In the same period more than 100,000 litres of diesel and hydraulic oil spilled from its truck and earthmoving equipment fleet. In 2022, there were 137 spills with an average volume of 125 litres.

Three state government ministers met with Alcoa in December over concerns about its mining close to the dam.

An Alcoa spokeswoman said it was improving its water management and monitoring and had cut reportable drainage failures to 19 in 2022, only one of which affected a dam. She said all spill areas were cleaned, and it had detected no hydrocarbons downstream from its mining.

“If Alcoa is doing this in one of the most sensitive and public areas, what are they doing further from public view?”

WA Forest Alliance director Jess Beckerling

“Alcoa voluntarily has agreed to stringent reporting of water management-related events that occur in our mining areas,” she said.

“We remain committed to being a sustainable miner and using leading management and monitoring practices in our operations.”

Loading

WA Forest Alliance director Jess Beckerling said the risk to Perth’s water supply was unacceptable and immediate action was required.

“If Alcoa is doing this in one of the most sensitive and public areas what are they doing further from public view?” she said.

How Alcoa shapes the land it mines determines whether sediment-laden water reaches the dam or not.

An agreement between Alcoa and Water Corporation requires water from a one in 20-year downpour be retained in sumps for at least five minutes and then drain into bushland, not into streams that feed the dam.

A spokeswoman for Water Minister Simone McGurk said the government had told Alcoa it must appropriately manage risks to dam water quality from its activities.

A review of water monitoring of Perth’s dams for the three years to June 2022 showed all dams, including Serpentine, were very good, the spokeswoman said.

The Alcoa spokeswoman said the company had cleared 7700 hectares in WA in the past decade and rehabilitated 6370 hectares.

The Pittsburgh-based company mines bauxite from a vast 12,600-square-kilometre lease that covers much of the northern jarrah forest from inland of Perth to Collie, 165 kilometres to the south.

Loading

In 2022 the UN Intergovernmental Panel on Climate Change concluded with high confidence the biodiverse region was at risk of ecological collapse due to hotter, drier conditions and more fires.

In January, Alcoa chief executive Roy Harvey told Wall Street analysts WA, where the company employs about 4000 people, was critical.

It has three refineries in the state to process the bauxite into aluminium oxide that is shipped to aluminium smelters around the world.

The WA operation is operated by Alcoa and owned by a joint venture, 60 per cent owned by Alcoa and 40 per cent by ASX-listed Alumina Limited, that also has a stake in Victoria’s Portland aluminium smelter as well as overseas operations.

The state-owned Water Corporation estimates facilities to make contaminated water drinkable could take five years to build at a cost up to $2.6 billion for all dams affected by mining.

The Morning Edition newsletter is our guide to the day’s most important and interesting stories, analysis and insights. Sign up here.