In August, roughly 100 fish turned up dead in a creek below the large Kensington gold mine north of Juneau.

Seven months later, state regulators and Coeur Mining, the multinational company that owns and operates the mine, still haven’t determined what killed them.

Samples taken from mine wastewater and Sherman Creek, where the dead fish were found, showed no obvious signs of pollution. That’s according to a technical memo that Coeur submitted to state regulators in November, which Northern Journal obtained through a public records request.

“Based on the findings of the investigation, the event remains inconclusive,” wrote Pete Strow, an environmental manager at Coeur in Juneau. His comments came in a note added to the company’s analysis, which was conducted for Coeur by WSP, a global environmental consulting group.

The lingering uncertainty reveals just how hard it can be to determine the cause of sudden fish die-offs, and it “typifies the importance” of frequent water sampling near mines, said Chris Sergeant, a watershed ecologist at the University of Washington who studies impacts from mining.

It’s possible that pollution killed the fish and then quickly flushed through the creek before it could be detected, said Sergeant, who reviewed the memo submitted to regulators by Coeur. A state official previously suggested that the die-off stemmed from a mine-related water quality problem.

“It is very easy to miss measurements during acute toxic events,” Sergeant said in an email.

Coeur acknowledged a potential link to an explosive used at the mine the day before the fish were found, according to their consultants’ report — though Strow also raised the possibility of “naturally occurring toxicity events.”

Neither regulators nor Coeur have provided a detailed theory about how the fish died.

In the months following the discovery, state regulators asked Coeur for extensive data and an explanation for the incident. They now say their focus has shifted to working with the company to prevent future incidents.

“Everybody who’s involved in this is always interested in trying to find out: ‘How did this happen? What was it that we didn’t anticipate?’ So we can anticipate it better,” said Gene McCabe, the director of the Alaska Department of Environmental Conservation’s water division, which oversees the state’s wastewater discharge permitting. “But we don’t want to spin our wheels too long.”

He added, “Sometimes, you’re just never going to have data that says, ‘Yes, that’s what it was.’”

State biologists found the dead Dolly Varden char, slimy sculpin and one pink salmon in Sherman Creek during a routine inspection Aug. 9. The biologists observed hemorrhaging on multiple dead Dolly Varden and “no obvious body decomposition or cloudiness in the eyes, suggesting recent death,” according to their September report.

They found the dead fish downstream of the mine’s wastewater treatment plant, including at the spot where the mine discharges treated water into the creek and at other locations more than a mile downstream, near where the creek flows into the ocean. Upstream of the treatment plant, the biologists saw live Dolly Varden.

A state pathologist later wrote that the circumstances — dead fish below the treatment plant but not above — “strongly suggest water quality or toxic issues” stemming from Coeur’s operation. He ruled out infection as a cause of death.

The scientists hired by Coeur said that ongoing monitoring of the creek supports the state’s conclusion that the die-off was tied to “a rapid event which passed quickly.”

But, the scientists added, Coeur’s field monitoring, surface water analysis, and other observations did not detect any “suboptimal or acutely toxic water quality before, during, or following” the day the dead fish were discovered.

The state biologists reported finding the fish during a survey that started at 10:00 a.m. on Aug. 9. Coeur took several water samples that day, according to lab reports included in the company’s memo to regulators.

Mine staff sampled mine wastewater at 9:00 a.m. and at a few spots in the creek between 2:15 p.m. and 4:20 p.m, the documents showed.

Strow, with Coeur, said the analysis suggests the die-off “may not be associated” with discharge from the treatment plant. A company spokeswoman, Rochelle Lindley, declined to elaborate or answer questions about the incident.

“Coeur Alaska is committed to continuous improvement, and our team continues to work with state regulatory agencies on best practices for our operation,” she said in a statement. “At this time, we don’t have any publicly available updates beyond what has already been provided.”

The investigation into the die-off scrutinized an explosive called RocKracker, according to documents reviewed by Northern Journal.

The product is “acutely toxic to some marine organisms” and generally shouldn’t be placed in bodies of water, according to its safety information.

Mine workers used the explosive the day before the dead fish were discovered, according to the consultants hired by Coeur. They said the company described “the potential for wastewater containing the explosive to reach the treatment plant discharging” into Sherman Creek.

A federal official, in a report to a local tribe obtained by Northern Journal, described the use of an “unapproved” explosive at Kensington on Aug. 8. Coeur, in documents submitted to the state, acknowledged having used RocKracker on a “one-time” basis.

Potentially toxic compounds from the product weren’t detected in the water samples taken from the creek the day the dead fish were found, according to Coeur’s report.

Allan Nakanishi, a state engineer and regulator who works with McCabe, said his office would forward information about the die-off, including Coeur’s use of RocKracker, to state compliance officers “to determine if further action is needed.”

Regulators say Coeur was compliant with the sampling requirements of its federal Clean Water Act permit. The permit calls for weekly tests for pollutants in the treated wastewater that the company releases directly into Sherman Creek.

In general, Coeur’s sampling program “is adequate to detect pollution,” said McCabe, the regulator.

But he wouldn’t rule out the possibility that pollution had occurred, undetected, between sampling times.

“What can always happen is something completely unexpected. The unpredictable, unaccounted-for event is always a factor,” McCabe said. “To build a program to operate every day of its life expecting the unexpected creates an unmanageable amount of data.”

In December, McCabe’s division asked for additional information from Coeur. Among the requests: an alternative explanation if the die-off was not conclusively related to the mine, a description of pollutants that could have killed the fish, and a diagram showing how those pollutants could have ended up in the creek.

Regulators also asked the company to suggest improvements to its wastewater treatment plant and monitoring program to ensure that it’s “capable of detecting potential releases in the future.” And they asked Coeur to discuss limitations of the data they collected.

Strow, in his letter, said the analysis “did not consider all potential environmental factors, such as naturally occurring toxicity events,” and he added that “some of the compounds tested during the investigation are not routine, and therefore we do not have baseline results for all parameters.”

Lindley, from Coeur, would not elaborate on what “naturally occurring” events might have killed the fish.

The company has already taken a number of steps in response to the incident, according to a preliminary document shared with DEC in February. Those include stopping the use of RocKracker, fine-tuning their water sampling, and creating a new position to oversee blasting operations at Kensington.

McCabe said Coeur has worked “very well” with regulators.

• Northern Journal contributor Max Graham can be reached at max@northernjournal.com. He’s interested in any and all mining related stories, as well as introductory meetings with people in and around the industry. This article was originally published in Northern Journal, a newsletter from Nathaniel Herz. Subscribe at this link. Alaska Beacon, an affiliate of States Newsroom, is an independent, nonpartisan news organization focused on connecting Alaskans to their state government.