Acid Sulfate Soils (ASS)
What is Defining Acid Sulfate Soils?
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:
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.
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Acid Sulfate Soils Management Advisory Committee (ASSMAC)
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National Acid Sulfate Soils Sampling and Identification Manual
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National Acid Sulfate Soils Guidance (NASSG)
Identification and Analysis of Acid Sulfate Soils
Acid sulfate soils include both Potential Acid Sulfate Soils (PASS) and Actual Acid Sulfate Soils (AASS). Potential Acid Sulfate Soils contain Reduced Inorganic Sulfur (RIS), such as pyrite, and are typically water-saturated in their natural state. These soils often have an alkaline field pH when undisturbed. However, when drained or disturbed, oxidation of the RIS occurs, producing sulfuric acid and causing the soil pH to drop below 4, classifying them as Actual Acid Sulfate Soils.
When this transformation happens, the resulting acidity can severely impact ground and surface waters, disrupt ecosystems, and damage infrastructure such as bridges, drains, pipes, and roads. It’s essential to avoid remediation attempts in naturally acidic environments, as these ecosystems rely on stable acidic conditions for survival.
Identifying Acid Sulfate Soils
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Characteristics of Undisturbed Acid Sulfate Soils
- Always wet and saturated.
- Difficult to walk on.
- Steely blue-grey color (anaerobic state).
- Strong rotten egg smell from hydrogen sulfide gas.
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Features of Oxidized Acid Sulfate Soils
- Dry, strong blocky structure.
- Dark to pale brown coloration.
Understanding Soil Acidity
It’s important to note that not all acidic soils are acid sulfate soils. While some naturally occurring soils have low pH levels and high acidity, they are not considered environmental hazards as they support acidophilic ecosystems. Field investigations are necessary to determine whether acidic materials qualify as acid sulfate soils or belong to other naturally acidic soil types.
Hazard Assessment
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Determining Net Acidity with Acid Base Accounting (ABA)
An Acid Base Accounting (ABA) approach is used to calculate the net acidity of acid sulfate soils. This assessment helps understand the soil’s potential environmental impact and informs management strategies. The net acidity is calculated using the following equations:
Net acidity = Potential Sulfidic Acidity + Actual Acidity + Retained Acidity – Acid Neutralising Capacity (when corroborated by other data)
Net acidity = Potential Sulfidic Acidity + Actual Acidity + Retained Acidity (when not corroborated by other data)
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Using Acid Sulfate Soil Risk Maps
Acid sulfate soil risk maps are essential tools for assessing potential risks in a proposed area. These maps indicate the distribution, depth, and likelihood of acid sulfate soil occurrence. Available across most Australian states and territories, they can be accessed through Geographical Information Software (GIS) and the Australian Soil Resource Information System (ASRIS). It’s important to note that these maps are intended for broad-scale planning and should be supplemented with field investigations for site-specific assessments.
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Have Questions? Let’s Clear Them Up
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The objective of an acid sulfate soil assessment is to assess the presence or absence of acid sulfate soils on the site through sampling and laboratory testing, and to determine whether an acid sulfate soil management plan is required for the site.
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When acid sulfate soils are disturbed, oxidation of reduced inorganic sulfur often occurs, creating actual acid sulfate soils. This acidity can present a significant hazard within infrastructures, bridges, drains, pipes and roads due to the dramatic change of soil properties.
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The development of significant acidity in Actual Acid Sulfate Soil materials has a detrimental impact on receiving ground and surface waters, and the ecosystems dependent on formerly non-acidified environments. The acid attacks soil minerals, releasing metals like aluminium and iron.
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Acid Sulfate is formed when pyrite in the soil reacts with oxygen and oxidises. Actual Acid Sulfate Soils can be prevented by determining where it might occur and avoid exposing affected soils to oxygen. If soils are exposed to oxygen, professional consultant advice shall be sought out for appropriate treatment methods.
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Acid Sulfate Soil assessment costs may vary depending on the extent of the assessment, for further information visit our contact us page and fill out the enquiry form.
