Acid sulphate soils
Acid sulphate soils (ASS) are of concern all along the NSW coastline and around much of Australia. ASS can cause major damage to plants, animals, waterways and structures. If you live in an area of potential ASS, it is critical that you know where the soils are, how to avoid disturbing them and what to do if they cause problems.
What is an acid sulphate soil?
During the last major sea level rise at the end of the last glaciation, around 13 000 years ago, rapid inundation of gently sloping coastlines led to the deposition of new sediments in newly flooded estuaries. (Consequently, ASS occur only within about 5 m above sea level.) The process continues today.
The sediments, deposited by slowing rivers, contain a number of components, crucially organic matter and iron. After deposition, bacteria in the sediments feast on the organic matter, using the sulphate in the sea water (in the absence of oxygen) and the iron as a source of energy. The by-product of this reaction is iron sulphide, a black mineral that smells like rotten eggs.
How do I recognise an acid sulphate soil?
Actual ASS are identified by an acidic pH (<4), rusty red colours, and the presence of a yellowish mineral called jarosite, which is a complex sulphate of iron and potassium.
How do I recognise a potential acid sulphate soil?
Potential ASS are characterised by poorly drained, low-lying swampy ground, a silty texture, black or grey coloration, and the characteristic smell of swamps. A mangrove mud is an excellent example. They are never associated with freshwater environments.
Do not disturb
On exposure to air, iron sulphide quickly oxidises to form iron oxides (rust) and sulphuric acid. This sulphuric acid dissolves minerals, releasing soluble aluminium, which is very dangerous to living organisms, particularly those with gills. A sudden release of acidic drainage water can cause widespread fish kills downstream. The sulphuric acid also corrodes concrete and steel, and plants struggle to grow.
If left alone so that they remain wet and anoxic, potential ASS are rarely a problem. If drained quickly, however, they oxidise and cause numerous problems. The natural release of such acids is usually slow enough for the estuary to absorb any problems (seawater is alkaline), but human activities can be too rapid for the estuary.
How bad is the problem?
Estimates suggest that there are more than 2 million ha of ASS in Australia, containing about 1000 million t of iron sulphides (Sammut and Lines-Kelly). Because of the incorporation of oxygen, 1 t of iron sulphides can produce about 1.5 t of sulphuric acid. That’s a potential 1500 million t of sulphuric acid.
The amount of acidity potentially developing depends mainly on how much iron sulphide is present in a soil, but also on the presence of minerals such as calcium carbonate shell grit that could neutralise the resulting acidity.
In some places in Australia, ASS excavated for drainage are still releasing sulphuric acid after a century. These soils tend to have a high clay content, which restricts the entry of oxygen into the soil and thus draws out the process. Each time it rains, the acid is then flushed out.
Consequences for farming
The sulphuric acid dissolves iron, aluminium, manganese and cadmium in the soil, which then become available to plant roots in toxic quantities. Crops, pasture and native vegetation all die, leaving bare earth, which is then exposed to further erosion and the release of more acid.
Consequences for fish
The sulphuric acid kills fish, dissolving their gills and suffocating them. The dissolved aluminium is also poisonous. Fish generally need a pH above 6 to survive. Releases of acid from ASS can reduce the pH to 2.
This is a particular problem after droughts, during which humans with bores and deep-rooted plants lower the water table. The exposed iron sulphates are oxidised, and the arrival of heavy rains then washes out the sulphuric acid.
Constant exposure to low levels of acid also takes its toll on marine organisms, not only fish but also oysters and prawns, which fail to thrive, threatening both natural populations and the industries that depend on them.
Where are the acid sulphate soils?
The first step in identifying the likelihood of encountering ASS problems is to obtain an “acid sulphate soil risk map” from the NSW DECCW or relevant agency in your state or territory. You will note from the maps that the risk of encountering ASS decreases with increasing elevation above sea level and is highest on flat inundated plains associated with estuaries.
Management plans
To avoid creating a problem or to remediate an existing one, you will need a careful sampling and testing program and a management and monitoring plan. Qualified consultants are essential. The management plan will need to be approved by the relevant authorities (usually the state EPA) before going to the planning approval stage. Careful science and good planning resulting in a convincing case will avoid objections.
The consultants’ report will quantify the problem and help to guide management decisions. Don’t turn a single sod until you know where the ASS are and are not.
Management options
Both potential and actual ASS can be managed. There are several strategies:
- Avoidance. If there is any way the development can be rearranged or modified to avoid disturbing the potential ASS on site, then this is nearly always the cheapest option.
- Reburial or encapsulation. Where undisturbed soil needs to be removed, it is best if it can be placed back into a waterlogged location so that it does not later oxidise.
- Reflood the area. Keeping the soil anoxic will halt any further oxidation, but care is needed. First, a raised water table will make it easier for flooding to wash acid offsite. Second, previous drainage may have allowed the soil to shrink, so previous dryland areas may end up under salt water.
- Shallow drainage. Deep drains allow deep penetration of the soil by oxygen. Redigging drains to make them wider and shallower will allow continued drainage without oxidation.
- Treatment. ASS can be treated in three ways:
- Liming to neutralise acidity. This involves adding enough lime (calcium carbonate) to get the final pH of the oxidised soil up to about 5.5. In some cases land can need prodigious amounts of lime to neutralise the acid being generated. Farmers cannot usually afford such amounts but developers may be able to.
- Extracting the iron sulphide. A cyclonic separator can be used to separate the iron sulphide from sandy soil. This is expensive.
- Managed oxidation. The material is removed and stockpiled in a controlled area where acid leaching from it can be captured and treated with lime before release to the river.
Further reading
NSW DECCW. Acid sulfate soils.
Sammut J, Lines-Kelly R. 2000. An Introduction to Acid Sulphate Soils. Environment Australia, Canberra.