NASHVILLE, Tenn. (AP) — Environmental groups are taking the Tennessee Valley Authority to trial over waste ash from an aging coal-fired power plant northeast of Nashville, saying it polluted the Cumberland River in violation of the Clean Water Act.

In a bench trial starting Monday in federal court in Nashville, the Tennessee Clean Water Network and Tennessee Scenic Rivers Association say the pollution is due to the TVA’s faulty storage of coal ash, a byproduct of burning coal for energy.

The nation’s largest public utility, which powers 9 million customers in parts of seven Southern states, says it has abided by the law and hasn’t sullied drinking water sources.

But the environmental groups contend coal ash storage ponds at the 1950s-era Gallatin Fossil Plant have been illegally seeping toxic pollutants into the groundwater and the Cumberland River for years. The groups also say in the lawsuit filed in 2015 that state regulators didn’t require sufficient changes at the plant to safeguard against contamination, according to court documents.

TVA spokesman Scott Brooks said decades of data show the plant hasn’t had any impact on potable water sources, adding the utility heeded all regulations and permits.

The plant sits on a bend of the river, which stretches almost 700 miles from eastern Kentucky headwaters through Tennessee to meet up with the Ohio River in western Kentucky. Nearby residents have private wells and the Cumberland River supplies drinking water to Nashville, about 40 miles away, among other areas.

In 2015, state environmental officials informed Albert Hudson, whose home is near the plant, that his well water met U.S. Environmental Protection Agency drinking water standards. But they also said the water showed levels of the harmful chemical hexavalent chromium – typically resulting from an industrial process – were slightly above EPA risk levels.

The Southern Environmental Law Center, representing the other environmental groups, cites test of two water wells – including Hudson’s – in court documents along with results that found the chemical in the Cumberland River near where the plant takes in water.

Hudson is slated to testify for the environmental groups.