DALLAS — Hurricane Katrina rapidly is 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.

New Orleans' neighborhoods are awash with dangerous levels of bacteria and lead, and with lower but still potentially harmful amounts of mercury, pesticides and other chemicals. Much will wind up in the soil or in Lake Pontchartrain.

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 gasoline from gas stations and the more than 300,000 flooded cars, likely to add an additional 1 million to 2 million gallons. Nor does it count oil from hundreds of smaller or undiscovered spills.

More than three-quarters of the oil from the Katrina spills had not been recovered by yesterday, 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 Johnson acknowledged the scope of the problem during a news conference in Washington, D.C. He wouldn't speculate on when residents could return or on whether the EPA might sanction lesser cleanups in some residential areas.

"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 Johnson described as "enormous."

Thomas 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," LaPoint said. "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."

New Orleans' air, too, is a source of danger. An EPA plane 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, Johnson said.

For now, the task is to pump billions of gallons of polluted water into Lake Pontchartrain, a brackish body of water that 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 — 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 also were 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. Tests also show that toxic substances will enter the food chain.

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 in the Gulf of Mexico.

But it was in the neighborhoods of New Orleans, steering a rescue boat past the roofs of ruined homes, where 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 Ewing, who works for the Texas General Land Office's oil-spill program. "This time, nobody wanted to put their foot in the water unless they were wearing rubber boots."