SHUTESBURY — Gasoline contamination at the Fire Station on Leverett Road, first discovered 15 years ago during a drainage ditch installation and seemingly remedied by a subsequent cleanup, has returned.

The town recently received a letter from the state Department of Environmental Protection, following tests completed by engineering consulting firm Tighe & Bond in June, showing the presence of contamination. In some cases, the contamination levels were higher than before the cleanup was completed in 2012.

“We were surprised and disturbed to discover that there were reportable levels of gasoline in the test wells at the fire station,” Selectboard Chair Rita Farrell said at Wednesday’s meeting. “We thought the whole thing had been dealt with a number of years ago, when that cleanup happened.”

Jeffrey Arps, a vice president of Tighe & Bond, said as the licensed site professional, he will be providing the response to the violation letter.

The hope, Arps said, is that he will be able to inform DEP that concentrations rebounded in a test well, but that there was no exposure to people, no vapor intrusion to the building and no impact on drinking water.

“Basically it’s there, but it’s not hurting anyone,” Arps said.

Arps said O’Reilly, Talbot & Okun did substantial remediation, taking a lot of contaminated soil from the property, including 2,000 yards using a $200,000 DEP grant. But because the building itself couldn’t be removed, the company did injections of 300 gallons of a sodium persulfate/calcium peroxide solution through the Fire Station’s floor to oxidize and remediate any gasoline there.

The company also considered drilling holes in the floor and removing the gasoline through a vacuum-like ventilation system.

The project was then complete, even knowing some gasoline remained.

“We’re not doing any more active remediation. We think we’ve done enough and Mother Nature will take care of what’s left,” Arps said of that approach.

The levels of aliphatic hydrocarbons in the tests saw a significant reduction in May 2017, but “fast forward six years” and the rebound occurred, Arps said.

He said this is not easily explained. The original contamination came from an 850- to 1,000-gallon underground gas tank that hadn’t been used since the 1970s.

“There’s some residual source that got missed,” Arps said. “Under the Fire Station is the only thing I could think of that is a continuing source.”

Arps said it is unclear what additional remediation should be done, but more monitoring wells might be needed.

Town Administrator Becky Torres said the town has been paying $980 a year for regular testing and reports, and there is about $7,000 left from an original account related to dealing with the contamination.

Scott Merzbach can be reached at smerzbach@gazettenet.com.