Significant environmental hazards have occurred from acid sulfate soil (ASS) disturbance when exposed to oxygen, which may include, severe acidification of soil, deoxygenation of water bodies, and scalding. This guide is intended to assist in identifying the presence of acid sulfate soil and its associated hazards. Specialist advice is always needed from a qualified Environmental Engineer.

Acid sulfate soils are naturally occurring soils containing iron sulfide minerals, principally pyrite. Acid sulfate soils form in waterlogged anaerobic conditions, where sulfate reducing bacteria convert dissolved sulfate into Reduced Inorganic Sulfur (RIS). Acid sulfate soils are distinguished from other soils by having either of the following properties:

  • Soils affected by the oxidation of RIS, or
  • The capacity of being affected by the oxidation of RIS constituents.
National guidance relating to acid sulfate soil can be found in the following documents:
  • Acid Sulfate Soils Management Advisory Committee (ASSMAC)
  • National Acid Sulfate Soils Sampling and Identification Manual
  • National Acid Sulfate Soils Guidance (NASSG)

These guidelines apply in certain activities, such as excavations or disturbance of acid sulfate soil, lowering the watertable, use of acid sulfate soil, and physical habitat modification for mosquito control.

Identification and analysis of Acid Sulfate Soils

Acid Sulfate Soils include Potential Acid sulfate Soils (PASS) and Actual Acid Sulfate Soils (AASS). Potential Acid Sulfate Soils are soil materials which contain RIS such as pyrite. The field pH of these soils in their undisturbed state is commonly alkaline. These soil materials are generally saturated with water in their natural state. Actual acid sulfate soils are where RIS contained within the PASS material have undergone oxidation. This oxidation occurs when PASS soils are drained, and pyrite oxidises into sulfuric acid resulting low pH usually below 4.

When PASS materials are disturbed, oxidation of RIS often occurs, resulting in dramatic changes in their properties, notably rapid and substantial pH decreases to below 4, which then classifies them as Actual ASS. The development of significant acidity in Actual ASS materials has a detrimental impact on receiving ground and surface waters, and the ecosystems dependent on formerly non-acidified environments. Furthermore, acidity can present a significant hazard infrastructure including, bridges, drains, pipes, and roads. Remediation should not be attempted within naturally acidic environments which could destroy their ecosystems.

Undisturbed acid sulfate soils can be identified through multiple features and characteristics, which include:
  • Always wet.
  • Entirely saturated.
  • Difficulty when walking on.
  • Distinctive steely blue-grey colour due to their anaerobic state.
  • Strong smell of rotten eggs due to the breakdown of sulfur and organic materials, creating hydrogen sulfide gas.

Caution is required when working in soil pits and confined spaces as prolonged exposure to hydrogen sulfide gas may cause severe health effects.

Oxidised acid sulfate soils on the other hand have relatively different features and characteristics, they feature a dry, strong blocky structure, with dark to pale brown colours.

Soil materials that are strongly acidic due to processes other than RIS oxidation are not considered acid sulfate soils. Soil materials such as actual acid sulfate soil and sulfuric soil are considered to be acidic, however, not all acid soil materials are actual or sulfuric acid sulfate soil materials. Many naturally occurring soils tend to have low pH levels and high acidity, however, they are not considered an environmental hazard as they are part of acidophilic ecosystems, where an acidic environment must be maintained. Field investigations can assist with determining whether an acidic material is an acid sulfate soil or not.

Hazard assessment

An Acid Base Accounting (ABA) approach is taken to determine the net acidity of acid sulfate soil, this is accomplished by using the following equations:

  • Net acidity = Potential Sulfidic Acidity + Actual Acidity + Retained Acidity – Acid Neutralising Capacity

Where the Acid Neutralising Capacity has been corroborated by other data.

  • Net acidity = Potential Sulfidic Acidity + Actual Acidity + Retained Acidity

Where the Acid Neutralising Capacity has not been corroborated by other data.

  • Verification Net Acidity = Potential Sulfidic Acidity + Actual Acidity + Retained Acidity – (post treatment Acid Neutralising Capacity – initial Acid Neutralising Capacity)

Where the addition of liming materials is being verified post treatment through calculating the Verification Net Acidity.

Acid sulfate soil risk maps are consulted to determine if there is a known acid sulfate soil risk in a proposed area. Risk maps are available in most states and territories within Australia, the maps indicate the distribution and depth of acid sulfate soil materials and the probability of their occurrence. Acid sulfate soil risk maps can be downloaded via Geographical Information Software (GIS) and the Australian Soil Resource Information System (ASRIS). These maps are used in broad-scale planning only.