What are acid sulfate soils and can I build on them?

Acid sulfate soils are natural soils and sediments that hold iron sulfides, and you can usually build on them. They sit mostly on low coastal land and stay harmless while they stay wet and undisturbed. Dig them up or drain them and air reaches them, which turns the sulfides into sulfuric acid. What matters for a build is how far the work disturbs the ground.

Where are acid sulfate soils found?

They formed in waterlogged, salty ground such as estuaries, mangrove flats and coastal wetlands. Many coastal suburbs in Queensland and New South Wales sit on them, they occur along other parts of the coast, and some turn up inland around wetlands. A lot can look dry and high because it was filled years ago, with the soil still lying underneath.

What goes wrong when they are disturbed?

The acid leaches out with iron and aluminium from the ground and can reach drains and creeks, where it harms fish and plants. It also attacks concrete and steel left in the ground, so footings, slabs and buried pipes can wear out sooner.

How do I read an acid sulfate soils map?

A map shows likelihood, not a result for your yard. In New South Wales the council’s local plan carries a map of classes, and the class says what depth or kind of work brings the question. In Queensland the mapping comes from the state, and some councils carry it as an overlay. Other states publish their own risk maps. A lot off the map can still hold the soil. The post on planning overlays covers how mapped controls work.

What work brings the question?

a pool or anything else dug below the water table
deep footings, and long trenches for drains or services
pumping out groundwater so a dig stays dry, or a new drain that lowers the water table
filling or cutting the block, since fill on low ground can press into the soil below

A light shed on a slab with shallow footings often disturbs very little, though the council or the certifier decides whether a test is still wanted.

What happens if the lot is mapped?

A soil consultant or geotechnical engineer tests the ground at the depth of the work. If the soil is confirmed and the work reaches the trigger the state or council sets, a management plan says how the soil is dug, treated, often with lime, and disposed of. The footings and the concrete are then designed to the test. The council assesses the plan where it is needed, and the certifier reads the result into the building approval.

Reading it for your own lot

RapidSiteplan reads the council and building rules for your lot from its address, checks the design against them as you place it, lets you edit it as often as you like, and prints the site plan. Site plans and assessments in minutes. Where a state or council publishes the mapping, the class at your lot shows with the lot, so you know to ask for the soil test before the design settles.

Search your address, or see what the plan shows for a shed, carport or other outbuilding. The Moreton Bay page lists what we hold for that council, and site plans by council covers the rest of Australia.

Common questions

What are acid sulfate soils and can I build on them? They are natural soils holding iron sulfides, mostly on low coastal land, and they make acid when air reaches them. You can usually build on them. Where they are mapped, the soil is usually tested first.

How do I read an acid sulfate soils map? The map shows how likely the soil is at that place, and it varies by state. It is a reason to test, not a finding about your yard. The council can tell you which map it applies.

Does building on acid sulfate soils cost more? It can. The soil test is an added cost, and a management plan, treatment of the dug soil and tougher footings add more where the test confirms the soil.