Soil Classified as PASS and How to Treat It On Site or In Situ

While PASS may appear harmless when undisturbed, it can become highly problematic once exposed to air. Understanding what PASS is and how to treat it on site or in situ is critical for construction, demolition, and infrastructure projects. Improper handling can lead to environmental damage, project delays, and compliance issues.

In this guide, we break down what Potential Acid Sulfate Soil is, where it’s found, and the correct methods for managing and treating it safely on construction sites.

What Is PASS (Potential Acid Sulfate Soil)?

PASS refers to soils that contain iron sulfides (mainly pyrite) but have not yet been exposed to oxygen. In their natural, waterlogged state, these soils are typically stable and non-acidic.

However, when PASS is disturbed through excavation or dewatering, it reacts with oxygen and water to form sulfuric acid, turning into Actual Acid Sulfate Soil (AASS).

This chemical reaction can cause:

• soil acidification
• corrosion of concrete and steel
• contamination of groundwater and waterways
• damage to infrastructure
• harm to aquatic ecosystems

Because of these risks, PASS must be carefully managed during construction works.

Where PASS Is Commonly Found

PASS soils are typically located in coastal, estuarine, and low-lying floodplain areas. In Queensland and New South Wales, they are commonly encountered in:

• coastal developments
• port and marina projects
• riverbanks and wetlands
• reclaimed land
• areas with high groundwater tables

Any excavation below the natural water table in these regions increases the likelihood of encountering PASS.

Why PASS Is a Major Risk on Construction Sites

PASS becomes hazardous when exposed to air during:

• excavation works
• trenching
• piling
• dewatering
• site preparation

Once oxidised, the resulting acid can:

Damage infrastructure

Acidic conditions can rapidly corrode:

• concrete structures
• steel reinforcement
• pipelines
• underground services

Impact the environment

Acid runoff can enter waterways, affecting aquatic life and causing long-term environmental damage.

Trigger compliance issues

Australian environmental regulations require proper identification and management of acid sulfate soils. Failure to comply can result in:

• project shutdowns
• fines and penalties
• remediation costs

How PASS Is Identified on Site

Before excavation begins, PASS is typically identified through:

Geotechnical and environmental investigations

Specialist consultants conduct soil testing to determine:

• presence of sulfides
• potential acidity levels
• soil classification (PASS vs AASS)

Acid sulfate soil testing

Laboratory analysis helps determine how much acid could be generated if the soil is exposed.

These assessments guide how the soil must be managed during construction.

How to Treat PASS Soil On Site or In Situ

When PASS is encountered, there are several accepted methods to manage and treat it safely.

1. Avoid Disturbance Where Possible

The most effective method is to leave PASS undisturbed.

If the soil remains below the water table and is not exposed to oxygen, it will not generate acid.

This approach is suitable when:

• excavation can be redesigned
• structures can be built above the affected layer
• minimal disturbance is required

2. In Situ Treatment (Neutralisation)

When disturbance is unavoidable, PASS can be treated in situ using neutralising agents such as lime.

How it works:

• lime (or similar alkaline material) is mixed into the soil
• the lime neutralises potential acidity
• prevents acid formation when exposed to oxygen

Common neutralising agents:

• agricultural lime
• quicklime
• hydrated lime

The treatment rate is determined through laboratory testing to ensure effective neutralisation.

3. Controlled Excavation and Treatment

In many construction projects, PASS must be excavated and treated during removal.

This involves:

• staged excavation to minimise exposure
• immediate treatment with neutralising agents
• controlled handling of soil
• preventing stockpile oxidation

Excavated material may be treated:

• on-site before reuse
• or transported to approved facilities

4. Moisture Control and Covering

To prevent oxidation, PASS soils should be kept wet or sealed from air exposure.

Methods include:

• covering stockpiles with plastic sheeting
• keeping soil saturated
• limiting exposure time during excavation

This is often used as a temporary control measure before treatment.

5. Water Management and Dewatering Control

Dewatering can trigger PASS activation by lowering groundwater levels and exposing sulfides to oxygen.

To manage this risk:

• control dewatering rates
• treat discharge water if contaminated
• monitor groundwater levels
• use sediment and treatment systems

Proper water management is critical for preventing acid generation and environmental contamination.

Environmental and Compliance Requirements

Managing PASS is regulated under environmental guidelines in Australia, particularly in Queensland and NSW.

Construction projects involving PASS typically require:

• Acid Sulfate Soil Management Plans (ASSMP)
• environmental approvals
• monitoring and reporting
• controlled disposal procedures

Failure to follow these requirements can result in serious compliance issues and environmental harm.

Best Practices for Managing PASS on Construction Sites

To safely deal with PASS soils, contractors should follow these best practices:

Conduct early investigations

Identify PASS risks before excavation begins.

Engage environmental specialists

Work with consultants to develop treatment strategies.

Minimise soil disturbance

Avoid unnecessary excavation in affected areas.

Apply correct treatment methods

Ensure neutralisation is done at the correct rates.

Monitor site conditions

Regularly check soil, water, and environmental impacts.

Coordinate with demolition and early works teams

Ensure all contractors understand the management plan.

The Role of Specialist Contractors

Managing PASS soils requires coordination between environmental consultants, engineers, and contractors experienced in earthworks, demolition, and site preparation.

Specialist contractors can assist with:

• controlled excavation in sensitive environments
• coordination with environmental management plans
• safe handling of contaminated soils
• integration with broader construction works

This ensures the project progresses safely while meeting environmental and regulatory requirements.

Conclusion

Potential Acid Sulfate Soil is a hidden but significant risk on many construction sites across Australia. While harmless when undisturbed, it can cause serious environmental and structural issues once exposed.

By understanding how to identify and treat PASS soils on site or in situ, project teams can minimise risk, maintain compliance, and avoid costly delays.

Proper planning, correct treatment methods, and experienced contractors are essential for managing these conditions effectively.

Need support managing complex site conditions? Contact Perfect Contracting for expert demolition, excavation preparation, and early works solutions across challenging construction environments.