‘Equivalent to sewage’: Qld islands contaminated with dangerous sludge

Dangerous amounts of muddy sludge are contaminating the once pristine waters off popular Moreton Bay with experts warning of health risks to swimmers and marine life.

Muddy sludge is contaminating once pristine waters off Moreton Bay.

Dangerous amounts of muddy sludge are contaminating the once pristine waters off popular Moreton Bay islands, with experts warning of health risks to swimmers and dire consequences for marine life.

A University of Queensland study reveals “insidious” sediment from recent floods now covers 98 per cent of the bay, with ammonium levels “equivalent to 180 years of sewage plant discharges”.

Dr Alistair Grinham, from the School of Civil Engineering, said there was now little “clean sand” left and a muddy “dead zone” fast developing.

“It’s very difficult to work out whether there’s sludge in the water because you can’t see it, some areas may look lovely and white, but the flood sediment settles into a thin layer and is insidiously changing the nature of the seabed and affecting water chemistry,” Dr Grinham told The Courier-Mail.

Dr Elvis Okoffo, from UQ’s Queensland Alliance for Environmental Health Sciences, measured plastic stored in 50 surface sediment samples.

“I wouldn’t be swimming in it or letting my kids do that either.”

Flood sediment – which has entered the bay via the Brisbane River mouth near Lytton and Port of Brisbane and spread widely to Moreton, North Stradbroke and Bribie islands – contains clay, organic matter and nutrients which microbes break down to release nitrogen.

“This (mud) causes stress across the whole bay, and excess nutrients feed algal blooms which can cause skin rashes (in humans),” Dr Grinham said.

“In 1970, Moreton Bay had about 400 square kilometres of clean sand and now it has just 30 square kilometres.”

He said seagrass, coral and other marine life could be smothered by the mud, endangering the nursery grounds of fish.

Collecting a mud sample raised from the seabed of Moreton Bay, Brisbane Water

The UQ study analysed sediment from 47 sites around Moreton Bay three days after the 2022 flood peak and then at more than 200 sites throughout that year.

Core samples of mud pulled from the seabed were also collected and compared to results of studies from 1970, 2015 and 2019.

“Over the past 50 years, 300 million cubic metres of mud has been washed into Moreton Bay – that’s enough to fill 300 Suncorp stadiums,” Dr Grinham said.

He said a “big mud zone” was growing in the central bay and climate change would bring more extreme weather events, worsening the problem.

Dr Grinham said restoration of river banks, particularly in the Lockyer Valley and Bremer River sub-catchments, were desperately needed to save the bay.

Dr Alistair Grinham, from the School of Civil Engineering, said there was now little “clean sand” left and a muddy “dead zone” fast developing., Brisbane Water

“From the steep cliffs we have now, if you return the channel shape with a much gentler slope and plant large trees to slow the flow of water and armour the banks with their root system, that will help, but it all costs money.”

The bay is also under threat from up to 7000 tonnes of microplastics – rubbish such as food wrapping and plastic bottles.

A separate study by Dr Elvis Okoffo, from UQ’s Queensland Alliance for Environmental Health Sciences, measured plastic stored in 50 surface sediment samples.

“The level of plastic contamination we found is equivalent to three Olympic swimming pools full of plastic or 1.5 million single use plastic bags,” Dr Okoffo said.

“The main types of plastic detected were polyethylene (PE) and polyvinyl chloride (PVC).

“PE is used for single-use items such as plastic food wrapping, bags and bottles and PVC is used in pipes, building materials, electronics, and clothing.”

Both research papers were published in Science of the Total Environment.

The City of Moreton Bay is home to around half a million people, and last year attracted 4.6m visitors, according to Tourism Research Australia.

Katelyn Collins, 17, Manly, and Alex Gordon, 17, Wynnum, at Wellington Point. Picture: Liam Kidston