Acid sulfate soils are soils and sediments that naturally contain iron sulfides. Left undisturbed and below the water table, they’re stable and harmless. The problem starts when they’re exposed to air, through excavation, drainage or a dropped water table, and the sulfides react to form sulfuric acid. That acid can corrode concrete and steel structures, stunt plant growth, and pollute creeks, rivers and estuaries. In NSW, acid sulfate soils have been found in every coastal estuary and embayment, which is why so much low-lying coastal development runs into them.
There are two forms:
- Actual Acid Sulfate Soils (AASS), soils that have already been exposed to air and have produced sulfuric acid.
- Potential Acid Sulfate Soils (PASS), soils that still contain unreacted sulfides and will produce acid if they’re disturbed and exposed.
When you need an assessment
Acid sulfate soils are managed through the planning system. Most coastal NSW councils carry an acid sulfate soil map in their Local Environmental Plan, and where your site is mapped, you generally need an acid sulfate soil assessment before works that disturb the soil or lower the water table beyond a set trigger. It’s commonly done as part of a contaminated land assessment for a development application, or before soil can be moved off site. If you’re not sure whether your site is affected, call us on 1300 888 338 and we’ll check.
How we assess acid sulfate soils
We run an acid sulfate soil assessment to the methodology in the NSW Acid Sulfate Soils Manual. In short:
- We review the site and its acid sulfate soil mapping, and design a sampling program for the site.
- Our consultants recover the soil samples on site, and send them to an external NATA-accredited laboratory for analysis, including chromium reducible sulfur testing to measure the acid-generating potential. Using an independent third-party laboratory adds to the assurance that the results are unbiased.
- We report the findings, and where acid sulfate soils are confirmed above the action criteria, we prepare an Acid Sulfate Soil Management Plan setting out how to handle, treat, neutralise or reuse the material so the works can proceed without harming structures or waterways.
Our reports and management plans are prepared to comply with NSW EPA and council requirements, and are signed off by Michael Edwards, our Certified Environmental Practitioner (Site Contamination Specialist), recognised by the NSW EPA. For the full contaminated land assessment process, see our Contaminated Land Assessments page.
We assess and plan. A licensed contractor does the works.
Airsafe is an independent consultant. We assess the soil, write the management plan and can oversee and validate the works, but we don’t carry out the earthworks, treatment or disposal ourselves. A separate licensed contractor does that, to the plan we’ve written. Because we don’t sell the works, our assessment has no stake in the outcome.
Acid sulfate soils also affect how soil can be moved: material containing acid sulfate or potential acid sulfate soils can’t be classified as VENM or ENM for clean reuse. We sort out the classification, see our Waste & VENM Assessment page.
From our case files
A recent example: a residential development application at Batemans Bay, on the NSW south coast, with Eurobodalla Shire Council. Airsafe completed a preliminary site investigation together with an Acid Sulfate Soil Management Plan, signed off by Michael Edwards, our Certified Environmental Practitioner (Site Contamination Specialist), so the application could go to council with both questions answered at once.
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Frequently asked questions
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The first check is your council’s acid sulfate soil map, usually part of the Local Environmental Plan, it shows whether your land is mapped and at what class. If it is, and your works will disturb the soil or lower the water table beyond the trigger, you’ll generally need an assessment. We can check the mapping for you and tell you what’s required, call us on 1300 888 338.
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Actual acid sulfate soils (AASS) have already been exposed to air and have generated sulfuric acid. Potential acid sulfate soils (PASS) still hold their sulfides unreacted, they’re stable until disturbed, but will produce acid once they’re dug up or drained. Both matter for how a site is managed; the assessment tells you which you’re dealing with and how much.
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Where acid sulfate soils are confirmed above the action criteria, the management plan sets out how the material will be handled during the works, how it’s excavated, treated or neutralised (commonly with lime), stockpiled, and either reused on site or disposed of, so the works can go ahead without releasing acid into structures or waterways. It’s written to comply with council and EPA requirements.
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No. Airsafe assesses the soil, writes the management plan and can oversee and validate the works. The earthworks, treatment and disposal are carried out by a separate licensed contractor working to our plan. Keeping the assessment separate from the works keeps it independent.
Talk to us about acid sulfate soils
If your site’s mapped, or you’ve hit sulfide-smelling soils mid-project, Airsafe will assess them and give you a management plan that satisfies the council and lets the job continue.
To organise an assessment, call us on 1300 888 338.
Last updated: June 2026
