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) and/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 otherwise exposed to air, the sulfides react with oxygen to form sulfuric acid.Identification & Investigation of Acid Sulfate Soils (2006), Department of Environment, Western Australia. Retrieved from portal
Release of this sulfuric acid from the soil can in turn release iron, aluminium, and other heavy metals and metalloids (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 groundwaterMosley LM, Palmer D, Leyden E, Fitzpatrick R, and Shand P (2014). Changes in acidity and metal geochemistry in soils, groundwater, drain and river water in the Lower Murray River after a severe drought. Science of the Total Environment 485–486: 281–291. and surface water bodies, killing fish and other aquatic organisms, and degrading concrete and steel structures to the point of failure.
Formation
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.Acid Sulfate Soil Technical Manual 1.2 (2003), CSIRO Land & Water, Australia. Retrieved from CSIRO
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 (e.g., Gillman wetlands, South Australia).Thomas, B.P., 2010. Coastal acid sulfate soil processes in Barker Inlet, South Australia, The University of Adelaide, Australia.
Generalised Pyrite Oxidation Reaction When drained, soils containing reduced inorganic sulfides such as pyrite may become extremely acidic (pH < 4) due to the oxidation of sulfides into sulfuric acid (H2SO4):
4 FeS2 + 15 O2 + 14 H2O -> 16 H+ + 8 SO4^2- + 4 Fe(OH)3 (v)D. Dent, 1986. Acid sulphate soils: a baseline for research and development. Publ. 39, ILRI, Wageningen, The Netherlands. . Free download from :
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 foul-smelling 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 Iron sulfate compounds (e.g., jarosite, schwertmannite, gypsum, and epsomite) 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 in a relatively wide range of pH conditions whereas the ferric form is not soluble except in an extremely acidic environment such as muriatic acid rust remover. 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.
Geographical distribution 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.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, NSWFitzpatrick, R., Marvanek, S., Powell, B., Grealish, G., and Gilkes, R. (2010). Atlas of Australian Acid Sulfate Soils: recent developments and future priorities. In "Proceedings of the 19th World Congress of Soil Science: Soil solutions for a changing world. Brisbane, Australia, 1–6 August 2010" (R. Gilkes and N. Prakongkep, eds.), pp. 24-27. Published on DVD; ; http://www.iuss.org ; Symposium WG 3.1 Processes in acid sulfate soil materials. 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.
Impact 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:Sammut, J & Lines-Kelley, R. (2000) Acid Sulfate Soils 2nd edition, Environment Australia, 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.Mosley, L., Fleming, N., 2010. Pollutant Loads Returned to the Lower Murray River from Flood-Irrigated Agriculture. Water Air Soil Pollut. 211, 475-487. 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.
Agricultural Impacts
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.Rice Polders Reclamation Project, Guinea Bissau. In: Annual Report 1980, p. 26–32, International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands. Download from web page , under nr. 12, or directly as PDF : 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,Agricultural Land Drainage: A wider application through caution and restraint. In: Annual Report 1991, p.21–35, International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands. Download from web page : , under nr. 3, or directly as 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: Annual Report 1992, p. 8-26. International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands. Download from web page : , under nr. 2, or directly as PDF :
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 rice, peanut and soybean.Review of water management aspects in Pulau Petak (near the town of Bandjermasin, Kalimantan, Indonesia). Mission Report 39, Research Project on Acid Sulphate (Sulfate) Soils in the Humid Tropics. International Institute of Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands. Download from web page : , under nr. 7, or directly as 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 acid sulfate soils have a well-developed soil structure thanks to the abundance of trivalent cations (mainly Al+3) which have a very strong flocculating effect; 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 (again, due to the flocculating effect of Al+3), devoid of silt and life. The soils can be colorful, though.
Construction 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.
Restoration and Management 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: Year 60 61 62 63 64 65 66 67 68 69 70 71 Yield 17 14 15 12 8 2 4 8 14 19 18 19
Drainage depth and intensity were increased in 1962. The water table was raised again in 1966 to counter negative effects.