Acid mud (= ‘sulfidic sediments’)

In many wetlands along the Murray and Darling Rivers, sediments flooded for decades by locks and weirs, are being exposed to air as drought-affected water levels fall. Inland sulfidic sediments have been found in NSW, Victoria, South Australia and Western Australia. Waterlogged soils often contain sulphides produced by bacteria decomposing organic matter, but if these sediments are allowed to build up and are then exposed to oxygen, they form sulphuric acid – hence the names ‘acid mud’ and ‘sulfidic sediments’. These are usually naturally occurring events but are worsened by human activities, particularly in inland aquatic ecosystems, and by sustained drought.

I will learn more about ‘sulphide sediments’ from colleagues at La Trobe University Albury-Wodonga Campus this week but, before this, I inspected the web to find some background facts and what follows are some notes. I would be interested if readers had links to further information.

For example Bottle Bend, near Mildura, was once a healthy wetland but it is now it’s a toxic waste site where nothing but micro-organisms can survive the acid water given its steel-eating pH of 1.6. This caused the death of all fish in the wetland and all the trees surrounding it. Thousands more wetlands – particularly those in the lower reaches of the Murray - could be brewing the same deadly formula and if the rivers again flow enough to re-flood the exposed acid mud, toxic baths will pose a major threat to towns and cities downstream. This slide show by the Murray-Darling Freshwater Research Centre was useful in getting an overall picture.

Constructed wetlands are potentially at more risk of producing sulfidic sediments than other wetlands because such wetlands are often designed to help improve water quality. The water, whether it is from storm water, treated sewage, industry or other sources is often of poor quality. If the levels of sulfate in the feed water is high (greater than about 10-20 mg S/l) then there is a real likelihood that the wetland will develop sulfidic sediments over time.

Not a lot is known about treating inland sulfidic sediment problems. Essentially they need to be first identified and then regularly flushed out. The flushing out required however impacts on water availability upstream from the sites of sulfidic sediment.

For example as pointed out by Kenneth Davidson, in Lake Alexandrina at the mouth of the Murray River, the soil in the lake is laced with sulphides that turn into sulphuric acid with prolonged contact with the air. The pH of the lake, which measures the acid/alkaline balance, is already bad enough to make it toxic to animals. Moreover, without flushing as a result of heavy rains upstream, the combination of salt and acid will move upstream and progressively contaminate the lower Murray. The danger is immediate. Murray Bridge, 38 km from the mouth of the Murray, is only two metres above sea level at the mouth of the river — a drop of less than 0.5 cm/km — which means that the salt and acid can move relatively easily upstream. The lower Murray is more akin to a series of interconnected ponds rather than a free-flowing river.

This is serious because Adelaide and South Australia's main provincial towns depend on the Murray for most of their water. Without flushing rains or 200 GLs from the Dartmouth Dam on the upper Murray, the water that Adelaide pipes from the Murray below Murray Bridge will be undrinkable. But if the water that is available from Dartmouth is allocated to the environment, it won't be available to irrigators further up the Murray.

According to Davidson, this is why the Brumby Government delayed for 15 months signing up to the Murray-Darling Basin Authority, which is supposed to give ultimate authority for allocation of the water to Federal Water Minister Penny Wong. Either the system must get well above average rainfall during the coming winter, sufficient to flush out the lower reaches of the Murray, supply Adelaide and keep irrigators alive or Wong will have to choose between Mildura and Adelaide as to who gets the 200 GL of water held in reserve in the Dartmouth dam. But there is no real choice. If the southern river Murray system dies, Mildura and the other irrigators along the southern Murray will die as well.

If salt and sulphuric acid damage is limited to Lake Alexandrina, the irrigators and the towns along the southern Murray can be kept on life support until there is a permanent increase in the flow of water into the Murray system.

Among the groups studying sulfidic sediments one with very useful data and articles is CSIRO Land and Water.