The most vivid description of Katrina's floodwater comes from a mystery set in the hurricane's aftermath, The Tin Roof Blowdown by New Orleans writer James Lee Burke.

"The smell was like none I ever experienced. The water was chocolate-brown, the surface glistening with a blue-green sheen of oil and industrial chemicals. Raw feces and used toilet paper issued from broken sewer lines. The gray, throat-gagging odor of decomposition permeated not only the air but everything we touched. The bodies of dead animals, including deer, rolled in the wake of our rescue boats. And so did those of human beings, sometimes just a shoulder or an arm or the back of a head, suddenly surfacing, then sinking under the froth."

This would certainly make you think twice before planting lettuce seeds in the sediment.

The floodwater was indeed dangerous. (Though locals got really tired of hearing outside media refer to "toxic gumbo" and "hazmat soup.") NBC anchor Brian Williams got seriously ill after inadvertently drinking some floodwater.

The surprising good news is that overall, the city's soil is no more contaminated after Katrina than it was before. (Overall because there were isolated oil spills.) The bad news is that, like many older cities, New Orleans has unsafe levels of lead in its soil. Both before and after the flood, nearly half of New Orleans soils exceed Environmental Protection Agency lead cleanup standards.

As for other toxic stuff, the city's soil took about a month to become clear of dangerous bacteria after the flood. Within three months, the salt from Lake Pontchartrain's brackish waters washed out of the soil, according to the agriculture school at Louisiana State University (The lake is essentially an estuary of the Gulf of Mexico.)

There were concerns in the first six months, said Dan Gill, a horticulturalist at Louisiana State University Agriculture Center and author of Month By Month Gardening in Louisiana. During those months, gardeners were advised to wear gloves and masks.

The surge of storm water was relatively clean, and it was preceded by inches of rain, so the contaminants were diluted in a huge volume of water. "The relatively safe condition of the soil was a relief. The city that was so unfortunate in so many ways was fortunate in that," Gill explained.