Two environmental groups have filed suit against the owners of a coal-fired power plant outside Concord.
The Conservation Law Foundation complaint says Merrimack Station in Bow is polluting the Merrimack River with excess hot water.
The nonprofit first announced its intent to file the Clean Water Act suit, along with the Sierra Club, last November.
The groups say the plant has been operating with an out-of-date federal permit for the hot water discharge for years.
They say it needs new cooling systems to protect the ecosystem of the river.
Merrimack Station was sold by Eversource to a Connecticut-based firm, Granite Shore Power, in early 2018.
Eversource denies the allegations in the new suit, saying it worked closely with the EPA on fulfilling environmental requirements before selling the plant. GSP did not immediately respond to a request for comment.
Merrimack Station is the largest coal-burning power plant left in New England, and one of two slated to remain online past 2021.
The other is the much smaller Schiller Station, in Portsmouth, also owned sold by Eversource to GSP in 2018.
This story has been updated to include comment from Eversource.