Dye could not wash up on beaches

30 Jul 2010

“The volume discharged is negligible given the volume of the water it went into,” Power and Water Corporation General Manager Water Services Steve McKenzie said.

“It was released 2m and 4m down, where the water was 11 metres or more deep, on an outgoing tide. It dispersed very quickly.

“Scientists measured and tracked the dye as it dispersed over the next five hours.

“It was so well diluted within five hours it was barely visible and within hours you would not have been able to see it at all.

“It is quite clear from aerial photographs taken during the study that it did not spread towards the coast.”

Rhodamine WT was specifically developed for use as a fluorescent dye for water tracing.

Scientists released 25L of the dye over a five-minute period from 1.5km further out from the existing East Point outfall on Friday, 16 July and Saturday 17 July.

Mr McKenzie said scientists from Charles Darwin University and the Australian Nuclear Science and Technology Organisation would spend several weeks interpreting the results from the two days of study.

“They measured the concentrations of the tracers at a variety of GPS-mapped locations over a number of hours and will use this data to calibrate a hydrodynamic model of the area,” he said.

“CDU has been working on a hydrodynamic and dispersion model of Darwin Harbour for about five years to study the dispersion of treated effluent and plan for the future.”

The study was one step in the program of works to close Larrakeyah outfall.

“This study and the Harbour model will be used to prepare the Public Environment Report on the proposed East Point outfall, which we will submit at the end of this year,” Mr McKenzie said.