As the floodwater levels continue to drop in the Greater New Orleans area, concern is rising about Katrina's environmental consequences. Search and rescue efforts are evolving into the grim task of corpse recovery, identification and disposal. Much of the city remains a ghost town, as police and military authorities maintain a dusk-to-dawn curfew. Heavy equipment operators are pushing heaps of rubble-the current Louisiana Dept. of Environmental Quality estimate is 22 million tons-into piles for eventual disposal. LaDEQ officials are now selecting sites. Crews are spraying standing water with pesticides to suppress mosquitoes.

The most immediate threat is from the polluted floodwaters that are now flowing back into Lake Pontchartrain, the Mississippi River and the marshes in battered St. Bernards and Plaquemine parishes. It is a nasty bouillabaisse of chemicals, raw sewage and spilled petroleum products.

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One Superfund site is under water, says an EPA official. So are several wastewater treatment plants and a number of industrial and chemical plants. High fecal coliform counts also suggest that sewer pipes may have ruptured, but this is one of many questions that cannot be answered for certain until the water goes down.

For years the U.S. Environmental Protection Agency and New Orleans have been at odds over discharges of effluent, mostly from combined sewer overflows during storm events. In the late 1990s, a consent decree forced a multimillion-dollar, 10-year improvement program. The city was roughly halfway through the program and had sunk about half a billion dollars into sewer pipelines and wastewater treatment plant expansions. Water quality in Lake Pontchartrain and other nearby estuaries improved almost immediately. Early last month a Lake Pontchartrain Basin Foundation press release hailed the return of manatees in large numbers, a sign that the lake was healthier than it had been in years.

That was before Katrina punched five holes in the northern floodwalls of the Crescent City, allowing millions of gallons of water to flood up to 80% of the bowl within the below-sea-level perimeter. With two months left in the hurricane season and no realistic treatment option available, the U.S. Environmental Protection Agency granted a discharge waiver, allowing the return discharge to flow into the lake without treatment. It was either that or discharge directly into the river, said EPA Administrator Stephen Johnson.

Discharge to the lake is the lesser of two evils for three reasons: New Orleans draws its drinking water from the river. Existing river flow is connected to a large hypoxic zone in the Gulf of Mexico and dumping an additional slug of pollution surely would not improve matters. Finally, it is better to contain the contamination in a single basin until the extent and the nature of contamination can be characterized. If remediative measures are possible, treatment in the lake would be easier than in the river.

A huge state and federal monitoring effort is under way, says Sam Coleman, the senior federal official for EPA. "We have more than 150 people deployed here to work with the state and determine the type of (contaminant) loading within the lake," he says. The agencies are working in tandem, Coleman says, with LaDEQ sampling the lake and EPA monitoring the standing water in New Orleans. EPA found a lead spike 56 times the agency's action level at one sample take Sept. 3. Pesticides and industrial chemicals also turned up, but not above the action threshold. Biological samples taken from Sept. 3-5 also revealed levels of fecal coliform and E. coli that were at least 10 times what EPA considers acceptable limits. Test data are online at www.epa.gov.

Edward Laws, dean of Louisiana State University's Coastal Science Institute, says he wouldn't be surprised to see fish kills from dropping dissolved oxygen levels in Lake Pontchartrain. DO normally drops as decaying organic matter consumes oxygen. LSU took samples at the 17th St. Canal Sept. 9, Laws says. The salinity level is 5-8 parts per thousand, higher than normal. "Two percent of the lake's volume got into the city. That's a dilution factor of 50, even if it were mixed thoroughly. When it is pumped back into the lake, if it remains anoxic, we can expect a fish kill," he says.

Sulfates could turn into hydrogen sulfide-sulphuric acid-and cause algal blooms. In the long term, worst-case scenario, this could generate neurotoxins that can cause respiratory problems in humans. If the contamination is bioaccumulative, it could wreck the oyster industry in Lake Borgne. Once the water is gone, he predicts, the mold issue will move to the fore. "Those wood houses that have been standing in water all this time will be extremely prone to mold," he says. Many will be razed, he says. But who will make that determination is a thorny issue. President Bush reiterated Sept. 12 that the city will take the lead in its reconstruction.

EPA's Coleman is more optimistic about the lake's chance of recovery. "Nature is pretty resilient," he says. Water quality samples should indicate if there will be more persistent problems. "The state, thanks to their own work and the Lake Pontchartrain Basin Foundation, has an excellent baseline of data," he says. EPA and LaDEQ will be adding more of their own samplers and hiring more private firms as well, he says. Officials are working from U.S. Geological Survey GIS overlays that show chemical and industrial facilities, wastewater treatment plants and, so far, one Superfund site in inundated areas.

EPA and the Centers for Disease Control have issued warnings to avoid contact with the water. Police, military workers and volunteers were trying to do that, for the most part, but many of those that did get wet quickly rinsed themselves with disinfectant bleach solution. Medical facilities in the city and surrounding areas...