The rulemaking follows a damning report by the US Environmental Protection Agency in September last year which pointed to Elk Valley coal mines as the source of elevated selenium levels in downstream water courses.

Selenium is a micronutrient most living things need to function, but at higher concentrations it can cause reproductive effects, reduced growth and mortality in fish populations.

Teck Resources, the biggest coal producer in Canada, has said it could face delays in permitting, or restrictions on mining in Elk Valley as a result of an unexpected and substantial reduction in populations of certain fish near the coal mines.

The Montana board of environmental review started the process to establish site-specific water quality standards for selenium following a request to do so from the environmental quality department (DEQ) since high selenium levels have persisted since Lake Koocanusa was listed as threatened by selenium in 2012. Montana currently has no permitted selenium discharge.

As established in 1987, the current selenium standard for water bodies stands at five micrograms per litre. In 2016, the EPA developed updated recommended national criteria at a value of 1.5µg/L for lakes and reservoirs and 3.1µg/L for rivers. The board now suggests pinning those figures at 0.8µg/L for Lake Koocanusa and 3.1µg/L for the river. The national recommended criteria is 1.5µg/L.

The Montana government said the DEQ had worked closely with BC's Environment & Climate Change Strategy ministry (ECCS), local tribes, and federal and state representatives to come up with the new standards with the goal for both Montana and British Columbia to adopt aligned standards that protect aquatic life.

The BC government confirmed Monday it had participated in the process to establish "a selenium-level objective", but stopped short of selecting a water quality objective, saying it was committed to "a science-based process informed by the best data available".

"A selenium-level target will only be established once BC is fully confident that the process has met this high standard and after seeking consensus with the Ktunaxa Nation Council on a recommended standard for selenium for this transboundary water body," said the ECCS in a statement.

Teck ran into trouble with its permit application for the Castle Mountain metallurgical coal expansion project last month when the federal government decided to join a provincial environmental review of progress on the project.

Teck spokesman Chad Pederson told Mining Journal the company was implementing recovery actions this year to rectify the elevated downstream selenium levels.

"Substantial technical effort is underway to determine whether the westslope cutthroat trout reductions were associated with water quality issues, flow conditions and habitat availability, or predation or other natural causes, and to develop a response plan. Concurrently, we are moving forward with recovery actions for implementation in 2020," he said.