Exactly how does someone clean toxic mercury deposits out of a section of a tidal river more than 30 miles long?
That’s the main question a federal judge is expected to decide next year as part of a court-ordered cleanup of mercury dumped over decades into the Penobscot River, by operators of the former HoltraChem chemical plant in Orrington.
In 2015, U.S. District Court Judge John Woodcock ordered Mallinckrodt, the last company to operate the defunct plant, to fund both the development of a river remediation plan and the anticipated river cleanup project. Mallinckrodt already is on the hook for a projected $130 million cleanup effort of the former HoltraChem property in Orrington, which was ordered by the state in 2010. Until the court decides what needs to be done, the total cost of cleaning the river won’t be known.
An engineering firm appointed to identify possible solutions has said the river cleanup could include digging up and removing the polluted sediments from the river bed; capping the sediments where they are; adding an inert bonding agent to reduce the toxicity of the mercury; building a berm to trap the sediment; collecting, burying and then capping the contaminated sediment more deeply in the river bottom; or letting the mercury disperse naturally into the ecosystem over time. Some combinations of those options also may be considered.
But any cleanup effort will likely include leaving some mercury in the river since removing it altogether could be too complicated and expensive compared to other, more cost-effective methods, according to scientists familiar with river pollution remediation projects, but who have not specifically studied the Penobscot River project.
Rivers cannot be turned off like a faucet, or drained like a bathtub, and the overall rate of diffusion of mercury in the river has been slow. If left as is, it would take 30 to 60 years for the mercury levels in the river to decrease enough to be considered safe.
The river cleanup will have to take into account the tidal and seasonal changes of the river between the Veazie Dam, which marks the definitive boundary between the freshwater river and the polluted tidal portion, and Perkins Point in Castine, where the brackish estuary fully opens up into the marine expanse of Penobscot Bay.
“It is a pretty large area,” Sean Smith, an assistant professor at University of Maine with expertise in river morphology and sediment transport, said Wednesday.
Digging up pollution and keeping it contained at the same time can be a challenge, Smith said. Dredging the bottom could create a plume of contaminants that flows away with the current. Erecting a containment barrier around a dredge area may be possible, he added, but keeping it stable as the river’s current intensity fluctuates or as the tide changes direction would be difficult.
Capping the mercury on the river bottom might be an option, but it may not be that practical. Capping a broad expanse of river bottom likely would be costly and could present more technical challenges than it would solve, said David Courtemanche, a former Maine Department of Environmental Protection scientist who now works for The Nature Conservancy. The two-way nature of the changing tides and the likelihood in winter of ice chunks scouring the bottom of shallow sections also could making capping ineffective, he added.
“I don’t know how well capping would work,” Courtemanche said. “It’s not an easy project, for sure. It’s just a really complex [estuary] system.”
With either capping or dredging “you’re dealing with a big problem and you want to make sure you don’t create other problems along the way,” Smith said.
Such potential problems could include getting dredging equipment to the desired remediation sites, according to Susanne Miller, director of the Bangor region office of Maine Department of Environmental Protection.
Even if they can get it there, project engineers will have to consider how dredging might impact protected species such as shortnose sturgeon or Atlantic salmon, she said. Federal permits to conduct such work might not be approved if those impacts are too severe.
“When you start to mess with the shape of [the bottom of] the river, it can affect fish as well,” Miller said Friday.
State officials continue to monitor the presence of mercury in the river. Some birds in marshes along the river contain “very high levels” of mercury, according to court-ordered studies of the river, and the state has banned fishing for crab or lobster anywhere upstream from the southern tip of Cape Jellison in Stockton Springs and Perkins Point in Castine.