Acid sulfate soils (ASS) are soils and other soft sediments that contain iron sulfides (mostly pyrite (FeS2; photo 1) with typically smaller quantities of iron monosulfides (FeS)). If the iron sulfides are exposed to atmospheric oxygen they can be oxidised. Pyrite oxidation produces a cocktail of sulfuric acid, aluminum, iron and other heavy metals that can move into coastal waterways, and this can have significant environmental and economic impacts.
Photo 1. Cluster of pyrite surrounded by framboids from the Pimpama River (SE Qld) pyritic sediments  (Photo: Micaela Preda)
The ASS of most concern have mostly developed during the Holocene period of high sea level. Between 18,000 and 6,500 years ago, sea level rose from 120 m lower than today to approximately its present position. Extensive, very shallow coastal waterways formed as the sea reached approximately its present level. During the last 6,500 years these shallow coastal waters have been infilled with coastal sediments, forming coastal plains that today extend up to tens of kilometres inland from the coast (Figure 1).
Sediments deposited in these environments can be high in sulfides such as pyrite, providing that certain conditions are met. Pyrite is the end product in a chain of reactions that require anoxic conditions, and sources of sulfur, iron and organic material. The first step is promoted by sulfate-reducing bacteria and involves the reduction of sulfate from seawater to form hydrogen sulfide gas. This reduced form of sulfur can then react with the available iron to form pyrite (Photo 1 and Figure 2). As a consequence of these conditions prevailing in the Holocene period, many of our low-lying coastal plains now form tracts of ASS.
Similar sediments laid down in previous interglacial high sea level stands (e.g. Swan Valley Coastal Plain) are also potential acid sulfate soils (PASS) . Sulfide-rich sediments continue to form today in tidal flats, salt marshes, mangroves and other coastal wetlands. Australia has roughly 50,000 km2 of ASS containing in excess of a billion tonnes of iron sulfides .
Figure 1. A map of a typical depositional coast. The dashed line indicates the mid-Holocene shoreline. The shoreline has extended seawards (prograded) due to coastal sedimentation during the last ~6,500 years of high, relatively stable sea level.
Some known adverse impacts of ASS in coastal lowlands:
The public health implications of disturbing acid sulfate soils are not well understood. However, acidified coastal wetlands may provide predator-free habitat for species of mosquito that transmit arboviruses (e.g. Ross River Fever) . Acid dust mobilised during ploughing and construction activities may also cause dermatitis and eye irritation .
Iron sulfides are stable under oxygen-free (typically waterlogged) conditions. However, the disturbance of ASS for agriculture, urban development, flood mitigation or other land uses can expose iron sulfides to air, causing them to oxidise and produce sulfuric acid. Coastal waterways with rivers in acid hazard zones are most at risk of being polluted by acid sulfate drainage.
Sulfide-bearing dredge materials deposited adjacent to open waters are also a potential source of acid drainage .
The following changes in biophysical parameters may indicate that a coastal waterway is affected by acid leaching from pyritic sediments:
Figure 2. Chloride/sulfate ratios vs. log salinity for seawater, seawater diluted with increasing amount of precipitation, and acidified creeks and drains. The chloride/sulfate ratios of acidified creeks and streams are much lower than those of seawater, precipitation (rainwater + dust) and seawater diluted with precipitation . The seawater dilution model is from Radke (2000) .
Photo 2. This pond in southeast Queensland is typical of acid sulfate soil disturbance. The blue-green colour and clarity is an indicator of the presence of aluminium. The high levels of aluminium cause suspended particles in the water to clump together and drop to the bottom, resulting in clear water. The acidity of water like this is often too high (pH less than 4) to support diverse aquatic life. Note also the iron staining on the banks of the pond. Not all clear dams have high aluminium, however, when murky water goes clear over a short period of time following soil disturbance, it is a good indicator that acid sulfate soils have been exposed.('QASSIT, Qld Department of Natural Resources and Mines').
Figure 3. Stability field diagram for dissolved and solid forms of iron as a function of pH and Eh at 1 atm and 25oC. Note that more iron is in a dissolved form (Fe2+) at the pH of acid sulfate drainage than the pH of seawater. (from Elder, J.F. 1988). Reproduced with permission of J.F. Elder.
More information on pH (changed from natural).
Brendan Brooke, Geoscience Australia
Bernie Powell, Queensland Department of Natural Resources and Mines
Micaela Preda, Queensland University of Technology