But the water - and the muck it is leaving behind � also owes its contamination to a source as mundane as it is unexpected: Toxins common in most urban environments that made their way en masse into the water as it stagnated atop the city.

So says a University of Florida professor who has spent years studying the harmful contaminants that turn up in urban runoff, or rainwater that washes across streets and other hard surfaces in cities.

Environmental engineering professor John Sansalone's perspective is especially relevant because it is based on field research in New Orleans and Baton Rouge, where he was a professor at Louisiana State University before taking a job at UF this summer.

"What we see in New Orleans is that when you put a lot of water in contact with the urban environment, all the potential contaminants that stayed around in that environment are now back in the water - definitely, to our horror," Sansalone said.

Federal and Louisiana officials continue to sound alarms about the contaminated waters and scum left behind by the retreating flood. Early September test results released late last week showed high levels of bacteria, lead and harmful levels of chemicals including arsenic, according to the Environmental Protection Agency.

While the sources of these and other contaminants remain under investigation, public scrutiny has focused on broken sewer pipes and other major failures in the city's infrastructure attributed to Hurricane Katrina.

Though these are certainly real problems, it's also highly likely that the stagnant waters are contaminated because they've soaked up "legacy" pollutants that accumulated during normal conditions on the city's streets, sidewalks, roofs and other impermeable surfaces, Sansalone said.

These pollutants, which normally appear in urban runoff, are more toxic than commonly understood, he said. In a study published last month in Water Environment Research, Sansalone and three co-authors report that runoff from an elevated section of Interstate 10 in Baton Rouge contained some contaminants at levels "greater than those found in untreated municipal wastewater from the same service area," according to the study.

The findings were based on periodic analysis of runoff that drains off Interstate 10 into Baton Rouge's City Park Lake just below the highway. Based on data first gathered in 1999, they revealed high levels of particulates, or microscopic- to millimeter-sized particles of material, as well as high chemical oxygen demand, an indicator of the presence of organic chemicals in oil, gas, grease, cigarette filters and other pollution.

Other research on urban runoff, meanwhile, has detected high levels of toxic metals and nutrients including phosphorus thought to leach from building materials, Sansalone said.

Organic chemicals are particularly dangerous to fish and other aquatic life because they reduce the levels of oxygen in the water, impinging on its ability to support life. Particulates cloud water, reducing sunlight penetration and plant growth. Once they cross a certain threshold, organic chemicals and metals also can be harmful to people.

New Orleans officials remain extremely concerned about bacterial contamination in the flood waters. Typically the result of contamination from untreated sewage, bacteria also can come from urban runoff, Sansalone said. Although it was not measured as part of his published study, other studies have found that such runoff contains heightened levels of bacteria stemming from bird and animal droppings, among other sources.

Sansalone said based on his studies of urban runoff alone, it's critical that environmental officials scour the city of flood residue. "How we clean up this residual matter - which will not be easy - will be a chronic issue to the health of the city," he said.

He said the contamination in New Orleans also highlights the need for other cities nationwide to do more to remove the toxins in urban runoff before, rather than after, it gets washed into waterways.

There are several good strategies, he said. Increasingly affordable "permeable pavements" allow runoff to be stored, evaporate or percolate through pavement and into the ground, where soil and microorganisms can help filter the contaminants.

Planting vegetation and especially trees also creates aesthetically pleasing buffer zones, providing storm water flooding control and other benefits. Finally, cities can use high-tech street sweeping equipment that is very effective at capturing pavement contaminants.

"If you pick up this potentially toxic material before it gets into the hydrological cycle, it is far more economical than if you try to take it out of the water after the fact," he said.

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