What are Acid Sulfate Soils?
Acid sulfate soils contain naturally occurring iron sulfides (principally iron sulfide or iron disulfide or their precursors). The exposure of soil sulfides to oxygen by drainage or excavation leads to the generation of sulfuric acid which can damage both the natural environment and built structures. The requirement for an acid sulfate soils assessment is usually triggered by the lodgment of a Development Application within an area previously known or mapped as potentially containing acid sulfate soil materials. There are two classifications of acid sulfate soils, actual and potential. Typically, both types are found in the same soil profile, with actual acid sulfate soils generally overlying potential acid sulfate soil horizons.
What is the difference between actual and potential acid sulfate soils?
Actual acid sulfate soils are soils containing highly acidic soil horizons or layers resulting from the aeration of soil materials that are rich in iron sulfides, primarily sulfide. This oxidation produces hydrogen ions in excess of the sediment’s capacity to neutralize the acidity resulting in soils of pH of 4 or less when measured in dry season conditions. These soils can usually be identified by the presence of pale yellow mottles and coatings of jarosite.
Potential acid sulfate soils are soils which contain iron sulfides or sulfidic material which have not been exposed to air and subsequently oxidized. The field pH of these soils in their undisturbed state is pH 4 or more and may be neutral or slightly alkaline. However, they pose a considerable environmental risk when disturbed, as they will become severely acidic when exposed to air and oxidized.
Acid sulfate assessment process
During a site investigation representative soil samples are collected from each soil layer and analysed for Suspension Peroxide Oxidation Combined Acidity & Sulfur (SPOCAS), chromium reducible sulfur and texture. Results are compared against relevant guidelines to determine whether a soil exceeds safe limits. Proposed earth works in soil that exceed the exposure limits may trigger the requirement for an Acid Sulfate Soils Management Plan.
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