Ash from wildfires in Southern California's residential neighborhoods poses a serious threat to residents and ecosystems, according to a new study published by The Los Angeles Times on Wednesday.
Such ash is extremely caustic and contains high levels of arsenic, lead and other toxic metals, federal geologists said.
Scientists from the U.S. Geological Survey (USGS) 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, said the study.
Metals, particularly arsenic, were found in such high concentrations in the ash that they would violate federal standards for cleaning up hazardous waste sites, according to the study.
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.