Maryland environmental officials said they are investigating a chemical leak from a recently shuttered paper mill on the Upper Potomac River that has discolored part of the river and could harm aquatic life.

State officials suspect the seepage contains “black liquor,” a byproduct of paper manufacturing that is about as caustic as lye, along with other chemicals from Verso’s former plant in Luke, Md. The Maryland Department of the Environment (MDE) has been working with the publicly traded company since April to pinpoint the source of the leak and develop a plan to clean it up, MDE spokesman Jay Apperson said.

Verso said it’s hired a consultant to investigate.

“It is not certain that the seepage is black liquor or that the mill is the source,” Verso spokeswoman Kathi Rowzie said in an email. "We are working with the environmental consultant and the state agencies to identify the substance and where it originated. Until those questions are definitively answered, any further comment would be speculation.”

Brent Walls, the head of Upper Potomac Riverkeeper, a nonprofit environmental organization, expressed alarm over what he sees as a low-key response from the state and the company since the leak was reported last spring. He said he learned about the leak in early September from another fisherman near the popular trout-fishing area in Allegany County.

Water samples collected by his organization found alarming levels of toxic materials, including arsenic, boron and methyl mercury, in addition to other chemical compounds that are consistent with black liquor, Walls said. He said the chemicals build up in the tissues of fish and other creatures, and pose a threat to human health, particularly to anyone who eats fish caught there.

“That combination just makes a soup for disaster,” Walls said. “We’re really concerned about it. It is a severe source of pollution.”

Walls criticized the state and the company for failing to notify the public or take swift remedial action, such as deploying booms to confine the leak.

Walls, citing a state inspection report, said there is also little doubt the leak emanated from the Luke Mill property — an inference also reached by the company’s environmental manager when he was contacted by a state environmental inspector in April. The inspection report suggests the company has known about the problem for years, possibly decades.

“They’re still keeping everything hush-hush,” Walls said.

The Luke mill created paper and jobs on that bend of the river through Appalachia from 1888 until its closure this year. The 228-acre property straddles the river, with a manufacturing complex on the Maryland riverbank and a limekiln and a million-gallon storage tank on the West Virginia side.

In its prime, the plant produced pulp, paper and specially coated paper used in Campbell’s Soup labels and glossy magazines. Over the years, the plant also generated its own power by burning the black liquor, a common practice in the industry since the 1930s.

In 2009 and 2010, U.S. paper companies even received a refundable tax credit by using black liquor as an alternative biomass fuel — a provision that angered environmentalists, who saw the tax break as a perversion of efforts to promote clean energy such as solar power. Similar laws in the District and Maryland meant that local utilities paid millions to paper mills that burned black liquor.

Verso, an Ohio-based company, closed the plant in late May, eliminating 675 jobs. The closure sent ripples through other businesses and nearby towns, prompting politicians from both parties and on both sides of the river to try to save the plant, including Maryland Gov. Larry Hogan (R) and members of Congress.

About a month or so before the closure, a person fishing on the North Branch of the Potomac River on the West Virginia side saw “black waste” leaking into the river and alerted the state’s environmental officials, an MDE inspection report says.

The MDE report, dated April 9, says Verso’s environmental manager, identified as Ron Paugh, said the company had been aware of the issue for some time and stopped storing black liquor on the West Virginia side of the river about 15 years ago.

MDE directed the company to investigate and determine the source of the leakage so it can be stopped, the inspection report says.

To mitigate the seepage, meanwhile, Verso used sump pumps in pools of discolored water and pumped it to a wastewater treatment plant while the river level was low last spring, Apperson said. When the river rose, the company had to remove the pumps, but it has now reinstalled them. He said a boom would not be effective because the material is found throughout the water column, not just at the surface.

“MDE has sampled water quality in the river and found elevated levels of sulfates and sodium and high pH, all of which can be harmful for aquatic life and are markers for liquors,” Apperson said. But the inspections also suggest the impact has been local, with “good water quality immediately downstream,” as the volume of water there dissipates the seepage quickly.

“We still cannot say with certainty there is a threat to human health,” Apperson said. He said that the agency, out of “an abundance of caution,” was taking steps to post advisories near the leak.

The company’s plan to investigate and monitor the leak was approved by West Virginia and Maryland officials in August. A cleanup plan will be developed based on what the company finds, Apperson said.

Walls, the region’s riverkeeper, said he suspects the state has been slow to press the company or call attention to the leak because it would complicate efforts to sell or redevelop the site when the region has already lost a lot of industry.

“You have this pollution source that has unknown levels of contaminants,” Walls said. "It’s going to be a huge expense.”