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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 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 (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〔Mosley 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.〕〔Mosley LM, Palmer D, Leyden E, Fitzpatrick R, and Shand P (2014). Acidification of floodplains due to river level decline during drought. Journal of Contaminant Hydrology 161: 10–23.〕 and surface water bodies,〔Mosley LM, Zammit B, Jolley A, and Barnett L (2014). Acidification of lake water due to drought. Journal of Hydrology. 511: 484–493.〕〔Mosley LM, Zammit B, Jolley A, Barnett L & Fitzpatrick R (2014). Monitoring and assessment of surface water acidification following rewetting of oxidised acid sulfate soils, Environmental Monitoring and Assessment 186: 1–18〕 killing fish and other aquatic organisms, and degrading concrete and steel structures to the point of failure.〔
== Acid sulfate soil 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.〔Sammut, J & Lines-Kelley, R. (2000) Acid Sulfate Soils 2nd edition, Environment Australia, ISBN 0-7347-1208-1〕
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