Catalyst: Toxic sediments

MARK HORSTMAN:

The levels of dioxins in fish from Sydney Harbour are now so high that even eating just one mullet like this in an entire year is enough to put me over the safe limit. Hidden beneath the sparkling waters, an environmental catastrophe is unfolding at the bottom of the harbour.

DR EMMA JOHNSTON:

If we lost 40% of our mammals in a national park, for example, or 40% of our birds, we'd be devastated. It'd be a national calamity. But we can lose 40% of the invertebrates in Sydney Harbour in the sediments and no-one seems to notice. So it's really a forgotten world.

MARK HORSTMAN:

So who's responsible? The legacy of industrial pollution usually gets the blame. But it's been illegal to dump toxic waste directly into the harbour for over 30 years. And, still, toxins in the sediment remain dangerously high. There has to be another culprit.

PROFESSOR GAVIN BIRCH:

The other source of contaminants is stormwater.

DR EMMA JOHNSTON:

We've analysed sediments from core samples down different embayments. So, right near the stormwater outlets the contaminants are generally highest.

MARK HORSTMAN:

When it rains, the heavy metals and other chemical residues on our roads and buildings are washed into the drains.

PROFESSOR GAVIN BIRCH:

Up to 4kg of zinc occurs in some of these very large truck tyres.

MARK HORSTMAN:

Toxic hydrocarbons called 'PAHs' come from car and truck exhausts. Pesticides and fertilisers are lifted away with the topsoil. Fine clay particles in the run-off are electrostatically charged. Like tiny magnets, these particles attract contaminants, such as heavy metals, dioxins and organochlorines.

PROFESSOR GAVIN BIRCH:

Our work has shown that the concentrations of contaminants in the stormwater hasn't improved over time. The risk, obviously, is that these contaminants ARE being transported around the harbour.

MARK HORSTMAN:

So we know that currents move the toxic silt around. But what does it do to the marine life? That's what ecotoxicologist Emma Johnston wants to find out.

DR EMMA JOHNSTON:

In Sydney Harbour we've observed almost the world's highest levels of metals in some of our brown algae. Now, the problem with that is that brown algae is food and it's shelter for a lot of other animals. And when these animals feed on contaminated seaweed they are basically poisoned.

MARK HORSTMAN:

And when these animals are eaten, the toxins are passed along the food chain, becoming more concentrated as they go.