The company planned that treated water discharged into McKnoes Brook – up to 72 tonnes an hour – would meet drinking water levels of safety in case it was accidentally ingested by swimmers.

Alcoa first filed its plan with the Department of Water and Environmental in March 2022 and after including additional information requested by the regulator, released it for public comment in September.

The proposal was not referred to the independent Environmental Protection Authority as Alcoa considered a significant impact to the environment unlikely.

The proposal was described as “an alarming prospect” in recent internal state government documentation obtained by this masthead that said Water Corporation and DWER did not support it and instead wanted Alcoa to deal with its PFAS on the contaminated sites.

Alcoa contaminated its Willowdale mine, which includes Orion and Arundel, by using firefighting foam with PFAS from 2014 to 2021.

The WA Department of Fire and Emergency Services stopped using firefighting foam containing PFAS in 2003.

As Alcoa spokeswoman said the presence of PFAS was not mentioned in information from the manufacturer of its firefighting foam.

“We learned in 2019 that the product did in fact contain traces of PFAS,” she said.

“We acted promptly to implement alternatives and notified DWER.”

PFAS, or per- and poly-fluoroalkyl substances, allowed the foam to better smother a fire and are used in many other products including nonstick cookware and waterproof clothing.

Residents of Bullsbrook north of Perth are engaged in a class action against the Department of Defence for contamination of their groundwater from firefighting drills at nearby RAAF Pearce.

PFAS are known as “forever chemicals” as they persist in the environment for such a long time. They have been shown to be toxic to some animals.

A DWER spokesman said piping contaminated water directly across a drinking water reservoir was inconsistent with public drinking water source area policies.

He said the regulator would consider these risks and any controls proposed by Alcoa and expected to reach a conclusion by April.

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Alcoa has reported to DWER two other mine sites – McCoy and Myara, plus a tank farm near its Kwinana alumina refinery – as suspected to have PFAS contamination. Decontamination at Alcoa’s Wagerip refinery is underway.

Alcoa interim vice president of operations in Australia Rob Bear said the company acted swiftly and decisively once it became aware of traces of PFAS at its operations.

“We continue to work closely with DWER to manage this issue, which we know is affecting many organisations that used then-best-practice materials to fight fires,” Bear said.

“Independent testing continues to show our mining operations have not negatively impacted on the supply of public drinking water.”

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