HANOVER — The U.S. Army Corps of Engineers is working to “fully isolate” its Hanover laboratory from the surrounding environment after a boiler malfunction over the weekend caused the release of 350 gallons of heating oil into the Connecticut River.

Engineers are working to “evaluate the labyrinth of drains” that run under the 1960s-era buildings of the Corps’ Cold Regions Research and Engineering Laboratory, along Route 10 and the river north of downtown Hanover, to prevent further problems, CRREL spokesman David Marquis said in an email Tuesday.

They ultimately hope to cut off connections to a central line that carries groundwater used for refrigeration and air conditioning into the river.

Two floor drains in CRREL’s boiler room allowed oil to enter that drainage system, which then carried the waste to the river and resulted in an “iridescent sheen” that flowed at least 2 miles south to near the Ledyard Bridge, according to the New Hampshire Department of Environmental Services.

The spill, which Hanover fire officials said involved a newly installed boiler, required firefighters from several Upper Valley towns to converge on the river Saturday as they attempted to contain the oil spill with a “deflection” boom just north of the Ledyard Bridge.

CRREL’s announcement comes after a regional nonprofit committed to protecting the river called on the federal government to overhaul the drainage system.

Andy Fisk, executive director of the Connecticut River Conservancy, said it is “appalling” that oil was able to leave the facility so easily.

“A facility like the CRREL should never have floor drains connected directly to a surface water source,” he said in a phone interview. “That’s really surprising and problematic.”

Fisk, whose group was previously known as the Connecticut River Watershed Council, went on to say that while the oil spill will affect the river’s ecology, it didn’t amount to a “large volume of oil” and first responders acted quickly.

The Hanover Fire Department was dispatched to CRREL around 4 p.m. Saturday after a security officer discovered the fuel oil leak.

Firefighters then worked with neighboring departments to deploy 550 feet of the rigid boom across the river, and Massachusetts-based firm Clean Harbors was called to perform a cleanup.

“I think we were very, very fortunate,” Hanover Fire Chief Martin McMillan said Tuesday while praising the on-site security for catching the problem. “Had that not been the case, I think we would have a lot larger problem.”

The boom also was “instrumental” in halting the oil’s advance farther down the river, he said.

“It’s really, really difficult to pull product out of moving water,” McMillan said. “That is one of the things we were up against.”

Hanover resident Kevin Mabey encountered the oil spill about half an hour after the leak was reported while hiking with friends in Pine Park.

The group came across a roughly 12-foot-wide patch of oil flowing south as well as a strong smell as they came up to the river, he said.

“It just had that really horrific smell to it, almost like gear oil or some other heavyweight oil,” he said Tuesday. “And anyone else who was walking around Pine Park, you could see they could smell it and they immediately went the other way. It drove you away from the river it was so bad.”

Mabey said he reported what he saw after getting home, and Hanover officials informed him they were working to contain the spill.

Marquis, the CRREL spokesman, said a system meant to prevent possible flameouts caused the initial leak. After the spill, the assembly was removed entirely and the fuel line was reconnected directly to the new boiler.

Meanwhile, he said, the two drains near the boiler have been sealed.

Hanover officials say there’s little they could have done to enforce better practices. Because the facility is owned by the federal government, it is exempt from local planning review and building inspections, according to Ryan Borkowski, Hanover’s building inspector.

“In all my time, I’ve been asked to go to the building once or twice to look at something for the fire department,” he said Tuesday. “But, to my knowledge, I’ve never done any permitting up there.”

However, Borkowski said, it’s not uncommon for buildings built through the 1960s to have an unfinished basement with a drain “that goes out to daylight.” But those were more common in residential lots.

“Nowadays, with modern plumbing codes, anything internal to the building would be connected to a sewer system,” Borkowski said.

Meanwhile, New Hampshire DES spokesman Jim Martin said the agency “is reviewing applicable regulations pertaining to floor drains and oil storage, as well as NH Water Quality Standards as it relates to the oil impact on the River.”

Tim Camerato can be reached at tcamerato@vnews.com or 603-727-3223.