US bauxite miner Alcoa has been ordered to urgently clean out a pipeline it built without approval over a southwest drinking water dam which is likely filled with toxic chemicals.

Pollution from the pipeline is likely, according to a prevention notice issued to Alcoa on Thursday by the Department of Water and Environmental Regulation.

Last month WAtoday revealed Alcoa built the 10-kilometre pipeline over the Water Corporations’ Samson Dam east of Waroona to move water contaminated with the toxic fire-fighting chemical PFAS to a treatment facility.

The company later applied for regulatory approval in August 2022, but its submission made no mention that the pipeline and ponds to hold the contaminated water were already built.

When this masthead revealed state government agencies had labelled the pipeline plan “an alarming prospect”, Alcoa issued a lengthy media statement but did not say it had already implemented much of the construction and used the pipeline.

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Read WAtoday’s exclusive Alcoa investigation here

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.

The pipeline that was used for two weeks in 2022 is likely to contain PFAS-contaminated water, according to the regulator’s preliminary investigation, so any leak near the dam is a threat to southwest WA’s drinking water supply system.