Acid Sulfate Soil

Rachel Jie Jiang
Acid Sulfate Soil

What is Acid Sulfate Soil (ASS)?

Acid sulfate soil is the common name given to soils and sediments containing iron sulfides (mainly iron sulfide, iron disulfide or their precursors). When acid sulfate soil is exposed to oxygen, sulfuric acid is generated. The majority of acid sulfate soils formed through natural process in the Holocene geotechnical period (approximately 10,000 years ago). Significant deposits of pyritic sediments formed in estuarine mud, where coastal water (containing sulfur) mixed with river water outflows which supplied a large amount of organic matter containing iron. ASS usually exists in mangroves, salt marshes, low-lying areas of coastal floodplains, estuaries and lakes.


Effect of ASS

- ASS has the potential to corrode or weaken concrete, iron, steel and certain metallic alloys.
- ASS can dissolve certain essential minerals from the soils resulting in toxicity to plants.
- ASS can cause severe bio-degradation of ecological habitat, soil and water quality, potentially resulting in the disease or death of fish or other aquatic organisms.


Types of ASS

Actual Acid Sulfate Soils (AASS)

Actual acid sulfate soils (AASS) or sediments contain highly acidic soil layers (sulfuric material) resulting from the aeration of soil materials that are rich in iron sulfides, primarily sulfide. Sulfuric material is composed of organic soil material which has pH < 4 can be usually identified by the occurrence of pale yellow jarosite mottles.

Potential Acid Sulfate Soils (PASS)

Potential acid sulfate soils (PASS) are soils which have not been exposed to air and oxidized. This material contains high iron sulfides or sulfidic material, which are waterlogged and rich in pyrite. PASS tend to be very dark and soft, blue grey or dark greenish grey coloured.

AASS and PASS are generally found in the same soil profile with AASS overlying PASS.



Acid sulfate soils have been classified based on the percentage of acid sulfate soil present in particular areas and within certain depths. According to NSW Acid Sulfate Soils Assessment Guidelines 1998, there are five classifications (see Table 1 below).

Table 1 Acid Soil Classifications

Class of land as shown on Acid

Sulfate Soils Planning Maps



 Any Works.


Works below natural ground surface;

Works by which the water table is likely to be lowered.


Works beyond 1 metre below natural ground surface;

Works by which the water table is likely to be lowered beyond 1 metre below natural ground surface.


Works beyond 2 metres below natural ground surface;

Works by which the water table is likely to be lowered beyond 2 metres below natural ground surface.


Works within 500 metres of adjacent Class 1, 2, 3, or 4 land which are likely to lower the water table below 1 metre AHD on adjacent Class 1, 2, 3 or 4 land.


ASS on site screening

Benbow Environmental (BE) has carried out several ASS assessments across Australia which include on-site screening to provide the most optimum remediation solutions to minimise the risk of exposure to ASS.

Following NSW ASS assessment guidelines, a number of acid sulfate soil samples were collected at various depths from boreholes and screened for indicators. The on-site screening involved the testing of each sample for pH and peroxide pH, using 30% hydrogen peroxide (H2O2) to oxidize the soils and comparing both results in order to obtain any positive reaction from the soil when mixed with 30% hydrogen peroxide.

The on-site pH test was undertaken using 1:5, Soil:Deinonised water. Field pH readings of ≤4 indicate that actual acid sulfate soil is present. Field pH value >4 and ≤5.5 are acidic and have high percentages of actual acid sulfate soils. Field pH values >5.5 are non-acidic and identify that soil as not being actual acid sulfate soil.

Soil samples from the same borehole are required to test pH value for presence of potential acid sulfate soil (un-oxidised sulfides). For instance, a positive PASS test may include one or more one of the following reactions:

- Soil solvent changes colour from grey tones to brown tones.
- Effervescence.
- Sample releases sulfurous odours.
- The lowering pH values of a final pH <3.5.

The soil samples which were indicated as positive in the on-site screening test were placed into sealed clean jars within 2 hours of sampling. The samples were then delivered to National Association of Testing Authorities Australia (NATA) certified laboratories for detailed analysis by using the Suspension Peroxide Oxidation Combined Acidity and Sulfate (SPOCAS) method.

The results of this study allowed the site to determine the extent of its acid sulfate soils on site, as well as provide correct remediation actions to minimise the effect of ASS. This is documented as an Acid Sulfate Soil Management Plan.

For further information please contact Rachel Jie Jiang: rjiejiang@benbowenviro.com.au