A federal judge has approved the details of a study that will examine whether it would be better to remove toxic mercury downstream from the former HoltraChem factory or to allow the Penobscot River to flush the pollution naturally.

Earlier this year, Senior U.S. District Judge Gene Carter agreed with a team of researchers who determined that mercury pollution in the lower Penobscot posed a threat to the health of wildlife and humans.

At the time, Carter directed the research team to put together plans for a more detailed study of how best to clean up the mercury from Orrington, where the HoltraChem facility was located, to Fort Point Light south of Stockton Springs.

Carter now has approved this second phase of the river study, which could begin this summer and likely will continue for at least two years. Both phases of the study are being paid for by Mallinckrodt Inc., the sole surviving former corporate owner of the chlor-alkali factory.

“The recent court approval means we can now look forward to the next phase of the downriver study and to ensuring Mallinckrodt continues to be held accountable for threatening our health and environment,” Adam Goode, a community organizer with the Maine People’s Alliance, said in a statement.

From the 1960s until it closed in 2000, the HoltraChem factory used mercury to produce chemicals for paper mills and other industries. Tons of mercury – a powerful neurotoxin particularly dangerous to infants and fetuses – ended up in the soils at the site or in the adjacent Penobscot.

Mallinckrodt has spent more than $35 million cleaning up the 235-acre factory site, which has been reduced largely to concrete pads and a few building skeletons. But hundreds of thousands of tons of contaminated soil remain on-site and untold amounts are still in the river.

The Maine People’s Alliance and the Natural Resources Defense Council won a landmark legal victory against the company in 2002. In recent years, the company has funneled millions of dollars into the river study but also has fought to delay beginning the next phase.

The price tag for physically cleaning up contamination in the river could far exceed the on-site remediation.

Mallinckrodt spokeswoman JoAnna Schooler said Monday that the company remains committed to a “thorough assessment of all pertinent data related to the HoltraChem site … and the Penobscot River.” Schooler pointed out that the company continues to work with state environmental officials and already has spent nearly $40 million on the cleanup.

“Consistent with being a good neighbor in this community, Mallinckrodt takes its responsibility toward the cleanup of this site very seriously and remains committed to a diligent and thoughtful approach to completing the environmental remediation of the former manufacturing site,” Schooler said.

The court-appointed research team, which is being led by biologist Drew Bodaly, plans to:

. Examine patterns of mercury methylation, which is when inorganic mercury is converted by bacteria into the form most dangerous to humans and wildlife.

. Examine mercury contamination levels in wildlife, with particular focus on lobsters and other wildlife consumed by humans. The mercury then builds up in the body, with higher rungs on the food chain accumulating higher levels of the toxin.

. Determine ongoing mercury discharges into the river.

. Estimate the time it would take for mercury levels to drop appreciably in the river.

. Determine whether it is better to let the mercury levels diminish naturally or to remove mercury from the river bottom, taking into account the risks of stirring up the pollution.

kmiller@bangordailynews.net

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