GALLATIN, Tenn. — The hulking Gallatin Fossil Plant sits on a scenic bend of the Cumberland River about 30 miles upstream from Nashville. In addition to generating electricity, the plant, built in the early 1950s by the Tennessee Valley Authority, produces more than 200,000 tons of coal residue a year. That coal ash, mixed with water and sluiced into pits and ponds on the plant property, has been making its way into groundwater and the river, potentially threatening drinking water supplies, according to two current lawsuits.

Coal ash, the hazardous byproduct of burning coal to produce power, is a particularly insidious legacy of the nation’s dependence on coal. Unlike the visible and heavily regulated airborne emissions from power plant smokestacks, coal ash is largely unseen unless there is a major spill and, until recently, far less effectively regulated.

More than 100 million tons of coal ash is produced every year, one of the nation’s largest and most vexing streams of toxic waste. The hazardous dust and sludge — containing arsenic, mercury, lead and other heavy metals — fill more than a thousand landfills and bodies of water in nearly every state, threatening air, land, water and human health.

The Gallatin power plant is facing citizens’ complaints and two major lawsuits over its handling of coal ash. One suit, filed in 2015 by an environmental advocacy group in federal court, says the utility violated the Clean Water Act by allowing toxic leaks from its coal ash disposal ponds. A second, also filed in 2015, by the state’s attorney general and its environmental enforcement agency, asserts that the Tennessee Valley Authority broke state pollution laws and endangered public health.

The state case is scheduled to go to trial this year; in the federal case, which went to trial at the end of January, the parties were scheduled to file proposed findings of fact on Friday.

A new rule regulating the monitoring, safe storage and disposal of coal ash went into effect in 2015. This past week, however, Scott Pruitt, the administrator of the Environmental Protection Agency, said in a letter to a Minnesota environmental official that the agency would reconsider the rule and delay the 2018 compliance deadline for states.

The plant is among the Tennessee Valley Authority’s fleet of power stations, which have used fossil fuels, nuclear energy and renewable sources like hydropower to bring reliable electricity to parts of seven states across the Southeast. The authority, created in 1933 as part of President Franklin D. Roosevelt’s New Deal, provides 99.7 percent of Tennessee’s electricity.

The Gallatin plant, like all others that burn coal, produces a steady and difficult-to-control stream of coal ash. Its disposal poses problems across the country, but particularly in the Southeast, which is highly dependent on coal for electricity.

“Gallatin is in many ways the worst site we’ve seen,” said Frank Holleman, a senior lawyer with the Southern Environmental Law Center, the nonprofit legal organization that filed the federal lawsuit on behalf of two state conservation groups. The group has filed several other suits claiming environmental harm from improper handling of coal ash in other states.

Mr. Holleman said vigorous federal enforcement of laws governing coal ash disposal was crucial because utilities like the Tennessee Valley Authority, which are responsible for managing coal ash storage, held so much political sway at the state level. He said that support for protecting water supplies from coal ash contamination cut across party lines and that efforts by the Trump administration to curb enforcement would be opposed even by residents who had voted for the president.

“We have to hope and pray that, regardless of any official’s political outlook, they can recognize that coal ash does not wear a political button,” he said.

Officials from the state’s attorney general’s office and Department of Environment and Conservation declined to comment for this article because of the continuing litigation. But in court filings, the state identified at least 10 places where unpermitted and illegal seepage from Gallatin’s coal ash ponds may have occurred, and said the utility’s own reports showed that the groundwater around the site was contaminated with heavy metals at levels exceeding state health standards.

On March 14, the Tennessee Valley Authority filed a motion in state court seeking to have the case dismissed, since, it argued, all seepage from the coal ash ponds is legally permitted by the state.

Coal ash gets far less attention than toxic and greenhouse gas emissions from power plants, but it has created environmental and health problems — every major river in the Southeast has at least one coal ash pond — and continuing legal troubles and large cleanup costs for the authority and other utilities.

The Gallatin site is pockmarked with ponds that serve as storage for millions of tons of coal ash slurry. As at other sites in the region, these ponds were built on top of karst, porous limestone prone to cracks and sinkholes that can let poisonous ash seep into groundwater and threaten drinking water — in this case, possibly affecting more than a million people.

At other sites in the Southeast, coal ash ponds were built out into rivers, making them vulnerable to spills or dam collapses.

Even though coal use is declining in the United States, it is still the second-biggest source of greenhouse gas emissions, according to the federal Energy Information Administration, and coal ash is one of the largest waste streams in the country.

Coal is used to generate power almost everywhere, but problems associated with coal ash and coal ash slurry disproportionately affect low-income and minority communities, experts say.

The environmental law center that brought the federal suit does not claim a civil rights violation on behalf of the downstream residents whose water was polluted. But Beth Alexander, one of the center’s lawyers who argued the Gallatin case in court, said the problem had not received much attention until recently because its victims were powerless and voiceless.

“If there were coal ash in East Hampton,” she said, “we would hear about it.”

She and her colleagues have outlined what they say is decades-long neglect of the coal ash issue. From 1970 to 1978, coal ash from one of the Gallatin slush ponds seeped into the groundwater and made its way to the Cumberland River beyond, the center’s lawyers and several scientists say. The slow-motion coal slurry spill totaled 27 billion gallons, more than 100 times the size of the Deepwater Horizon oil spill.

The Tennessee Valley Authority denied that amount of coal ash water had entered the river (though it did acknowledge some historical contamination during a recent site visit). Walter Kutschke, an engineer for Aecom, an engineering firm contracted by the authority, confirmed the 27 billion-gallon figure, however, under cross-examination.

While the plant’s history is not at issue in the current trial, lawyers for the environmental law center cited it in arguing that smaller coal ash leaks were still occurring at Gallatin.

The authority has had more recent challenges in dealing with coal ash: In 2008, an ash pond dike at its Kingston Fossil Plant in eastern Tennessee collapsed, releasing just over a billion gallons of coal ash water into the Emory River, which flows into two other rivers, including the Tennessee.

The slurry released in that spill, which has been called the largest environmental disaster of its kind, buried 300 acres of land in toxic sludge. That sludge was taken to an unlined landfill in Alabama, just outside a predominantly African-American community, prompting challenges under federal civil rights law.

The spill helped spur the E.P.A. to create new rules regulating coal ash storage, and the Tennessee Valley Authority has begun to comply by draining and capping many of its ash ponds and storing future coal ash in dry landfills.

Gallatin’s coal ash problems do not approach the scale of earlier disasters, but people and communities are still affected. The City of Gallatin, which gets its drinking water from the river, at a spot about a mile downstream from the plant, said in a 2014 report that the coal plant represented a threat to its water supply.

John Kammeyer, the Tennessee Valley Authority’s vice president for civil projects, who manages its coal ash storage, said the state had acknowledged the issue of seepage from coal ash ponds, which is allowed in certain circumstances.

“If we thought we were doing anything unsafe or untoward, we wouldn’t do it,” Mr. Kammeyer said.