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Received evidence for deterioration in water quality in the River Murray

By Jennifer Marohasy - posted Wednesday, 20 August 2003


I queried my findings directly with the MDBC. In response, Dr Pradeep Sharma, Senior Modelling Engineer, replied, "Thanks to major investments in salinity mitigation works undertaken in the Murray Darling Basin over the last decade, I would like to concur with the conclusion that average salinity in the River Murray has in effect improved during the last decade."

Turbidity

Turbidity is a measure of the suspended sediment load. Turbidity levels generally rise with increased discharge (eg. increased rainfall). Australia's inland river systems are considered to be naturally relatively turbid.

Since European settlement the most significant change to water quality in many inland river systems is thought to be an increased sediment input from the early years of land clearing and the introduction of sheep, cattle and rabbits. As a result of improved management practices over recent decades erosion is likely to have stabilized or reduced to pre-European levels.

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According to plots from data sourced directly from the MDBC, turbidity levels (NTU) at both Morgan and Swan Hill appear to be relatively stable. Turbidity has been measured at both sites since 1978. Average yearly turbidity levels have not increased over this period.

Mean daily turbidity levels at Morgan exceeded 400 nephelometric turbidity units (NTU) in July 1983 but the relatively high turbidity levels may have been a consequence of drought breaking rains carrying higher than usual sediment loads because of increased erosion from reduced vegetation cover as a consequence of the drought in the early 1980s.

During years of low mean turbidity, mean daily values for both Morgan and Swan Hill are typically in the 20 - 40 NTU range.

Nitrogen and Phosphorus

It is generally believed that algal blooms in inland rivers are due to elevated nutrient levels, particularly phosphorus. While it was thought that the major sources of these nutrients was agricultural fertilisers, sewerage treatment plants and feedlots, the most recent and relevant
report on the MDBC website suggested that a large proportion of the phosphorus may come from natural sources in particular basalt-derived soil.

Whatever the origin of the phosphorus, a plot of yearly average phosphorus levels (mg/l) for key sites in the middle and lower basin show levels have been stable since data was first collected in 1978 (data sourced directly from the MDBC).

High nitrate levels can be an indication of excess runoff from agricultural fertilisers. Nitrate levels also appear to be stable for key sites, at least since levels were first measured by the MDBC in 1978.

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In Conclusion

On 16th July 2003 the Australian newspaper published some hard data showing actual water quality trends for the Murray River. The article was titled "Murray salinity tipped to rise", yet began "salinity levels in the Murray River have been halved since 1982" and the graph showing declining salt levels at Morgan did not accord with the title of the story. The journalist, Richard Sproull, was quoting from Matthew Kendall, of the MDBC, who was apparently quoting from a 1999 drainage strategy report when he said that he was expecting salinity levels to rise.

According to the reports on the website, this has been the prediction since at least 1998 - and it has been consistently proven wrong. Perhaps the computer models need an overhaul? Salinity levels at Morgan are now at pre World War II levels.

We have spent billions of dollars over the last 2 decades on environmental programs; it should not be surprising that the condition of our rivers is improving.

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Article edited by Ian Spooner.
If you'd like to be a volunteer editor too, click here.

This article was first published at the IPA Water Forum No. 2, Canberra, 25 July 2003. Click here for the full text with graphs.



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About the Author

Jennifer Marohasy is a senior fellow with the Institute for Public Affairs.

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