The vast majority of ponds and landfills holding coal waste at 250 power plants across the country have leaked toxic chemicals into nearby groundwater, according to an analysis of public monitoring data released Monday by environmental groups.

The report, published jointly by the Environmental Integrity Project and Earthjustice, found that 91 percent of the nation’s coal-fired power plants reported elevated levels of contaminants such as arsenic, lithium, chromium and other pollutants in nearby groundwater.

In many cases, the levels of toxic contaminants that had leaked into groundwater were far higher than the thresholds set by the Environmental Protection Agency, the groups said.

The examples span the country. At a family ranch south of San Antonio, a dozen pollutants have leaked from a nearby coal ash dump, data showed. Groundwater at one Maryland landfill that contains ash from three coal plants was contaminated with eight pollutants. In Pennsylvania, levels of arsenic in the groundwater near a former coal plant were several hundred times the level the EPA considers safe for drinking.

The voluminous data became publicly available for the first time last year because of a 2015 regulation that required disclosures by the overwhelming majority of coal plants.

“At a time when the EPA — now being run by a coal lobbyist — is trying to roll back federal regulations on coal ash, these new data provide convincing evidence that we should be moving in the opposite direction,” Abel Russ, lead author of the report and an attorney for the Environmental Integrity Project, said in a statement.

The report acknowledges that the groundwater data alone does not prove that drinking-water supplies near the coal waste facilities have been contaminated. Power companies are not routinely required to test nearby drinking water wells. “So the scope of the threat is largely undefined,” the report states.

However, according to the EPA, nearly 90 million people rely on groundwater for their drinking supplies. Groundwater is also widely used in agriculture for irrigation. Monday’s report also details multiple instances, largely in rural areas, in which residential tap water has been affected by coal ash.

Coal ash ranks among the nation’s largest industrial waste streams. According to the EPA, in 2012, coal-fired electric utilities burned more than 800 million tons of coal in the United States, generating about 110 million tons of coal ash.

The EPA said Monday that it was reviewing the new report and could not yet comment on its contents. But the agency noted that the groundwater monitoring required by the 2015 rule was merely a “first step” in a process intended to assess and address any contamination from coal ash storage sites.

“Where contamination is detected above specified levels, the regulations require the owner or operator of the facility to initiate measures to clean up the contamination,” spokesman John Konkus said in a statement, adding that companies also are required to be transparent about what actions they are taking.

The Trump administration has sought to overhaul portions of the Obama-era requirements for handling the toxic waste produced by burning coal. For instance, the agency last year put in place changes aimed at providing more flexibility to state and industry officials in implementing the 2015 restrictions.

The 2015 regulations dictated how coal ash must be stored across the country and were finalized in the wake of two high-profile spills in Tennessee and North Carolina, which collectively contaminated waterways and damaged nearby homes. The Obama administration negotiated for years with environmental groups, electric utilities and other affected industries about how to address coal waste, which can poison wildlife and poses health risks to people living near storage sites.

Changes made under President Trump would extend the life of some existing ash ponds, empower states to suspend groundwater monitoring in some cases and allow state officials to certify whether a facility meets adequate standards. EPA officials estimate the rule changes will save the industry tens of millions of dollars a year in compliance costs.

The administration also has said it plans to make an additional proposal that would further water down existing coal ash regulations.

But that effort has been complicated by an August ruling by the U.S. Court of Appeals for the District of Columbia Circuit, which found that parts of Obama-era regulation around coal ash storage were not stringent enough to adequately protect public health. For instance, the court tossed provisions that would have allowed some unlined or clay-lined coal ash pits to continue operating, as long as testing revealed no leaks.

Monday’s report, which included information from thousands of groundwater monitoring wells, suggests serious questions remain about the long-term safety of coal ash ponds and landfills that dot the country.

The analysis found that a majority of the country’s more than 250 coal plants have unsafe levels of at least four potentially toxic substances, including arsenic, which the EPA has classified as a human carcinogen. In addition, the report found that few coal ash waste ponds have waterproof liners to prevent harmful substances from leeching into groundwater, and more than half are built beneath the local water table or within five feet of it.

Lisa Evans, an expert on coal ash and a senior attorney for Earthjustice, said in an interview that the findings raise only more questions about the impact of the leaks.

“With all of these, the contamination is really not in dispute. It’s the industry’s own numbers,” Evans said. “The question now is, where is the contamination going? Who’s in the path of a plume? Is it people? A waterway?”

Among the most striking examples cited in Monday’s report were near the San Miguel power plant located an hour south of San Antonio. The groundwater samples from beneath a family ranch there showed that at least 12 pollutants had leaked from the electric coop’s coal ash dumps.

The report also found that in Belmont, N.C., Duke Energy’s Allen Steam Station’s coal ash dumps were built beneath the water table and had leaked cobalt — which can cause health problems at high exposures — into groundwater at concentrations well above those considered safe. The power plant also reported unsafe levels of eight other pollutants.

At recent meetings, some residents have pushed for Duke to remove coal ash from the site.

The issue flared up last year when Hurricane Florence unleashed flooding at coal ash sites alongside Duke Energy’s L.V. Sutton power plant, carrying coal ash components into a cooling lake and then into the nearby Cape Fear River. The company at one point estimated that flooding, which caused the collapse of a dam separating the lake from the river, washed away the equivalent of more than 150 dump trucks full of coal ash.

“Drinking and recreational water supplies around Allen Steam Station remain safe from coal ash impacts, and our modeling shows they’ll continue to be safe in the future,” Duke Energy spokesman Bill Norton said in an email.

James Roewer, executive director of the Utility Solid Waste Activities Group, which lobbies on coal ash issues on behalf of electric utilities, said the groundwater monitoring disclosures are a sign the industry is adhering to EPA regulations.

“I’d look at these reports as a visible, public demonstration of the industry working to comply with these rules and protect the environment,” Roewer said.

He said that where there is evidence of contamination that exceeds EPA standards, companies are required to take corrective measures. “It’s going to be a very public and transparent process,” he said. Roewer added that just because contaminants are detected near a coal ash storage site, “that doesn’t necessarily translate into people’s drinking water being contaminated.”

Cleaning up coal ash is costly. In December, a member of the Virginia State Corporation Commission said that it could cost ratepayers as much as $3.30 a month over 20 years — between $2.4 billion and $5.6 billion — to clean up Virginia-based Dominion Energy’s 11 coal ash ponds and six coal ash landfills in the state. And five Dominion facilities continue to churn out coal ash.

Monday’s report relies on data that was made public starting in March 2018 as required by a 2015 regulation known as the coal ash rule. The information was collected by a variety of environmental groups including the Sierra Club and Prairie Rivers Network. The data covers 265 coal plants and includes more than 550 coal ash ponds and landfills that were monitored by over 4,600 groundwater wells.

About a quarter of coal plants did not register data because they either closed their coal ash dumps before the regulation took effect or because they received extensions or exemptions.