How Acid Sulfate Soils Are Investigated and Managed in NSW

A good acid sulfate soil investigation does more than establish whether ASS is present. It determines where acid-generating materials occur, how the proposed works will interact with them and what management is actually required.

That distinction is important.

Acid sulfate soils can remain relatively stable while waterlogged and undisturbed. Risk develops when excavation, drainage, dewatering or other works expose sulfidic materials to oxygen and allow acidity or dissolved metals to affect surrounding soil, groundwater or surface water. NSW Government guidance identifies excavation, drainage and development as common mechanisms capable of triggering these impacts.

For development and infrastructure projects, the assessment therefore needs to connect three things:

the ground conditions + the proposed disturbance + the potential environmental consequences.

This is the basis of a proportionate acid sulfate soil investigation.

Start With the Ground Model

An ASS investigation should not begin by simply selecting a number of boreholes from a standard table.

The first step is to understand why acid sulfate soils might occur at the site and where within the soil profile they are likely to be encountered.

This typically involves review of:

• Acid Sulfate Soil Risk Mapping;

• the applicable Local Environmental Plan and ASS class;

• surface and Quaternary geology;

• soil landscape mapping;

• topography and site elevation;

• geomorphology;

• aerial photography;

• groundwater conditions;

• surrounding estuarine, wetland or floodplain environments; and

• previous geotechnical or environmental investigations.

NSW ASS risk maps themselves were developed using interpretation of the geomorphic environment supported by aerial photography, field investigation and laboratory testing. They are intended to predict the likelihood, approximate depth and landscape setting of acid sulfate soils rather than confirm conditions at an individual property.

This makes geology, geomorphology and elevation particularly important.

For example, low-lying Holocene estuarine clay has a very different ASS potential from residual weathered sandstone at the same property.

The specialist ASS training undertaken by Confluence similarly identifies geology, ASS mapping, soil landscapes, topography and aerial photography as key inputs when developing the initial investigation model.

Understand the Proposed Disturbance Before Designing the Investigation

The next question is:

What is the project actually going to do to the soil and groundwater?

The investigation should consider:

• maximum excavation depth;

• horizontal extent of earthworks;

• estimated volume of material disturbed;

• cut and fill requirements;

• trenching and service installation;

• piling or drilling;

• temporary stockpiling;

• whether groundwater will be intercepted; and

• whether temporary or permanent dewatering is proposed.

This matters because the presence of ASS below a project does not necessarily mean it presents a material risk.

If sulfidic material occurs at 2.5 m below ground level but excavation stops at 500 mm and groundwater conditions remain unchanged, management may be very different from a project involving a 3 m basement and sustained dewatering.

The investigation is therefore designed around the zone of influence of the works, not simply the property boundary.

How Deep Should an ASS Investigation Go?

Investigation depth should extend sufficiently beyond the proposed disturbance to establish what lies beneath it.

The 2018 National Acid Sulfate Soil Guidance provides current good-practice advice for the identification and sampling of ASS materials and for defining their extent in the landscape.

The specialist training provided to Confluence uses the following general investigation principles:

• where groundwater levels will not change, investigate approximately 1 metre below the proposed depth of disturbance; and

• where groundwater will be lowered, investigate approximately 1 metre below the estimated final groundwater drawdown.

It also identifies high-resolution vertical sampling, commonly at approximately 0.25 m intervals, with increased sampling where thin or changing soil horizons are encountered.

The important principle is not the number itself.

The investigation needs to capture the vertical variability of the soil profile, because ASS can occur within relatively discrete geological horizons.

Stratigraphy Is Critical

ASS results should not be interpreted as a disconnected spreadsheet of laboratory numbers.

They need to be considered against the stratigraphy.

During drilling or augering, the consultant should be looking for changes in:

• soil texture;

• colour;

• organic content;

• shell material;

• moisture conditions;

• groundwater;

• sedimentary structures;

• iron staining;

• jarosite; and

• other evidence of oxidised or reduced conditions.

Coastal sulfidic materials commonly occur as grey or dark-grey saturated clays, silts and sands associated with former estuarine environments, although appearance alone cannot confirm ASS.

A useful investigation therefore builds a profile such as:

fill → alluvium → tidal sand → estuarine clay → older residual or weathered material

and then interprets field and laboratory results against those geological units.

That allows the consultant to determine whether ASS is confined to one particular horizon rather than treating the entire excavation profile as chemically identical.

Field Screening Helps Identify the Important Horizons

ASS investigations commonly include field screening such as:

• field pH;

• peroxide-oxidised pH;

• reaction characteristics following peroxide addition;

• soil colour and texture;

• jarosite or other oxidation features; and

• observations of reduced or sulfidic materials.

Field screening can help identify changes within the profile and assist with selecting samples for laboratory analysis.

However, field screening is not definitive laboratory confirmation of ASS.

A low pH or strong peroxide reaction can indicate a potentially significant horizon, but results need to be interpreted alongside geology, soil characteristics and appropriate laboratory analysis.

This is especially important when distinguishing true acid sulfate soil conditions from naturally acidic residual soils.

Laboratory Testing: More Than Just “Does It Contain Sulfur?”

Modern ASS laboratory assessment is intended both to identify ASS materials and to quantify their potential acidity hazard. The National Laboratory Methods Manual provides current good-practice methods for that purpose.

Common analytical parameters can include:

Chromium Reducible Sulfur

Chromium Reducible Sulfur, generally reported as S₍Cr₎, measures reduced inorganic sulfur associated with sulfidic material.

It is particularly useful for assessing the potential acidity associated with unoxidised sulfides.

Titratable Actual Acidity

TAA assesses existing acidity already present within oxidised materials.

This can be important where Actual Acid Sulfate Soil or sulfuric material has already developed.

Acid Neutralising Capacity

Some soils contain minerals capable of neutralising acidity.

However, the presence of measured Acid Neutralising Capacity does not necessarily mean that all of it will react sufficiently quickly or effectively to offset acidity generated during disturbance.

The 2018 guidance introduced greater caution around automatically crediting ANC within net acidity calculations, with corroborating evidence potentially required where effective neutralisation is being relied upon. This issue was specifically covered in Confluence's specialist ASS training.

Net Acidity

Net acidity brings together relevant acidity and sulfur components to quantify the potential acid hazard associated with the material.

This can then be compared with applicable action criteria and used to inform treatment requirements.

It is more accurate to describe this as quantifying the acidity hazard than simply saying that the laboratory is testing whether ASS is “present”.

What About SPOCAS?

The Suspension Peroxide Oxidation Combined Acidity and Sulfur method, commonly referred to as SPOCAS, has historically been widely used for ASS investigations.

However, the national guidance recognises limitations associated with peroxide oxidation methods, particularly in soils containing significant organic matter.

Chromium Reducible Sulfur testing is therefore commonly important when assessing sulfidic materials, with laboratory method selection based on the characteristics of the soil and the question being investigated.

This is another reason ASS assessment should not simply consist of sending every sample for the same analytical suite.

Hazard and Risk Are Not the Same Thing

One of the most useful distinctions in ASS assessment is between hazard and risk.

A soil may contain enough reduced inorganic sulfur to represent a significant acidity hazard if fully oxidised.

But whether that hazard becomes an environmental risk depends on factors such as:

• whether the soil will actually be excavated;

• how much will be disturbed;

• how long it will be exposed;

• groundwater conditions;

• natural buffering;

• surrounding receptors;

• drainage pathways; and

• the proposed management controls.

For example, a high-sulfur layer several metres below the proposed excavation may represent a geological hazard but little immediate project risk if it remains saturated and undisturbed.

The same material exposed across a large excavation and stockpiled for several weeks could present a substantially different risk.

This is why modern ASS assessment is best approached through a Conceptual Site Model, linking the ASS source, disturbance mechanism, transport pathways and environmental receptors.

Groundwater Can Be More Important Than the Excavation

ASS risk is not limited to soil physically removed from an excavation.

Lowering groundwater can expose sulfidic soils to oxygen beyond the excavation itself.

Projects involving:

• basement dewatering;

• trench dewatering;

• deep excavations;

• drainage infrastructure;

• groundwater pumping; or

• long-term groundwater lowering

may therefore require assessment of the zone of groundwater drawdown.

National ASS guidance specifically addresses dewatering in shallow groundwater environments and recommends considering the hydrogeological setting, ASS hazards, receptors, pathways and the extent of groundwater influence.

Confluence's specialist training similarly emphasises development of a sound groundwater Conceptual Site Model and consideration of the zone of influence and cone of depression when assessing dewatering risk.

This is particularly important on low-lying coastal and estuarine sites where groundwater and ASS conditions are closely connected.

Turning Investigation Results Into an ASS Management Plan

Where ASS will be disturbed and cannot reasonably be avoided, the investigation provides the technical basis for an Acid Sulfate Soil Management Plan (ASSMP).

A useful ASSMP should not simply state that lime will be added.

It should explain how the earthworks will actually be undertaken.

The NSW ASSMAC framework identifies key elements including:

• the environmental setting;

• geology and hydrogeology;

• vertical and horizontal extent of ASS;

• proposed excavation;

• relevant field and laboratory data;

• treatment and mitigation measures;

• monitoring and validation; and

• contingency procedures.

For a construction project, this can translate into practical requirements for:

• excavation sequencing;

• identification and segregation of ASS materials;

• temporary stockpile locations;

• treatment pad construction;

• soil moisture management;

• neutralising agent application;

• mixing methodology;

• verification sampling;

• runoff and leachate controls;

• groundwater management;

• material reuse or disposal; and

• unexpected finds.

The objective is to integrate the chemistry with the construction methodology.

How Is ASS Treated?

Agricultural lime is commonly used to neutralise acidity because it provides a relatively safe and effective source of calcium carbonate.

Treatment requirements are generally calculated from the measured acidity hazard with an appropriate safety factor rather than applying an arbitrary quantity of lime.

Treatment needs to consider more than the calculated application rate.

Effective treatment also depends on:

• uniform mixing;

• soil texture;

• material thickness;

• moisture content;

• treatment-pad conditions;

• contact between soil and neutralising material; and

• sufficient verification.

Clay-rich soils, for example, may require substantially more effort to achieve effective mixing than loose sandy material.

The objective is not simply to add lime. It is to demonstrate that the treated material meets the adopted management criteria.

Verification Is Part of the Treatment Process

Treatment should generally be followed by verification appropriate to the management strategy.

Verification sampling can establish whether the treatment process has achieved the required outcome before soil is reused, placed or otherwise managed.

Depending on the project, verification may include consideration of:

• residual net acidity;

• soil pH;

• adequacy of neutralisation;

• treatment homogeneity;

• stockpile volumes; and

• compliance with the ASSMP.

This closes the loop between:

characterisation → treatment → verification → reuse or placement.

Without verification, there may be little evidence that calculated treatment rates were actually achieved throughout the soil mass.

Acid Sulfate Soils and Excavated Material

ASS management also intersects with soil reuse and waste regulation.

Material that can remain and be appropriately managed on the development site presents a different regulatory pathway from material proposed for off-site reuse or disposal.

NSW now has a general Acid Sulfate Soil Resource Recovery Order and Exemption 2025, which commenced on 5 December 2025 and establishes requirements for the supply and application of qualifying ASS for use as earthworks or engineering fill.

The Order and Exemption contain specific eligibility, sampling, testing, treatment, record-keeping and reuse requirements.

ASS should therefore be considered alongside the project's broader waste classification and material reuse strategy, particularly where substantial earthworks are proposed.

When Can ASS Be Avoided Rather Than Managed?

Treatment is not automatically the preferred response.

Where practicable, the lowest-risk option may be to avoid disturbing the affected horizon altogether.

Design responses could include:

• reducing excavation depth;

• modifying footing design;

• altering service alignments;

• avoiding unnecessary groundwater lowering;

• retaining sulfidic soils below groundwater;

• changing construction sequencing; or

• minimising the volume requiring excavation.

Good ASS assessment can therefore influence design before construction starts.

That can be substantially more effective than developing a complex treatment strategy after the earthworks methodology has already been fixed.

Common Problems With ASS Investigations

Several recurring issues can make an investigation less useful.

Relying on Mapping Alone

ASS risk mapping is an important screening tool, but it does not confirm conditions at an individual property.

Sampling Without Understanding the Geology

A laboratory result means far more when it can be related to a defined soil or geological horizon.

Stopping the Investigation at the Excavation Depth

Sampling should provide sufficient information below the disturbance to understand whether deeper ASS or groundwater changes could affect the project.

Treating Field Screening as Confirmation

Field pH and peroxide screening are useful investigative tools, but should not be used as a substitute for appropriate laboratory characterisation.

Automatically Crediting ANC

Measured neutralising capacity needs to be interpreted carefully before being relied upon to reduce calculated acidity.

Ignoring Groundwater Drawdown

A project may disturb ASS without directly excavating it if dewatering exposes sulfidic materials to oxygen.

Preparing the ASSMP Without the Contractor in Mind

A technically correct treatment calculation is of little value if the treatment area, excavation sequencing, stockpile volumes or mixing method cannot actually be implemented on site.

A Better Way to Think About ASS Assessment

The most useful way to approach an acid sulfate soil investigation is:

Develop the ground model → understand the proposed disturbance → investigate the relevant soil horizons → quantify the acidity hazard → assess how the project creates risk → design proportionate management controls → verify the outcome.

That is fundamentally different from asking only:

“Did the laboratory find acid sulfate soils?”

The purpose of the investigation is to provide enough information to make decisions about the development.

Acid Sulfate Soil Assessment and Management in NSW

Confluence Environmental provides acid sulfate soil assessment, investigation, testing, Acid Sulfate Soil Management Plans, construction support and verification across NSW.

Our investigations are developed around the site's geology and geomorphology, proposed excavation, groundwater conditions and construction methodology rather than applying the same sampling program to every site.

Environmental personnel undertaking ASS work have completed specialist acid sulfate soil training delivered by a Registered Soil Practitioner – Acid Sulfate Soils (RSP-ASS) covering NSW planning requirements, investigation design, sampling and laboratory interpretation, hazard and risk assessment, dewatering and ASS management.