A trial starts Monday on whether Merrimack Station in Bow releases too much warm water into the Merrimack River in violation of its EPA permit, a case that could decide the fate of the last coal-fired power plant in New England.

The lawsuit, brought by Sierra Club and Conservation Law Foundation against the plant’s owner Granite Shore Power, begins Monday in U.S. District Court in Concord. It’s a bench trial with no jury before Judge Joseph Laplante, and could last all week or longer. Both sides have a number of experts on their witness list.

If they win, plaintiffs are asking that Granite Shore Power install cooling ponds or other equipment to lower the temperature of water returned to the river and also pay a penalty. It’s unclear what the effect of those costs would be on a decision to keep it open. All of New England’s other coal-fired plants have shut because they weren’t economical to run.

Merrimack Station generates electricity only during periods of peak demand such as heat spells or cold snaps because its electricity is too expensive to be used regularly. The plant receives millions of dollars a year from ratepayers in return for guaranteeing to be available during peak periods through a process known as the forward capacity auction.

The issue in the lawsuit is straightforward, even if technical details can be obtuse.

Like most thermal power plants, those which burn a fuel to make steam that spins turbines and create electricity, Merrimack Station uses water to remove “waste heat” that wasn’t used in power production. This is why most such plants are built alongside rivers, lakes or the ocean.

When running, Merrimack Station takes in water from the Merrimack River and then releases it at a higher temperature, known as thermal discharge. The temperature differential and amount of release are regulated by the Environmental Protection Agency in a 1992 permit. A renewal permit was issued in 2020 under the Trump administration that changed thermal discharge requirements but it was challenged in court by environmental groups and is only partly in effect.

Water temperature is an issue because it’s extremely important to some aquatic wildlife, including fish that migrate up or down it. A release of warm water can act as a barrier.

“If shad are having a migration run and there’s a plume in the way they can die, or spend excess energy … be subject to predation,” said Reed Super, lead counsel for the Sierra Club. “The river favors thermally tolerant fish as opposed to coldwater fish it’s supposed to have.”

Granite Shore Power has argued that they are not violating the permits and are operating within legal limits.

Granite Shore Power is an investment group that bought Merrimack Station from Eversource in 2017. The also own Schiller Station in Portsmouth, a much smaller coal-fired plant that was shut two years ago.

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