Acid sulfate soil myths: 5 things QLD buyers get wrong

Acid sulfate soils are real, they sit under much of coastal Queensland, and the trouble they cause comes almost entirely from disturbance: excavation, dewatering or draining that lets air reach sediments which have stayed waterlogged and harmless for thousands of years. Beyond that, most of what buyers and first-time developers believe about them is half right. The belief usually has a sound core, then breaks down on a detail that matters once you price earthworks or lodge a development application. Here are five of them.

"If my block isn't on the water, there's no acid sulfate soil"

Partly true, but the boundary is elevation and geology, not whether you can see the sea. Acid sulfate soils in eastern Australia form at low elevations in estuarine and marine sediments of Holocene or Pleistocene age (Walker 2012). Holocene means deposited in roughly the last ten thousand years, as sea level rose and rivers dropped their load across the coastal plain. That includes land well back from the shore: old tidal flats, infilled channels and former mangrove ground now sitting behind levee banks and canal estates.

A lot on the Nerang River floodplain on the Gold Coast, or on the lower Pine and Caboolture deltas in Moreton Bay, can be a kilometre or more from open water and still sit on the same estuarine sequence. What matters is how low the land is, not the view. Queensland's acid sulfate soil mapping is drawn to an elevation threshold for this reason, commonly 5 metres AHD and below, with a second band to about 20 metres for older material. Check the mapped elevation of the lot, not its distance to a beach.

"The soil test for my slab will pick it up"

It will not. A standard AS 2870 site classification and the soil reactivity test behind it are about shrink-swell clay and footing design. They answer a different question from acid sulfate testing, which measures reduced sulfur, usually pyrite, and the acid that sulfur can generate once exposed to oxygen (Walker 2012).

The two investigations use different samples, different field handling and different laboratory suites. Acid sulfate sampling has to minimise oxidation between the ground and the laboratory, which is why samples are chilled below about 4 degrees or oven-dried and sealed quickly, and why the testing pairs indicative and quantitative methods to cross-check each other (Walker 2012). None of that happens in a routine slab classification. If the ground is low-lying estuarine, you need acid sulfate sampling as a separate scope. A desktop report flags whether the mapping and geology make that likely before anyone books a rig.

"Acid sulfate soil only matters for big canal and marina projects"

The large projects get the attention because they move the most sediment, but the trigger is disturbance of any sulfide-bearing ground, and a house footing job can reach it. The acid-generating reaction starts when pyrite is exposed to water and oxygen, producing sulfuric acid and dissolved iron, and the acid can keep propagating once it starts (Walker 2012). A pool excavation, a strip footing dug below the water table, or dewatering for a basement on a low lot is enough contact.

Disturbance does not have to be deep to count. Lowering the water table drains air into ground that was permanently saturated. On the delta estates around Moreton Bay, where soft estuarine deposits sit close to the surface, the material at footing depth can be the acidic layer itself. The scale of the works sets the scale of the management plan, not whether a plan is needed.

"The ground and the problem are uniform across the lot"

Sulfide distribution is uneven, which is why a single test tells you little. Published practice for acid sulfate material favours systematic sampling across the full sequence, sometimes blending crushed core to average out the patchiness, precisely because the sulfide and the acid-neutralising minerals vary from point to point (Walker 2012).

The reason is in the geology. Holocene estuarine infill is layered and variable. Case work on Holocene coastal sequences describes soft estuarine deposits with undrained shear strengths as low as 15 to 30 kPa filling old channels, interbedded with firmer material (Australian Geomechanics 2019). One borehole can hit a soft, sulfide-rich channel fill while the hole next to it finds stiffer ground. For a development site this is why the investigation is designed around the mapped channels and former tidal flats, and why reading groundwater bore logs near the lot earns its place in a desktop study.

"If the overlay flags it, the land is unbuildable"

The acid sulfate soil overlay is a management trigger, not a prohibition. It means a disturbance above a threshold volume or depth requires an acid sulfate soil management plan with the development application, setting out how material will be excavated, neutralised, limed, reburied or treated. It rarely means you cannot build.

The practical cost is in handling. Lime treatment, controlled reburial below the water table and column leach testing to confirm field performance are routine parts of managing the material (Walker 2012). Lime treatment of these soils is a researched and workable method (Australian Geomechanics 2015). The budget item is earthworks and compliance, not the land itself. Where a desktop study helps is in telling you early whether the overlay applies, how low the lot sits and what the nearby geology suggests, so the management cost is in your feasibility rather than a surprise after settlement.

What to do with these five

For any low coastal lot in Queensland, check the mapped elevation and the geological unit first, then treat acid sulfate testing as a separate scope from the slab classification, and read the overlay as a cost to plan for rather than a wall. The common thread is that the mapping and the geology tell you where to look before anyone drills.

To see what the published mapping, elevation and nearby bore records say for a specific low-lying address, generate a desktop report for the address or see a sample report first.

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

• Chris Walker (2012). Acid Sulfate Rock Management In Earthworks For Roads. Australian Geomechanics, 2012 Sydney Symposium (2012). geomechanics.org.au
• Australian Geomechanics Society (2019). Proceedings of the 13th Australia New Zealand Conference on Geomechanics. Australian Geomechanics, 13th Australia and New Zealand Conference on Geomechanics (2019). geomechanics.org.au
• Australian Geomechanics Society (2015). Proceedings of the 12th Australia New Zealand Geomechanics Conference. Australian Geomechanics, 12th Australia and New Zealand Conference on Geomechanics (Feb 2015). 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.

← All articles