Following the heavy rain of March – April we have had two months of relatively low rainfall (198 mm compared with 667 mm), consequently the river has stabilised and most test parameters are now within their normal range. Salinity has risen throughout the river as less water has been flowing. pH has also risen, particularly in the upper river where the water is fresher and changes in the composition of the groundwater are more evident than they are after dilution with more saline water downstream. Turbidity has been more variable; upstream of the ferry the river has become a little clearer, but downstream the turbidity has increased, and a lot of very fine silt is disturbed by wading into the river.
We often make the confusing observation that water flowing out of small creeks and drains is a very brown colour, but it has surprisingly low turbidity measured, in other words the brown colour is not due to silt or mud. That leaves two possible causes for the brown coloration, iron extracted from soil minerals or tannin from decaying leaves. There have been some claims that acid sulphate soils are contributing undesirable breakdown products into the water, including iron leached from the soil. This particularly causes lower pH and also produces slimy surface films of iron consuming bacteria and iron oxide. We have collected water samples from a number of locations in the river and have found quite acceptably low levels of iron except following heavy rain when there is a flush of water from tributary creeks. Water Quality Guidelines give acceptable iron concentrations for recreational water of 0.3 ppm, our tests for river water generally fall between 0.1 and 0.5 ppm. Iron levels are greater in all the creeks, particularly in Eenie Creek which measures up to 4 ppm when there is only very slow flow occurring. However such iron concentrations are not high enough to contribute significant yellow/brown colour to the water, so it must be concluded that the brown colour observed is largely due to tannin being extracted from decaying leaf litter on the ground.
A question has been asked, whether there is any difference in water quality parameters, especially salinity, between surface water and water near the bottom of the waterway. We have taken measurements on various occasions and locations to test for differences between surface and stream bottom. In most places the depth available was only a little more than 1 m, but 2m deep measurements were obtained at Noosa Sound, Tewantin Marina, and Upper Noosa River. The differences found in general were quite small and largely within the range observed between duplicated measurements taken at every site.
* Temperature. The temperature differences were within the range of duplicate measurements, so generally considered insignificant.
* pH. At most sites there was negligible difference, however at Tewantin Marina pH was lower at the surface, whereas at Upper Noosa River (almost fresh water) pH was higher at the surface.
* Salinity . Salinity is measured most accurately as conductivity. At all sites the salinity was marginally lower at the surface, although the differences were not significant at Noosa Waters Canal, Lake Cootharaba (stirred up by strong wind), and Upper Noosa River (low salinity). In the river estuary downstream of the marina there was up to 10% difference in salinity between surface and bottom of the river at both 1 and 2 m depths.
* Turbidity and Dissolved Oxygen. Both were too subject to disturbance by wind and waves, so comparisons are inconclusive.
We have continued collecting water samples for nutrient testing by Noosa Council. With reduced rainfall all nutrient residues have remained low, and within the same ranges as previous tests.
Thanks to the Waterwatch team members who have contributed to all the above work.
By Bruce McConkey