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The US Environmental Protection Agency (EPA) has added seven new sites, including the Copper Bluff mine in California, to its Superfund National Priorities List (NPL), a catalogue of the “most serious uncontrolled or abandoned releases of contamination” in the US.

While the Copper Bluff mine ceased operations in 1962, acidic waste has seeped into the Trinity River from the entrance of the mine, which produced copper and zinc during its lifetime. The acidic quality of the waste is the result of groundwater reacting with minerals in the abandoned mine, producing sulphuric acid which can then dissolve metals from surrounding areas of rock. The acid and the dissolved metals then pollute the water, with the Copper Bluff mine in particular releasing up to 500 gallons per minute of water containing cadmium, copper and zinc into the river.

The pollution is particularly damaging as the Hoopa Valley tribe, a Native American group in the region, depend on the Trinity River as source of water and food.

“The Hoopa Valley Tribe and the tribal fishery are still affected by this mine, despite its closure decades ago,” said EPA Pacific Southwest regional administrator Mike Stoker. “Adding the site to the National Priorities List is an important step towards cleaning up this toxic legacy.”

The EPA first considered adding the mine to the Superfund NPL in September 2018, and announced its addition on Monday. The agency will now complete an investigation into the mine and its pollutants, including monitoring the project’s discharge rate over a number of seasons, to determine how to most effectively contain the pollution.

Alongside the Copper Bluff mine, the EPA added six other sites to the Superfund NPL, including one belonging to the McLouth Steel Corporation in Michigan and another belonging to Magna Metals in New York state, as the agency aims to tackle the long-term environmental damage caused by industry.

The EPA Superfund programme is responsible for clean-up operations at the most polluted land in the US, and responding to natural disasters. Since its creation in 1980, the project has treated 1,507 sites across the US, and improved public health by a number of metrics, including a 25% reduction in the number of birth defects reported among children living near affected sites.