What are acid sulfate soils?
Scanning Electron Microscope image of microscopic pyrite crystals
Acid sulfate soil (ASS) is the common name given to soils and sediments containing iron sulfides, the most common being pyrite. When exposed to air due to drainage or disturbance, these soils produce sulfuric acid, often releasing toxic quantities of iron, aluminium and heavy metals.
- Acidification process
- Potential acid sulfate soils
- Actual acid sulfate soils
- Acid sulfate soils
- Land uses on acid sulfate soils
Although some acid sulfate soils (ASS) were formed millions of years ago and occur in ancient marine rocks, those of most concern were formed after the last major sea level rise, within the past 10 000 years (the Holocene epoch). They commonly occur on coastal wetlands as layers of Holocene marine muds and sands deposited in protected low-energy environments such as barrier estuaries and coastal lakes. In similar environments, they are still being formed.
Acid sulfate soils are formed when seawater or sulfate-rich water mixes with land sediments containing iron oxides and organic matter in a waterlogged situation, in the absence of oxygen.
Under these anaerobic conditions, certain bacteria that help form pyrite (the reactive component of ASS) flourish. Up to a point, the warmer the temperatures, the more favourable are the conditions for these bacteria, and the greater the potential for formation of iron sulfides. Thus ASS found in tropical areas of Queensland may contain higher levels of iron sulfides than those formed in the cooler southern states.
Though the pyrite in acid sulfate soils is microscopic, it has a very large surface area and therefore reacts rapidly when exposed to oxygen. Pyrite particles in mining situations are larger and will still react with oxygen, but the reaction times are much slower.
Formation of pyrite
For pyrite to form, it requires:
- a supply of sulfur (usually from seawater)
- anaerobic (oxygen free) conditions
- a supply of energy for bacteria (usually rotting organic matter e.g. mangrove leaves)
- a system to remove reaction products (e.g. tidal flushing of the system)
- a source of iron (most often from terrestrial sediments)
- temperatures greater than 10ºC
The pH scale (see diagram) is used to measure the acidity of a solution. It is actually a measure of the hydrogen ion (H+) activity and is a logarithmic scale ranging from 0 (strongly acidic) to 14 (strongly alkaline). Neutral solutions are pH 7. Because it is a logarithmic scale, a soil with a pH of 4 is 10 times more acidic than a pH 5 soil and 1000 times more acidic than a pH 7 soil.
When ASS are exposed to air, (that is, no longer in a waterlogged anaerobic state), the iron sulfides in the soil react with oxygen and water to produce a variety of iron compounds and sulfuric acid. Initially a chemical reaction, the process is accelerated by bacteria such as Acidithiobacillus ferrooxidans. A simplified chemical equation summarises the oxidation process in acid sulfate soils.
Many other reactions and products such as jarosite KFe3(SO4)2(OH)6 can be produced. Both major products in the above equation are detrimental to the environment. The problem is exacerbated as the generated acid attacks the fine clay particles present in the soil, resulting in the release of soluble forms of aluminium (Al) which can then move into groundwater, drains and water bodies. The acid can also solubilise manganese and other heavy metals, resulting in a toxic brew being released into the environment.
Potential acid sulfate soils
Potential acid sulfate soils being excavated from depth, in contrast with the brown, well-drained surface soil
ASS are not always a problem. Under the anaerobic reducing conditions maintained by permanent groundwater, the iron sulfides are stable and the surrounding soil pH is often weakly acid to weakly alkaline. Such soils are called potential acid sulfate soils (PASS) as they have potential to produce sulfuric acid when disturbed or exposed to air.
Potential acid sulfate soils:
- often have a pH close to neutral (6.5–7.5)
- contain unoxidised iron sulfides
- are usually soft, sticky and saturated with water
- are usually gel-like muds but can include wet sands and gravels
- have the potential to produce acid if exposed to oxygen.
Actual acid sulfate soils
Actual acid sulfate soil containing jarosite
When PASS are disturbed or exposed to oxygen, the iron sulfides are oxidised to sulfuric acid and the soil becomes strongly acidic (usually below pH 4). These soils are then called actual acid sulfate soils (AASS) (that is, they are already acidic).
Actual acid sulfate soils:
- have a pH of less than 4 (i.e. they are already acid)
- contain oxidised iron sulfides
- vary in texture
- often contain jarosite (a yellow mottle produced as a by-product of the oxidation process).
Acid sulfate soils
Actual and Potential acid sulfate soils are often found in the same profile
The term acid sulfate soils generally includes both actual and potential acid sulfate soils, which often occur in the same soil profile. AASS usually overlie PASS.
In the soil profile pictured there is a dark organic-rich surface soil from 0 to 0.15 m; an oxidised horizon with orange mottling from 0.15 to 0.35 m; and an AASS horizon from 0.35 to 0.90 m, characterised by the presence of yellow jarosite. The PASS horizon, which extends from 0.9 m downwards consists of dark grey, wet, marine mud.
Land uses on acid sulfate soils
An estimated 2.3 million ha of ASS occur along 6500 km of the Queensland coastline. Many of these areas are under pressure for agricultural and urban development. This can disturb ASS and release sulfuric acid, which may drain into adjacent waterways after heavy rain.
The problems associated with disturbance of ASS are often long-term and difficult, if not impossible, to reverse. Since 700 ha of land within sight of Cairns was drained in 1976, CSIRO scientists estimate 72 000 tonnes of acid have been washed into Trinity Inlet. The implications are enormous for marine life, recreational and commercial fisheries industries, the recreation industry, urban usage, agricultural land development on riverine or delta areas and aquaculture. A government funded project is now underway to rehabilitate the East Trinity site.
Fitzpatrick RW, Fritsch E and Self PG (1996). Interpretation of soil features produced by ancient and modern process in degraded landscapes; V. Development of saline sulfidic features in non-tidal seepage areas. Geoderma 69,1–29.
Last reviewed 16 December 2011
Last updated 30 October 2007