At a power plant in Memphis, Tennessee, coal ash waste that built up over decades has been leaching arsenic and other toxic substances into the groundwater.

The contamination, ranked as a top problem in a new national assessment of water testing at coal ash sites, is in a shallow aquifer for now. But below that lies a second aquifer that provides drinking water to more than 650,000 people, and there are concerns that the contamination could make its way into the deeper water supply the city relies on.

"We have one of the purest drinking water sources in the whole country, and now we'll have arsenic and other coal ash compounds leaking into our water supply if things don't get cleaned up," said Scott Banbury, a Memphis resident and representative of the Sierra Club. "So getting rid of that ash is important."

The scale of the Memphis problem emerged from industry reports on groundwater testing near ponds and landfills that store coal-burning wastes from power plants. These reports, required by recent regulations, show that polluted groundwater is a widespread problem, with unsafe levels of toxic contaminants linked to more than nine out of every 10 coal-fired power plants with monitoring data, about 91 percent. The Environmental Integrity Project and other advocacy groups compiled and analyzed the data in a report released Monday.

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The worst contamination, according to the analysis, was at the San Miguel Power Plant south of San Antonio, Texas, with 12 pollutants above safe levels in groundwater. In Memphis, the Tennessee Valley Authority's Allen Fossil Plant, which was shut down last year, is in the Top 10.

The report covers water testing at 265 existing and retired coal plants, comprising more than 550 individual coal ash ponds and dumps that have groundwater monitoring wells. That represents about three-fourths of the coal power plants in the country, according to the authors. Some plant owners were not required to make groundwater testing results public because they closed their ash dumps before the U.S. Environmental Protection Agency's first national rules on coal burning waste took effect in 2015, or because they were eligible for an exemption or extension of reporting deadlines.

"It's everything we could get our hands on," said the report's lead author, Abel Russ, a senior attorney with the Environmental Integrity Project, which worked with Earthjustice on the report. The contamination is "widespread," Russ said, and in many places, "groundwater may be unusable for decades or hundreds of years."

In all, unsafe levels of contamination were reported in groundwater at sites in 39 states and Puerto Rico. Illinois has the most power plants with polluted ash storage sites with 16, followed by Texas, Indiana, Kentucky, Michigan, North Carolina and Missouri, all with more than 10.

Trump EPA Is Considering Weakening Standards

The results come as President Donald Trump's EPA, led by former coal and energy industry lobbyist Andrew Wheeler, is weighing further changes to the Obama-era EPA rules with the goal of cutting utility compliance costs. Earlier, the Trump administration relaxed EPA coal ash rules in part by extending the deadline to stop using some coal ash ponds from next month to Oct. 31, 2020.

"The fact that we have now analyzed the data and found significant contamination at almost every coal ash site provides a strong record to oppose any future rollbacks," said Lisa Evans, a senior attorney at Earthjustice.

Earthjustice last year won a lawsuit against EPA that will require the closure and cleanup of some 100 additional coal ash ponds left out of the 2015 rule. EPA is appealing.

An EPA spokesman on Friday did not comment on the agency's plans for changing coal ash rules. On Monday, EPA said it was reviewing the environmental groups' report and would not comment on it yet.

For decades, wastes from burning coal went unregulated by the federal government, leaving a patch of inconsistent state rules. The Obama administration in 2015 put in place national rules, favoring dry storage in landfills over wet storage in ponds. Those rules require utilities to conduct groundwater monitoring at ponds and landfills, close leaking ash ponds, clean up polluted groundwater and disclose their studies and actions.

The ash contains contaminants like selenium, mercury, cadmium, and arsenic, which are associated with cancer and other serious health effects, according to the EPA.

Posting water quality monitoring reports is just the start of a cleanup process. Earthjustice reported in January that utilities had acknowledged pollution at unsafe levels at 70 power plants in 22 states. By last week, those numbers had climbed to 88 plants in 25 states, Evans said.

Some utilities are blaming the pollution on other sources, and future disputes are expected over utilities' cleanup plans, she said.

Trouble in Texas and Memphis

The new report ranked the 10 most contaminated sites, based on the extent to which pollution—including all potentially unsafe pollutants—exceeded safe levels.

Brian McGovern, a spokesman for the Texas Commission on Environmental Quality, declined to comment on how his agency was responding to reports of groundwater pollution from the San Miguel plant, the nation's worst case. However, he said Texas is working on a new statewide coal ash management rule.

At the Memphis plant, the utility reported arsenic levels 350 times higher than drinking water standards, according to the report. TVA's Allen Fossil Plant, which burned coal for six decades along the banks of McKellar Lake, near the Mississippi River, is about five miles southwest of downtown Memphis.

The contaminated groundwater is above the deeper aquifer that the city uses for drinking water, most of it separated by a layer of clay.

In an annual report required by the coal ash rules and posted on the TVA website Friday, the TVA acknowledged that an underground area near its coal ash pond has no clay barrier. The report follows a 2018 U.S. Geological Survey study that found the two aquifers were connected and places the location of the connection near the leaking ash ponds, said Amanda Garcia, an attorney with the Southern Environmental Law Center.

The site set off alarm bells while TVA was testing water wells for a new natural gas power plant on the property. Pollution from the upper aquifer moved toward the lower aquifer during the testing.

TVA stopped using those wells and the utility plans to start pumping out and treating the contaminated groundwater at the Allen plant later this year, said Scott Brooks, a TVA spokesman. It is also studying the best way to stop the leaking and close its ash pond at Allen and other coal ash sites across the utility's seven-state system, he said.

No decision has been made yet on whether the ash can be safely managed on the spot or will need to be hauled away, Brooks said, adding that the utility intends to base the decision on the best available science.

For now, he stressed, none of the pollution documented under the plant had been detected in the city's drinking water supply.

For Banbury, the Memphis Sierra Club leader, that pollution is too close for comfort.

"This was something that nobody ever thought was a concern," he said. "Now it's a huge concern."