In May we reported on a major turbidity event in Merri Creek in our Muddy Merri Troubles article. The intense yellow colour of Merri Creek generated high levels of community concern. A month later, in mid-June, the same thing happened again. This time we decided to measure turbidity levels of the creek in Brunswick East.
In brief we found:
(1) The highest level of turbidity - 500 NTU - was exceptionally high, and higher than any other recorded level in the lower urban reaches of Merri Creek over the last 18 years.
(2) It took 15 days of steady decline for the turbidity to drop below 20 NTU.
(3) Merri Creek was perceived as 'very muddy' for 14 days; it was only when turbidity dropped to 22 NTU that it was seen as ‘improving and clearing’.
You can read the detailed results of our investigation by following the prompt below or download the full report as a pdf.
Mid-June turbidity event
After heavy rain over the weekend of 16th and 17th June 2018, MCMC undertook daily (or as near as possible) turbidity measurements, using a turbidity tube, of samples taken from Merri Creek just upstream of Blyth St, Brunswick East. Our intent was to track the rise and subsequent fall of turbidity at this downstream location on Merri Creek after heavy rain in the middle and upper parts of the catchment.
Turbidity is a measurement of water clarity, based on the passage of light through a water sample. It gives an indication of the amount of suspended and colloidal particles in the water. It is measured in units called Nephelometric Turbidity Units, abbreviated as NTU.
We also took photos, just before taking each of the turbidity samples. The purpose of the photos was to determine how well visual impressions of turbidity levels correlated with actual measurements of turbidity and to consider these in relation to reports from local community of continuing turbidity.
The results of our turbidity measurements from 18 June to 4 July 2018 are shown below. The peak of 500 NTU was followed by a decline over the following 16 days to a low level of 17 NTU.
Other Merri Creek turbidity results
Merri Creek Stream Team The Friends of Merri Creek Stream Team monitor Merri Creek monthly at St Georges Rd, North Fitzroy. This site is about 1 km downstream of the MCMC East Brunswick site. The group’s data is available through the Victorian Waterwatch data-base and is below. Data for 2018 are not yet available.
Melbourne Water’s monthly monitoring data for Merri Creek at Roseneath St Clifton Hill is available through the Yarra and Bay Report Cards . Except in instances of localised pollution events, turbidity levels in the lower Merri at Clifton Hill are unlikely to differ in any significant way to those measured in North Fitzroy and Brunswick East. Hence it is reasonable to compare these results.
These Melbourne Water results are presented in comparison with the State Environment Protection Policy (SEPP) objective of less than 20 NTU for the annual 50th percentile (red line). The Annual 50th percentile = median, that is, the objective is that at least half the measurements are at or below 20 NTU.
Discussion - Measured Turbidity
The Melbourne Water data over the last 18 years show that turbidity levels in the lower Merri Creek have often exceed the 20 NTU objective of the SEPP but have rarely exceeded 200 NTU. The Merri Creek Stream Team data are consistently below 200 NTU. The peak in the Melbourne Water data, of 400 NTU in 2010, coincides with a period of high turbidity that generated considerable public attention in a manner similar to the recent events in May and June 2018. In response to the 2010 event, MCMC undertook an investigation of development sites in the upper Merri and identified a number of sites around Wallan that were generating high levels of turbidity.
The June 2018 measurement by MCMC of 500 NTU in Merri Creek Brunswick East is higher than any of the recorded data from the last 15 years and is more than 250 times higher than the SEPP 50th percentile objective of 20 NTU. This is an indication of the extremity of the event.
In comparing monthly monitoring data with MCMC’s daily monitoring data, we acknowledge that monthly monitoring can easily miss the extreme peaks in a turbidity event, and even miss the peaks entirely, for example if the return to a level of 20 NTU takes two weeks or less.
Photo record of Merri Creek Clarity
The visual perception of degree of turbidity/clarity as shown in the photos does not follow the clear, declining trajectory that the NTU levels show. Merri Creek does not appear be significantly improving until the 2 July when NTU had reached 22 NTU.
This photographic impression is consistent with verbal reports conveyed to MCMC on an almost daily basis that “..the creek’s still looking bad today”. From the 2 July, when the NTU level had dropped to 22 NTU, the same people started reporting that “the creek’s getting clearer at last.”
This was also the perception of the author who took the samples and photographs, and whose visual impression of the creek was that its clarity was barely improving even as the measured turbidity dropped. This may be because the yellowish sediment deposited on the creek bed became more visible and dominated the perception of clarity of the creek, as water column cleared.
The level of turbidity recorded in Merri Creek, Brunswick East on 19 June 2018 of 500 NTU was exceptionally high, and higher than any other recorded level in the lower urban reaches of Merri Creek from the last 18 years.
Actual turbidity and visual perceptions of ‘turbidity’ changed at different rates. Measured turbidity peaked two days after an intense rain event in the upper Merri and declined steadily over 16 days. After 15 days turbidity was below the SEPP objective of 20 NTU.
Visual perceptions of apparent turbidity showed a far less clear cut decline. For at least 14 days the creek ‘looked bad’ i.e. it lacked clarity, and it was only when turbidity had dropped to 22 NTU that the visual appearance of the creek was perceived to be ‘improving and clearing’.