When hurricane Katrina roared ashore along the Gulf Coast 100 days ago, it stirred up tons of toxic substances, much of which had been stored safely or buried benignly beneath the earth. But when floodwaters subsided or were pumped out of low-lying areas, they left behind sediment and sludge that oozed and then dried into layers of muck and dust that in some places remain hazardous.
Just how hazardous may not be known for months or years. But as rebuilding begins and as officials allow residents to return to the worst-hit neighborhoods, government and independent scientists are getting a better idea of the risks - though they aren't always agreeing on the severity of the problem.
The US Environmental Protection Agency has tested sediments in more than 430 sites in the New Orleans area alone. The EPA found organic compounds, hydrocarbons, pesticides, and heavy metals, but in most cases the levels are "similar to the historical levels found in these parishes before Katrina and to other urban areas throughout the nation."
While some "localized areas" contain arsenic and other substances above levels the state and federal governments deem acceptable, the EPA reported last week, "the majority of chemicals detected were below levels of health concern."
Independent investigators this week said the situation along the Gulf Coast is more urgent.
Scientists who tested soil and sediment in Alabama, Mississippi, and Louisiana - on behalf of the Natural Resources Defense Council and the Sierra Club - found what they say are dangerous levels of arsenic, heavy metals, dioxin, and E. coli bacteria.
"There's a real need to get out there and get it removed now, get it removed and contained," says environmental chemist Wilma Subra, who heads the consulting firm in New Iberia, La., that conducted the testing.
"Some government officials feel that once the sludge dries up, the organisms are dead. But that isn't the case here," says Ms. Subra, who won a MacArthur Award for some of her previous research and has served on EPA advisory committees. "When people are out walking in their yards and streets, they are inhaling these particles that contain microorganisms that are still unsafe."
At most of the sites tested, levels of arsenic were above federal and state safety standards. Near the DeLisle Elementary School, which is not far from a DuPont chemical plant in Mississippi, for example, arsenic levels were found to be more than twice the EPA standard, Subra reports.
The school now has 1,200 students in Grades K-12, many of them from schools not yet reopened. Many classes are held in portable buildings, requiring students to walk outside several times a day. Rain has washed away some of the sediment, but lately it's been dry and dusty.
"Once the sludge dries, it can become airborne dust, carrying with it the metals and pathogens," says Peter deFur, a biologist who conducts research on environmental health and ecological risk assessment at the Center for Environmental Studies at Virginia Commonwealth University in Richmond. Those most at risk, says Dr. deFur, are children, who are lower to the ground, and people who work outside.
Dita McCarthy's daughter is in the 10th-grade class now being taught at DeLisle Elementary. "She has often described to me the clouds of dust the children are going through as they go from class to class," says Ms. McCarthy. "We really need more testing of the site around the school."
Testing at DeLisle Elementary School also detected barium, chromium, lead, and mercury. While levels of those heavy metal weren't above EPA limits, says Subra, they add to the cumulative impact of the toxic substances.
EPA officials have yet to fully examine the report of the environmental groups. "It's a little too early for us to comment on it," says Carl Terry, regional spokesman for the EPA.
"We've been doing comprehensive sampling, and we will continue to do that," says Mr. Terry. "If at any time there is information that we need to take action on, we will do that."