Hurricane Katrina is rapidly becoming the worst environmental calamity in U.S. history, with oil spills rivaling the Exxon Valdez, hundreds of toxic sites still uncontrolled, and waterborne poisons soaking 160,000 homes.

Across southern Louisiana, the Coast Guard reported seven major oil spills from refineries or tank farms that totaled 6.7 million gallons, or 61 percent as much as the 11 million gallons that leaked into Alaska's Prince William Sound from the Exxon Valdez in 1989.

The total does not count the gasoline from gas stations and the more than 300,000 flooded cars, which was likely to add another 1 million to 2 million gallons. Nor does it count the oil from hundreds of smaller or undiscovered spills. Altogether, 396 calls had come in to the Coast Guard's national oil-spill hotline by Wednesday afternoon.

More than three-quarters of the oil from the Katrina spills had not been recovered by Wednesday, the Coast Guard said.

The magnitude of the oil spills came into focus with word that laboratories trying to test sediment from newly drained areas were having a problem: There was so much petroleum in the dirt that they couldn't test for anything else.

The Exxon Valdez became the benchmark for U.S. oil spills by leaking North Slope crude into Alaska's cold isolation. This time, the danger includes untreated sewage, cancer-causing compounds, nameless black gunk from rail yards, chemicals used to kill plants or insects, substances that are poisonous even in the tiniest amounts, and decomposing remains.

Environmental Protection Agency Administrator Stephen L. Johnson acknowledged the scope of the problem during a Washington press conference. He wouldn't speculate on when residents could return or whether the EPA might sanction lesser cleanups in some residential areas, in effect turning them into industrial zones.

He said the agency had learned from its post-Sept. 11 experiences in Manhattan not to downplay risks or declare an area safe too soon.

"All of us ... want New Orleans to return to the thriving city that it was before Katrina," he said, but only if the job is "done right and [is] proactive of public health."

Besides the water, the city must deal with a mass of hazardous debris that Mr. Johnson could describe only as "enormous."

Thomas W. LaPoint, an aquatic biologist who heads the Institute for Applied Sciences at the University of North Texas, said history's infamous toxic sites might prove simple by comparison.

"This is pretty much unprecedented," said Dr. LaPoint. "At other toxic sites, such as Love Canal and Times Beach, there was a point source. Here, the potential for contamination is pretty widely spread throughout the area.

"I can't imagine that they could sample, clean up and open up before next summer – half a year, nine months away."

Throughout the Gulf Coast hurricane area, crews were trying to check 466 industrial facilities that had highly dangerous chemicals before the storm.

The EPA also has visited four Superfund toxic waste sites near New Orleans, looking for obvious damage, but hadn't tested yet to see what happened there.

Another Superfund site, the Agriculture Street landfill in eastern New Orleans, hadn't been inspected. The site, where low-income housing and a school were built on or near the waste years ago, is still under water.

Workers were cruising the flooded streets and the drained areas for hazardous material, retrieving more than 5,000 containers so far, including gas cylinders and a medical waste container that they found floating.

The air, too, is a source of danger in New Orleans. An EPA airplane equipped with electronic sensors to spot air pollution detected a plume of chloroacetic acid, an industrial agent and defoliant that poses extreme toxic risks when inhaled.

Ground crews found the source, an open, 55-gallon drum, and secured it, Mr. Johnson said.

Some say the toxic flood has revealed a chasm in agencies' ability to cope with a huge disaster.

"To deal with anything the size of the New Orleans metropolitan area, it's beyond their comprehension," said Willie Fontenot, who worked for the Louisiana attorney general's office for 27 years, helping local communities battle the state's politically powerful polluters.

"We don't have any kind of system set up in the country to do that," said Mr. Fontenot, who retired in April and lives in Baton Rouge. "And it's needed."

For now the task is to pump billions of gallons of highly polluted water into Lake Pontchartrain. The nation's second largest saltwater lake had just begun recovering from ecological collapse.

At the EPA's request, the Army Corps of Engineers put out floating barriers to try to stop some oil and gasoline before it enters the lake. But they won't stop the two most immediate threats in the water, high levels of bacteria and lead.

One site sampled Sept. 3, an Interstate 10 interchange north of the French Quarter, had lead 56 times higher than the amount that would be allowed in drinking water. Other samples taken days later across a much wider area were also high, but not near that mark.

Officials haven't pinpointed a source, but a likely suspect is the lead paint that for decades covered the city's huge stock of old houses. If that proves true, it could reveal problems in New Orleans' performance in lead paint removal, a major public health priority.

So far the EPA has called the most attention to test results that exceeded safe drinking water limits, which are generally the strictest that the government enforces. But that approach obscures dozens of instances in which the water contained lesser amounts of pesticides, metals or other harmful substances that could still cause problems.

For example, at several test sites, the floodwater contained 2,4-D, a widely used weed killer. All were around 3 micrograms per liter, well below the 70-microgram limit for that chemical in drinking water.

However, that federal limit was set not because it's the safest level for people, but because current water treatment technology can't "reasonably" be required to achieve lower levels, the EPA says.

For many of the chemicals in the New Orleans floodwater, the agency hasn't established how much should be allowed in drinking water, so there's no way for the public to tell easily if the measured amounts might hurt people.

Tests also show that toxic substances in the floodwater will enter the coastal food chain.

Several water samples had mercury, a powerful nerve poison, above the amount allowed in saltwater environments in order to protect the long-term health of people eating fish or shellfish.

The results also show gaps in the current knowledge. Tests so far did not look at TCCD, the most widely studied form of dioxin.

Dr. Arnold Schecter, one of the world's foremost authorities on dioxin, said the tissues of fish or people, not floodwater, would be the best place to look for dioxin.

"These are fat-soluble compounds, so you're not going to see much in water or soil or air," said Dr. Schecter, professor of environmental science at the University of Texas School of Public Health at Houston's Dallas campus

Serious dioxin levels have been found in the southwest Louisiana town of Lake Charles, and Dr. Schecter said he'd be surprised if biological monitoring did not reveal a similar problem in New Orleans.

Another concern, he said, is that long-lasting pollutants will remain in higher concentrations and higher toxicity when the water dries up. "The question will be how much will get into people by the three routes: respiratory, gastrointestinal, and dermal or skin."

Blood tests might be necessary, he said.

Those who have been working in the floodwater understand the danger all too well. One is J.T. Ewing, who for his living deals with some of the world's most toxic muck, the pungent and flammable stuff that leaks out of oil tankers.

But in the neighborhoods of New Orleans, steering a rescue boat past the roofs of ruined homes, he didn't want to touch the water.

"Normally, you get your boat stuck on top of a car, which does happen, or on top of a fence, you just put your foot down on it and push off," said Mr. Ewing, who works for the Texas General Land Office's oil spill program.