Acid sulfate soils in QLD and NSW: how to check a site before you disturb the ground
You're planning to dig. Maybe it's a slab, maybe a basement, maybe just cutting a batter to level a sloping block. Somewhere in the planning scheme you spot two words that stop you: acid sulfate soils. Now you're wondering what that means for your build, and whether it's going to cost you.
Here's the short version. Acid sulfate soils are perfectly stable while they stay wet and undisturbed. Dig them up, drain them, or lower the water table, and they turn nasty. Knowing whether you're on them, before you break ground, saves you from a very expensive surprise.
What acid sulfate soils actually are
Acid sulfate soils (often shortened to ASS) contain iron sulfides, mostly pyrite, laid down over thousands of years in low-lying coastal and estuarine ground. While they sit below the water table, they're harmless.
The problem starts when they meet air. Excavation, dewatering, or drainage exposes those sulfides to oxygen, and they react to form sulfuric acid. That acid can leach into groundwater and waterways, kill vegetation, corrode concrete and steel footings, and release metals like aluminium and iron into the soil.
There are two flavours worth knowing:
- Actual acid sulfate soils — already oxidised and acidic.
- Potential acid sulfate soils — still wet and inert, but primed to turn acidic the moment they're disturbed.
Most coastal QLD and NSW sites that flag as risky fall into the potential category. That's the one that catches builders out, because the ground looks and behaves normally until you dig it.
Where you find them in QLD and NSW
Acid sulfate soils sit in a fairly predictable spot: low-lying land near the coast, generally below about 5 metres AHD, and often much lower. Think old floodplains, backswamps, mangrove flats, drained wetlands, canal estates and estuarine margins.
So the usual suspects are:
- Coastal strips from the Gold Coast up through Brisbane, the Sunshine Coast and further north.
- Low ground around the Logan, Brisbane, Pine and Caboolture rivers.
- The NSW coast — the Tweed, Richmond, Clarence and Hunter floodplains are well known for it, along with much of the low-lying Sydney basin fringe.
Both states map this. In Queensland, acid sulfate soil risk is captured in state and council overlays and in the state ASS mapping. In NSW, most coastal councils carry an acid sulfate soils planning map with risk classes, and there's state-wide risk mapping as well. If your block is low, flat and near tidal water, assume it's worth checking.
Why it matters for footings and earthworks
This isn't just an environmental box to tick. Acid sulfate soils touch the parts of a build that cost real money.
Footings and concrete. Acidic ground and acidic groundwater attack concrete and corrode reinforcing steel. That can push you toward sulfate-resistant concrete, thicker cover to steel, or a different footing system entirely.
Earthworks and dewatering. Any cut, trench or basement that goes below the water table can trigger oxidation. That usually means an acid sulfate soils management plan, testing, and treatment of the spoil with lime — none of which is free, and all of which needs to be priced before you commit to a design.
Approvals. Disturbing acid sulfate soils above certain volumes or depths can require a permit or a management plan as part of your development approval. Finding that out late can stall a project for weeks.
Combine this with the fact that these are low, often soft, often reactive soils, and you can see why the ground here deserves early attention. A wet, low, potentially acid site is rarely a simple Class A or S under AS 2870.
When a desktop check is enough (and when it isn't)
A geotechnical desktop study won't hand you a lab certificate. What it does is tell you, quickly and cheaply, whether acid sulfate soils are a live risk for your address, and how seriously to take it.
A good desktop check pulls together:
- Whether the site sits in a mapped acid sulfate soils overlay or risk class.
- The site's elevation and how far it sits above sea level.
- Local geology and soil landscape mapping.
- Nearby groundwater bore records, which hint at how shallow the water table is.
- Flood and low-lying context, since these soils and flood-prone land go hand in hand.
If the desktop work comes back clean — the block is elevated, well drained, on residual soils away from the coast — you can usually move on with confidence.
If it flags risk, the desktop report tells you before you spend on design. From there, if you're excavating below the natural water table or disturbing large volumes, you'll want a site investigation with actual soil sampling and lab testing (a pH and peroxide test) to confirm what's really there and shape a management plan.
Think of the desktop report as the triage step. It stops you drawing footings for a site you don't yet understand, and it tells you whether the next dollar should go to a driller or straight to the slab.
Common questions
How do I know if my block has acid sulfate soils? Start with elevation and location. If your site is low-lying (roughly below 5 metres AHD) and near the coast or a tidal river, it's a candidate. Then check the council and state acid sulfate soils mapping for your address. A geotechnical desktop report brings the mapping, elevation, geology and bore records together so you get a clear yes, no, or worth-testing answer.
Do I need a permit to disturb acid sulfate soils? Often, yes. In both QLD and NSW, disturbing acid sulfate soils beyond certain depths or volumes can trigger a permit or require an acid sulfate soils management plan as part of your approval. The thresholds vary by council and by risk class, so check your local planning scheme early — before you finalise earthworks.
Will acid sulfate soils affect my footing design? They can. Acidic ground and groundwater are aggressive to concrete and steel, which may call for sulfate-resistant concrete, extra cover, or a revised footing system. Because these soils are usually low, soft and reactive, they also influence your AS 2870 classification. Sort the ground understanding out first, then design.
Want to know whether acid sulfate soils are a risk for a specific address? Pop in your address or see a sample report.
LayeredGeo compiles geology, soils, groundwater and site data into an automated geotechnical desktop report for any address in Queensland and New South Wales.