ACID SULFATE SOILS

Is your worksite in a coastal, low lying area?

What are Acid Sulfate Soils?

Acid Sulfate Soils formed at the end of the last ice age when the sea level rose. Soil that was rich in iron or organic material became waterlogged with the sea water and sulfide from the water reacted with the iron from the soil creating iron sulfides. As long as the soil remains in its waterlogged state the pH remains neutral.

When do problems arise?

Problems arise when waterlogged Iron Sulfide rich soils are exposed to the atmosphere. The Iron Sulfide oxidises in the presence of air to form Sulfuric Acid (H2SO4). This process typically occurs during construction work, especially the construction of canalled waterfront developments from low lying land, where soil is excavated to form canals, and the soil is then piled on top of the nearby land. As the soil dries out it oxidises on interaction with the air.

An apparently small quantity of soil can, on oxidation, generate a large quantity of sulfuric acid which runs into the water and kills aquatic life. The resultant pH can become very low.

What other problems are caused?

Acid from acid sulphate soil can quickly attack concrete, and if building in acid sulfate soils it may be necessary to use acid resistant concrete.

What does it look like?

Acid Sulfate Soils can be classified into Potential Acid Sulfate Soils and Actual Acid Sulfate Soils. Potential Acid Sulfate Soil is not necessarily acidic but has the potential to become acidic. These soils are in a waterlogged state where the water prevents the oxidation of the Iron Sulfide. Actual Acid Sulfate Soils are Potential Acid Sulfate Soils that have been exposed to the atmosphere via a dropping water table or other mechanical disturbance.

Potential Acid Sulfate Soil looks like wet dark grey mud. Actual Acid Sulfate Soil can be yellow in appearance. The yellow colouring is formed by a mineral called jarosite (Potassium Iron Sulfate Hydroxide). Its presence is indicative of sulfates in the soil oxidising.

Can I tell if the area is likely to have Acid Sulfate Soils?

Naturally low lying coastal areas, especially those with casuarinas and melaleucas, are those likely to have Acid Sulfate Soils. In addition, Getex has maps of the coastal regions with areas likely to have Acid Sulfate Soils at various depths shown in graded colours.

I want to develop land that may be affected by acid sulfate soils - what requirements might I have to follow?

If you propose to carry out works where it is likely that

You will disturb more than 1 ton of soil, during for example the excavation of soil for the creation of basement parking and/or building foundations, drain maintenance or construction and dredging operations.
The works are expected to be lower than the water table.
Works are expected to lower the water table, such as occurs in the construction or maintenance of drains and the use of pumps to lower the natural groundwater level within and/or around a development site. Note: if your property is in “Class 5” land then activities on your property (such as those indicated above) may have an impact on Class 1-4 lands that adjoin your property, i.e. alter the ground water level below 1 metre Australian Height Datum (AHD).

If your site has been identified as likely to be affected by Acid Sulfate Soil on an Acid Sulfate Soil map

Then Council approval, as far as Acid Sulfate Soils are concerned, will likely to be required.

What can be done?

Getex can test for the presence of Acid Sulfate Soil. Holes are drilled or dug to a minimum of 1 metre below the maximum planned depth of excavation. Samples are taken and field tested by oxidation with concentrated peroxide. The samples which field test positive are sent to a lab for more testing.

If the results are positive the soil must be managed correctly if disturbance is required. The excavated soil must be limed before disposal. Getex can advise on the correct procedures and the correct quantities of lime and can provide Management Plans for handling the soil.

Contact GETEX for assistance

GETEX OPERATES 24/7

02 9889 2488