Acidity
In the coastal zone of Australia, the impacts of disturbing acid sulfate soils (ASS) are a significant land and water management issue. Acid sulfate soils are based on sediments deposited under estuarine conditions that contain iron sulfides called pyrite. These sulfides were formed by bacteria combining iron in the organic-rich waterlogged sediments with sulfate in the brackish tidal waters. It has been estimated that acid sulfate soils occur across 40,000 km2 of the Australian coastal zone, containing about a billion tonnes of pyrite (Sammut and Lines-Kelly, 1996).
When maintained in a waterlogged condition these sediments are stable and are termed potential acid sulfate soils (PASS). However, activities such as road construction, drainage, cropping, urban development or mining can expose these sediments to air. This allows the pyrite to oxidise, which is a chemical reaction that generates sulfuric acid. The soil profile can acidify (pH < 4) to form actual acid sulfate soils (AASS). Severe soil acidification can lead to the development of denuded acid scalds. These areas are associated with greatly reduced agricultural productivity or degraded ecological values. Deterioration of soil fabric can be caused by the flocculation of clays, resulting in soil shrinkage and ground subsidence (White et al, 1997).
Following rainfall, the store of acid can migrate into drains and be exported to estuaries and wetlands. The acid water can dissolve iron, aluminium and other metals to levels that are toxic to aquatic life. Increased dissolution of compounds such as silica can also initiate algal blooms (Tulau, 1999). Secondary precipitates of iron and aluminium can smother benthos and aquatic plants. Secondary oxidation of iron also consumes oxygen, with deoxygenated water also responsible for fish kills. The chronic effects on aquatic life also include fish disease outbreaks, reduced food resources, limitations on the migration potential for fish, reduced fish recruitment and altered ecology including weed invasion by acid-tolerant plants (Sammut et al, 1996). The acid water can also cause infrastructure damage with the corrosion of concrete and metal.
Interactions between groundwater and surface water systems are fundamental to the acidification process. Declining shallow watertables can expose sulfides in the soil profile to air, initiating the generation of sulfuric acid. Rising watertables can bring acid products to the land surface. How the shallow acid groundwater system interacts with surface water features such as drains, streams and wetlands is of critical importance. Seepage of acid groundwater occurs when the shallow watertable is higher than the drain water level. Acid export can result with tidal drawdowns causing low drain levels and a high hydraulic gradient towards the drain. This is exacerbated if the acid sulfate soil has high hydraulic conductivity, mainly due to cracks and macropores caused by soil shrinkage or biological activity. Hence, a key strategy to limit acid export is to maintain high water levels in the drain to reduce or even reverse the hydraulic gradient towards the drain. This can be achieved by installing a retention structure within the drain such as a weir, dropboards or a sluice gate.
Case Studies
Relevant Links
Department of the Environment and Heritage Coastal Acid Sulfate Soils
Natural Heritage Trust Coastal ASS Program
OzEstuaries Acid Sulfate Soils
NSW Department of Natural Resources Acid Sulfate Soils in NSW
NSW Department of Primary Industries Acid Sulfate Soils
Queensland Department Natural Resources, Mines and Water Acid Sulfate Soils
References
Sammut J, Lines-Kelly R, 1996. An introduction to acid sulfate soils. Department of Environment, Sport and Territories (Cth), Australian Seafood Industry Council.
Sammut J, White I, Melville MD, 1996. Acidification of an estuarine tributary in Eastern Australia due to drainage of acid sulfate soils. Marine and Freshwater Research 47(5)
Tulau MJ, 1999. Acid sulfate soil management priority areas in the lower Richmond floodplain. Department of Land and Water Conservation, Sydney.
White I, Melville MD, Wilson BP, Sammut J, 1997. Reducing acidic discharge from coastal wetlands in eastern Australia. In: Wetlands Ecology and Management. 5: 55-72. WJ Streever (ed). Wetlands Rehabilitation Australia. Kluwer Academic Publishers, Netherlands