By Patrick Jackson
The filthy floodwaters that have engulfed much of New Orleans are posing a fresh challenge for the city - where should the toxic mess be deposited?
Fears are growing that the wrong choices now could spark environmental problems for decades to come.
Lake Pontchartrain, the large water mass north of New Orleans, is the focus of many of these fears.
The lake is long used to pollution but was getting cleaner
Engineers need to pump out the water which swept in when Hurricane Katrina's storm surges from the lake brought down sections of its floodwalls on 29 August.
But the last thing the lake and the delicate wetlands of Louisiana and Mississippi need is a tide of urban filth.
The areas have already suffered decades of seeping pollution and erosion.
The Mississippi River might seem a more obvious channel than the lake for the mess, carrying it out to sea.
Yet the lake is the city's traditional drain, and it is impractical to try to pump all the water out to the south.
Sewage and unknown amounts of industrial chemicals float in the stagnant water - along with the unrecovered bodies of the victims. Oil, diesel and petrol from vehicles are adding to the mix.
And the facilities to treat the contamination before pumping the water away are just not there in a city without power.
Scientists cannot yet say for sure how poisonous the water actually is, and city officials have described reports of a "toxic soup" as exaggerated.
New Orleans has no large industrial base, says John Day, a professor at Louisiana State University's (LUS'S) Department of Oceanology and Coastal Studies - but for now scientists "just don't know" what a full analysis of the waters will show.
If no major new source of toxins emerges, the biggest areas of concern will organic waste and oil slicks.
While they may have a short-term impact, these elements should largely break down in the lake water in a matter of months, says Professor Day.
Scientists from LSU have already begun field trips to New Orleans to collect samples for monitoring the level of toxins in the water.
Aerial photographs are also helping them to establish the volume of floodwater.
These images suggest the quantity of floodwater in downtown New Orleans on 2 September was 95 billion litres (21bn gallons, 25bn US gallons), Hassan Mashriqui of the LSU Hurricane Center told the BBC News website.
That represents about 2% of the volume of the lake.
Covering 1,632 sq km (630 sq miles), Pontchartrain is home to more than 125 species of aquatic life, from anchovies to alligators.
1,632 sq km (630 sq miles) - second-largest US saltwater lake
Home to 125 aquatic species including anchovies and sharks
Named in 1699 after a French minister
Wildlife in the wetlands of the lake's basin includes otters and wild boar, ducks and eagles.
The lake is no stranger to pollution from its big city neighbour, but it had actually been getting cleaner in recent years. Six decades of dredging its shell beds to make asphalt and cement came to an end in 1991.
Pontchartrain's ecosystem may have been hit directly by Katrina at the very beginning, when surges of seawater from the Gulf of Mexico arrived, dangerously increasing its salt content.
Certainly, the hurricane itself did serious ecological damage further north, along the Gulf Coast, where a storm wave with a peak of nine metres (30ft) was recorded.
"On the Mississippi coast, the water went in and went out - in New Orleans, it went in and sat there," said Professor Day.
The wetlands, which act as a natural brake on hurricane surges, have been reduced by about 25% over the last century by development.
As a rule of thumb, for every mile of wetlands that a storm surge passes, it reduces the flooding by a foot, the professor says.
He argues that if the US federal authorities had heeded ecological warnings and spent $20-25bn on restoring wetlands in the Mississippi Delta, America would not now be facing a bill of $100bn.
Washington, Professor Day says, must finally take global climate change seriously as the rising sea level and more frequent hurricanes many associate with it impact directly on low-lying areas like New Orleans.