Dive Brief:

• The Tennessee Valley Authority (TVA) on Thursday reached a settlement agreement with environmental groups and the state, requiring it to excavate 12 million tons of toxic coal ash from its Gallatin Plant ponds.

• The settlement follows a five year battle over the utility's coal ash management practices and their impact on nearby waterways. In 2014, the Southern Environmental Law Center (SELC) filed a notice of intent to sue on behalf of two clean water groups in the state, and Tennessee filed suit in 2015, alleging the utility's practices violated state water quality and solid waste laws.

• Almost all the coal ash, except one pond, at the plant will be recycled or placed in a lined landfill, at an estimated cost of around $640 million over about 20 years, TVA spokesperson Scott Brooks told Utility Dive in an email.

Dive Insight:

TVA's coal ash practices remain controversial a decade after its Kingston coal plant released over a billion gallons of the toxic waste into surrounding waterways — the country's largest coal ash disaster to date.

Of the hundreds of workers who cleaned up the damage, 36 are dead and more than 300 are sick or dying from exposure to the toxins.

That disaster's legacy, paired with the 2014 Duke Energy spill, has led to greater public scrutiny over the dangers of the pollutants, and more states are taking action. North Carolina ordered Duke to completely excavate all its ponds in April, and Virginia required the same of Dominion Energy in January. 

The 976 MW Gallatin plant became a source of concern for the Tennessee Clean Water Network and Tennessee Scenic Rivers Association in 2014. The plant is located on a peninsula in the Cumberland River, and the groups feared the ash could be leaking into the river through groundwater near the "complex" of ponds, which are all unlined, Amanda Garcia, managing attorney at SELC, told Utility Dive.

An initial investigation found the ponds were located on a porous rock formation, karst, known for causing sinkholes.

"Looking back at TVA's historical records, there was a period of time in the 1970s where they were putting coal ash into the pits and the coal ash was basically just falling out through the bottom into groundwater, and from groundwater into the river," said Garcia. "TVA eventually tried to plug some of those sinkholes. But, that's just the type of terrain it is. You can't plug all the holes."

Additional testing around the impoundments ​found coal ash water was seeping through earthen berms on the side of the pits. There was also evidence of contamination in nearby surface water, causing SELC to file the notice of intent.

The state case was pending, until Thursday's settlement agreement.

"After a thorough review of the scientific evidence, and with the availability of an onsite lined landfill, TVA worked with [Tennesee Department of Environment and Conservation] to determine that it is the best interest" of all involved to excavate the ponds, TVA President and CEO Jeff Lyash said in a statement. The "complex geology and groundwater flows," played a role in the decision, said the utility.

One of the Gallatin plant ponds, inactive since the 1970s is exempt from the settlement cleanup requirements. The utility has five years to prove it can effectively address groundwater contamination around that basin, without moving the ash.

The utility said the one exempt pond has not shown reason for concern.

"The only issues we've found in years of groundwater monitoring at the non-registered site are with pH levels," said Brooks. "There are no exceedances of constituents, nor karst issues, in that location. We will conduct additional studies and work with TDEC on our options for that site per the order."

The agreement also requires TVA to propose a mediation plan that would clean up additional contaminants left in the soil or surrounding groundwater after the sites are emptied.