What are coastal acid sulfate soils?

Coastal acid sulfate soils are naturally occurring sediments deposited under estuarine conditions. Much of this material was deposited after sea levels rose close to their current levels about 6500 years ago. In the process, many open coastal embayments were inundated as shallow estuaries, within which sediments were gradually laid down.

The lower Macleay River floodplain is an example of this process. Following a rise in sea level in the early Holocene epoch, the lower Macleay was a broad estuary (below, left). However in the several thousand years since, it has gradually infilled with estuarine (acid sulfate) and alluvial sediments, a process which continues to the present (below, right).

Lower Macleay early and late holocene

Click photo to enlarge

Potential acid sulfate soils

In their natural condition, these acid sulfate sediments contain iron sulfides (most commonly as pyrite), which was formed by bacterial activity in oxygen-starved (anaerobic) and waterlogged conditions. In this state, the soils are known as 'potential acid sulfate soils'. They are also known as 'sulfidic' materials because they contain sulfidic material that has not been exposed to air and oxidised.

These materials have a field pH of more than 4.0, which is acid.

Actual acid sulfate soils

When these materials are exposed to oxygen due to drainage or disturbance, they produce sulfuric acid in excess of the sediment's capacity to neutralise the acidity. This makes the soil more acidic with a pH below 4.0.

The overall equation for the complete oxidation of pyrite can be written as:

FeS2 + 15/4O2 + 7/2H2O → Fe(OH)3 ↓ + 2SO42- + 4H+

(iron pyrite + oxygen + water → iron hydroxide + sulfate + acidity)

These 'actual acid sulfate soils' (also known as 'sulfuric' materials) often release toxic quantities of iron, aluminium and heavy metals.

Actual and potential acid sulfate soils are often found in the same soil profile, with actual acid sulfate soils generally overlying potential acid sulfate soil horizons.

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Page last updated: 11 October 2013