Cooking oil, faeces, soap scum and vet drugs: Scientists reveal gross truth behind mysterious Sydney ‘tar balls’

Scientists have revealed the “disgusting” truth behind the origin of the mysterious “tar balls” that washed up on Sydney beaches and sparked a health panic.

A multidisciplinary team of scientists at the University of NSW have spent weeks analysing the mysterious balls – first spotted on the shores of Coogee Beach last month.

Associate Professor Jon Beves, from the university’s School of Chemistry, said the material was far closer to “human generated waste” and was not solely attributable to waste run-off from an oil spill, as originally thought.

The sticky balls contained hundreds of different components, including molecules that derive from cooking oil and soap scum, faeces, PFAS chemicals, steroidal compounds, antihypertensive medications, pesticides and veterinary drugs, Professor Beves said.

Radiocarbon dating of the samples indicated the balls contained about 70 per cent modern carbon and 30 per cent fossil carbon.

Professor Beves said modern carbon included any carbon derived from plants and animals, as opposed to fossil fuels, while the surface was made up of about 85 per cent modern carbon and 15 per cent fossil carbon. Modern carbon includes carbon derived from plants and animals, as opposed to fossil fuels.

“The higher concentration of modern carbon at the surface may result from the loss over time of components that evaporate more easily,” he said.

An elemental and X-ray analysis by the team revealed the balls contained “significant” levels of calcium but lower levels of other metals.

Sydneysiders have been warned to stay away from a popular eastern suburbs beach as authorities move to clean up the hundreds of black tar-like balls that have washed ashore.

“While we’re not yet certain about the exact form of calcium in the samples, their presence suggests they might be combining organic components to form stable, water insoluble, solid masses,” Professor William Alexander Donald, an analytical chemist involved in the research, said.

The presence of fats, oils and high calcium levels in the black balls suggests they could potentially be related to fat, oil, and grease (FOG) blobs – or “fatbergs” – commonly formed in sewage systems.

The exact origins of the black balls remain unknown.

“Carbon-14 dating suggested they weren’t purely from an oil spill, as the material was mostly modern carbon, loaded with fats, oils and greasy molecules,” Professor Donald said.

“Detecting markers of human faecal waste – like coprostanol – alongside recreational drugs like THC and industrial PFAS ‘forever chemicals’ pointed us to sewage and other sources of urban effluent as the most consistent origins.”

Thousands of the black balls were found washed up across beaches in Sydney last month, including on the shores of Bondi, Bronte, Tamarama and Maroubra.

The NSW Environment Protection Authority (EPA) had also been working closely with NSW Maritime, Randwick City Council and scientists to determine the origin of the balls.

In a statement, it said it considered possible causes like a shipping spill or wastewater overflow.

However, the balls’ exact origin could not be determined.

“As a result, authorities have so far been unable to trace the source, but final results are due in the coming weeks.”

Sydney Water confirmed soon after the balls were reported that there were no issues with the operation or maintenance of the Bondi or Malabar water resource recovery facilities.

Transport for NSW Maritime closely examined weather patterns in the days leading up to the event, but nothing conclusive could be determined.