Officials in Houston are just beginning to grapple with the health and environmental risks that lurk in the waters dumped by Hurricane Harvey, a toxic stew of chemicals, sewage, debris and waste that still floods much of the city.
Flooded sewers are stoking fears of cholera, typhoid and other infectious diseases. Runoff from the city's sprawling petroleum and chemicals complex contains any number of hazardous compounds. Lead, arsenic and other toxic and carcinogenic elements may be leaching from some two dozen Superfund sites in the Houston area.
Porfirio Villarreal, a spokesman for the Houston Health Department, said the hazards of the water enveloping the city were self-evident.
"There's no need to test it," he said. "It's contaminated. There's millions of contaminants."
He said health officials were urging people to stay out of the water if they could, although it is already too late for tens of thousands.
"We're telling people to avoid the floodwater as much as possible. Don't let your children play in it. And if you do touch it, wash it off," Mr. Villarreal said. "Remember, this is going to go on for weeks."
Flooding always brings the danger of contamination and disease. This inundation, which put nearly 30 percent of the nation's fourth-largest city underwater, will pose enormous problems, both immediately and when the waters finally recede.
Dr. David Persse, Houston's director of Emergency Medical Services, said officials were monitoring the drinking water system and the sewer system, both of which he said were intact so far. But hundreds of thousands of people across the 38 Texas counties affected by Hurricane Harvey use private wells, according to an estimate by Louisiana State University researchers, and those people must fend for themselves.
"Well water is at risk for being contaminated," Dr. Persse said, "and the well owner is really the one who is responsible. In the City of Houston, we have folks that use well water but we strongly recommend against it — and this will sound awful — we don't take responsibility for it."
Harris County, home to Houston, hosts more than two dozen current and former toxic waste sites designated under the federal Superfund program. The sites contain what the Environmental Protection Agency calls legacy contamination: lead, arsenic, polychlorinated biphenyls, benzene and other toxic and carcinogenic compounds from industrial activities many years ago.
Kathy Blueford-Daniels grew up just a block away from one of those sites, a wood-treating facility that used cancer-causing creosote and other toxins. As a young girl, she would try to avoid the plant and the pungent, oil-like goo that lined the ditches around it.
Now 60, Ms. Blueford-Daniels still lives on the same block, in Houston's Fifth Ward. So when Harvey's rains started to pour into her neighborhood, she immediately began to wonder what the rising waters would carry off the old industrial property.
"I wasn't so fearful of the storm. But I'm scared of that site," she said. "I thought: This is going to be a travesty. The contamination could be going anywhere."
An E.P.A. spokesman, David Gray, said in a statement that the agency would inspect two flooded Superfund sites in Corpus Christi, but he did not specify which ones or say whether additional sites elsewhere in Texas would be checked.
Houston also lies at the center of the nation's oil and chemical industry, its bustling shipping channel home to almost 500 industrial sites. Damaged refineries and other oil facilities have already released more than two million pounds of hazardous substances into the air this week, including benzene, nitrogen oxide and volatile organic compounds, according to a tally by the Environmental Defense Fund of company filings to Texas state environmental regulators.
"We're very concerned about the long-term implications of some of the emissions," said Elena Craft, a senior health scientist and toxicologist at the Environmental Defense Fund in Texas.
"As well as the flooding and the impact on pipelines, there's underground and above-ground storage tanks," she said. "It's a suite of threats."
Houston's sewer systems have also long struggled with overflows, drawing scrutiny from federal regulators who worry about raw sewage seeping into groundwater. Like dozens of cities across the country, Houston has been negotiating a consent decree with the E.P.A. that would require the city to upgrade its pipes and overhaul its maintenance regime.
"Houston's had problems with their sewer system in the past. They already had cracks and leaks that were allowing storm water to get into the sewers," said Erin Bonney Casey, research director at Bluefield Research, a water-sector consultancy based in Boston.
"When it rains, the sewer pipes get infiltrated with storm water. The pipes exceed their capacity and you get discharge of a mix of sewer water and storm water," she said. "As you can imagine, this raises major concerns around disease and contamination of local water supplies."
Marc Edwards, the Virginia Tech professor who helped identify the Flint water contamination crisis, said Houston's abundance of private water wells added the city's woes. People who evacuate return home and use them, to their great risk.
"Poop from animals and humans that normally does not get into the water supply is present" in the wells, he said, and "if they are present in water that you drink it would cause massive epidemics in a matter of days."
He added: "Everything else, as horrible as it is, is really a more chronic secondary concern. It's pretty rare that those things are present in flood water, short term, in levels that can kill you.
But he also worried about people who had three or four feet of water in their houses and would not realize that all the pesticides and hazardous products they keep under the sink would now have contaminated their houses.
"After Katrina, when the floodwaters receded, we had to go around to assist communities to pick up debris and leftover chemicals, like propane tanks, pesticide containers that were compromised and household hazardous waste," said Mr. Meiburg, now director of graduate programs in sustainability at Wake Forest University.
"The water is going to be polluted," he said. "You know that from the get-go."