wear protective gloves, masks and long-sleeved clothing. The hazardous substances also might reach drinking water supplies. Although the alkalinity will diminish over time and be diluted by heavy rain, USGS officials said they do not know how quickly it will neutralize.
High-alkaline water could be poisonous to wildlife and vegetation essential to its survival.
Some animals in burn areas are endangered species.
Data from wilderness areas is still being analyzed. The pH of ash from those fires was lower, which suggests that the residential fires burned hotter.
USGS teams conducted similar tests after other disasters: the collapse of the World Trade Center, the eruption of Mt. St. Helens and Hurricane Katrina.
Hazardous substances remained in the wake of all of them. Lead levels were much higher in post-hurricane New Orleans than in Southern California's fire areas.
Times staff writers Tony Perry and Janet Wilson contributed to this report.