Why is acid leaking out of Tunks Park?

Cammeray residents have raised concerns about a murky substance flowing through their local waterways. 

For more than a year, acid has been leaching out of the ground in Tunks Park and washing into the adjacent Long Bay.

North Sydney Council, which is responsible for the park, has been working since 2024 to find out what is causing the contamination. 

At this stage, council is certain it is coming from acid sulfate soils. But in order to nail down the source of the leakage and find potential solutions, councillors have endorsed spending nearly $330,000 on an investigation.

What are acid sulfate soils?

• Acid sulfate soils occur naturally in coastal settings. They are a type of soil, containing iron sulfides, which forms underground, away from oxygen. 

• While buried, they are harmless, but when exposed to open air, they can turn acidic and release heavy metals into an ecosystem.

Acid sulfate soils become a problem when the earth they are buried in is disturbed. Flooding, heavy rainfall or human interference can lead to this.

Are they dangerous?

According to the University of Adelaide, these acids pose a severe risk to aquatic fauna, water quality and plant life. 

The release of the acid can also smell terrible.

What do we know? 

The working theory is that acids are leaking from either tidal pumping (waves driving water in and out of coastal soil) or rainwater seeping through the soils into the bay.

The investigation has been led by the NSW Department of Climate Change, Energy, the Environment and Water. 

History

Tunks Park is a relatively new feature of the North Shore, having been artificially constructed in the 1950s. 

Previously, Long Bay stretched out under the Long Gully Bridge, meeting with Flat Rock Gully, before it was filled in and converted to parkland.

Note: Article amended to accuratel