Acid mud (= ‘sulfidic sediments’): A repost

I got several things wrong in my earlier effort on this. This is a rethink which probably is still not error free but it is an improvement.

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 learnt more about ‘sulphide sediments’ from colleagues at La Trobe University Albury-Wodonga Campus last week. The following notes are based on this meeting together with an earlier inspection of the Web to find some background facts. Again I would be interested if readers had links to further information.

The basic idea is that while water resource analysts have paid attention to the need for periodic flooding of river systems and wetlands to maintain ecological health (see here) some wetlands need to be periodically and regularly drained dry to inhibit the formation of sulfidic sediments. 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. There are fairly high salt levels at sewerage treatment works such as the Western Treatment Plant and I would be interested to know how sulfidic mud issues are avoided there or why, indeed, they do not arise.

Coastal problems of sulfidic sediment can be dealt with by adding lime to soils but this type of policy response is ruled out in inland waterways simply because of the scale of the problem. Indeed 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 needs to be repeated and will, of course, have impacts on water availability upstream from the sites of sulfidic sediment.

One potential approach is to utilise triage arguments to isolate areas where sulfidic sediments exist to prevent them interacting with other components of a river system. Then the emphasis becomes one of attempting to prevent the development of acid sulphide problems elsewhere in river systems with appropriate management policies such as periodic drying out of wetlands. 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 have turned into sulphuric acid. 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.

One proposal is to flood Lake Alexandina with salt water from the ocean to flush it out. This would help disperse the acidic liquids but would damage the lake if it is judged to be primarily a body of fresh water. Evidence from diatoms in the lake floor has been used by different spokespeople to claim different things – some have claimed that historically it is a freshwater lake while others claim it is saltwater. History here matters if the policy objective is to retain levels of environmental authenticity.

Among the groups studying sulfidic sediments one with very useful data and articles is the CSIRO’s Land and Water. Staff members of this group provided me with useful information last week but should not be held responsible for the tentative remarks made above.