Granny Flats and Acid Sulphate Soils

Acid sulfate soils

Acid sulfate soils (ASS) occur naturally in many parts of Australia and are harmless when left in a waterlogged, undisturbed environment. However, when exposed to air, through drainage or excavation, the iron sulfides in the soils react with oxygen and water to produce iron compounds and sulfuric acid.  This acid can release other substances, including heavy metals, from the soil and into the surrounding environment and waterways.

Activities with the potential to disturb ASS must be managed carefully to avoid serious environmental harm. When acid sulfate soils are disturbed, they can generate large amounts of sulfuric acid, iron, aluminium and sometimes heavy metals. This can cause major impacts to the environment and to infrastructure.

When we build Granny Flats we research your property for ASS.
More info and how to manage ASS here:


Impacts of acidity

When acidity builds up to high levels in water, it poisons plants in and around affected creeks and ponds. It can also kill fish and other aquatic creatures if they are unable to escape.

Lower levels of acidity will simply make aquatic plants and animals weaker and more vulnerable to disease, and make it harder for young organisms to reach adulthood. Over time, sensitive species may be driven out and replaced by stronger, acid-tolerant invaders.

One example is mosquitoes, which can tolerate acidic water much more easily than the insects that prey on them. Acidified wetlands can therefore be a source of mosquito plagues. Acidic water is unhealthy for drinking and can cause skin irritation.

In Queensland any incident involving a fish kill should be reported by calling the Queensland Government on 1300 130 372 and selecting option 2.

Sulfuric acid can also attack concrete and steel, slowly destroying pipes, roads, bridges, and building foundations. In areas where acid sulfate soils are not treated properly before construction, (expensive) repairs may be required, or infrastructure may need to be replaced well before the end of its intended lifespan.

Impacts of iron

While iron is not toxic in itself, iron-laden water smells and tastes foul.

Iron can be released both by pyrite breakdown and by acid attack on the soil, and can travel long distances in groundwater.

Iron minerals precipitate out of acid-sulfate-affected surface water as it flows downstream, forming an orange scum that smothers vegetation on banks and stains built structures. The scum can also clog water pumps and damage boats.

When iron is complexed with organic matter, it can flow out to sea and fuel blooms of toxic algae.

Impacts of aluminium

Aluminium is the most common element in the earth’s crust. While it is safe when bound up in rocks and soil minerals, it can be damaging when released into water by acid.

Aluminium hydroxide compounds are toxic to fish, affecting their gills and their ability to absorb oxygen.

Aluminium ions also hamper plant growth, damaging root systems. Aluminium toxicity can affect both natural ecosystems and crops like sugarcane, which is often grown on low-lying coastal land.

Impacts of heavy metals

As acid attacks the soil structure and releases iron and aluminium, it will also release any other metals attached to soil minerals.

Many elements that are stable at neutral pH become mobile under acidic conditions, and can be toxic to plants and/or animals, including humans. Arsenic is one example, as are zinc, lead and manganese.

Impacts on the physical soil

Construction projects in acid sulfate soils areas can be difficult to manage even when the soil is not acidified.

As many acid sulfate soils are unconsolidated estuarine muds and clays with gel-like properties and low load-bearing capacity, foundations or earthworks built on these materials may settle or subside unevenly and slowly.

Careful engineering is required to avoid problems with subsidence, which can cause roadways to slump and foundations to crack.

Impacts on health

Though not yet fully documented, the possible health impacts of acid sulfate soils are under investigation. Effects could include:

  • stunted growth, poor health and mental impairment caused by drinking or bathing in aluminium-rich waters
  • dermatitis as a result of skin contact with acid soil materials.

Heavy metals in acid sulfate soils can also become soluble when sulfuric acid is produced, ending up in toxic quantities in leachate and nearby waterbodies.

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