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 sulfideminerals (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 oxygento form sulfuric acidIdentification & Investigation of Acid Sulfate Soils (2006), Department of Environment, Western Australia. Retrieved from [http://portal.environment.wa.gov.au/portal/page?_pageid=53,84383&_dad=portal&_schema=PORTAL portal] ] .
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 groundwaterand water bodies, killing fishand other aquatic organisms, and degrading concreteand steelstructures to the point of failure .
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, lithotrophicbacteria such as "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 mangroveswamps or estuaries, may contrain higher levels of pyrite than those formed in more temperate climates.Acid Sulfate Soil Technical Manual 1.2 (2003), CSIRO Land & Water, Australia. Retrieved from [http://www.clw.csiro.au/staff/FitzpatrickR/barker_inlet_reports/Final_App1_coastal_ASS_tech_manual_v1.2.pdf CSIRO] ]
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.Sammut, J & Lines-Kelley, R. (2000) Acid Sulfate Soils 2nd edition, Environment Australia, ISBN 0-7347-1208-1. Retrieved from [http://www.environment.gov.au/coasts/cass/booklet.html booklet] ]
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 reactionis as follows:
:2 FeS2 + 9 O2 + 4 H2O → 8 H+ + 4 SO4= + 2 Fe(OH)3 (solid) D. Dent, 1986. Acid sulphate soils: a baseline for research and development. Publ. 39, ILRI, Wageningen, The Netherlands. ISBN 9070260 980. Free download from : [http://www2.alterra.wur.nl/UK/ilri/Products/ILRI+download.htm ILRI-Alterra] ]
The product Fe(OH)3, iron (III) hydroxide (orange), precipitates as a solid, insoluble mineral by which the
alkalinitycomponent is immobilized, while the acidityremains active in the sulfuric acid. The process of acidification is accompanied by the formation of high amounts of aluminium(Al+++, released from clay mineralsunder 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
Haematite(Fe2O3), a red solid
Goethite(FeO.OH), a brown mineral
Schwertmannitea brown mineral
# Iron compounds (e.g.
# H-Clay (
hydrogenclay, with a large fraction of adsorbed H+ ions, a stable mineral, but poor in nutrients)
The iron can be present in bivalent and
trivalentforms (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 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 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
salinesulfate-rich groundwater in some agricultural areas. In Australia, coastal acid sulfate soils occupy an estimated 80,000 km2, underlying coastal estuaries and floodplains near where the majority of the Australian population lives [Fitzpatrick R. W., Davies P.G., Thomas B. P., Merry R. H., Fotheringham D. G and Hicks W. S. (2002). Properties and distribution of South Australian coastal acid sulfate soils and their environmental hazards. 5th International Acid Sulfate Soils Conference, Tweed Heads, NSW] . Acid sulfate soil disturbance is often associated with dredging, excavation dewatering activities during canal, housing and marina developments.
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:
* Ecological damage to aquatic and
riparianecosystems through fish kills, increased fish disease outbreaks, dominance of acid-tolerant species, precipitation of iron, etc.
* Effects on estuarine fisheries and
aquacultureprojects (increased disease, loss of spawning area, etc).
* Contamination of groundwater with
arsenic, aluminiumand other heavy metals.
* Reduction in agricultural productivity through metal contamination of soils (predominantly by aluminium).
* Damage to
infrastructurethrough the corrosion of concrete and steel pipes, bridges and other sub-surface assets.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 drainageof these soils is normally not advisable.
When cultivated, acid sulfate soils cannot be kept wet continuously because of climatic dry spells and shortages of
irrigationwater, 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 [ R.J.Oosterbaan, 1981. Rice Polders Reclamation Project, Guinea Bissau. In: ILRI Annual Report 1980, p. 26–32, ILRI, Wageningen, The Netherlands. ] . The indigenous population of Guinea Bissauhas 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 [ R.J.Oosterbaan, 1992. Agricultural Land Drainage: A wider application through caution and restraint. In: ILRI Annual Report 1991, p.21–35, International Institute for Land Reclamation and Improvement, Wageningen, The Netherlands. View on line : [http://www.waterlog.info/pdf/caution.pdf] . ] 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 [ H.S. Sen and R.J. Oosterbaan, 1993. Research on Water Management and Control in the Sunderbans, India. In: ILRI Annual Report 1992, p. 8-26. International Institute for Land Reclamation and Improvement, Wageningen, The Netherlands. ] .
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. [ R.J.Oosterbaan, 1990. Review of water management aspects in Pulau Petak (near the town of Bandjermasin, Kalimantan, Indonesia). International Institute of Land Reclamation and Improvement, Wageningen, The Netherlands. View on line : [http://www.waterlog.info/pdf/pulaupetak.pdf] . ] . 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 leachingthat has occurred.
In the second half of the 20th century, in many parts of the world,
waterloggedand 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 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.
* Sammut, J., White, I. and Melville, M.D. (1996). Acidification of an estuarine tributary in eastern Australia due to drainage of acid sulfate soils. Marine and Freshwater Research 47, 669-684.
* Sammut, J., Melville, M.D., Callinan, R.B. and Fraser, G. (1995). Estuarine acidification: impacts on aquatic biota of draining acid sulphate soils. Australian Geographical Studies 33, 89-100.
* Wilson, B.P, White I. and Melville M.D. (1999). Floodplain hydrology, acid discharge and change in water quality associated with a drained acid sulfate soil. Marine and Freshwater Research. 50; 149-157.
* Wilson, B.P. (2005) Classification issues for the Hydrosol and Organosol Soil Orders to better encompass surface acidity and deep sulfidic horizons in acid sulfate soils. Australian Journal of Soil Research 43; 629-638
* Wilson, B.P. (2005) Elevations of pyritic layers in acid sulfate soils: what do they indicate about sea levels during the Holocene in eastern Australia. Catena 62; 45-56.
* [http://www.environment.gov.au/coasts/cass/index.html Australian Department of Environment & Heritage National Coastal Acid Sulfate Soil website]
* [http://www.nrw.qld.gov.au/land/ass/index.html Queensland Department of Natural Resources & Water Acid Sulfate Soils website]
* [http://www.dec.wa.gov.au/ass Western Australian Department of Environment Acid Sulfate Soils website]
* [http://www.waterlog.info/faqs.htm] Frequently asked questions, question 3: documentation on acid sulfate soils
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