An unexpected large boulder discovered during construction of a sewer pumping station on Parkwood Drive added $62,000 to the cost of adding new sewer lines to the area.

The Board of Selectmen approved the increased cost Tuesday during an update on ongoing sewer projects by Michael Guidice, of engineering firm Camp Dresser & McKee.

Selectman Cara Winslow said she was concerned about the increase, as it will "equal real dollars for the people who are going to pay the betterment."

The betterment fee is the amount each household along a new sewer line is required to pay to cover the cost of the extension. Parkwood property owners have not yet been given a final figure for how much the betterment fee will cost, which has been causing anxiety among residents.

"I'm a little bit surprised that the people who bid this job didn't understand the terrain" that they would be dealing with, Winslow said. "What I see is bordering on irresponsible on the part of the people who bid this contract."

Guidice said that while he understood Winslow's concerns, he negotiated a lower price. "We think this is a fair price for the work that they did," he said.

The increased cost is actually low in comparison to the total cost of the project, which is roughly $6 million, Guidice said. A 3.5 to 5-percent range for increased expenses is typical for construction projects of this size, he said.

Selectmen Holmes and Eckstrom disagreed with Winslow.

"[The contractors] didn't see the ledge. It was unfortunate. And it cost them a small fortune to do this, a lot more than $62,000," Holmes said.

Eckstrom added: "We got a pretty good deal on it."