NASHVILLE (AP) — Four state attorneys general are supporting a judge's order for a federal utility to clean up coal ash at a Tennessee plant.

In the 6th U.S. Circuit Court of Appeals brief Thursday, Democratic attorneys general for Maryland, California, Washington and Massachusetts said overturning the order would threaten the Clean Water Act and give polluters incentive to skirt regulation by rerouting discharges to groundwater.

In August, a judge ordered Tennessee Valley Authority's Gallatin Fossil Plant ash excavated and removed. He cited pollutants leaking into the Cumberland River, but said there's scant evidence of harm.

Eighteen other largely Republican states want the ruling overturned.

Tennessee officials are separately suing over TVA Gallatin's pollution. They wrote that they support the cleanup order.