Acid Sulphate Soils (ASS)

What is acid sulphate soil?

Acid sulphate soil (ASS), or acid sulfate soil, is the common name given to soils and sediments containing iron sulphides, the most common being pyrite. When exposed to air after being disturbed, soils containing iron sulphides produce sulphuric acid (or sulfuric acid) and often release toxic quantities of iron, aluminium and heavy metals. For more information refer to this CSIRO website with many reports on acid sulfate soils along the River Murray and Lower Lakes (Lake Alexandrina and Lake Albert).

DENR Acid Sulphate Soil Research Program - 2010

Acid-bearing, pyrite-rich acid soil on the surface of lower Currency Creek.

Recent rains bring sulphuric acid to the surface, Currency Creek
Pyrites on the surface, acid in the cracks.  Left, a piece of school chalk 
(calcium carbonate) dissolving in acid.

Why should I care about acid sulphate soils?

While acid sulphate soils do not pose a significant risk to human health or the environment when undisturbed, disturbance by drainage, dewatering or soil excavation can cause:
  • significant environmental and economic impacts including fish kills;
  • damage to estuarine fisheries and loss of biodiversity in wetlands and waterways;
  • contamination of surface and ground water resources by acids, arsenic, heavy metals and other contaminants;
  • loss of agricultural productivity; and
  • corrosion of concrete and steel infrastructure by acidic soil and water
Once exposed, winds can be expected carry acid sulphate soils over large distances, with toxic dusts likely to reach Adelaide and the Fleurieu Peninsula on south-easterly winds.

  Corrosion on new galvanized steel fence (click to enlarge image)

Explain the acidification of Lake Alexandrina countdown

The current modelled critical acidification threshold water levels are minus 1.5m AHD for Lake Alexandrina and minus 0.5m AHD for Lake Albert. It is predicted the water level of Lake Alexandrina will reach this trigger level by February 2010. These predictions may change as more data is collected and the acidification modelling is improved.

What is the acidification water level trigger?

This is the level at which acidity being formed in Lower Lakes sediments can no longer be neutralized by the alkalinity of the sediments and lakes waters. Any water levels below these will expose large areas of acid sulphate soils and any subsequent refilling (including future freshwater inflows or the introduction of 
seawater) will likely result in the acidification of the water body.

Do other states have problems with acid sulphate soils?

Yes, unfortunately South Australia is not alone. Read here about the problems other states have encountered.