Usually when agencies try to assess the environmental risks of an abandoned mine, they’re looking for a particular result: they want to hear that the risks aren’t serious enough to merit an expensive cleanup program. In the case of the Salt Chuck mine on southeast Alaska’s Prince of Wales Island, the jury is still out. The U.S. Forest Service will soon be considering a report by San Francisco-based URS Corp., geologist Nancy Darigo said in a presentation at the Northern Latitudes Mining Reclamation Workshop in Juneau May 15.
The Salt Chuck mine was active between 1919 and 1941 and produced about 300,000 tons of copper sulfide ore. Environmental investigations at the site began in the mid-1990s, with the Bureau of Land Management undertaking field efforts and identifying potential contaminant sources. In 2002 the Forest Service contracted URS to do further research. The company identified a larger area of tailings than had previously been suspected and a number of data gaps. A more focused investigation began in 2006.
Darigo, who is based in Anchorage with URS, was one of the people who worked at the Salt Chuck site. Mill ruins and an abandoned barge can still be seen there, and the mine has been recommended for the National Register of Historic Places. Meanwhile, Toronto-based Pure Nickel has staked claims in the area, anticipating that there could be more mining there in future.
Salt Chuck Bay intended for public useSalt Chuck Bay is intended for intensive public recreation use and for the harvesting of shellfish and wildlife by the local community, Darigo said. Butter clams, softshell clams, littleneck clams and blue mussels are some of the main shellfish species that live in the area. It is also a critical habitat for black bear, waterfowl and salmon. URS studied the ingestion of chemicals by saltwater fish and shellfish and compared the results with federal, state and Canadian agencies’ acceptable standards, but these standards often varied depending on the agency.
“As far as what we know about tailings, we know that copper and vanadium are well above eco-benchmarks in background,” Darigo said. “We know that the metals in the tailings themselves are not a big human risk, but if you look at tissue of clams that are actually living in the tailings, they do contain high levels of these metals compared to the EPA screening values for fish ingestion.” The tailings at Salt Chuck tend to be completely inundated at high tide, she added.
URS looked at data from a laboratory test called TCLP, for toxicity characteristic leaching procedure, which showed that there is some leaching of the tailings, but it isn’t necessarily a hazardous waste. “This test is designed to mimic fairly acidic conditions. ... What’s actually going on in the natural conditions,” Darigo said. “And the million-dollar question ... is if the site risk interpretation indicates that a cleanup is warranted, how do you decide on the cleanup levels that are appropriate, given that the submerged media are not subject to real black-and-white regulations?”
Another test, called AVS/SEM for acid volatile sulfide/simultaneously extractable metals was used to find out whether metals in the tailings are being ingested by the creatures that live in them. The synthetic precipitation leaching procedure was also used to subject the tailings samples to leaching in the laboratory using site water. Bio-assay tests were conducted using the estuary fish Menidia beryllina — the inland silverside — because this fish is used to surviving in a wide range of salinities. The tests went for a couple of weeks to see if the fish could survive in a solution of the contaminated media from the mine site and water.
“As far as our surface water program, we do know that we have several metals that are showing up high in the low-tide tailings seep between the two tailings piles,” Darigo said.
“We know that the results from different years are quite variable. ... Our proposed sampling program included additional downgradient samples, and a look at background intertidal water, which hadn’t been conducted previously. Also we went pretty far afield for another sort of background sample along the Kasaan Peninsula, there’s an area where there’s other abandoned mines, just to see if there’s a regional effect going on.”
Arsenic problem in southern Salt Chuck BayAn arsenic problem was identified in southern Salt Chuck Bay, but there is also high arsenic in the background levels, so it was difficult for URS to determine what proportion of the arsenic was naturally present and how much might be coming from the tailings. “As if this wasn’t confusing enough, we find that a couple of metals are higher in certain species of shellfish than others and they don’t always show the same pattern, but it looks like there is a pattern, so what does this have to do with how you interpret risk to people?” Darigo asked.
The conclusions drawn by URS will be partly based on practical considerations, such as the risks at the particular locations that are favorites for clamming. In the past month a draft engineering evaluation-cost analysis has been completed and submitted to Alaska’s Department of Environmental Conservation and the U.S. Forest Service. Later this year there will be a public meeting for the Prince of Wales community, and a final report should be available by the end of 2007, “after much gnashing of teeth,” Darigo said. Only then will a decision be taken on whether to implement a cleanup and what form a cleanup might take.