Is my block mapped for acid sulfate soils?
Written by Shea Cullen, architect, for Dudils.
Acid sulfate soils lie under much of the low lying coastal land in New South Wales. Left wet and undisturbed they cause no trouble, but once dug up or drained they release sulfuric acid that can attack concrete and steel and harm the waterways downstream. Type an address and the Site Checker reads the acid sulfate soils class mapped for your lot from the NSW Planning Portal, in the other constraints section of the report.
NSW addresses only. Free, and no sign up.
What are acid sulfate soils?
They are soils and sediments containing iron sulfides, mostly pyrite, that formed under estuaries, mangroves, swamps and tidal flats over roughly the last 10,000 years. While they stay waterlogged they are stable. When digging or a lowered water table lets air reach them, the sulfides oxidise into sulfuric acid, which can leach iron, aluminium and heavy metals into drains and creeks and corrode concrete and steel in footings and pipes.
They are mostly found on coastal land below about 5 metres above sea level, which is why coastal councils map them and inland councils mostly do not.
What does class 1 to 5 mean on my block?
The class says how deep works have to go before the rules are triggered, and the Site Checker shows the class mapped for your lot. The classes come from the Acid Sulfate Soils Map and the acid sulfate soils clause of your council's Local Environmental Plan. The clause is 6.1 in many LEPs, but the number varies, so search your LEP for the words acid sulfate.
Class 1: any works. Class 2: works below the natural ground surface, or works likely to lower the water table. Class 3: works more than 1 metre below the natural ground surface, or likely to lower the water table more than 1 metre below it. Class 4: the same as class 3 at 2 metres.
Class 5: works within 500 metres of class 1 to 4 land that sits below 5 metres AHD (the Australian Height Datum, roughly mean sea level), where the works are likely to lower the water table below 1 metre AHD on that land. Class 5 land itself rarely holds acid sulfate soils; it is mapped because works on it can drain the land next door.
Do I need an assessment before I dig footings or a pool?
Only if the works reach the depth your class sets. On class 3 land, strip footings 600 millimetres deep do not trigger the clause, but a pool, a basement, deep piers or a trench more than a metre down may. Dewatering, which means pumping groundwater out of an excavation, lowers the water table and can trigger it on its own, even when the dig is shallow.
Where the clause is triggered, the works need development consent and an acid sulfate soils management plan. The exception is where a preliminary assessment by a suitably qualified consultant shows the soils are not present, or will not be disturbed, and council confirms that in writing. Get that assessment early: it is quicker than a management plan and decides whether you need one.
What does the acid sulfate class not tell me?
The map is a regional prediction of where the soils are likely to be, drawn from landform and elevation, not a test of your lot. A class 2 lot can turn out to have none at the depth you dig, and land off the map can still hold them, particularly near estuaries. Only a soil test by a geotechnical or environmental consultant says what is actually in the ground.
The class also says nothing about other ground conditions such as reactive clay, old fill or a high water table, which the geotechnical report for your footings covers. On a low lying coastal block, check the flood reading on the same report as well, because the land that holds these soils is often the land that floods.
Will acid sulfate soils add to my building costs?
Only when the works go deep enough to disturb them. A house on shallow footings above the trigger depth usually costs the same as one elsewhere. Where the soils are disturbed, the costs are the assessment, treating excavated soil with agricultural lime before it is reused or taken away, and managing any water pumped from the dig. The structural engineer may also specify concrete and steel rated for acidic ground. Have the soil tested before a pool or a basement is priced, so the quote includes the treatment rather than adding it later as a contract variation.
Is my block mapped for acid sulfate soils? Check any NSW address, free.
NSW addresses only. Free, and no sign up.
What else do people ask?
Is class 5 acid sulfate soil a problem for a house?
Rarely. Class 5 is mapped because works there could lower the water table under nearby class 1 to 4 land. A house on ordinary footings that does not pump groundwater is unlikely to trigger the clause.
Does acid sulfate soil affect a house that is already built?
Not usually. The soils cause trouble when they are disturbed or drained, so the question arises when you dig, drain or add a pool, not while the house simply stands on them.
Who treats acid sulfate soils on a building site?
The builder or excavation contractor, following the management plan prepared by the consultant. The plan sets the lime rates, where treated soil can go and how the water is monitored.
Where does this information come from?
The Site Checker reads public maps. It is not a section 10.7 planning certificate, a flood certificate, a bushfire attack level assessment or a survey, and a map can be out of date for your lot. Confirm anything you rely on with council or the relevant authority before you buy, design or build. Nothing on this page replaces advice from your certifier, engineer, lawyer or your own designer.