BANGOR – Employees from Maine Test Borings Inc. were on a small barge in the Penobscot River this week gathering data needed to develop a game plan for cleaning up a coal tar deposit near the city docks at the Bangor Waterfront.

The testing company, based in Brewer, is among the firms tapped to help figure out how to address the problem, Bangor City Engineer Jim Ring said Wednesday.

Initial studies determined that Bangor Gas Works, which operated on the city’s west side from 1881 to 1963, was the source of the plume-shaped deposit, beginning at a former sewage outlet known locally as the “old stone,” or Davis Brook sewer, and extending 1,400 feet downstream.

The sewage outlet began at the gas plant and emptied into the river. The gas works generated coal tar as a waste product of its gas manufacturing operations near what is now Second Street Park, city officials believe.

For most of the year, the coal tar deposit on the riverbed is not much of a problem. But when the water warms in August, the deposits rise to the surface in small “blebs” that stick to objects such as boats and animals. Coal tar, when absorbed through the skin, is known to cause cancer.

The lead consultant for the work is RMT Inc., headquartered in Madison, Wis., Ring said. Besides Maine Test Borings, consultants involved in the research and planning phase include S.W. Cole Engineering, a Bangor-based environmental engineering and consulting firm.

Though a great deal of testing already has been done, much more needs to take place before a remediation plan can be submitted to the Maine Department of Environmental Protection, Ring said.

Planning for the cleanup of the coal tar deposits in the Penobscot River began over the summer, after a federal judge signed a consent decree that the city had agreed to earlier this year.

U.S. District Judge George Singal approved the document, which sets a timeline for remediation, including 30 days for public comment.

The consent decree is the result of a lawsuit the city filed more than four years ago against Citizens Communications Co., the Connecticut-based owner of the former Bangor Gas Works.

The DEP and the city are in the process of deciding the specifics of the cleanup operation, the cost for which has been estimated at $12 million to $20 million.

“Right now, we’re looking at some kind of a capping approach,” Ring said. “We’re at a point where we need to get more complete data.”

The studies and sampling now under way will resume in the spring, Ring said. The results will help determine specifically what cleanup method should be used.

DEP’s initial proposed remedy includes isolating the contaminated area, filling in a portion of the river to prevent migration of contaminants, then stabilizing the sediment permanently.

Dawn Gagnon may be reached at dgagnon@bangordailynews.net or 990-8189.