Toxicologists and public health experts warned yesterday that pumping billions of gallons of contaminated water from the streets of New Orleans back into the Gulf of Mexico - the only viable option if the city is ever to return to even a semblance of its former self -would have a crippling effect on marine and animal life, compromise the wetlands that form the first line of resistance to future hurricanes, and carry deleterious consequences for human health throughout the region.
The full extent of the danger is unknown and unknowable, but the polluted waters are known to contain human and animal waste, the bodies of people and animals, household effluence, and chemical and petrochemical toxins from the refineries that dot the Gulf coast in and around New Orleans.
Even before the pumping is complete, a process city officials said yesterday would take at least three weeks (some engineers believe it could last months), the consequences for all living creatures - humans, animals, fish and micro-organisms - are likely to be dire.
"We're talking about a mass of decomposing dead bodies and animals. This is going to produce a horrible festering of unknown consequences," said Harold Zeliger, a chemical toxicologist and independent consultant based in New York State.
The waters now swilling around the streets and neighbourhoods of New Orleans will probably end up either in the Mississippi River or in Lake Pontchartrain, just to the north of the city, where they are likely to react with the oxygen in the water and deprive all living creatures, starting with the fish, of the means to life.
"We're looking conceivably at zero-dissolved oxygen, which will lead to the death of fish and other organisms," Dr Zeliger said. "If the migratory birds who pass through the area find any fish to eat, they will be contaminated so the birds will start dying in large quantities ... Reptiles and snakes are going to be driven out of their nests and habitats, which has implications for human safety. We're going to see water moccasins [a highly venomous snake], which are nasty critters, and alligators threatening people."
The prospect of severe chemical fallout overshadowed the cautious progress made by army engineers and rescue workers yesterday. The flood waters began to recede in New Orleans after they successfully plugged the biggest breach in the city's levee system and managed to activate the first water pumps. The breach along the 17th Street Canal, at the eastern end of the city, was responsible for allowing the single biggest body of water to cascade into the city from Lake Pontchartrain, leaving 80 per cent of the city submerged.
But officials are fearful of what they will find once the water level goes down. The human death toll is expected to number in the thousands. Then there is the damage to buildings and artefacts, some of them of immense historical value, wrought by the storm. There is still no fresh running water or air conditioning, while daily temperatures are 90F (32C) or more.
Already, more and more bodies are appearing, floating in the water, or pointed out by people still being rescued from their homes. Some of the bodies have been loaded onto refrigerated trucks and mobile morgues before being identified.
"It's going to be awful, and it's going to wake the nation up again," said Ray Nagin, Mayor of New Orleans, who has suggested that 10,000 may have died in his city alone.
Mr Nagin said that once pumping was completed, it would take several weeks more to clear the debris. Some military engineers are measuring the process in months rather than weeks, and are warning that it could take a year or more before New Orleans was once again habitable in any meaningful sense.
The 17th Street Canal is, moreover, only one of the major problems facing the recovery operation. Other smaller levee breaks in New Orleans defences are still defying the best efforts of the army, which is dropping giant 16,000lb sandbags on damaged sections. Restoring electricity could take up to eight weeks.
The toxic consequences of the disaster will have a profound impact on New Orleans even after the initial clearing is done. Dr Zeliger pointed out that the only way to make the water remotely potable would be to chlorinate it, but given the degree of contamination, this would create its own devastating side effects.
"If one chlorinates poor-quality water, it creates categories of trihalmethanes and other compounds that produce their own nightmarish effects on human health, such as spontaneous abortions," he said. "You'll see the formation of chloroform and bromoform and other toxins. It will be a long time before decent potable water can be drawn - my prediction would be a minimum of one year."
Such warnings have not deterred a minority of New Orleans residents - perhaps 10,000 people - from trying to remain in their homes, or wherever they can find shelter. "We can't force them out," said a member of the Kentucky National Guard, one of thousands of armed guardsmen patrolling the streets alongside army and marines units.
Mr Nagin said it was essential that everyone left so that the repair and recovery could begin. "It's just not safe, leave for a little while," he urged.
Slowly but perceptibly, progress is being made by both federal and state agencies, helped by churches and charities. The emergency medical airlift from New Orleans airport is complete. In some areas, the first financial resources are being distributed to refugees who have lost everything. Across the ravaged region, some communications are starting to be restored.
But thousands of families, if not tens of thousands, remain separated. Many still search for missing relatives. And New Orleans is only one casualty of a storm that, in varying degrees, devastated a region almost as large as Britain. From south-eastern Louisiana across coastal Mississippi, towns and villages have been all but obliterated.
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