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Acid Sulfate Soils (ASS) are naturally occurring soils, sediments or organic substrates (e.g. peat) formed under waterlogged (anaerobic) conditions that contain iron sulfide minerals (predominantly as the mineral pyrite) or their oxidation products. In an undisturbed state below the watertable, these soils are benign and not acidic. However if the soils are drained, excavated or exposed to air by lowering of the water table, the sulfides will react with oxygen to form sulfuric acid (often in harmful quantities).

This release of sulfuric acid and iron into the soil can in turn release aluminium, nutrients and heavy metals (particularly arsenic) held within the soil into groundwater. Once mobilised in this way, the acid, metals and nutrients can seep into waterways, killing fish, other aquatic organisms and vegetation, and can degrade concrete and steel pipes and structures to the point of failure.

Contents 1 Impacts of Acid Sulfate Soil 2 Acid Sulfate Soils Formation 3 References 4 External links

Flushing of acidic leachate to groundwater and surface waters can cause a number of impacts, including:

• Ecological damage to aquatic and riparian ecosystems through fish kills, increased fish disease outbreaks, dominance of acid-tolerant species, precitation of iron, etc.
• Effects on estuarine fisheries and aquaculture projects (increased disease, loss of spawning area, etc).
• Contamination of groundwater with arsenic, aluminium and other heavy metals.
• Reduction in agricultural productivity through metal contamination of soils (predominantly by aluminium).
• Damage to infrastructure through the corrosion of concrete and steel pipes, bridges and other sub-surface assets.

Source: Sammut & Lines-Kelly, 2000.

The soils/sediments which are of most concern are those which formed within the last 10,000 years, after the last major sea level rise. When the sea level rose and inundated the land, sulfate in the seawater mixed with land sediments containing iron oxides and organic matter. Under these anaerobic conditions, lithotrophic bacteria such has Thiobacillus ferrooxidans form iron sulfides (pyrite). Up to a point, the warmer the temperatures, the more favourable are the conditions for these bacteria, and the greater the potential for formation of iron sulfides. Tropical water logged environments, such as mangrove swamps or estuaries may contain higher levels of pyrite than those formed in more temperate climates.

The pyrite is stable until it is exposed to air (that is, no longer in a waterlogged, anaerobic conditions), at which point the pytrite oxidises and produces sulfuric acid. The impacts of ASS leachate may persist over a long time, or peak seasonally (after dry periods with the first rains). In some areas of Australia, ASS drained 100 years ago is still releasing acid (Sammut & Lines-Kelly, 2000).

Acid Sulfate Soils are widespread around coastal regions, and are also locally associated with freshwater wetlands and saline sulfate-rich groundwater in some agricultural areas.

Sammut, Jesmond; Lines-Kelly, Rebecca (2000). Acid Sulfate Soils 2nd edition. Environment Australia. ISBN 0-7347-1208-1. 

• Australian Department of Environment & Heritage National Coastal Acid Sulfate Soil homepage
• Queensland Department of Natural Resources & Water Acid Sulfate Soils page
• Western Australian Department of Environment Acid Sulfate Soils page