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Acid Sulfate Soils (ASS)

Site identification, management and treatment of naturally occuring soils, sediments and peats that contain pyrite or similar materials.


Acid Sulfate Soils (ASS)

Acid sulfate soils (ASS) are soils, sediments, and peats comprising iron sulfides. They are predominantly in the form of pyrite material and are naturally occuring.

Allthough harmless, oxidation of iron sulfides in the soils can produce iron compounds and sulfuric acid. When these oxidised soils are next saturated, the sulfuric acid can aid in the release of other bound minerals in the soil matrix, such as heavy metals. These contaminants can then easily migrate into the surrounding environment impacting sensitive receptors such as conservation category wetlands.

ASS are widespread across Western Australia and are more common along low lying coastal waterlogged areas and environments dependant on groundwater. The Department of Water and Environmental Regulation have produced several risk maps based on geological data and are available online at However these maps should be used as a guide only and further investigation and assessment should be considered.

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So, What do Enveng do?

During the design and construction of major projects in WA, ASS can present a significant risk to our clients. While the majority consider regulatory approval and compliance to be the highest risk, costs associated with practical identification, treatment and/or disposal tend to overshadow the regulatory component. We have found that regulatory non-compliance is predomonantly associated with poor planning, lack of understanding and, more commonly, insufficient budget.

Tendering on MCPs requires an understanding of the risks and costs associated with managing ASS during construction and is an essential component to project success. Where there is a lack of baseline data, assumptions and project outcome driven modelling (based on years of practical hands on experience) becomes vital for decision making by Commercial and Project Managers.

Engineering Solutions for ASS

We consider managing ASS on construction projects an engineering and sustainability challenge rather than an environmental issue. Where possible, ASS materials should remain onsite and treated if required. However it is more likely that geotechnical and engineering requirements may result in the material being assessed as unfit for re-use on site and removal off-site would need to be considered.

Material can be sent off site for re-use if it meets the criteria for uncontaminated fill as per the WA Landfill Waste Classification and Waste Definitions 1996 (as amended 2019), however there are additional requirements that will have an impact on costs and the project will need to consider the financial implications should the material not meet the reuse criteria and has to be disposed to Landfill (we generally deal with this in the R&O register during tender).

If projects have the space and time, onsite management, treatment and classificaiton for reuse remains the most cost effective and sustainable outcome.

Do we get our hands dirty?

We sure can, Enveng Group can supply the necessary resources to identify, assess and manage ASS and have several key supply chain partners that specialise in the onsite treatment.

We can support your project in a multitude of ways with a focus to reduce time and effort (i.e. cost) while maintaining regulatory and operational compliance. Activities such as:
Tender support - Early works investigations - regulatory engagement - assessments - management and identification during excavation works - treatment - reuse options - disposal - validation and close out reporting.


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