PORTLAND – A trial over the extent and potential dangers of mercury pollution in the Penobscot River that pits two environmental groups against a former owner of the now-defunct HoltraChem Manufacturing Co. plant in Orrington wrapped up in federal court Thursday.

At issue is whether the mercury from the former chemical plant that for years was dumped into the Penobscot River poses a threat to the animals and people who live near the river and the bay into which it drains.

During the two-week trial in U.S. District Court in Portland, the Maine People’s Alliance and the Natural Resources Defense Council sought to show that mercury in the river and bay sediment does pose a grave danger to the environment and human health. They filed suit two years ago seeking more extensive studies of the degree of mercury contamination of the Penobscot River from Orrington to Stockton Springs and into Penobscot Bay.

In addition, the two groups said such a study should examine the impact of the contamination on shellfish and other wildlife in the region. If it is found that the mercury poses a risk to such organisms, it all should be cleaned up, they said. Mercury is known to interfere with reproduction and fetal neurological development.

The former plant owner, Mallinckrodt Inc., sought to build a case that contamination of the river had been studied adequately and that the mercury there does not pose a risk to humans or the environment.

The St. Louis-based pharmaceutical company has not owned the plant for more than 20 years. But as the only former owner still in business, the company has worked with the Maine Department of Environmental Protection and the U.S. Environmental Protection Agency to develop a plan for cleaning up the facility that until September 2000 made chlorine and other chemicals, mainly for paper mills.

The company has committed verbally to clean up mercury contamination in the river next to the plant, including dredging a small portion of the Penobscot River. Mallinckrodt has not, however, signed a formal agreement with the agencies to undertake such a cleanup.

That agreement could be signed within a matter of days, Mallinckrodt spokeswoman June Fowler said during an interview Thursday.

The company already has spent millions of dollars on planning and undertaking cleanup activities at the Orrington site, Fowler said.

“It’s more than a verbal commitment. It’s a commitment in action,” she said.

The six witnesses called by the company, which owned the plant from 1967 to 1982, repeatedly said the mercury contamination had been studied adequately and that it was found to pose no imminent threat. The witnesses included officials from the DEP and EPA as well as private consultants. No company officials took the stand.

“While this litigation has shown that there is disagreement among scientific experts, neither the U.S. EPA nor the Maine DEP would approve a work plan that did not protect the environment or public health,” Fowler said after the trial concluded.

A plan to clean up the HoltraChem plant currently is being developed by the EPA, DEP and Mallinckrodt but has yet to be completed.

The studies of the river to date have been inadequate, said witnesses called by the plaintiff environmental groups. They only looked at a small section of the river downstream from the plant and did not assess the impact of contamination on wildlife in the area.

Any comprehensive study should include Penobscot Bay because that is where mercury dumped in the river ultimately ends up, said the groups’ key witness, Robert Livingston, a professor at Florida State University who specializes in the pollution of aquatic systems.

Such a study not only must determine the amount of mercury present in the river and ocean bottom but also must find out if the mercury has broken down into a form that makes it readily available to be absorbed by organisms that live in and feed from the water, he said. In addition, animals that live near the river and bay should be studied to see if their bodies contain abnormally high levels of mercury, said Livingston, who was the first and last witness to take the stand.

During Livingston’s testimony Thursday, Judge Gene Carter asked if Mallinckrodt’s plans to clean up 6 kilometers of the river were enough.

No, said Livingston, that wouldn’t solve the problem. That’s because the full extent and impact of contamination farther down river is not known.

After the trial, MPA co-executive director John Dieffenbacher-Krall said the proceedings made it clear that state and federal regulators had lost control of the situation. He said it was shocking that the agencies allowed thousands of pounds of mercury to be dumped into the river without studying what the heavy metal was doing to the ecosystem.

“It’s a public resource that belongs to all of us,” Dieffenbacher-Krall said of the Penobscot River. “It’s not their private sewer to dump waste into.”

Although both parties rested their cases Thursday, Carter asked that instead of oral closing arguments both parties submit written post-trial briefs. The judge may ask both sides to make oral arguments at a later date. A ruling is not expected for several months.