The Tennessee Valley Authority will drain old coal-fired power plant ash storage ponds in three states and cap, cover and monitor the remaining residuals under a final environmental impact statement, issued on June 10. But environmental groups say they will sue to have materials at some sites excavated and moved to lined storage.

“The Environmental Protection Agency agreed there was no more or less environmental concern with leaving the dry ash in place versus moving it,” says a TVA spokesman. The utility, which operates in seven southern states, has 10 legacy impoundments in Tennessee, Alabama and Kentucky that date to the 1950s. They vary in size from 10 to nearly 400 acres and contain millions of cubic yards of fly-ash, bottom-ash, boiler-slag and flue-gas desulfurization materials.

TVA compared dewatering and stabilizing the residuals and installing a cover system with an alternative plan of excavating, drying and shipping materials to lined landfills. Closure in place would not exceed $200 million; removing the materials could cost more than $2 billion. Moving the materials would reduce the risk of contaminating ground and surface water, but risks would rise during transport, the statement says. TVA’s active storage sites are being converted to dry storage under a 2009 pact and are not included in the current plan.

A 2015 rule sets new federal requirements for utility coal-ash disposal after a retaining wall at TVA’s Kingston, Tenn., plant collapsed in 2008. It sent 5.4 million cubic yards of coal-ash slurry across 300 acres and into the Emory and Clinch rivers. The Southern Environmental Law Center, Charlottsville, Va., has filed a notice of intent to sue the utility for Clean Water Act violations at the Cumberland plant in

Tennessee and the Colbert plant in Alabama. “TVA is owned by the public and should be setting an example. But, instead, it is using a retrograde and primitive technique,” says senior attorney Frank Holleman, adding that the states’ karst geology makes groundwater pollution likelier.

“They are refusing to do what is widely being done in North Carolina and all of South Carolina,” Holleman says, noting successful suits against South Carolina Electric & Gas and Santee Cooper, which agreed to excavate ash from unlined pits. Duke Energy agreed to excavate two sites in South Carolina and is under court order to excavate seven in North Carolina after a 2014 pipe collapse dumped 30,000 tons of toxic coal ash into the Dan River. That state is soon to issue final cleanup priority of 32 impoundments. But on June 13, the center also sued Duke Energy for coal-ash pollution at a storage site in Roxboro, one of seven not under court order. The utility stores 7 million tons of ash in an unlined, leaking pit filled with water.