Does my block have acid sulfate soils?

Acid sulfate soils lie under low lying land right along the NSW coast, including the Central Coast. Left alone they are harmless. Dig into them, or drain them, and they can produce acid that eats concrete and steel and damages waterways. Type your address below to see whether your block is mapped, and what class it is.

What are acid sulfate soils?

They are natural sediments that contain iron sulfides. While they stay wet and undisturbed there is no problem. Once they are exposed to air, through excavation or by lowering the watertable, the iron sulfides react with oxygen and create sulfuric acid. The NSW Government's acid sulfate soils guidance lists the effects: acid and metals washing into waterways, plants dying in very acidic soil, and corrosion of concrete, iron, steel and some aluminium alloys.

Where are acid sulfate soils found on the Central Coast?

The NSW Government says acid sulfate soils are found in every coastal estuary in the state, usually less than 1 metre above sea level, and that they are common in mangrove and saltmarsh areas and underlie floodplains. Brisbane Water, the Tuggerah Lakes and Lake Macquarie are all estuaries, so on the Central Coast the low lying land around them, and along the creeks that feed them, is where to expect them.

The map is what applies to your lot. The Central Coast LEP 2022 Acid Sulfate Soils Map gives land a class from 1 to 5, and a block on higher ground can still carry a class if it sits close to low land.

What do acid sulfate soil classes 1 to 5 mean?

The class tells you which works need development consent under clause 7.1 of the Central Coast LEP 2022.

• Class 1: any works.
• Class 2: works below the natural ground surface, or works likely to lower the watertable.
• Class 3: works more than 1 metre below the natural ground surface, or likely to lower the watertable more than 1 metre below it.
• Class 4: works more than 2 metres below the natural ground surface, or likely to lower the watertable more than 2 metres below it.
• Class 5: works within 500 metres of Class 1, 2, 3 or 4 land that is below 5 metres AHD (Australian Height Datum, the national reference for ground levels), where the works are likely to lower the watertable below 1 metre AHD on that land.

Works that disturb less than 1 tonne of soil and are not likely to lower the watertable do not need consent under this clause.

When do I need a preliminary assessment or a management plan?

When your works reach the trigger for your class. Clause 7.1 then says consent must not be granted unless an acid sulfate soils management plan has been prepared in line with the Acid Sulfate Soils Manual and given to council.

There is a way around the full plan. If a preliminary assessment, also prepared in line with the manual, shows a management plan is not needed, and council confirms that in writing, consent is not required under this clause. In practice that means sampling the soil before you design the footings rather than after, so you know which path you are on.

What does it mean for footings, pools and excavation?

It is the depth that matters. On Class 3 land a slab on ground may sit above the trigger while a pool, a basement garage or deep piers go well past 1 metre. Dewatering an excavation, such as pumping out a pool hole or a basement, can lower the watertable, which is a trigger on its own. On Class 1 or 2 land almost any digging counts.

I check the class before I set floor levels or choose a footing system, because it can change the design. Moving a pool, making it shallower or leaving out a basement can keep the works under the trigger altogether. Where acid sulfate soils are disturbed, the acid can attack concrete and steel, so your engineer needs to know the class. Low lying land is also often flood prone, so check that mapping at the same time.

How do I check if my block has acid sulfate soils?

Use the checker above, which reads the Central Coast LEP map for your lot. Or check it yourself on the NSW Planning Portal Spatial Viewer: search your address and expand the acid sulfate soils result. For a finer picture of likely depth, the NSW Government's acid sulfate soil risk maps are on eSPADE.

For the rest of the maps on your block, run the full Site Checker, or book a free chat and I will go through the result with you.