Acid sulfate soils: what they are and what class 1 to 5 means

Acid sulfate soils (also spelt acid sulphate soils) are coastal and estuarine soils containing iron sulfides, mostly pyrite. Left wet and undisturbed, they are harmless. Dig them up, drain them or lower the water table, and the sulfides meet air and produce sulfuric acid, which attacks concrete and steel, damages vegetation and waterways, and brings approval obligations with it.

The maps are where you start, not where you finish. Batchelder (2012) describes a proposed development at Bulli, north of Sandon Point. The acid sulfate soils map put the high-risk ground beside the site, near Hewitts Creek, not on it. The geological map showed mostly coal measures rock. The soil landscape map put the whole site on alluvial plain. A walkover and hand-auger samples to 1 m settled it: a remnant patch of swamp on the site was acid sulfate soil, and the development needed a management plan.

What is the difference between actual and potential acid sulfate soils?

Actual acid sulfate soils have already oxidised and are acidic. Potential acid sulfate soils are still waterlogged and unoxidised, and turn acidic once disturbed or drained. The potential ones cause the trouble on building sites, because the ground looks and behaves normally until it is dug or dewatered. Once the acid forms, it can leach into groundwater and waterways and release metals such as aluminium and iron.

Where do acid sulfate soils occur in QLD and NSW?

On low-lying land near the coast, generally below about 5 metres AHD (Australian Height Datum, roughly mean sea level) and often much lower: old floodplains, backswamps, mangrove flats, drained wetlands, canal estates and estuarine margins. Well-known areas include:

• The coastal strip from the Gold Coast 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, and low-lying parts of Sydney around Botany Bay and the Georges and Parramatta rivers.

Both states map them: Queensland through state mapping carried into council planning scheme overlays, NSW through council LEP maps and state-wide risk mapping. If a block is low, flat and near tidal water, check it.

What does the acid sulfate soils class mean in NSW?

Most NSW coastal councils map acid sulfate soils in their local environmental plan (LEP) as Class 1 to Class 5, under a clause usually numbered 6.1 or 7.1. The class describes where the soils are likely to sit in the profile, and it sets which works need development consent.

Class	Where the soils are likely to sit	Works that need consent
1	On and below the natural ground surface	Any works
2	Below the natural ground surface	Works below the natural ground surface, or works likely to lower the water table
3	Beyond 1 metre below the natural ground surface	Works more than 1 metre below the natural ground surface, or works likely to lower the water table more than 1 metre below the natural ground surface
4	Beyond 2 metres below the natural ground surface	Works more than 2 metres below the natural ground surface, or works likely to lower the water table more than 2 metres below the natural ground surface
5	Not expected on the land itself, but it is within 500 metres of Class 1 to 4 land	Works within 500 metres of adjacent Class 1 to 4 land that is below 5 metres AHD, where the works are likely to lower the water table below 1 metre AHD on that adjacent land

What does Class 5 acid sulfate soils mean?

Class 5 is the one that catches people out. The lot is not mapped as containing acid sulfate soils, yet a basement that needs dewatering can still draw the water table down on neighbouring Class 1 to 4 land. Where consent is triggered, the clause generally requires an acid sulfate soils management plan unless a preliminary assessment shows one is not needed. Each council's LEP holds the operative wording, so read the local clause rather than assuming the standard text. A section 10.7 planning certificate shows which LEP and maps apply to the lot.

How does Queensland decide when acid sulfate soils need assessment?

Queensland does not use the NSW classes. Its mapping shows where acid sulfate soils are likely from landform and elevation, and council planning schemes set triggers by what the earthworks do. A common form, carried over from the former state planning policy, is excavating or removing 100 cubic metres or more of soil or sediment, or placing 500 cubic metres or more of fill at an average depth of 0.5 metres or more, on land at or below 5 metres AHD where the natural surface is below 20 metres AHD. Councils word it differently, so check the acid sulfate soils overlay code in the local planning scheme.

What do acid sulfate soils change for footings and earthworks?

Concrete and steel. Acidic ground and groundwater attack concrete and corrode reinforcing steel, so the design engineer may call for a more durable concrete, thicker cover to the steel or a different footing system. Road projects show how fast it happens, because some rock carries the same sulfides. Bridgement (2017), writing for NSW Roads and Maritime Services, describes a 1978 Hume Highway embankment at Paddys River where corrugated steel culverts had corroded through within 18 months. On the Federal Highway near the ACT border, the NSW cuttings were designed to manage acid runoff and had no reported problems. The adjoining ACT section had deteriorating batters, damaged concrete, surface pH below 3 and fish kills in a nearby farm dam.

Earthworks and dewatering. Any cut, trench or basement that exposes the soil to air or goes below the water table can start the reaction, which usually means a management plan, soil testing and liming of the excavated soil. Batchelder (2012) describes land south of Nowra beside a drained swamp, with an acid scald visible from the air. Test pits found the alluvium was acid sulfate soil, soft and highly variable, with the acid in the silt and clay rather than the sand. The plan dug the central alluvium out as a pond, put the development on the firmer residual ground around it, and kept the excavated soil on site as neutralised landscaping, because trucking it away was costly even once limed.

Approvals. Disturbing acid sulfate soils above the depth or volume triggers brings a management plan into the development approval. Finding that out late holds a project up.

The same estuarine ground often has soft clay and a shallow water table, and AS 2870 lists soft soils among the conditions that make a site Class P (a problem site needing specific engineering design). Whether that applies to a particular lot is for the site investigation to decide.

Can a desktop check tell you whether a block has acid sulfate soils?

It shows whether they are a likely issue before design starts. A geotechnical desktop study (a review of published mapping and records, with no drilling) does not test the soil. It brings together:

• Whether the site sits in a mapped acid sulfate soils overlay or class.
• The site's elevation in metres AHD.
• Local geology and soil landscape mapping.
• Nearby groundwater bore records, which indicate how shallow the water table may be.
• Flood and low-lying context, since these soils and flood-prone land often go together.

Batchelder's view was that the acid sulfate soils maps are better called susceptibility maps: the best public information for planning, but not complete, and to be confirmed on site. Where the maps disagree, as at Bulli, is where the field work should go.

An elevated, well-drained block on residual soils away from the coast is unlikely to trigger the planning rules, though it still needs its own investigation for the footing design. Where the desktop work flags a risk and the works will dig below the water table, dewater or move large volumes, the next step is sampling: field pH tests, and laboratory tests that measure how much acid the soil can generate. Those results shape the management plan.

Common questions

How do I know if my block has acid sulfate soils? Start with elevation and location. If the site is low-lying (roughly below 5 metres AHD) and near the coast or a tidal river, it is a candidate. Then check the council and state acid sulfate soils mapping for the address. A geotechnical desktop report brings the mapping, elevation, geology and bore records together so you can see whether testing is likely to be needed. Only soil testing confirms whether the soils are there.

What does Class 5 acid sulfate soils mean? Class 5 land is not expected to contain acid sulfate soils itself, but it lies within 500 metres of Class 1 to 4 land. Consent is only needed for works that are likely to lower the water table below 1 metre AHD on that nearby land, such as dewatering for a basement. A typical house slab on Class 5 land does not trigger it.

Do I need a permit to disturb acid sulfate soils? Often, yes. In NSW the class on the LEP map sets which works need consent, and in Queensland the planning scheme sets triggers by the volume excavated or filled below 5 metres AHD. Either can require an acid sulfate soils management plan as part of the approval, so check the local planning scheme 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 more durable concrete, extra cover, or a different footing system. The soft, wet estuarine ground they occur in can also make a site Class P under AS 2870. The site investigation decides the classification, and the footing design follows from it.

To see whether acid sulfate soils are mapped for a specific address, generate a desktop report for the 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, New South Wales, Victoria and South Australia.

Sources

• Bob Batchelder (2012). Acid sulphate soils and rocks – occurrence and management in the Illawarra Region. Australian Geomechanics, Volume 47, Number 1 (Mar 2012). geomechanics.org.au
• Nicholas Bridgement (2017). Managing the risks associated with acid sulfate rock in NSW road projects. Australian Geomechanics, Volume 52, Number 4 (Dec 2017). geomechanics.org.au

About this article. Published by LayeredGeo and written from the published research cited in the Sources section above. It is general information about how property and ground conditions are assessed in Australia, not engineering, planning, legal or financial advice, and it is not specific to any property. Check anything that matters against the source dataset or a suitably qualified professional before you rely on it. If you spot something wrong, tell us at hello@layeredgeo.com.au and we will fix it.

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