Three weeks after Hurricane Katrina, New Orleans is still uninhabitable.  It's a city without clean water or electricity or sewage. As cleanup crews try to bring the city back to life.  Environmental crews are trying to determine what kind of chemicals are being left behind by the receding floodwaters. 

Technicians taking soil samples in New Orleans

Federal and state environmental officials are taking water samples to find out what was released into the floodwaters in New Orleans. Early on, the U.S. Environmental Protection Agency found high levels of lead in the water.

Professor Edward Bouwer, with Johns Hopkins University, studies hazardous substances in urban environments.  He says other contaminants could also remain after the flood.

Edward Bouwer

"Examples would be petroleum compounds, grease, gasoline, even, polycarbon biphenyl compounds and heavy metals like mercury, arsenic, chromium.  Once the water disappears, we will have those in the sediments."

A number of these contaminants cause cancer.  Lead can cause brain damage.

Some residents have been allowed back into the city to salvage their belongings.

Professor Bouwer says parts of the city may be too contaminated to be rebuilt.

"They've detected chromium 6.  They've detected arsenic -- reported to be above drinking water standards -- in the water,” says Professor Bouwer. “These chemicals ultimately will end up in sediments, and if it is high in the water, it will be even higher in the sediments because sediments tend to accumulate.  So that certainly indicates that there are going to be "hot spots" (toxic sites) and the toxic metals are the ones that will persist for a very long time."

The Louisiana Department of Environmental Quality expects to clean up the soil once the water is pumped out and pay special attention to oil spills.

Donald Boesch

Scientists will have to monitor the toxins in Lake Pontchartrain, where the floodwater is being pumped.  But Donald Boesch, head of the Center for Environmental Science at the University of Maryland, says the contaminants shouldn't have much impact on the Gulf of Mexico because very little water flows between Lake Pontchartrain and the Gulf.

"So by and large, I think most experts would expect that most of the contaminants that would escape into the lake will stay there."

But Professor Boesch says if people want protection from hurricanes and storm surges, they will have to preserve natural barriers.

"The biggest concern is what do we do to reconstruct the wetlands that surround New Orleans, which traditionally over the long run have provided this ‘speed bump,’ if you will, that the storm surges would come and slow down, a lot of which has been lost.” Professor Boesch then raised an important question. “How do we restore that and do that in a way that allows this habitable space that we can protect from floods in consort with the environmental restoration."

Canals have been built throughout the wetlands in Louisiana making them ineffective as a barrier to storms.   Professor Boesch says a decision to restore the wetlands will require political will and some changes in the way people live, but he also says those who live in low-lying coastal areas have to be realistic about natural threats and natural protection.