Acid sulfate soils are naturally occurring soils, sediments or organic substrates (e.g. peat) that are formed under waterlogged conditions. These soils contain iron sulfide minerals (predominantly as the mineral pyrite) or their oxidation products. In an undisturbed state below the water table, acid sulfate soils are benign. However, if the soils are drained, excavated or exposed to air by a lowering of the water table, the sulfides react with oxygen to form sulfuric acid.
Release of this sulfuric acid from the soil can in turn release iron, aluminium, and other heavy metals (particularly arsenic) within the soil. Once mobilized in this way, the acid and metals can create a variety of adverse impacts: killing vegetation, seeping into and acidifying groundwater and surface water bodies, killing fish and other aquatic organisms, and degrading concrete and steel structures to the point of failure.
The soils and sediments most prone to becoming acid sulfate soils formed within the last 10,000 years, after the last major sea level rise. When the sea level rose and inundated the land, sulfate in the seawater mixed with land sediments containing iron oxides and organic matter. Under these anaerobic conditions, lithotrophic bacteria such as Desulfovibrio desulfuricans obtain oxygen for respiration through the reduction of sulfate ions in sea or groundwater, producing hydrogen sulfide. This in turn reacts with dissolved ferrous iron, forming very fine grained and highly reactive framboid crystals of iron sulfides such as (pyrite). Up to a point, warmer temperatures are more favourable conditions for these bacteria, creating a greater potential for formation of iron sulfides. Tropical waterlogged environments, such as mangrove swamps or estuaries, may contain higher levels of pyrite than those formed in more temperate climates.
The pyrite is stable until exposed to air, at which point the pyrite rapidly oxidises and produces sulfuric acid. The impacts of acid sulfate soil leachate may persist over a long time, and/or peak seasonally (after dry periods with the first rains). In some areas of Australia, acid sulfate soils that drained 100 years ago are still releasing acid.
When drained, pyrite (FeS2) containing soils (also called cat-clays) may become extremely acidic (pH < 4) due to the oxidation of pyrite into sulfuric acid (H2SO4). In its simplest form, this chemical reaction is as follows:
+ 9 O2
+ 4 H2
O → 8 H+
+ 4 SO42−
+ 2 Fe(OH)3
The product Fe(OH)3, iron(III) hydroxide (orange), precipitates as a solid, insoluble mineral by which the alkalinity component is immobilized, while the acidity remains active in the sulfuric acid. The process of acidification is accompanied by the formation of high amounts of aluminium (Al3+, released from clay minerals under influence of the acidity), which are harmful to vegetation. Other products of the chemical reaction are:
- Hydrogen sulfide (H2S), a smelly gas
- Sulfur (S), a yellow solid
- Iron(II) sulfide (FeS), a black/gray/blue solid
- Hematite (Fe2O3), a red solid
- Goethite (FeO.OH), a brown mineral
- Schwertmannite a brown mineral
- Iron sulfate compounds (e.g. jarosite)
- H-Clay (hydrogen clay, with a large fraction of adsorbed H+ ions, a stable mineral, but poor in nutrients)
The iron can be present in bivalent and trivalent forms (Fe2+, the ferrous ion, and Fe3+, the ferric ion respectively). The ferrous form is soluble, whereas the ferric form is not. The more oxidized the soil becomes, the more the ferric forms dominate. Acid sulfate soils exhibit an array of colors ranging from black, brown, blue-gray, red, orange and yellow. The hydrogen clay can be improved by admitting sea water: the magnesium (Mg) and sodium (Na) in the sea water replaces the adsorbed hydrogen and other exchangeable acidic cations such as aluminium (Al). However this can create additional risks when the hydrogen ions and exchangeable metals are mobilised.
Acid sulfate soils are widespread around coastal regions, and are also locally associated with freshwater wetlands and saline sulfate-rich groundwater in some agricultural areas. In Australia, coastal acid sulfate soils occupy an estimated 58,000 km2, underlying coastal estuaries and floodplains near where the majority of the Australian population lives. Acid sulfate soil disturbance is often associated with dredging, excavation dewatering activities during canal, housing and marina developments. Droughts can also result in acid sulfate soil exposure and acidification.
Acid sulfate soils that have not been disturbed are called potential acid sulfate soils (PASS). Acid sulfate soils that have been disturbed are called actual acid sulfate soils (AASS).
Disturbing potential acid sulfate soils can have a destructive effect on plant and fish life, and on aquatic ecosystems. Flushing of acidic leachate to groundwater and surface waters can cause a number of impacts, including:Ecological damage to aquatic and riparian ecosystems through fish kills, increased fish disease outbreaks, dominance of acid-tolerant species, precipitation of iron, etc.
Effects on estuarine fisheries and aquaculture projects (increased disease, loss of spawning area, etc.).
Contamination of groundwater and surface water with arsenic, aluminium and other metals.
Reduction in agricultural productivity through metal contamination of soils (predominantly by aluminium).
Damage to infrastructure through the corrosion of concrete and steel pipes, bridges and other sub-surface assets.
Potentially acid sulfate soils (also called cat-clays) are often not cultivated or, if they are, planted with rice, so that the soil can be kept wet preventing oxidation. Subsurface drainage of these soils is normally not advisable.
When cultivated, acid sulfate soils cannot be kept wet continuously because of climatic dry spells and shortages of irrigation water, surface drainage may help to remove the acidic and toxic chemicals (formed in the dry spells) during rainy periods. In the long run surface drainage can help to reclaim acid sulfate soils. The indigenous population of Guinea Bissau has thus managed to develop the soils, but it has taken them many years of careful management and toil.
In an article on cautious land drainage, the author describes the successful application of subsurface drainage in acid sulfate soils in coastal polders of Kerala state, India.
Also in the Sunderbans, West Bengal, India, acid sulfate soils have been taken in agricultural use.
A study in South Kalimantan, Indonesia, in a perhumid climate, has shown that the acid sulfate soils with a widely spaced subsurface drainage system have yielded promising results for the cultivation of upland (sic!) rice, peanut and soybean. The local population, of old, had already settled in this area and were able to produce a variety of crops (including tree fruits), using hand-dug drains running from the river into the land until reaching the back swamps. The crop yields were modest, but provided enough income to make a decent living.
Reclaimed acid sulfate soils have a well-developed soil structure; they are well permeable, but infertile due to the leaching that has occurred.
In the second half of the 20th century, in many parts of the world, waterlogged and potentially acid sulfate soils have been drained aggressively to make them productive for agriculture. The results were disastrous. The soils are unproductive, the lands look barren and the water is very clear, devoid of silt and life. The soils can be colorful, though.
When brickwork is persistently wet, as in foundations, retaining walls, parapets and chimneys, sulfates in bricks and mortar may in time crystallise and expand and cause mortar and renderings to disintegrate. To minimise this effect specialised brickwork with low sulfate levels should be used. Acid sulfates that are located within the subsoil strata has the same effects on the foundations of a building. Adequate protection can exist using a polythene sheeting to encase the foundations or using a sulfate-resistant Portland cement. To identify the pH level of the ground a soil investigation must take place.
By raising the water table, after damage has been inflicted due to over-intensive drainage, the soils can be restored. The following table gives an example.
Drainage and yield of Malaysian oil palm on acid sulfate soils (after Toh Peng Yin and Poon Yew Chin, 1982)
Yield in tons of fresh fruit per ha:
Drainage depth and intensity were increased in 1962. The water table was raised again in 1966 to counter negative effects.
In the "millennium drought" in the Murray-Darling Basin in Australia, exposure of acid sulfate soils occurred. Large scale engineering interventions were undertaken to prevent further acidification, including construction of a bund and pumping of water to prevent exposure and acidification of Lake Albert. Management of acidification in the Lower Lakes was also undertaken using aerial limestone dosing.