According to a study conducted by federal geologists released Tuesday 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.

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.