Acid Sulfate Soils Explained for Home Builders

Acid sulfate soils are a naturally occurring feature of many coastal and low-lying areas in Australia. They are not a problem when left undisturbed, but when excavated or drained during construction, they can produce sulfuric acid that damages buildings and the environment. If you are building on a coastal or low-lying block, understanding acid sulfate soils and the potential management costs is essential.

What Are Acid Sulfate Soils?

Acid sulfate soils are soils that contain iron sulfide minerals (primarily pyrite, FeS₂). These minerals form naturally in waterlogged, oxygen-free environments — typically in coastal lowlands, estuaries, tidal wetlands, and former marine or brackish environments. The sulfides are stable when the soil is waterlogged, but when exposed to air, they oxidise and produce sulfuric acid.

There are two types of acid sulfate soils:

• **Potential Acid Sulfate Soils (PASS)** — Soils that contain sulfides but have not yet been oxidised. These are the soils that are at risk of producing acid if disturbed. They are typically found below the water table in waterlogged conditions.
• **Actual Acid Sulfate Soils (AASS)** — Soils that have already been oxidised and are producing acid. These are typically found in areas where the water table has been lowered (through drainage, groundwater extraction, or drought) or where the soil has been excavated and exposed to air.

Where Are Acid Sulfate Soils Found?

Acid sulfate soils are found in coastal and low-lying areas across Australia, particularly in:

• Coastal lowlands and floodplains (typically below 5m AHD)
• Estuaries, tidal wetlands, and mangrove areas
• Former marine or brackish environments (areas that were once under seawater)
• Areas with high groundwater tables near the coast
• Reclaimed land (areas that were once wetland and have been drained and filled)

Most states have acid sulfate soil risk maps that show the areas where ASS is likely to be found. These maps are typically available from the state's environmental or planning authority.

How Acid Sulfate Soils Affect Construction

If your block is in an acid sulfate soil risk area and the construction involves disturbing the soil (through excavation, site cut, dewatering, or foundation construction), you may need to:

How to Check If Your Block Is at Risk

1. Check the state's acid sulfate soil risk map — most states have an online mapping tool that shows ASS risk areas.
2. Check the council's planning scheme for an acid sulfate soil overlay or provision.
3. Check the elevation of the block — ASS is typically found below 5m AHD.
4. Check the groundwater level — if the groundwater table is high, the risk of ASS is greater.
5. If the block is in a risk area, commission a preliminary ASS assessment from a geotechnical engineer.
6. If ASS is confirmed, commission an acid sulfate soil management plan before construction begins.

Acid Sulfate Soil Testing

If your block is in an ASS risk area, the council may require an acid sulfate soil assessment as a condition of the building approval. The assessment involves:

• Drilling boreholes at the site and collecting soil samples from different depths
• Laboratory testing of the samples for sulfide content (typically using the SPOCAS or chromium-reducible sulfur method)
• Classification of the soils as PASS, AASS, or non-ASS based on the test results
• Recommendations for management if ASS is confirmed

The assessment typically costs $1,500–$5,000, depending on the number of boreholes and the depth of testing required.

How Builders Handle ASS Costs in Contracts

Builders handle acid sulfate soil costs in one of three ways:

• **Included in the site costs** — The builder has conducted a preliminary assessment and included the cost of ASS management in the fixed site costs. This is rare, as most builders do not test for ASS until the site works begin.
• **Provisional sum** — The builder includes an estimate for ASS management but notes that the actual cost may vary. This is the most common approach.
• **Exclusion** — The builder explicitly excludes all ASS-related costs from the contract. You are responsible for testing, management, and any additional construction costs.

What to Check Before Buying Land

Common Mistakes and Hidden Costs

• Not checking whether the block is in an acid sulfate soil risk area before buying.
• Assuming that because the block is in a developed area, ASS has already been addressed — this is not always the case.
• Not budgeting for the cost of ASS testing and management — the cost can be $5,000–$30,000 or more.
• Forgetting that ASS can affect the long-term durability of the foundations if not managed — the acid can corrode concrete and steel over years.
• Not checking whether the builder has included ASS-related costs in the contract.
• Assuming that a shallow foundation will avoid ASS — even shallow foundations can disturb ASS if the soil is excavated or the groundwater is lowered.

Frequently Asked Questions