WATERVILLE — A 200-gallon heating oil spill from a Fairfield home storage tank is suspected by Waterville fire officials as the cause of a gaseous odor reported Friday by neighbors on Water Street in the city’s South End, fire Chief David LaFountain said.

A combination of the spilled oil, which is believed to have entered the sewer system about 6 a.m. Friday, and the breezy, humid weather created a situation in which a sewer clear-out pipe at one building and small holes in sewage pipes in the area allowed an odor to escape into the air.

The fumes alarmed Water Street-area residents, who complained about a dangerous odor coming from the area, according to Waterville fire Lt. Shawn Esler.

“The sewage pipe system created a draft, which found its way through any opening it could,” Esler said.

State officials said more investigation is needed into the cause of the odor.

Peter Blanchard, director of the Division of Response Services in the state Department of Environmental Protection, said Friday evening that it is unclear what caused the odor. Blanchard said he spoke with a DEP supervisor who said the Fairfield spill was not related to the Waterville incident.

However, LaFountain isn’t so sure.

“I’m not convinced that it wasn’t,” he said.

LaFountain acknowledged that the incident presents a mystery that needs to be investigated.

“To me, it’s an issue that the DEP and the sewerage district have to try to figure out.”

He said someone from the Huhtamaki plant on College Avenue called fire headquarters to report a paint thinner odor in the sewer system, in the area where the fuel oil spilled.

Sewage carried through the Waterville sewer system ultimately leads to the Kennebec Sanitary District treatment plant downstream. The complaints came from people in the area of 27 Water St., about two miles south of the Huhtamaki plant.

After responding to those complaints of a strong odor, Esler said the Fire Department performed meter readings in the Water Street building’s basement and found readings that suggested the air in the basement was flammable and toxic.

While searching, the Fire Department, along with members of the Maine Army National Guard 11th Weapons of Mass Destruction — Civil Support Team, which assists civil authorities with chemical incidents, found two old containers in the basement of 27 Water St. One of the cans had been used to hold kerosene, while the other held paint thinner, Esler said. Shortly after fire crews arrived, the complex was evacuated.

“In situations like this, you don’t know what the chemical is,” Esler said. “You take every necessary step to ensure safety for the responders.”

LaFountain said he does not think the odor originated entirely at 27 Water St.

“It was discovered there because of an uncapped sewer drain, four inches in diameter, that allowed air in the sewer pipes to vent into the house,” he said.

One resident was treated at the scene after inhaling the fumes, according to Esler. The building, which is across from the Hathaway Creative Center, was opened again about 1:15 p.m., Esler said.

The DEP is investigating the incident, Esler said.

“The DEP is looking into the infiltration of the oil into the system,” Esler said at 4 p.m. “They haven’t found the source as of yet.”

LaFountain said neighbors originally thought the odor was caused by Summit Natural Gas, which was working just down the road on Water Street, but that was not the case.

“We were called for what was described as a natural gas leak, but the leak is not related,” LaFountain said.

Tim LeVasseur, superintendent of the Kennebec Sanitary Treatment District, which controls several sewage pump stations along the Kennebec River, said one of his employees smelled something Friday morning that was a combination of paint thinner and kerosene.

If the heating oil believed to have come from Fairfield did seep into the sewer drain, LaVasseur said, the volume of oil isn’t enough to cause any problems to the drainage system, which handles slightly less than 8 million gallons a day.

Waterville Fire and Rescue, Delta Ambulance, the DEP and the Maine Army National Guard responded to the incident.

“It was a lot of cooperation between agencies,” Esler said.

Jesse Scardina — 861-9239

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Twitter: @jessescardina