Understanding Acid Sulphate Soils: A Practical Guide for Landowners

Acid sulphate soils can become an important consideration for property development, excavation and construction projects, particularly in low-lying coastal areas of Australia.

In their natural, undisturbed state, acid sulphate soils may present little environmental risk. Problems can arise when these soils are excavated, drained or exposed to oxygen, potentially generating sulfuric acid and increasing soil acidity.

For landowners, developers and builders, the key question is not simply whether acid sulphate soils may occur on a property. It is whether the proposed development is likely to disturb them and, if so, what investigation or management is required.

This guide explains what acid sulphate soils are, how they are identified and when an Acid Sulphate Soils Management Plan may be needed.

What Are Acid Sulphate Soils?

Acid sulphate soils are naturally occurring soils and sediments containing iron sulfide minerals, commonly pyrite.

They are most often associated with waterlogged, coastal and estuarine environments where low-oxygen conditions allow sulfide minerals to form and remain relatively stable.

When these soils are disturbed and exposed to oxygen, the sulfides can oxidise and generate sulfuric acid.

This can result in:

• increased soil acidity

• acidic groundwater or surface water

• mobilisation of metals

• impacts to vegetation and aquatic ecosystems

• corrosion or deterioration of some infrastructure.

The important distinction is therefore between the presence of sulfide-bearing soil and the disturbance of that soil.

A site may contain acid sulphate soil without presenting a significant problem if the material remains undisturbed.

Potential Acid Sulphate Soils and Actual Acid Sulphate Soils

Two terms commonly arise during acid sulphate soil investigations.

Potential Acid Sulphate Soils

Potential Acid Sulphate Soils, or PASS, contain sulfide minerals that have not yet substantially oxidised.

These soils may appear relatively normal and may have a near-neutral pH when first encountered.

The risk occurs when excavation, drainage or groundwater lowering exposes the material to oxygen.

Actual Acid Sulphate Soils

Actual Acid Sulphate Soils, or AASS, have already undergone oxidation and generated acidity.

These materials may exhibit:

• low soil pH

• yellow or orange staining

• iron precipitates

• acidic drainage

• visible signs of oxidation.

The distinction matters because a soil does not need to be strongly acidic at the time of sampling to represent a future acid generation risk.

Where Are Acid Sulphate Soils Found?

Acid sulphate soils are common throughout many coastal parts of Australia.

They are particularly associated with:

• estuaries

• floodplains

• wetlands

• former marine environments

• mangrove areas

• low-lying coastal land

• alluvial and estuarine sediments.

In New South Wales, acid sulfate soil risk mapping is commonly incorporated into Local Environmental Plans.

A property may therefore be identified as being within a mapped acid sulfate soil area even where there are no obvious signs of acidic soil at the ground surface.

This is particularly relevant where proposed works involve deeper excavation.

Why Do Acid Sulphate Soils Matter During Development?

Many development activities can disturb acid sulphate soils.

Common examples include:

• swimming pool excavation

• basement construction

• service trenches

• foundations and piling

• road construction

• drainage works

• bulk earthworks

• dredging

• groundwater dewatering.

Excavation can directly expose sulfide-bearing soil to oxygen.

Groundwater dewatering can also be important because lowering the water table may expose soils that were previously saturated.

This means a project does not necessarily need to remove large quantities of soil before acid sulphate soil management becomes relevant.

Environmental Impacts of Acid Sulphate Soils

If disturbed acid sulphate soils are not appropriately managed, the resulting acidity can affect both the development site and surrounding environments.

Soil Acidity

Oxidation can substantially reduce soil pH.

Strongly acidic conditions can affect vegetation, soil biology and future land use.

Water Quality

Acidic runoff or groundwater can migrate from an excavation or stockpile into drains, groundwater systems, wetlands and waterways.

This can create broader environmental degradation beyond the immediate excavation area.

Metal Mobilisation

Acidic conditions may increase the mobility of metals such as iron and aluminium.

These metals can then migrate with groundwater or surface water and potentially affect receiving environments.

Construction Impacts

Acidic soil and groundwater can also affect some construction materials and may need to be considered when planning excavation, drainage and below-ground infrastructure.

How Are Acid Sulphate Soils Identified?

Acid sulfate soil mapping is useful for identifying potential risk, but mapping alone does not confirm whether acid sulphate soil is present within the proposed excavation area.

A site-specific Acid Sulphate Soil Investigation may therefore be required.

An investigation typically considers:

• acid sulfate soil mapping

• site geology

• soil landscape

• groundwater conditions

• proposed excavation depth

• proposed disturbance volume

• potential dewatering

• soil profile and field observations

• laboratory testing.

The objective is to determine whether the proposed works are likely to disturb acid sulphate soil and whether management measures are required.

Acid Sulphate Soil Testing

Acid sulphate soil testing generally involves collecting soil samples from the depths likely to be affected by the proposed works.

Field screening may include:

• field pH testing

• oxidised pH testing

• observations of soil colour and texture

• identification of potential oxidation indicators.

Laboratory testing may then be undertaken to more accurately assess the acidity characteristics of the soil.

Depending on the project, testing may include methods such as Chromium Reducible Sulfur (CRS) analysis.

Laboratory interpretation can help determine:

• whether sulfide minerals are present

• whether existing acidity is present

• the potential for future acidity generation

• whether adopted management criteria are exceeded.

Importantly, field screening alone does not always determine whether treatment is required.

The final decision should be based on the overall soil conditions, laboratory results and proposed disturbance.

What Does Acid Sulphate Soils Class 5 Mean?

Acid sulphate soil mapping in NSW commonly includes Classes 1 to 5.

These classes broadly relate to the likelihood and depth of acid sulfate soils and the types of works that may trigger planning requirements.

Acid Sulphate Soils Class 5 generally relates to land located near higher-risk acid sulfate soil areas rather than land where acid sulfate soils are expected to occur close to the surface.

However, Class 5 does not necessarily mean acid sulfate soil issues can be ignored.

Depending on the relevant Local Environmental Plan, works that lower the groundwater table in nearby Class 1, 2, 3 or 4 land may still require assessment.

The relevant Council planning controls should therefore be reviewed before undertaking excavation or dewatering.

When Is an Acid Sulphate Soils Management Plan Required?

An Acid Sulphate Soils Management Plan, commonly referred to as an ASSMP, sets out how acid sulfate soils will be managed during construction.

Whether an ASSMP is required depends on factors such as:

• mapped acid sulfate soil risk

• excavation depth

• volume of soil disturbance

• laboratory testing results

• groundwater conditions

• dewatering requirements

• Local Environmental Plan requirements.

An ASSMP may include requirements for:

• excavation controls

• soil segregation

• temporary stockpile management

• neutralisation or lime treatment

• verification testing

• groundwater management

• runoff and leachate controls

• water quality monitoring

• reuse or off-site disposal.

Importantly, being located within a mapped acid sulfate soil area does not automatically mean an ASSMP will be required.

A properly designed investigation may demonstrate that acid sulphate soil is not present within the proposed excavation depth or that the proposed works do not trigger significant management requirements.

How Are Acid Sulphate Soils Managed?

The preferred management strategy depends on the material and the proposed works.

Where practical, avoiding unnecessary disturbance is often the simplest approach.

Where excavation is unavoidable, management options may include:

Minimising Disturbance

Reducing excavation depth or changing construction methodology may reduce the quantity of acid sulphate soil requiring management.

Soil Treatment

Where appropriate, alkaline materials such as agricultural lime may be used to neutralise acidity.

Treatment requirements should be based on laboratory results rather than assumed application rates.

Controlled Stockpiling

Excavated acid sulphate soil may require controlled storage, covering and runoff management while awaiting treatment or disposal.

Groundwater Management

Where dewatering is required, groundwater quality may need to be assessed and managed before discharge.

Waste Classification

Where excavated material is transported off site, waste classification and lawful disposal requirements may also apply.

Do Acid Sulphate Soils Prevent Development?

No.

The presence of acid sulphate soils does not automatically make land unsuitable for development.

Many residential, commercial and infrastructure projects are successfully completed in acid sulfate soil areas.

The key is understanding:

• where the material occurs

• how deep it occurs

• whether the proposed works will disturb it

• how it will behave once disturbed

• what management is actually required.

Early assessment can prevent unnecessary delays and reduce the risk of discovering acid sulphate soils after construction has already commenced.

When Should You Arrange an Acid Sulphate Soil Investigation?

You should consider obtaining environmental advice where:

• your property is mapped as acid sulfate soil risk

• Council has requested an acid sulfate soil assessment

• excavation extends into mapped risk depths

• a basement or swimming pool is proposed

• deep trenches or foundations are planned

• groundwater dewatering may be required

• unusual acidic soil or water is encountered during construction.

The scope of investigation should reflect the proposed development.

A small residential excavation does not necessarily require the same level of investigation or management as a major infrastructure project.

Acid Sulphate Soil Assessment in NSW

Confluence Environmental provides acid sulphate soil investigation, testing and management advice for residential, commercial, development and construction projects across NSW.

Our services include:

• Acid Sulphate Soil Investigations

• acid sulphate soil testing

• PASS and AASS assessment

• Chromium Reducible Sulfur testing and interpretation

• Acid Sulphate Soils Management Plans

• excavation and soil management advice

• groundwater and dewatering assessment

• waste classification

• environmental monitoring.

If your property is mapped within an acid sulfate soil risk area, early investigation can determine whether the proposed works will actually encounter acid sulphate soils and what management measures, if any, are required.

Planning excavation, a swimming pool, basement or development in an acid sulphate soil area? Contact Confluence Environmental for practical advice on acid sulphate soil investigation, testing and management across NSW.