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Acid sulfate soil

Where are acid sulfate soils?

Acid sulfate soils are found in every coastal estuary and embayment in NSW. There are over 260,000 ha of high risk areas, including about 150,000 ha under agricultural production. The largest of these areas are on the coastal floodplains of northern NSW, particularly the floodplains of the Tweed, Richmond, Clarence, Macleay, Hastings, Manning and Hunter Rivers.

Conditions for the development of acid sulfate soils commonly occur in coastal lagoons and in the estuarine parts of coastal rivers. Acid sulfate soils have been forming in coastal estuaries since the sea level rose to near the present level in the early Holocene period (after the end of the last ice age). Sea levels rose to about 2 metres above present levels around 6,000 years ago, with a subsequent gradual decline. At the time, coastal floodplains were largely open estuaries, but they have since infilled with estuarine and alluvial sediments.

Acid sulfate soils are common in areas of mangroves and salt marsh as well as underlying large areas of rivers and deltas, levees, backswamps and other formerly brackish seasonal or permanent freshwater swamps, and their coastal flats. The environment at the time of sediment deposition can be ascertained by palynological methods (i.e, by studying pollen grains and other spores found in the sediments).

A plume of acidic aluminium-rich water in the Manning River from Ghinni Ghinni Creek. Photo: Mike Dove.

Due to its estuarine origin, the upper surface of acid sulfate soil is usually close to sea-level, generally lower than 1 m AHD and often 0-0.3 m AHD. Translocation of the products of pyrite oxidation may extend acid sulfate soils above this elevation.

Therefore, areas with high risk acid sulfate soils close to the soil surface, including acid sulfate soil scalds, are generally wetlands, degraded wetlands, or were previously wetlands. In their natural (pre-drainage or disturbance) range of hydrologic states, the native vegetation of backswamp (extending to backplain) sites would have varied from woodland around swamp margins, through to sedgeland or rushland in the generally wettest sites, which are usually treeless. Areas of sulfidic sands may also occur, particularly in higher energy, lower estuarine and coastal locations.

Melville et al. (1991) estimated that the soils of the coastal areas of the Tweed may contain 500,000 tonnes of pyrite, potentially representing more than three-quarters of a million tonnes of sulfuric acid. Measurements across McLeods Creek floodplain have shown that an average of about 50 tonnes/ha of actual acid sulfate soils occur. It has also been shown that the Tweed River floodplain can contribute at least 2,600 tonnes of sulfuric acid to the river in a single wet season ( Wilson et al. 1999 ).

Referring to the Tuckean in the Richmond, Sammut (1996) found that these drains may store 25 tonnes of acid, with up to 40 tonnes of sulfuric acid exported in a single day ( Sammut 1995). The static load of acid in the Broadwater was found to be about 16 tonnes. It was also found that over 90 km of the river and over 150 km of drains and other waterbodies are frequently acidified. During the 1994 flood, 950 tonnes of sulfuric acid and 450 tonnes of aluminium were discharged through the barrage ( Sammut 1996).