Acid Sulfate Soils - Explained

What causes acid sulfate soils?

Acid Sulfate Soils are naturally occurring soils and sediments.  The majority of acid sulfate sediments were formed by natural processes when certain conditions existed in the Holocene geological period (the last 10,000 years).

Acid Sulfate Soils need the following conditions to develop:

• The presence of iron-rich sediments;
• Sulfate (usually from seawater);
• Removal of reaction products such as bicarbonate;
• The presence of sulfate reducing bacteria; and
• A plentiful supply of organic matter.

These conditions exist in coastal alluvial (riverine) sediments such as those found in

mangroves, salt marsh vegetation or tidal areas, and at the bottom of coastal rivers and lakes. They can also be present in inland estuarine environments, stormwater detention basins and other man-made structures where organic matter can accumulate in anaerobic conditions.

Is Acid Sulfate Soil Dangerous? 

Acid sulfate soils pose a considerable risk to the environment and have a number of subsequent associated issues for land-owners, conservationists, developers and infrastructure managers.

Disturbance of acid sulfate soils can generate large amounts of sulfuric acid, aluminium, iron, and occasionally heavy metals. These minerals have the potential to cause significant harm to both the natural and built environment in a number of ways:

• Increase in acidification of water causing poisoning of flora and fauna; at lower levels this causes weakening of aquatic animals and plants making them more vulnerable to disease and destruction.
• Similar impacts on flora and fauna are caused by Aluminium compounds (which are toxic to fish) and Aluminium ions damage root systems limiting plant growth.
• Sulfuric acid also poses a risk to concrete and steel, where it can gradually cause decay to bridges, roads, building foundations and pipes. Failure to treat acid sulfate soils effectively before construction, (expensive) can result in the need for expensive repairs, reduced lifespan of buildings and infrastructure and more intensive maintenance and monitoring regimes.
• Although iron isn’t toxic in and of itself, heave concentration of iron in water results in foul smelling and tasting water.  
• High concentration of Iron minerals also negatively impact vegetation through the formation of orange scum that covers vegetation on water-courses, stains infrastructure and can also clog water pumps and damage boats.
• When Iron also combines with organic matter and flows out to sea it can fuel blooms of toxic algae.

What are some of the signs of Acid Sulfate Soils?

The indicators for Actual Acid Sulfate Soils and Potential Acid Sulfate Soils are quite different. The field inspection should investigate for the presence of both soils. Commonly, actual acid sulfate soils are found overlaying potential acid sulfate soils and both are usually covered by non-acid sulfate alluvial topsoil. 

Visual indicators of Acid Sulphate Soils include:

• Iron staining and iron dependent (oxidising) bacteria;
• Oil like slicks. Unlike oil though, this slick will break into smaller segments when agitated;
• Dark grey/green muds or dark grey sands from below the water table;
• Monosulfidic black oozes;
• Acid tolerant vegetation species, marine vegetation, mangrove, casuarina, melaleuca;
• Ponded water is often clear with a slight blue tinge.

How can Acid Sulfate Soils be identified? 

Acid Sulfate soils are often covered by non-acidic topsoil. If you are planning on developing land within an area that has already been identified as at risk of acid sulfate soils, you will need to have an investigation of the soil completed by an environmental professional with suitable experience.

A competent investigation would include the following elements:

• A desktop assessment and site inspection
• Soil sampling
• Laboratory analysis of samples
• A report on the findings including an Acid Sulfate Soil Management plan (ASSMP).

Suitably Experienced Environmental Professional

Investigating Actual Sulphate Soil sites can be a complex process requiring the assistance of a suitably experienced environmental professional. This person may be referred to as a suitably qualified person (SQP) or qualified person (QP) in various legislation/guidelines. This professional should satisfy the following:

• An appropriate tertiary qualification in one or more of the following: soil science, geology, hydrogeology, environmental science or environmental engineering;
• Accreditation under a recognised scheme such as Certified Professional Soil Science, Certified Environmental Practitioner Scheme and International Association of Hydrogeologists;
• At least 10 years’ experience in a relevant field of practice that includes assessment and management of acid sulfate soils in various land use settings.

It is important to note that there is no specialist category for an appropriate professional in the assessment and management of acid sulfate soils. The duration of experience is the primary means for determining competency.

What is an Acid Sulfate Soil Management Plan (ASSMP)? 

Following the investigation, you will be provided with an Acid Sulfate Soil Management Plan (ASSMP). This will set out the steps to safely manage any acid sulfate soils found. The plan will need to account for; impacts on the environment, a strategy for soil removal, a contingency plan if acidification does occur and an ongoing monitoring program before, during and after development. The ASSMP will need to be approved before any work that may disturb acid sulfate soils can begin. Once the project is completed, a closure report will need to be prepared detailing how the acid sulfate soils were managed and clearly laying out any ongoing management conditions.