As state fisheries officials examine the impact the mercury contamination is having on upper Penobscot Bay, Mallinckrodt continues its court-ordered work to clean up the former HoltraChem Manufacturing site.

Amec Foster Wheeler, a global engineering firm based in London, is developing a plan for how to clean up mercury that was deposited into the Penobscot River from the former chemical plant in Orrington.

A scientific study ordered by a U.S. District Court judge found that 6 to 12 tons of mercury were discharged from HoltraChem into the Penobscot River between 1967 and the early 1970s. Smaller amounts have been released since then, and at least 9 tons of the metal still remain in sediments of the upper and lower Penobscot estuary, the study concluded.

After hearing closing arguments in a lawsuit over the cleanup of the river, a federal judge indicated last summer that he would appoint a panel to recommend cost-effective ways to rid the waterway of mercury.

In September, Judge John Woodcock issued a ruling in federal court ordering that an engineering firm be hired to develop a plan to clean up mercury deposited in the Penobscot River. The remediation plan for the river is being developed now as the result of three work orders filed earlier this year in U.S. District Court in Bangor.

The most recent work order aims to develop information that will be used to determine how to best remediate contaminated sediments identified in two earlier engineering studies. Alternatives include dredging, capping and trapping sediments.

Six mercury-contaminated sites identified in the second phase of the research are undergoing field work this spring and summer. They are in the area of Harriman Cove in Bucksport, in the Frankfort Flats area, in Hampden, at Gross Point near the mouth of Orland River and at Fort Point Cove and Odom Ledge, both near Stockton Springs.

Those locations were chosen because they were identified as “mobile pools,” or unconsolidated riverbed sediments that accumulate, disperse and reaccumulate seasonally, with their movement affected by river currents, scour and salinity levels, according to a work order filed in May in U.S. District Court.

As for the land clean-up, trains continue to haul away tons of mercury-contaminated soil that have been excavated as part of the multimillion-dollar environmental cleanup at the Orrington site.

The land cleanup began in 2014 and is expected to wrap up in 2018 or early 2019, Stacy Ladner, an environmental specialist for the Department of Environmental Protection, said earlier this year.

The cleanup effort could cost Mallinckrodt Inc., the last remaining owner of HoltraChem still in existence, as much as $130 million, according to court documents filed in conjunction with litigation over the pollution.

Mallinckrodt has spent tens of millions of dollars over the years removing metallic mercury, mercury sludge and contaminated storage tanks and buildings from the site, according to company and town officials.

Previous work at the HoltraChem site included the installation of riprap on the riverbank next to one of the plant landfills to prevent erosion and river contamination, the addition of a groundwater collection system to capture contaminated groundwater discharging from the site for treatment and the construction of a new wastewater plant for the treatment of contaminated groundwater.