An introduction to acid sulfate soils
Date: 02 Nov 2004 Author: J. Sammut, Rebecca Lines-Kelly
Acid sulfate soils are the common name given to soils containing iron sulfides. In Australia, the acid sulfate soils of most concern are those which formed within the past 10,000 years, after the last major sea level rise.
Iron sulfide formation and oxidation
When the sea level rose and inundated land, sulfate in the sea water mixed with land sediments containing iron oxides and organic matter. The resulting chemical reaction produced large quantities of iron sulfides in the waterlogged sediments. When exposed to air, these sulfides oxidise to produce sulfuric acid, hence the name acid sulfate soils.
Potential acid sulfate soils
The iron sulfides 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 sulfides. This layer is commonly known as potential acid sulfate soil (PASS) because it has the potential to oxidise to sulfuric acid.
Actual acid sulfate soils
When the iron sulfides are exposed to air and produce sulfuric acid, they are known as actual acid sulfate soils. The soil itself can neutralise some of the sulfuric acid. The remaining acid moves through the soil, acidifying soil water, groundwater and, eventually, surface waters.
This soil pit shows both potential and actual acid sulfate soils together. The wet grey mud at the bottom of the pit is the iron sulfide layer. The mottled yellow layer above the wet mud is actual acid sulfate soil. The yellow colour is jarosite, a sulphur mineral. Its presence shows that the iron sulfides are oxidising and forming sulfuric acid. The oxidation has occurred because the watertable has dropped and exposed the top section of the iron sulfide layer to air.
Location of acid sulfate soils
Iron sulfide 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, estuaries and tidal lakes. In general, we expect to find iron sulfide layers where the surface elevation is less than five metres above mean sea level.
In Australia iron sulfide 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 sulfate soils in Australia containing about one billion tonnes of iron sulfides. One tonne of iron sulfides can produce about 1.5 tonnes of sulfuric acid when oxidised.
Natural Setting - low frequency, low magnitude, short duration acidity
Post Drainage - High frequency, high magnitude, persistant acidity
Under natural conditions iron sulfide layers are covered by water and colonised by native vegetation, as shown above. Any acid produced is usually neutralised by the tidal flows of alkaline sea water. The rest of the acid remains in the soil. In severe droughts plant roots can take up so much of the water in the soil that the watertable drops and exposes the iron sulfide layer to air. When this occurs, acid is generated and, in floods following dry periods, some acid can be released into streams.
Areas where iron sulfide layers occur are waterlogged and often drained for agriculture. Drainage and excavation of these areas expose the iron sulfide layers to air as shown opposite page below. Drainage greatly accelerates the natural rate of oxidation, so that large slugs of acid groundwater are released rapidly into estuarine streams. The concentrated acid can overwhelm the stream’s capacity to neutralise it. The acid can then affect the health of fish and other organisms.
When drains are dug in the iron sulfide layer, excavated heaps of iron sulfide muds are often left beside the drain as shown above. As this drain spoil oxidises it produces acid, a process which can continue for many years. The acid makes it difficult for plants to grow on the spoil; when it rains the acid leaches into the drain water. The problems associated with drainage of acid sulfate soils mean that drainage works need to be undertaken with extreme caution and in consultation with relevant authorities.
In some areas of Australia, acid sulfate soils drained 100 years ago are still releasing acid. In clay soils the oxidation process is very slow, possibly taking centuries, because it is difficult for air to circulate in the clay. Oxidation of sandy materials can be quite rapid, sometimes taking less than a year, because of the good air circulation within the sandy soil.
The acid produced by oxidation of iron sulfides affects both soil and water, and can damage the environment severely. As sulfuric acid moves through the soil, it strips iron, aluminium and sometimes manganese from the soil. In some cases it also dissolves heavy metals such as cadmium. In the soil this mixture can make the soil so acid and toxic that few plants can survive. In some cases, where peat overlying the iron sulfide layer has burnt away, the iron sulfide layer is completely exposed to air. It produces so much sulfuric acid that nothing will grow, giving the soil surface a bare, scalded appearance as shown.
Acid sulfate soils reduce farm productivity. The sulfuric acid lowers pH, which makes several soil nutrients less available to plants. The acid dissolves iron and aluminium from the soil so that they become available to plants in toxic quantities in soil water. These conditions reduce plant growth, and only acid-tolerant plants such as smartweed, shown below, can survive. This effectively means loss of drought-refuge swamp pastures used in the past by farmers.
Animal productivity is affected by acid sulfate soils. The acid discourages good quality pasture. Grazing animals may take in too much aluminium and iron by feeding on acid-tolerant plant species and drinking acid water.
When many of these waterlogged soils are drained their gel-like iron sulfide layer dries out and the soil can actually shrink and subside. This may make farmland more prone to flooding and waterlogging.
Sulfuric acid produced by acid sulfate soils corrodes concrete, iron, steel and certain aluminium alloys. It has caused the weakening of concrete structures and corrosion of concrete slabs, steel fence posts, foundations of buildings and underground concrete water and sewerage pipes.
When acid sulfate soils are used as landfill they can affect plant growth and landscaping, as shown on opposite page below, and create erosion problems. Developers of coastal subdivisions need to take particular care because disturbance and oxidation of iron sulfides during development may prevent establishment of gardens or lawns. Few plants will grow where acidity is as low as pH2.7.
Impact on fish and aquatic life
Drainage of coastal wetlands for agricultural and urban development constantly releases enough sulfuric acid and aluminium to affect the aquatic food chain, fish populations, and the health of fish. Sulfuric acid affects waterways and the aquatic life in them. Most aquatic life needs a minimum pH of 6 to survive. The pH of acid water can be as low as 2, and is often around 4. The lower the pH the more acid the water. Fish and crustaceans try to avoid acid water, but if they cannot escape, they may die. Plants, unable to escape the acidified water, are often killed.
Massive fish kills can occur when sulfuric acid is washed into waterways. This is a particular problem after droughts, when the watertable has dropped and the iron sulfide layer has oxidised.
Drought-breaking rains wash substantial quantities of sulfuric acid and aluminium into waterways, and massive fish kills can result.
However, fish kills are the most obvious effect of acid sulfate soils; the chronic, less visible effects such as reduced hatching and decline in growth rates are more common and widespread. Repeated flows of acid water prevent the fish population recovering. Not only are there fewer fish but there are more mosquitoes, because larvae-eating fish are unable to survive the acid water.
Acid water affects the health of fish and other aquatic life through damage to the skin and gills. Skin damage increases the susceptibility of fish to fungal infections which may lead to diseases such as epizootic ulcerative syndrome, also known as ’red spot’. Gill and skin damage reduce the ability of fish to take in oxygen or regulate their intake of salts and water. ’Red spot’ disease, shown right, causes significant economic losses to commercial fishermen.
Acid water affects the habitat of aquatic life. When acid water mixes with less acid stream water (above pH 4) the iron dissolved in the acid water precipitates and smothers plants and the streambed. These iron solids can move downstream and smother streambeds where there is no acid water.
The aluminium in acid water is toxic to most water organisms, because it damages their gills and at lethal levels can suffocate them. Cloudy green-blue water is an indicator of the presence of aluminium.
Sulfuric acid can also dissolve heavy metals in the soil such as cadmium. When these are washed into waterways after rain they can be absorbed by fish and other aquatic life.
Only a few acid-tolerant waterplants can survive in acid water: these include waterlilies and spike rushes. The acid-tolerant waterlilies can take over the drain or stream, even when pH returns to normal, preventing other species from re-establishing.
High levels of aluminium in acid water can cause particles floating in the water to clump together and drop to the bottom, leaving the water crystal clear. This clarity looks attractive but indicates that the water is too acid for aquatic life. The clear water can be up to 5 degrees C warmer than water with particles floating in it. The clear water allows acid-tolerant plants to saturate the water with oxygen which can kill fish through the ’bends’ or gas bubble disease. Clear water also increases the risk of fish suffering sunburn and melanomas.
Listed below are the short term and long term effects of acid water on fish and fish habitat.
Short term effects
- fish kills
- fish disease
- mass mortalities of microscopic organisms
- increased light penetration due to water clarity
- loss of acid-sensitive crustaceans
- destruction of fish eggs
Long term effects
- loss of habitat
- persistent iron coatings
- alterations to waterplant communities
- invasion by acid-tolerant waterplants
- reduced spawning success due to stress
- chemical migration barriers
- reduced food resources
- dominance of acid-tolerant plankton species
- growth abnormalities
- reduced growth rates
- increased predation
- changes in food chain and web
- damaged and undeveloped eggs
- reduced recruitment
- higher water temperatures due to increased light
- increased availability of toxic elements
- reduced availability of nutrients
The combination of fish kills, declining fish health and degraded habitat reduces fish populations in areas affected by acid water. Fish kills affect many ages of fish, and the loss of larvae and juveniles can be a regular occurrence. Some 70% of our commercial fish species spend part of their life cycles in estuaries, so the impacts of acid water raise major concerns for the future of commercial and recreational fishing industries, and the ecosystem. Researchers are currently looking at the long term impacts of acid water on fish populations.
Our understanding of acid sulfate soil chemistry and its effects has increased rapidly over the past five years, but there is still much to be learnt about management and rehabilitation of these soils.
The best technique for managing acid sulfate soils is to avoid disturbing or draining the iron sulfide layer in the first place. Iron sulfides are harmless while covered by water. To avoid disturbing the iron sulfide layer, it is important to know where it is likely to be found, and some states produce maps for this purpose. It is necessary to take soil cores to find out the exact location and depth of the iron sulfide layer on a particular site.
It is useful to know what the iron sulfide layer looks like so that it if is uncovered accidentally it can be re-covered with water immediately. The photograph below shows excavated iron sulfide soil dark grey and wet. It is also important to be able to recognise indicators of actual acid sulfate soils to prevent further acidification of land and waterways. These indicators include the cloudy green-blue water, excessively clear water, iron stains, poor pasture, scalded soil, and yellow jarosite described earlier.
Sulfuric acid can be neutralised with agricultural lime, but this is too costly for large areas of badly affected land. One technique that has had good results to date is liming of drains so that the sulfuric acid produced in the drain walls is neutralised by the lime as it is washed out. Acid water can also be neutralised by lime.
Covering land with water to prevent further oxidation may be one solution for badly affected areas where the land is scalded. The water cover encourages the growth of acid-tolerant grasses such as water couch and provides drought pasture for stock. However, it is important that the water remains on the affected area; if it evaporates the soil will oxidise again.
Wide, shallow drains like the one shown below allow surface water to drain quickly from the surface of low-lying land without exposing the iron sulfide layer beneath the soil. Deep, narrow drains are more likely to expose the iron sulfide layer and leak sulfuric acid into waterways.
Current research directions
Current research into acid sulfate soils includes:
- their occurrence and distribution
- effects of raising and lowering watertables
- development of accurate field testing techniques
- the role played by plant roots in lowering watertables
- behaviour of acid water in estuarine water bodies
- Future research directions include
- hydrology and watertable behaviour
- role of watertable control
- role of drainage and flood mitigation
- drain design
- grass species for wet areas
- standardised laboratory procedures
- standardised sampling procedures
- rehabilitation techniques
- economics of changed agricultural practices
Maps of acid sulfate soils in NSW are on the same grid as the 1:25,000 topographical maps and cost $10 each. They are available from the 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 Sulfate Soils are available by ringing the EPA’s Pollution Line 131 555.
Contact NSW DPI's Acid Sulfate Soils Advisory Officer to find out what other material has been produced.