Acid Sulfate Soils

See also: How Clean Is Clean?

Acid sulfate soils are the common name given to soils containing iron sulfides. In Australia, the acid sulfate soils of most concern are those which formed within the past 10,000 years, after the last major sea level rise.

When the sea level rose and inundated land, sulfate in the sea water mixed with land sediments containing iron oxides and organic matter. The resulting chemical reaction produced large quantities of iron sulfides in the waterlogged sediments. When exposed to air, these sulfides oxidise to produce sulfuric acid, hence the name acid sulfate soils.

The iron sulfides are contained in a layer of waterlogged soil. This layer can be clay or sand, and is usually dark grey and soft. The water prevents oxygen in the air reacting with the iron sulfides.

When the iron sulfides are exposed to air they oxidise and produce sulfuric acid. The acid moves through the soil, acidifying soil water, groundwater and, eventually surface waters.

It is estimated that there are more than two million hectares of acid sulfate soils in Australia containing about one billion tonnes of iron sulfides. One tonne of iron sulfides can produce about 1.5 tonnes of sulfuric acid when oxidised.

Iron sulfide layers were formed under tidal conditions, so they are found in low-lying areas near the coast. They are still being formed today in mangrove forests and salt marshes, estuaries and tidal lakes. In general, we expect to find iron sulfide layers where the surface elevation is less than five metres above mean sea level.

The main areas are the New South Wales, Queensland, Northern Territory and North Western Australian coastlines, with some near the other capital cities.

WHAT’S THE PROBLEM ?

There’s not one but many problems.

* Sulfuric acid produced by acid sulfate soils corrodes concrete, iron, steel and certain aluminium alloys. It has caused the weakening of concrete structures and corrosion of concrete slabs, steel fence post, foundations of buildings and underground concrete water and sewerage pipes.

* The release of sulfuric acid can poison large areas of soil and waterways, devastating fish stocks and other aquatic life.

* Disposal of acid sulfate soils is a big headache.

* Even when undisturbed, few plant species will grow in these soils.

* Planning approval is required for works in these soils under some conditions.

CAN YOU USE LIME ?

Yes but when one does the sums it usually takes a lot of lime. Mixing the lime into the soil is not that simple either.

HOW CAN NEW ENVIRONMENT HELP ?

New Environment can advise on which locations may have acid sulfate soils and can sample and test for acid sulfate soil if a problem is suspected. We can advise on soil management and the recently introduced regulatory requirements.