BANGOR – Nineteen years after a 1980 environmental assessment concluded that there wasn’t a connection between the former Bangor Gas Works and coal tar in the Penobscot River, a consultant’s report determined that the plant was the sole source of the pollution.

Those are just two conflicting pieces of evidence a federal judge will consider over the next three weeks in a jury-waived trial to determine who will pay the $12 million cost of cleaning up a 10-acre coal tar plume located along Bangor’s waterfront redevelopment.

U.S. District Court Judge George Singal, sitting in U.S. District Court in Bangor, on Monday heard opening arguments in the case that began nearly three years ago when the city of Bangor sued Stamford, Conn.-based Citizens Communications Co., the successor of a series of corporate entities that owned and operated Bangor Gas Works.

Citizens has denied that it is solely responsible for the tar plume. It has filed a dozen third-party lawsuits against companies it claims operated closer to the river and are more likely polluters.

On Monday, more than 20 boxes of files and dozens of binders that could be entered as evidence sat in a corner of the third-floor courtroom as the city’s attorney played a video downloaded onto a laptop computer. It showed a black gray film floating on top of blue-green water identified as the Penobscot River.

“This is the story of how the tar plume formed,” Bangor attorney William Devoe, representing the city, said in his opening statement. “It is the story of two evolutionary paths – one in the words of science, the other in the words of history. The history of this case began in 1851 when the city was younger, thriving and looking to take advantage of new technologies.”

One of those new technologies was gas. In 1952, one year after the city granted permits to the locally owned company, Bangor Gas Works began removing usable gas from coal on the site where Shaw’s Supermarket now sits.

Both sides agree that the residue left after the gasification process was a tar substance that was stored in tanks at the facility.

Devoe told Singal that as the evidence stacks up over the next few weeks, “the lines of history and science will come together.

“When history and science come together, the truth will be found at the bottom of the river,” he concluded.

Evidence to be introduced by the city, Devoe said, includes a 1999 report prepared by an environmental engineering firm in Madison, Wis., that excluded all firms but the gas works as the source of the tar plume in the river.

Citizens’ attorney Martha Gaythwaite of Portland countered in her opening statement that just because there’s tar in the riverbed doesn’t mean that Bangor Gas Works put it there.

She displayed poster-sized diagrams showing the locations of businesses along the riverfront that handled tar and coal.

“The city claims that the tar went down the sewer pipe [to the river],” she said. “What is significant evidence is that there is no tar sticking inside the old stone sewer in the plant location.”

The city and the Maine Department of Environmental Protection agreed in 1980 that there wasn’t a connection between the substance in the river and the coal tar left in tanks on the site, she said. That tar was taken to the paper mill in Old Town where it was burned.

“The city had it right in 1980,” Gaythwaite said.

Both sides have agreed that after the trial concludes, it will be at least a month before Singal issues his written verdict.