The Gulf Coast hurricane slammed into one of the most industrialized areas of the United States, home of more than 400 refineries, chemical plants and other facilities that produce, use or store hazardous material, according to Greenpeace, the environmental advocacy group.
The storm caused at least five major oil spills along the Mississippi River south of New Orleans, according the US Environmental Protection Agency.
It swept through 31 Superfund sites -- heavily polluted areas awaiting federal cleanup -- including five in New Orleans. As of Wednesday, the EPA had still been unable to visit all of them and one, in Crescent City, was still under water.
The floodwaters that covered 80 percent of New Orleans, including many of the poorest neighborhoods, may leave contamination from bacteria and pollutants as they withdraw and residents return, some environmentalists fear.
"My fear is that the people who suffered the most when Hurricane Katrina struck will be the people who become most exposed to toxins," said Eric Olson, a senior attorney for the Natural Resources Defense Council.
Federal and state officials say two weeks of sampling floodwaters has not raised any alarm bells, even though it is swimming with e. coli bacteria that may indicate the presence of other bacteria that could be harmful.
The water contains heavy metals such as lead and arsenic, and oil products such as diesel, but not huge amounts, and the evidence shows that people would have to eat the residue or drink the water to be poisoned.
"We are looking for this long-term risk and it doesn't seem to be there," said Dr. Tom Clark, an infectious disease specialist with the Centers for Disease Control in New Orleans.
Heavy concentrations of petroleum products in the muck have complicated the process of sample testing, EPA says.
Evacuated residents of St. Bernard Parish, where nearly 20,000 barrels of crude oil spilled out of a damaged tank, were told that when they returned, they would have to wear rubber gloves and boots and leave children behind to protect them from the residual sludge.
But officials say it is too soon to give a definitive evaluation of damage and even more difficult to say how long it will take to fix any problems.
"Until we have a better handle on what's the magnitude of the problem -- whether it's sediment, whether it's water, whether it's debris issues or whether it's air issues -- it really is impossible to speculate on what it's going to take and what time it's going to take," said EPA Administrator Stephen Johnson.
Questions remain in part because it took nearly a week after the storm struck on Aug. 29 for assessments to begin. Before that, EPA officials on the scene were too busy with rescue operations.
In addition, the heavy concentration of petroleum products in the muck has complicated the process of sample testing, EPA says.
The EPA and the Louisiana Department of Environmental Quality have flown 15 missions with aircraft equipped to collect environmental screening data. So far, nothing of acute concern has been found.
(Additional reporting by Ben Berkowitz in Baton Rouge and Maggie Fox in New Orleans)