Along with the sewage in the floodwater is a witches' brew of chemicals from a variety of sources, including leaking fuels and oils from gas stations and submerged cars, paints and solvents from small businesses and household cleaners and pesticides from peoples' homes.

But the biggest chemical plants and refineries to the south and east of the city were spared a direct hit by the hurricane. If that had happened, breaches in large tanks and other industrial facilities might have spewed heavy petroleum, hydrocarbons and chlorine gas.

"From the perspective of chemical or environmental contamination, it could have been much worse. One advantage is that we have so much water in the city and that dilutes out the chemicals," Pardue said. "People shouldn't have an irrational fear of chemicals in the water. I'm more concerned about the viral and bacterial things. There's going to be a lot of gastrointestinal and public health issues."

Sam Coleman, a regional director for EPA's Superfund toxic waste division in Dallas, said he could not predict how long it would take to clean, disinfect and then test the hundreds of small community drinking water systems that no longer work because of the loss of power.

"Personally, I've never seen anything like this," he said. "No one has quite seen it this bad."