Introduction Acid Sulphate Soil (ASS) is the common term given to naturally-occurring soils and sediments that contain iron sulphides such as pyrites (FeS2) which is not visible to naked eyes. Under undisturbed waterlogged conditions, these soils are relatively harmless. However, when drained by mean of excavation or dewatering the soils (lowering the water table), it is exposed to air which lead to the reaction of iron sulphides in the soils with oxygen and water to produce toxic quantities of metal compounds and sulphuric acid. This in turn releases metal ions such as aluminium, iron and heavy metals (particularly arsenic) within the soil, which can be easily mobilised by both surface and subsurface water flow into the surrounding soils and waterways.

Acid sulphate soils have been formed since millions of years ago and occur in ancient marine rocks, however those of most concern were formed after the last major sea level rise, within the past 10 000 years (the Holocene epoch). They commonly occur on coastal wetlands as layers of Holocene marine mud and sands deposited in protected low-energy environments such as barrier estuaries and coastal lakes. At present time, under similar environmental conditions, layers of iron sulphate are still being formed in the soils.

Deposits of acid sulphate soils can be found less than five metres above sea level, particularly in low-lying coastal areas. Ideal places for acid sulphate soil formation includes mangroves, salt marshes, floodplains, swamps, wetlands, estuaries and brackish or tidal lakes. As acid sulphate soils are regularly buried beneath layers of more recently deposited soils and sediments of alluvial or aeolian origin, their presence may not be obvious on the soil surface.

Schwertmannite, one of the oxidification product and the most common acid sulphate soils landscapes.
Pyrite – principal component of acid sulphate soils.

Acid Sulphate Soils exposed to the air

Formation

Acid Sulphate Soil usually form on low lying area of coastal flood plains or estuarine conditions where seawater or sulphate-rich water mixes with land sediments containing iron oxides and organic matter in a waterlogged situation, under anaerobic conditions. Certain bacteria that form pyrite flourish in this condition and up to a point, the warmer the temperatures, the more favourable are the conditions for these bacteria to react and therefore the greater the potential for iron sulphides to form. Due to this, tropical waterlogged environments (mangrove swamps or estuaries) may contain higher levels of pyrite than those formed in more temperate climates. Pyrites react rapidly when exposed to oxygen due to its large surface area.

Requirement for pyrite to form: * a supply of sulphur * anaerobic condition * energy supply for bacteria (organic matter) * a system to remove reaction products (tidal flushing) * a source of iron (land sediments) * temperature greater than 10°C

Types of acid sulphate soils
There are two types of acid sulphate soils:

a) Potential acid sulphate soils (PASS)

The undisturbed soils or sediments that contain unoxidise iron sulphides with the potential to oxidise and produce sulphuric acid when drained or excavated. It often show a pH close to neutral (6.5-7.5). PASS are usually soft, sticky and saturated with water.

b) Actual acid sulphate soils (AASS)

Soils or sediments that once contained sulphides but have already undergone oxidation to produce acid , therefore resulting a soil pH of less than 4. In soil profile, they will display yellow/orange mottling, characterised by the presence of jarosite, a by-product of the oxidation process. If these soils still contain sulphides and are allow to oxidise further, they have the tendency to produce more acids. The texture of AASS can vary.

Generally in soil profile, the AASS will overlie the PASS.

Impact of Acid