What Are Acid Sulphate Soils?
Acid sulphate soils are naturally occurring soils and sediments containing iron sulphides, mainly pyrite. They formed over thousands of years in waterlogged, oxygen-poor environments such as estuaries and coastal wetlands. (NSW government documents use the spelling “sulfate”. Same soils.)
The problem starts when these soils meet oxygen through drainage, dewatering or excavation. The sulphides oxidise and produce sulphuric acid. That acid can mobilise metals such as iron and aluminium, leach into groundwater and flow into waterways. The consequences include fish kills, damaged aquatic habitat, and corroded steel and concrete infrastructure.
Actual Acid Sulphate Soil (AASS)
AASS is soil where oxidation has already happened. Sulphuric acid is present or actively forming.
- Already oxidised: the sulphides have reacted with oxygen, usually because the area was drained or previously disturbed.
- Field indicators: a soil pH generally below 4 in field screening, and often pale yellow mottles of jarosite. Jarosite is a classic sign, but it isn’t always present, so don’t rely on appearance alone.
- Immediate management required: acid is already being generated, so disturbing AASS calls for a management response straight away.
- Higher immediate risk: the acid is already present and mobile.
Potential Acid Sulphate Soil (PASS)
PASS contains iron sulphides that have not yet oxidised, and on the surface it can look harmless.
- Unoxidised: the pyrite is intact and the undisturbed soil pH is typically near neutral.
- Stable while waterlogged: while it stays saturated and isolated from oxygen, it poses little risk.
- Becomes AASS when disturbed: draining, dewatering or excavating lets oxygen in and starts acidification.
- The key risk: a site that appears benign can turn acidic once exposed to air.
That’s why laboratory testing matters. Field pH alone can understate the risk of PASS, and confirmatory analysis is needed to establish its acid-generating potential.
AASS vs PASS: Key Differences
| Factor | Actual ASS (AASS) | Potential ASS (PASS) |
|---|---|---|
| Oxidation state | Already oxidised | Unoxidised, sulphides intact |
| Immediate risk | High: acid is present now | Latent: triggered by disturbance |
| Visual indicators | Low pH, often yellow jarosite mottles | Often no obvious signs; dark, waterlogged, sometimes a rotten-egg smell |
| Management approach | Neutralise and contain acidity | Prevent oxidation, or treat as soon as it is exposed |
| Regulatory obligations | LEP acid sulphate soil provisions apply; must be treated before disposal | LEP provisions apply equally; waste rules require it to be kept wet and moved quickly, or neutralised |
Different pH thresholds appear in different NSW documents. Field screening under the ASS Manual uses pH below 4 for actual ASS, while the EPA’s waste guidelines use 5.5. Always confirm which criteria your consultant and your waste facility are applying.
A single profile often holds both types, with AASS in the drained upper layers and PASS below. Classify based on testing at depth, not on surface appearance.
Where Are Acid Sulphate Soils Found in NSW?
Acid sulphate soil in NSW is concentrated in coastal floodplains, estuaries, backswamps and former wetlands. As a general guide, these soils occur in coastal lowlands below about 5 metres AHD (Australian Height Datum).
Acid sulphate soil mapping in NSW sits within Local Environmental Plans (LEPs), and councils are responsible for it. The maps divide land into five classes:
- Class 1: acid sulphate soils likely on and below the natural surface
- Class 2: likely below the natural surface
- Class 3: likely more than 1 m below the surface
- Class 4: likely more than 2 m below the surface
- Class 5: not typically present, but within 500 m of Class 1–4 land
You can view them through the NSW Planning Portal Spatial Viewer. The maps indicate risk and are not a substitute for site-specific testing.
Management and Regulatory Requirements
For acid sulphate soil construction in NSW, expect several layers of obligation:
- Development consent: whether consent is needed depends on the LEP class and the type of works. Some minor works, such as those disturbing very small volumes of soil and unlikely to lower the water table, may be exempt.
- Acid Sulphate Soil Management Plans (ASSMPs): where consent is required, the application must generally be accompanied by an ASSMP prepared in line with the NSW Acid Sulfate Soils Manual. The plan sets out how material will be handled, treated, monitored and validated.
- Treatment by liming: neutralisation typically uses fine agricultural lime at rates calculated from laboratory results. Thorough mixing and validation sampling are essential.
- Waste and disposal: soil that leaves the site is waste. AASS must be treated by the generator, then chemically assessed and classified, before going to a landfill licensed for that waste class. PASS has its own rules: it must be kept wet and delivered promptly (within 16 hours of excavation under EPA guidelines), or neutralised first. Planning your soil disposal pathway early avoids delays and unplanned costs.
Good PASS soil management comes down to prevention. Minimise exposure time, keep material moist, and have treatment and disposal arranged before excavation, not after.
Why This Matters for Contractors
Missing acid sulphate soils is one of the more avoidable ways to get into trouble on a coastal or low-lying site. The consequences can include:
- Environmental harm to waterways and groundwater
- Stop work orders from councils or regulators
- Remediation costs and project delays
- Liability for damage to neighbouring land or infrastructure
Depending on your LEP class and the works proposed, an assessment may be mandatory before you disturb the ground. Working out early whether your ASS PASS soil classification points to AASS, PASS or both lets you build the right controls into your program and budget.
Acid sulphate soils also rarely occur alone. Low-lying sites often carry fill of unknown origin or hydrocarbon and metals contamination, and EBH Environmental’s approach to contaminated soils covers how these combined issues are managed.
