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Media Release: Rapid Creek samples placed in context

10 Nov 2015

The NT EPA today expressed concern over the Save Rapid Creek lobby group's interpretation of the results of its water sampling from drains adjacent to Rapid Creek.

The Chair of the NT EPA, Dr Bill Freeland, said the reason for the group's reported copper level readings being interpreted as higher than recommended guidelines was because the samples had been taken from two drains leading from Darwin Airport's land to Rapid Creek, and because inappropriate guidelines had been used. The guidelines used to interpret the results (the ANZECC guidelines) should not be applied to such water, having been designed for flowing freshwater streams.

Dr Freeland said that while the recorded levels of copper and zinc in the filtered water samples were high, it was important that Save Rapid Creek's findings (copper levels at up to 7 micrograms per litre and zinc at up to 23 micrograms per litre) be seen in the context of Darwin stormwater and stormwater in general.

"The results of the sampling are more appropriately compared to water quality values taken from stormwater in the Darwin area," he said.

Samples taken from stormwater leaving the suburb of Moil throughout the 1995/1996 wet season have published average total metal levels of 7.87 micrograms of copper per litre and 37.8 micrograms of zinc per litre. These values for total copper and zinc are high, although these results cannot easily be compared to the Save Rapid Creek group's filtered samples. A possibly better comparison for the airport drain would be with results from the industrial area of Winnellie. The 1995/1996 values were maximum values of 14.1 micrograms per litre of copper and 177 micrograms per litre of zinc.

Standard guidelines for stormwater quality have higher levels of metals than the ANZECC guidelines used for flowing freshwater streams. For example, the Queensland stormwater quality guidelines indicate mean total metal levels for copper and zinc of 80 and 160 milligram per litre respectively.

Dr Freeland said a better measure of the current status of Rapid Creek was provided by data on sediments collected by Charles Darwin University and the Department of Land Resource Management and published in 2015. This study indicated that the sediments were pristine prior to WWII, when they became contaminated with metals but not to a level to cause concern. Inputs from the airport and adjacent suburbs had continued since then, however there had been no deterioration in the quality of the sediment.

The NT EPA developed the Darwin Harbour Stormwater Strategy in response to its concern about the quality of stormwater in Darwin.

Recommendations of the stormwater strategy are being implemented, including proposals for Rapid Creek along with, in the longer term, other creeks entering the harbour, Dr Freeland said.

Media contact: Helen Farquhar 0437 458 213