(Bloomberg) -- Harvey has moved on from the U.S. GulfCoast, leaving behind a toxic stew of human sewage, dead cattle,leaking chemical plants, spilled gasoline storage tanks andabandoned pickup trucks.

The cleanup will take patience, billions of dollars and fleetsof heavy gear to clear acres of muck and enough debris to fillhundreds of football stadiums in effort overseen by federal andstate authorities.

Related: Staying safe: A Hurricane Harvey survival guide forclaim adjusters

“When the flood waters recede is when you really have to look atthe damage,” Gina McCarthy, former administrator of the Environmental Protection Agency, said in aninterview. “It’s going to take considerable time.”

Refineries are spewing pollutants as they restart. Homeownersrisk dangerous mold and contamination from household chemicals. Andthere’s a heightened threat from a dozen or more polluted Superfundlocations around Houston that may have been under Harvey’swater.

Scores of people have been confirmed dead in one of thecostliest natural disasters in the country’s history. On Friday,many areas in Texas and Louisiana remained inaccessible and rescuescontinued as more than 21,000 federal staff worked on reliefefforts, according to the Federal EmergencyManagement Agency.

The American Red Cross had more than 2,000 disaster workers onthe ground, and more than 38,000 people sought refuge inshelters.

400 chemical, plastic, oil & gas plants/refineries

The parts of Texas slammed by Hurricane Harvey are host to morethan 400 chemical and plastics plants and oil and gas refineries.In Crosby, a chemical plant owned by Arkema SA was hit byexplosions after floods knocked out power supplies needed torefrigerate volatile chemicals. The EPA flew a chemical-sniffingplane in the area and said it didn’t find toxic concentrations awayfrom the facility.

"We will consider using any authority we have to further addressthe situation to protect human health and the environment," EPAAdministrator Scott Pruitt said in an emailed statement.

$5.95B initial disaster aid funding request

The White House is considering an initial $5.95 billion disasteraid funding request to Congress, with $5.5 billion to the U.S.emergency management agency and the remainder to the Small BusinessAdministration, according to two administration officials.

Related: House is said to plan vote on Harvey funds inmid-September

Residential flood insurance losses from Hurricane Harvey areestimated to reach between $6.5 billion and $9.5 billion, with mostof that drawn from the government-backed National Flood Insurance Program, according to theproperty data and analytics firm CoreLogic Inc./United States.

More than 311,000 Texans had already applied for federaldisaster relief funds as of Thursday and more than $530 millionalready had been granted, Vice President Mike Pence said. About100,000 homes were damaged by the storm, White House HomelandSecurity Adviser Tom Bossert said in a briefing.

Stay away from smoke plumes & flood water

Texas and U.S. officials have warned residents to stay away fromsmoke plumes and flood water. Leslie Fields, director ofenvironmental justice with the Sierra Club, ticked off a list ofhazards, including dead animals in flood water, gasoline fromsunken cars, and potentially leaks from a former paper plant thatcontains cancer-causing dioxin.

Related: Returning home after HurricaneHarvey

“It’s a bad situation,” said Fields. “This water is some of theworst ever.”

Refineries generate extra pollution as they shut down and thenturn back on, much as a cold automobile can spew clouds if it’sstarted after sitting idle for a time, according to Elena Craft, asenior health scientist with the Environmental Defense Fund. That’s led to 2 millionpounds of emissions in Texas since Aug. 23, the equivalent of 40%of last year’s total, according to state records, Craft said.

“We don’t really know what communities might have been exposedto,” Craft said. “Their risk is, overall, increased.”

Pollution already lying in and under the ground in Superfundsites — heavily contaminated places tagged by the EPA forcleanup — can be spread by floodwaters, said McCarthy, theagency’s former administrator.

“You can’t contain contaminants in a flooded area,” McCarthysaid. “EPA’s going to have to go back and look at those areas tosee what’s happened with existing contamination, as well as look atwhatever new Superfund sites are being created now.”

Top task: restore water treatment facilities

The top task is to restore water treatment facilities, said theEnvironmental Defense Fund’s Craft.

Flooding swamped municipal water pumps in Beaumont, Texas,leaving the city of more than 100,000 people without access todrinking water. Across the stricken area, 22 wastewater treatmentplants were inoperable due to flooding and power outages, accordingto a list maintained bymaintained by the Texas Commission onEnvironmental Quality, a state regulator. It listed 53 inoperablepublic drinking water systems.

Crews will flock to the Gulf Coast to help with the cleanup, andcompanies will need to establish camps with trailers, generatorsand showers to accommodate them, said Jody Cordaro, chief executiveofficer of SCE Environmental Group Inc., a Lake Ariel,Pennsylvania-based contractor that cleans up after environmentaldisasters.

Related: Once Houston's flood waters recede, what damageswill be revealed?

Workers will use an armada of gear including small loaders toclean streets, trucks with grappling hooks to lift debris fromcurbside, and giant grinders to chew through muck that can containtrees and housing remnants. “I would venture to say that everypiece of heavy equipment in Texas is already accounted for andrented,” Cordaro said.

Some of the waste will be riddled with fuel and othercontaminants, and will need to be trucked to landfills that couldbe several states away, Cordaro said.

Cordaro’s company sent 200 workers to help clean up afterHurricane Sandy struck New Jersey and New York in 2012.

They stayed on the job for more than seven months.

“I think this cleanup is going to be significantly longer,” hesaid.

Long-term health consequences unknown

Chemical spills and runoff unleashed by Harvey coulddisproportionately affect people of color and the poor, includingresidents living in the shadow of southeast Texas refineries.

“Refineries and petrochemical operations in Houston, almost toonumerous to count, have been venting a toxic mix of hazardous airpollutants those trapped by rising floodwaters are forced tobreath,” Michele Roberts, co-coordinator of the EnvironmentalJustice Health Alliance, a policy group, said in an email. “Thelong-term health consequences of this toxic air pollution areunknown.”

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