Acid Sulphate Soils
A Developer's environmental disaster waiting to happen
Over the past few years Acid Sulphate Soils (ASS) causing acid runoffs have only come to the attention of the community because of increasing releases of acid into the aquatic and marine environment which have killed many fish. Fish kills and certain types of algal blooms are a possible sign of acid release. ASS have been know to engineers and scientist for many years. Due to changing technology, equipment and the ever money hungry developer or council who insists on converting an inexpensive area of wetland into a multi-million dollar canal housing development, housing development, marina or Golf Course or farmers extending their farm and provision of services such as sewerage, water, phone, power and gas are exposing more and more ASS which are degrading or destroying sections of our marine and aquatic environment.
Until recently test carried out on soil samples had an inherent flaw. The ASS test procedure called for the grinding up of the soil sample. Because this also included the grinding up of any shells which were in the sample thus the results were flawed.
The calcium carbonate in the shell when ground allows all the calcium carbonate to be neutralised by any acid caused by oxidation of the soil. When sulphuric acid reacts with aquatic shells (calcium carbonate) in the ground it forms an insoluble layer of calcium sulphate on the surface and any further reaction is retarded if not stopped. Early ASS test were giving false results. Thus when lime was added to the soil to neutralise it insufficient was used and acid still leached from the soil.
Now before ASS tests are carried out all shells must be removed from the test sample.
Acid Sulphate Soils
From "an introduction to acid sulphate soils" by Department of the Environment, Sports and Territories. October 1997.
Acid sulphate soils are the common name given to soils containing iron sulphides. In Australia, the acid sulphate soils of most concern are those which formed within the past 10,000 years, after the last major sea level rise.
Iron sulphide formation and oxidation
When the sea level rose and inundated land, sulphate in the seawater mixed with land sediments containing iron oxides and organic matter. The resulting chemical reaction produced large quantities of iron sulphides in the waterlogged sediments. When exposed to air, these sulphides oxidise to produce sulphuric acid, hence the name acid sulphate soils.
Potential acid sulphate soils
The iron sulphides are contained in a layer of waterlogged soil. This layer can be clay or sand, and is usually dark grey and soft. The water prevents oxygen in the air reacting with the iron sulphides. This layer is commonly known as potential acid sulphate soil (PASS) because it has the potential to oxidise to sulphuric acid.
Actual acid sulphate soils
When the iron sulphides are exposed to air and produce sulphuric acid, they are known as actual acid sulphate soils. The soil itself can neutralise some of the sulphuric acid. The remaining acid moves through the soil, acidifying soil water, ground water and, eventually, surface waters.
Location of acid sulphate soils
Iron sulphide layers were formed under tidal conditions, so they are found in low-lying areas near the coast. They are still being formed today in mangrove forests and salt marshes, estuarine and tidal lakes. In general, we expect to find iron sulphide layers where the surface elevation is less than five meters above mean sea level.
In Australia iron sulphide layers are found along the coastlines of the Northern Territory, Queensland and New South Wales. They are also found along the northern coastline of Western Australia, and around Perth, Adelaide and Westernport Bay near Melbourne. Scientists have estimated that there are more than two million hectares of acid sulphate soils in Australia containing about one billion tonnes of iron sulphides. One tonne of iron sulphide can produce about 1.5 tonnes of sulphuric acid when oxidised..... pages 3 and 5.
Contacts:
Maps of acid sulphate soils in NSW at 1:25,000 at $10.00 ea from Department of Land and Water Conservation, Sydney (02) 9228 6315
Queensland Department of Primary Industries is currently mapping from the NSW border to Bundaberg.
The NSW Environment Protection Authority's guidelines Assessing & Managing Acid Sulphate Soils are available by ringing the EPA's Pollution Line 131 555.
ASSAY, a quarterly newsletter about acid sulphate soils is available through the NSW acid sulphate soils information officer on (02) 6626 1344 or fax (02) 6628 1744
The address is Wollongbar Agricultural Institute, Bruxner Highway NSW 2477 or e-mail: jon.woodworth@agric.nsw.gov.au NSW Agriculture has information about acid sulphate soils on its internet site at www.agric.nsw.gov.au
Queensland has an Acid Sulphate Investigation Team (QASSIT) situated at the Department of Natural Resources Block C. Gate2, Resource Sciences Centre, 80 Meiers Road Indooroopilly QLD 4068; telephone (07) 3896 9819; or fax (07) 3896 9782. The e-mail address is HeyK@dnr.qld.gov.au . QASSIT carries out mapping, site investigations, laboratory testing, education and ASS research.
last upgrade 23 Oct 1999
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