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Acid sulfate soil
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 will 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 water bodies, killing fish and other aquatic organisms, and degrading concrete and steel structures to the point of failure .
Additional recommended knowledge
Acid sulfate soil formation
The soils and sediments which are most prone to becoming acid sulfate soils are those which 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 has Thiobacillus ferrooxidans form iron sulfides (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 contrain higher levels of pyrite than those formed in more temperate climates.
The pyrite is stable until it is exposed to air, at which point the pyrite 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:
The product Fe(OH)3, iron (III) hydroxide (orange), precipitates as a solid, insoluble mineral by which the alkali 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 (Al+++, released from clay minerals under influence of the acidity), which are harmful to vegetation. Other products of the chemical reaction are:
The iron can be present in bivalent and trivalent forms (Fe++, the ferro ion, and Fe+++, the ferri ion respectively). The ferro form is soluble, whereas the ferri form is not. The more oxidized the soil becomes, the more the ferri forms will dominate. Acid sulphate 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 hydrogen adsorbed will be replaced by the magnesium (Mg) and sodium (Na) present in the sea water.
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 40,000 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.
Acid sulfate soils which have not been disturbed are known as potential acid sulfate soils (PASS); acid sulfate soils which have been disturbed are known as actual acid sulfate soils (AASS).
Impacts of acid sulfate soil
Disturbing potential acid sulfate soils can have a destructive effect on plant and fish life, and on coastal ecosystems. Flushing of acidic leachate to groundwater and surface waters can cause a number of impacts, including:
Source: Sammut & Lines-Kelly, 2000.
Potentially acid sulfate soils (also called cat-clays) are often not cultivated or, if they are, planted under 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. The article can be downloaded from his website].
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, pea nut and soy bean.  . 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 cat-clays 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.
Acid sulfate soil restoration
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 sulphate 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 raised again in 1966 to counter negative effects.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Acid_sulfate_soil". A list of authors is available in Wikipedia.|