A new problem emerging in the drought is sulfidic sediments drying out in inland wetlands.

The drying sediments produce foul odours, and in some cases there is a risk that the soils will produce damaging acid.

In coastal areas, acid sulfate soils have long been recognised as a problem for landholders and the environment.

The soils, old marine muds rich in iron sulfide, are fine while covered with water, but when they dry out, oxygen combines with the sulfide to produce sulfuric acid that acidifies the soil.

After rain, the acid washes into waterways and can kill fish.

Coastal landholders know the importance of maintaining high water tables to prevent the soils drying out, either through filling in drains, or holding water in drains.

Another technique is to flush out the acidic drain water with tidal flows because alkaline seawater can neutralise the acid.

Obviously, these management options are not relevant to the droughtstricken inland, since we still know very little about inland sulfidic soils or ways to manage them.

A recent CSIRO study of lower Murray River wetlands found sulfidic sediments in most inland wetlands appear to be a function of increasing salinity and the water regime of the wetlands.

They tend to accumulate where irrigation practices and river regulation increase salinity and waterlogging in the landscape.

Sulfide concentrations are highest in saline, perennial wetlands and lowest in wetlands that are fresh or have a more regular wetting and drying regime.

Interestingly, few Murray wetlands have potential acid sulfate soils, probably due to local geology and the fact most of the wetlands are closed hydrological systems, unlike coastal wetlands.

The survey found only one wetland with potential acid sulfate soil, located above the junction of the Murray with the Darling River.

CSIRO recommends inland sulfidic soils be described as sulfidic sediments rather than acid sulfate soils because of their low acidification potential.

The Federal Government’s Raising National Water Standards program is now funding research into appropriate wetting and drying strategies in inland wetlands to prevent the buildup of sulfidic sediments and to remediate affected systems.

If you’d like to know more about these sediments, read the 2006 paper Sulfidic materials in dryland river wetlands by Lamontagne et al in Marine Freshwater Research at www.publish.csiro.au/paper/MF06057.htm