"We have thousands of people who want to voluntarily evacuate at this time," Police Chief Eddie Compass said. "Once they are all out, then we'll concentrate our forces on mandatory evacuation."

'Unprecedented mess'

Breaks in the weather, nature's resilience and engineering ingenuity could mitigate the size and scope of the pollution and health problem, as they have with some previous natural disasters. But the environmental cleanup will be one of the nation's largest ever, experts say.

"This is an unprecedented mess for the U.S. in recent history, and it seems to be certainly affecting many more people than prior U.S. natural disasters," says Robert Pitt, professor of civil and environmental engineering at the University of Alabama.

Even discounting the area's unique geology and hydrology, officials and other experts say they're dealing with uncharted waters. "If we had never had a levee breach, we still would have had a tremendous amount of water in these sub-basins," says Don Basham, engineering construction chief for the Army Corps of Engineers.

Meanwhile, a warehouse explosion along the river in New Orleans and an oil spill several days after the hurricane passed through have added to the challenge.

"Everywhere we look there's a spill," said Mike McDaniel, secretary of Louisiana's Department of Environmental Quality, in the state's first major assessment of hurricane Katrina's environmental impact. "There's almost a solid sheen over the area right now."

While officials won't know the full extent of the problem until the floodwaters recede — and probably not until weeks or months after that — they do know that the cleanup of what has become an enormous chemical cesspool will be one of the costliest ever. The Corps of Engineers figures that just cleaning up millions of tons of debris — shattered buildings (some with lead paint or asbestos), washed-out motor vehicles, the sodden detritus of private life and commerce — will cost $1.5 billion.

For now, some environmental regulations are being waived in order to address immediate problems. The Louisiana Department of Environmental Quality issued the naval base in Chalmette a variance so it could burn debris, mostly food that spoiled after losing power. The Environmental Protection Agency has waived the need for Clean Water Act permits to allow the pumping of polluted water out of New Orleans into Lake Pontchartrain.

Health emergency

Health and Human Services Secretary Mike Leavitt has declared a public health emergency in five states: Alabama, Florida, Louisiana, Mississippi and Texas. There's no telling when wastewater treatment and other sanitary facilities will become functional. More than 500 sewage plants were damaged or destroyed in Louisiana, including 25 major ones.