Originally published in Castine Patriot, February 11, 2021 and The Weekly Packet, February 11, 2021
Callahan Mine cleanup continues
Major work set for this summer
As part of the tailings impoundment dam restructuring throughout last year, several tailing piles were re-situated to for permanent containment.
by Eli Forman
Last month the Environmental Protection Agency released its yearly update on the ongoing remediation of the former Callahan Mine in Harborside.
The report outlined specific progress on several areas of the mine site, including stabilizing the tailings containment and impoundment dam and construction of a new access road using waste rock from the site.
The remediation of the site began in 2004 and has been organized in three separate projects that the EPA refers to as “operable units.”
The first operable unit involved clean-up of arsenic and lead contamination in neighboring residential properties along with containment of polychlorinated biphenyl or “PCBs” in the former mine operation area.
This operable unit was completed in 2013, according to the EPA.
The operable unit currently under way involves stabilizing and covering mine tailings and dredging followed by long-term containment of contaminated sediments in a confined aquatic disposal “CAD” cell located in the former mine pit.
The final goal of the current operable unit will be the restoration of vegetation and habitat on the site.
The entire mine site occupies nearly 120 acres along the east side of Goose Falls Road, incorporating several salt marshes and estuary ponds.
Mine activity on the peninsula began in the late 19th century, as prospecting for copper, iron, lead, zinc and silver took off and a flurry of small speculative mines dotted the area.
Most of these mines operated only briefly as the metallurgical technology to efficiently extract zinc and copper rich ore did not exist at the time.
In the mid 1960s however, technological updates and rising commodity prices ushered in a new mining frenzy in the area, with the underground Black Hawk mine in Blue Hill and the open-pit Callahan Mine on Cape Rosier at the forefront of the boom.
According to the Maine Geological Survey, The Black Hawk mine in particular produced nearly 1,000,000 tons of zinc-copper-lead ore between 1972 and 1977, before closing permanently.
In its operable time, from 1968 to 1972, the Callahan Mine produced roughly 800,000 tons of copper and zinc ore.
In this process, a dam was constructed to drain the entire Goose Pond estuary in order to dig the mine pit, and around 1.7 million cubic yards of waste rock were excavated to access the ore. Ore processing also occurred on the site, involving a milling process of crushing and purifying zinc and copper using a variety of chemical leaching agents.
After mining operations ceased in 1972, the estuary was allowed to re-flood the mine pit and a basic tailings dam was built to impound the toxic chemical effluent of the milling process and remaining deposits of lead and cadmium across the site.
According to the EPA, portions of the estuary were also used as a settling basin for the milling effluent, resulting in contaminated sediment.
In total, the EPA estimates the portion of the site where active mining and milling operations occurred at 75 acres.
In 2015 the EPA completed installation of a specialized drain and passive treatment system that allows for safe drainage of the tailings impoundment.
Over the last year, crews have been working to further contain and stabilize the remaining tailings by reducing the height of the dam and adding buttresses where needed.
The tailings dam was lowered around 20 feet, and the entire tailings impoundment reshaped in order to fit the dam’s new footprint.
This required the re-situation of approximately 53,000 cubic yards of rock and 46,000 cubic yards of tailings.
Other activities performed at the site throughout 2020 include surface and groundwater monitoring, the installation of geotechnical monitoring instrumentation to constantly asses the stability of the tailings dam and pumping tests to determine the rate of groundwater flow into the tailings impoundment.
In 2021, according to the EPA, a cover system will be implemented over the tailings impoundment, effectively sealing contaminants from possible remobilization into the surrounding environment.
The cover will include a bentonite clay liner, a plastic membrane layer and a geocomposite drainage liner, overlaid by 12 inches of stone.
According to the EPA, this process is expected to take place throughout this coming summer and be completed sometime in August.
The final aspect of this operable unit, the dredging of contaminated sediments in the estuary and their disposal in an underwater CAD cell, has yet to receive necessary federal funding, although designs for the project were completed in 2019, according to the EPA.
Once that effort is complete, the current operable unit will be finished and the final operable unit of the cleanup phase will commence, addressing site-wide groundwater and the remaining waste-rock piles across the old mine.