They are ponds the size of city blocks: sewer pits that contain the dangerous by-products of coal. Lagoons full of diluted pig droppings. Huge pools on piles of radioactive debris.

The risks posed by such garbage ponds, found in thousands of industrial and agricultural locations across the country, have been greatly mitigated by a huge sewer pond in Piney Point, Florida that has been in jeopardy in recent days from catastrophic failure.

Officials said Monday the danger of collapse was over and residents were allowed to return home after an emergency pumped millions of gallons of water from the pond into local waterways. The environmental impact of such a large release of contaminated water was unknown. Last weekend, the specter of a flood caused authorities to evacuate hundreds of people from their homes.

Outdoor ponds are critical to important industries such as livestock and power generation. However, environmental groups say they pose a major risk to the environment, health and safety, be it due to mismanagement or, increasingly, the effects of climate change.

“They’re just an irresponsible way of dumping very hazardous waste,” said Daniel Estrin, general counsel for the Waterkeeper Alliance, a nonprofit group for clean water. “And with climate change, we will see more frequent and stronger storms hitting these locations.”

The emergency in Florida is particularly bad at a former phosphate mining plant south of Tampa. There is a pool that initially held more than 400 million gallons of sewage containing traces of heavy metals and other toxic substances on a pile of phosphogypsum residue that is at least 70 feet high. Residues are waste that is left behind when ores from phosphate mining are processed into phosphoric acid, a component of fertilizers.

For decades the residue, a radioactive damp slurry containing traces of radium along with arsenic, lead and other elements, was dumped in ponds and allowed to evaporate, leaving enormous stacks of phosphogypsum with water. The fear was that if the pond collapsed, it would wash away the debris and send a “wall of water” over nearby homes and businesses.

The mounds of such debris, scattered across more than two dozen locations in Florida, are among the tallest earthen structures in the state. Florida is the world’s largest phosphate-producing area, according to the EPA, accounting for about 80 percent of the country’s phosphate mining. The United States extracts and uses approximately 23 million tons of phosphate annually.

However, at the site of the current breach, evaporation has not kept pace with rainfall, which, according to the Bradenton Herald, has continued to contribute to the site’s ponds. Over the past year, the site’s owner, HRK Holdings, found tears in the plastic liner containing the sewage on numerous occasions and warned local officials that the ponds were quickly running out of capacity, the Herald reported.

Jeff Barath, general manager at HRK Holdings, was reached by phone and said he was the “boot-on-the-floor type” and not authorized to speak to the press. A number he gave for a speaker and a number listed on the company’s corporate website failed to connect.

To relieve pressure on the walls of the pools, workers dumped approximately 35 million gallons of sewage into nearby waterways every day. While the fear of a major breach appears to be over, the emergency release of the polluted water, which also contains nutrients that could stimulate harmful algal blooms, is likely to cause environmental damage, followed by fish deaths.

“When the highest point on our horizon is a toxic waste dump, it’s terrifying,” said Hannah Connor, a senior lawyer for the environmental health program at the Center for Biological Diversity. “And with more rain events and heavier storms, this will happen more often.”

While stacks of phosphogypsum residue like the one at the Piney Point, Florida site are concentrated, thousands of industrial and agricultural open-air sewage ponds dot the land. This includes at least 70 stacks of phosphogypsum, 700 ash ponds near coal-fired power plants, and thousands of agricultural facilities like the giant lagoons on large industrial animal farms.

These farming basins are generally a noticeable gum pink hue, a deceptively cheerful color that results from anaerobic bacteria that digest the foul-smelling slurry, a mixture of water, animal excrement, and chemicals.

When farming was done on a more humane scale, manure had value to farmers as fertilizer for corn, which then fed the next generation of pigs and cows. Now most of the corn is grown on an industrial scale using synthetic fertilizers. As a result, excrement is now collected and stored in cesspools.

These earthen pits, many of which are unlined, pose a risk of leaking into groundwater, said D’Ann Williams, researcher at the Center for a Better Future at the Johns Hopkins Bloomberg School of Public Health. The lagoons also leak gases or crusts can form on the top that trap the gas and then release bursts of hydrogen sulfide or ammonia, both of which affect air quality in the area.

“And when you have floods and can’t handle the amount of water that comes in, bacteria and chemicals can end up in surface water and on land,” Ms. Williams said.

Hurricane Florence, which brought record-breaking floods to the Carolinas in 2018, inundated more than 100 pig lagoons and released their contents into the flood. Excess nitrates in pig manure have also been linked to health problems, such as blue baby syndrome, which causes the blood to become unable to carry oxygen around a child’s body and can be fatal.

Various efforts to strengthen federal oversight of manure lagoons have stalled, and most ponds are state regulated. However, the Environmental Protection Agency has taken action in some of the most egregious cases and ordered dairy farms to shore up their lagoons after tests in residential drinking water wells showed elevated levels of nitrates that can harm human health.

In the early 2000s, agricultural giant Smithfield Foods promised to explore alternative ways of dealing with manure as part of an agreement with North Carolina. An expert at the world’s largest pork producer, now a wholly owned subsidiary of the Chinese meat and food processing company WH Group, has developed several options, including one that can solidify the faecal waste, but has found none to be economically feasible.

Environmental groups recently requested the state to review the agreement. Smithfield has stated that it has already fully complied with the terms of the agreement. The company did not immediately provide further comments.

“It’s a model that needs revision – this large-scale animal production model,” Ms. Williams said. “These are huge industries, but they are not regulated as industries. They’re still regulated like they’re small farms. “

When coal-fired power plants generate electricity, they leave hundreds of thousands of tons of a toxic residue called coal ash that is mixed with water and dumped into ponds on the factory premises.

Kemp Burdette, the Cape Fear Riverkeeper in North Carolina, saw what happens when the ash ponds, which contain arsenic, mercury, lead, and other hazardous heavy metals, are inundated by floods.

Hurricane Florence flooded Duke Energy’s Sutton facility in Wilmington, NC, which previously burned coal. (It burns gas today.) In response to a lawsuit from environmental groups, Duke had begun excavating the ponds and dumping the coal ash in lined landfills, but the floods eroded the site’s defenses and released coal ash.

“They had this stream of water that had picked up the coal ash that was just coming out,” said Mr. Burdette. “You could see great spirals of ash just floating down the river.”

Bill Norton, a spokesman for the Duke, said “a very small amount of ashes” had been stripped from the property and recovered.

More than 700 landfills and bodies of water in the United States still store coal ash. After an Obama-era rule, utilities should have started shutting down their coal ash ponds in 2018, but the Trump administration has attempted to weaken the rule in one of its many backward steps on environmental regulations. President Biden is currently reviewing the rollback.

However, North Carolina has begun requiring utility companies to excavate their coal ash stores under a new state law requiring all ponds to be closed, the ash secured, dried out, and removed from the water by 2029. The ponds of the Sutton Plant are now closed.

“The claim has always been: we can’t clean up this stuff, it’s impossible,” said Mr. Burdette. “But of course it is possible. You just have to spend the money on it. “

Jesus Jiménez contributed to the reporting.