The dank and putrid floodwaters choking this once-gracious city are so poisoned with gasoline, industrial chemicals, feces and other contaminants that even casual contact is hazardous, and safe drinking water may not be available for the entire population for years to come, state and federal officials warned Tuesday.

Mayor Ray Nagin authorized law enforcement officers and the military to force the evacuation of all residents who refuse to heed orders to leave. Nagin's emergency declaration, released late Tuesday, targets those still in the city unless they have been designated by government officials as helping with the relief effort.

It came as the U.S. Army Corps of Engineers gingerly pumped swill into Lake Pontchartrain, where rising water levels could increase pressure on levees that may have been damaged when Katrina hit but cannot be checked because they are under water on the city side.

As hundreds of police officers, emergency workers and volunteers waded through flooded neighborhoods trying to coax remaining residents from their ruined homes, health officials offered the first tentative assessments of the environmental damage wrought by Hurricane Katrina and its resulting floods: They ranged from contaminated water to the destruction of coastline that acts as a buffer against hurricanes and other severe weather.

The fallout from Katrina continued to buffet Washington; President Bush and members of Congress announced at least three separate probes into the faltering governmental response to the storm and its aftermath. Bush, reeling from bipartisan complaints about the slow federal reaction, promised to lead an investigation to "find out what went right and what went wrong" and informed congressional leaders of a request for as much as $40 billion in additional relief funds.

State officials released new tallies of Katrina's destruction, with up to 160,000 homes in Louisiana destroyed and nearly 190,000 public school students displaced by the storm and its aftermath.

Louisiana health officials reported 83 confirmed deaths but cautioned that the total is likely to soar into the thousands as corpses are uncovered in receding floodwaters. As of late Tuesday, 59 bodies had arrived at a temporary morgue in St. Gabriel, La., that is set up to handle more than 5,000 dead if necessary.

"It could take days, it could take years, it could take lifetimes" to identify some victims, said Bob Johannessen of the Louisiana Department of Health and Hospitals.

Some local officials in Louisiana were adamant in placing most of the blame on the Federal Emergency Management Agency (FEMA) and other federal agencies.

"Bureaucracy has murdered people in the greater New Orleans area," Aaron Broussard, president of Jefferson Parish, said on CBS's "Early Show." "So I'm asking Congress, please investigate this now. Take whatever idiot they have at the top of whatever agency and give me a better idiot. Give me a caring idiot. Give me a sensitive idiot. Just don't give me the same idiot."

Amid the detritus in New Orleans, officials began to shift their focus from rescue to recovery, affixing red tags to floating corpses and noting locations by global satellite positioning for retrieval later. Survivors were being talked out of staying.

Micheline Doley, 24, and three companions agreed to abandon their dry third-floor apartment on South Liberty Street after aid officials warned they would no longer drop off water for them. Doley said she had a battery-powered television, hot water and gas for cooking.

"I didn't want to leave New Orleans," Doley said, carrying a duffel bag after getting off a rescue boat with her friends. "As long as they kept bringing us water, we didn't want to leave. We kept telling them to go help other people."

With tens of thousands taken out of the city in recent days, rescuers said there was a dramatic drop in the number of survivors found Tuesday.

"We're just seeing fewer and fewer people," said Larry Gillian, a paramedic volunteer from the North Arkansas Regional Medical Center. "The two people we brought in today, we've been after for the last couple days to come in and they wouldn't come. . . . Then they saw their friends come in and decided to come in themselves."

Gregory J. Smith, director of the National Wetlands Research Center for the U.S. Geological Survey in Lafayette, La., who was helping in search-and-rescue efforts, warned that parts of the city will be uninhabitable once the waters recede.

"There are concerns about hotspots for disease and for environmental hot spots," Smith said. "Some people left before the storm, some people left right after the storm and there are some people who will only leave under desperation, and that's where we are now."

Although levels have continued to drop in some areas, authorities only began concerted efforts to pump floodwaters into Lake Pontchartrain after plugging the biggest levee breach on Monday.

Only three out of 148 pumps in the New Orleans pumping station are operating, and it could take 80 days before the city and its outlying suburbs to the east are dry, according to Gordon Nelson, an assistant Louisiana transportation secretary. In the city's 17th Street Canal, which had been the site of the most serious break, only 9 cubic feet per second is being pumped out, compared with a potential capacity of 4,600 feet per second, Gordon said.

U.S. Army Corps of Engineers representative Dan Hitchings said 34 portable pumps were being installed, with more on the way.

Tuesday brought the first sign that the number of evacuees housed in hundreds of shelters nationwide may begin to fall off, with the Department of Homeland Security reporting about 180,000 shelter residents, down from about 230,000 the day before. Officials in several states said evacuees are leaving shelters once friends and family members can take them in.

At the same time, the number of storm victims seeking assistance continued to climb. FEMA said more than 360,000 individuals and families had applied for state and federal disaster relief by Tuesday afternoon. That number includes about 270,000 in Louisiana, 70,000 in Mississippi and 26,000 in Alabama, according to FEMA spokeswoman Mary Margaret Walker.

In the first formal assessment of the environmental devastation wrought by Katrina, state authorities in Baton Rouge announced a litany of contaminants likely to be found in the floodwaters, including tens of millions of pounds of concrete, lumber, cars, animal carcasses and all the other solid waste of a major metropolitan area.

Most sewage-treatment plants in New Orleans were destroyed. Two major spills sent 78,000 barrels of oil into Lake Pontchartrain, and fuel has coated the city from 2,200 fuel tanks and leaking gasoline from flooded cars and boats.

"It is almost unimaginable what we're going to have to plan for and deal with," said Mike McDaniel, the state's secretary of environmental quality. "I don't think anyone has ever dealt with this. The tsunami comes to mind."

When residents return to the city, they will probably need to bring in bottled water and other sources of water while the city rebuilds water-treatment plants.

The mix of contaminants poses a serious disease risk to those wading through the filthy water on rescue and body-recovery missions, McDaniel and others warned. Rescuers were urged to get hepatitis and tetanus shots.

In a telephone news briefing Tuesday, Julie L. Gerberding, director of the Centers for Disease Control and Prevention, said "it would not be too surprising" if evacuees living in shelters experience somewhat higher rates of infectious illness.

Respiratory illnesses, including influenza when the season for it begins in the fall, are the most likely, she said. Diarrheal diseases are also possible, especially ones caused by norovirus and the bacterium Escherichia coli. The infections most likely to appear are ones already in the population before the flood, she said, noting that "in the city of New Orleans, cholera has not been present for years."

Other aspects of the environmental toll wrought by Katrina are obvious to rescuers and others traveling the increasingly empty city. On such grand boulevards as St. Charles and Napoleon avenues, the foliage that drapes the majestic oaks and magnolia trees is suddenly turning brown. Birdsong has largely disappeared, replaced with the whine of boat engines and the shouts of rescuers seeking survivors.

Lloyd Thornton, a volunteer from Kemah, Tex., cut the engine on his airboat Tuesday while floating along General Taylor Street. He and his companion, a New Orleans police officer, spotted a towel hanging out a broken window that could have been a cry for help.

"Is anybody in there?" Thornton yelled. "Is anybody home?"

There was no answer. Thornton restarted the engine and continued the search.

Salmon reported from Baton Rouge, La.; Eggen, from Washington. Staff writers David Brown, Juliet Eilperin, Michael A. Fletcher, Spencer S. Hsu and Shankar Vedantam and research editor Lucy Shackelford contributed to this report from Washington.

Firefighters try to contain an early-morning blaze on New Orleans's east side, one of several that were reported across the city. New Orleans artist Sheila Bass lost all her paintings in the hurricane.