Dye traces river currents with an aim to protect oyster leases from sewage spills.

A main aim is to ease pressure on oyster growers by developing a more targeted approach to future sewage overflows, avoiding the need to shut harvesting in entire river systems.

The project is a partnership between the University of NSW (UNSW) and the NSW Food Authority, with support from local councils.

"At the moment if there is a sewage spill they will close the whole estuary [to oyster harvesting] ," UNSW Water Research Laboratory engineer Yarran Doherty said. 

"The idea of this project is to develop a more targeted, data-informed approach.

"You can see where a spill is going, which oyster leases it is going towards, and you can decide which ones to close.

"The aim is to be more selective about it rather than be overly cautious."

Where does the dye flow?

The study involves 11 NSW river systems that support oyster growers from the Tweed in the north down to Pambula on the south coast.

The data gathered will be used to develop detailed computer models simulating how the river system currents move, and how a potential sewage or pollution spill would spread and dilute once in an estuary. 

The non-toxic red dye is released into the river systems and tracked by field teams.

Mr Doherty is part of a team currently studying the Camden Haven River on the Mid North Coast and said they were using a specialised instrument known as a fluorimeter.

"We have a device that when you put it in the water it shoots out a red beam and catches the light coming back, it shows the concentration [of the dye] , and how fast it's spreading," he said.

"We try to do cross sections, back and forwards, to see how wide it is.

"We are trying to find the estuary characteristics, so how the water is moving in and out of the system and if a pollutant goes in the system how fast that spreads where it's going."

Computer models show river systems:

The director of the UNSW Water Research Laboratory, Brett Miller, said it would be about another year before the river system computer models were complete and brought online to help inform future decisions.

"We've been building what we call pilot computer models of all of the estuaries, and now we come out and do the field work and gather more data so we can further refine those models," he said.

He said the models would be crucial decision-making tools and a more targeted response to sewage spills would help ease pressures on oyster growers.

"We know for some of the estuaries that they are quite compartmentalised in the way the currents move," he said.

"If it was a spill near the mouth of the estuary on a fast-outgoing tide, it might all go out to sea in the first five to six hours."

So far dye mapping has also been done in the Shoalhaven River and the Hastings River, and the project will continue to other regions in coming months.

Mr Doherty said residents living near the river systems would be unlikely to notice the red dye for very long.

"The dye disperses pretty quickly … after about half an hour to an hour," he said.

"It disperses and breaks down in the sun and in the next 24 hours it will be completely gone."

Original Story By | Emma Siossian & Fiona Poole