Maryland is fining the owner of two of the state's largest coal power plants $1 million for dumping too much nitrogen into the Potomac and Patuxent rivers, one of the largest penalties state environmental regulators have levied in years.

The Chalk Point Generating Station in Prince George's County and Dickerson Generation Station in Montgomery County were alleged to have violated the Clean Water Act from 2010 to 2013. As the power plants scrubbed more nitrogen from emissions into the air, they were discharging more of the harmful nutrient in their wastewater, state environmental officials said.

In agreeing to the settlement, the Houston-based NRG Energy Inc., which owns both the Chalk Point and Dickerson plants, did not admit to wrongdoing or liability.

Maryland Department of the Environment Secretary Ben Grumbles called it a "stiff penalty" that signals the state is holding polluters accountable, something some environmentalists question.

"It's a strong statement that environmental laws must be respected," Grumbles said.

"A million dollars is a significant fine," said Attorney General Brian E. Frosh, whose office helped negotiate the settlement. "I think this is appropriate."

But one water-quality advocate who helped alert regulators to the violations said he thinks the penalty is not enough. A group of environmentalists threatened to file a lawsuit against the power plants before former Gov. Martin O'Malley's administration stepped in with its own lawsuit in 2013.

"I think this is a slap on the wrist," said Fred Tutman, the Patuxent Riverkeeper. "It's a lot of money, but not in contrast to the extent of the violations."

The consent agreement, filed in federal court Friday, also requires the plants to invest in technology that would remove more nitrogen from their wastewater and spend another $1 million on water-quality improvement projects in the Patuxent and Potomac watersheds.

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In issuing nearly 8,000 enforcement actions during fiscal year 2015, the state levied $3.7 million in penalties. Similar totals have been reported going back to at least 2011, except for fiscal 2013, when penalties totaled $5.9 million.

Officials NRG Energy Inc. said the company is pleased to have resolved the case.

"We have worked with MDE on the best route forward," said NRG spokesman David Gaier. "We're really glad that a part of this settlement includes support for environmental projects because we're happy to help actually do some additional good."

Gaier said that in 2009, the Chalk Point and Dickerson plants installed technology known as "scrubbers," which remove nitrogen oxide and sulfur dioxide pollutants from emissions, to satisfy new state and federal regulations. But at that time, he said, "there was no history or experience" in removing nitrogen from wastewater at power plants.

Since 2010, the 50-year-old plants have exceeded the nitrogen discharge levels laid out in their Clean Water Act permits by more than 46,000 pounds, according to the consent agreement.

Nitrogen and phosphorus harm the bay by fertilizing algae blooms that block sunlight from reaching underwater grasses and strip the water of oxygen. Such blooms cause dead zones to form in the bay and can lead to fish kills.

While farms, wastewater treatment plants and failing septic systems are more commonly considered primary sources of those pollutants, coal power plants also bear some responsibility. The nitrogen oxide they emit into the air can be absorbed into the bay.

But in this case, technology that used water to remove that pollution from the air instead allowed it to accumulate in wastewater.

Grumbles said the penalty sends a message that power plants should not just be concerned with what they put in the air.

"They have an environmental responsibility to make sure the water is clean," he said.

The $1 million penalty will go toward Maryland Department of the Environment operations. Grumbles said it will be invested in "protecting the public health and the environment."

David Smedick, who leads the Sierra Club's Beyond Coal Campaign in Maryland, noted that the figure is about one-hundredth of what the state was authorized to charge NRG under the federal Clean Water Act.

"It's unfortunate that the only reprimand NRG has received for dumping excess nitrogen into the Chesapeake Bay is essentially a slap on the wrist," he said.

Some environmental advocates said they were glad to see a tough stance from the Hogan administration, which has emphasized a cooperative approach to pollution violations instead of a punitive one.

It's common that environmental enforcement spans multiple administrations because they often involve a lengthy litigation process. But fact that this case began under O'Malley does not change what it says about the Hogan administration, said Eric Schaeffer, executive director of the Environmental Integrity Project in Washington.

He called the settlement "a sensible outcome" and added, "If this is a harbinger of things to come, well, that's good."

But others said they remain concerned about how often the state is prosecuting new cases against polluters. A coalition of environmental groups led by the Center for Progressive Reform released a report in May showing that the number of cases the state is referring for criminal investigation has dropped by one-third over three years.

Frosh said in May the decline was concerning, and on Monday he said the flow of cases environmental regulators are referring to his office for law enforcement has not changed significantly in the months since.

Note: An earlier version of this story should have noted that NRG Energy Inc. did not admit to wrongdoing or liability as part of the consent agreement. It has been corrected here.

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