Acid Sulphate soils are naturally occurring and widely present in the south-west of Western Australia and are harmless when left in a waterlogged, undisturbed environment. However, when exposed to air, through water drainage, water extraction or sand excavation proposed to occur on Banksia Road, the iron sulfides in the soils react with oxygen and water to produce iron compounds and sulfuric acid. This acid can release other substances, including heavy metals, from the soil and into the surrounding environment and waterways .
Acid Sulphate soils are a result of the last major sea level rise that deposited sulphides that readily reacted with land material which resulted in iron sulphide deposits. The following images show the Acid Sulphate Soil Risk Map for the Swan Coastal Plain:
RED = high to moderate risk | YELLOW = moderate to low risk
Several high to moderate risk areas surround the Banksia Road site, these areas align to known wetlands which have not been identified and have not been assessed in the sand mining proposals. There is a risk to neighbouring properties and the biodiversity that surrounds the site. The cumulative impacts of both sites operating are not known as they have been reviewed in isolation.
These naturally occurring iron sulphides, particularly pyrites, in the soil remain benign when undisturbed and in waterlogged conditions however when these compounds are disturbed or drained, they mix with atmospheric oxygen which produces and releases sulphuric acid into the environment . Acidification of groundwater can cause a range of ongoing and permanent environmental issues including the release of heavy metals such as Arsenic, Aluminium and Chromium which can be harmful to human and animal health. In some areas of Australia, Acid Sulphate Soils drained 100 years ago are still releasing acid .
Potential impacts of Acid Sulphate soils include :
- soil and water acidification;
- adverse changes to the quality of soil and water including the release of heavy metals (groundwater, surface water, wetlands, watercourses and estuaries);
- degradation of wetlands, water-dependent ecosystems and ecosystem services;
- loss of habitat ecosystem complexity and biodiversity;
- invasion and dominance of wetlands and waterways by acid-tolerant water plants and plankton species
- reduction of soil stability and fertility;
- loss of/deterioration in quality of water sources for stock, irrigation and human use by increasing acidity and heavy metal concentrations;
- acid surface scalds in discharge areas;
- loss of visual amenity caused by rust coloured stains, scums and slimes from iron precipitates;
- risk of long-term infrastructure damage through acidic water corroding metallic and concrete structures (concrete cancer) such as roads, bridges, pumps, sub-surface pipes, retaining walls, brick course work and foundations;
- blocked reticulation systems and other small pipe systems due to iron precipitates;
- acidification of surface water bodies increasing mosquito breeding, which may increase the prevalence of mosquito-borne diseases such as Ross River virus; and
- increased financial burden of treating and rehabilitating affected areas and maintenance of infrastructure.
According to the Department of Environment and Regulation WA (now the Department of Water and Environmental Regulation), the best strategy for Acid Sulphate Soils is avoidance as this is the most economically viable and environmentally responsible option . Mining for sand or gravel is an identified major risk of Acid Sulphate soil. The proposed sand mines on Banksia and Boomerang Road in Casuarina/Wellard are located in moderate to low risk areas however high to moderate risk areas surround the sites which can be adversely affected.
CASE STUDY | INVESTIGATION OF SOIL AND GROUNDWATER ACIDITY, STIRLING
Concern about acidic groundwater in Stirling was raised in mid-December 2001 by a concerned resident whose garden was dying. The resident contacted the City of Stirling, and preliminary testing by the City of Stirling indicated that the groundwater was extremely acidic and contained high metal concentrations.
Investigations to date show that groundwater acidity issue is restricted to 49 domestic bores immediately downgradient in the direction of groundwater flow from the Roselea and Stirling Lakes residential estates, and the Spoonbill Reserve.
Download the Full Report (.pdf, 829KB)
It is important to note that these risk areas are a guide only and that Acid Sulphate Soils may be present due to natural occurrences or past land uses. The proposed area for mining has Banksia woodland on grey, sandy soils that are often highly leached with low clay content which gives them very low to zero natural buffering capacity. Further investigation is required to determine the presence of “coffee rock” in the nominated area. Coffee rock is the common name for cemented iron and/or organic rich sands which are a major contributor to acidification of soils upon changes to groundwater levels. Coffee rock can be located in shallow soil or may be found deeper in the soil profile, particularly in areas marked as moderate to low risk of Acid Sulphate Soils .
This is one of many deficiencies identified, further supporting our call for the Minister for Environment, Stephen Dawson, to use Section 43 of the Environmental Protection Act to have proposals rewritten and reassessed using current data.
 SAMMUT, J., 2000, An introduction to acid sulfate soils (2nd edition): Environment
Australia and Agriculture, Fisheries and Forestry Australia.