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Coastal Acid Sulfate Soils Animation Transcript

Coastal Acid Sulfate Soils

Scene one

The opening scene shows a coastal environment with the sea level higher than it is today. As the sea level drops, deposited sediments are exposed altering the coastal landscape and including an estuary. The deposited sediments include a layer of pyrite which remains waterlogged and protected by another layer of soil. The accompanying text reads “The sea level is always fluctuating. Eight thousand (8,000) years ago the sea was up to two metres higher than it is today. This provided suitable conditions for pyrite to form in the waterlogged sediments that are now above the high tide mark.”

Scene two

The animation zooms in to show a closer view of the layer of pyrite, and arrows demonstrate how pyrite is formed in waterlogged conditions. The accompanying text reads “Pyrite (FeS2) forms from sulfate (SO42-) and iron (Fe2+) when they are in an oxygen-free, waterlogged, organic matter rich soil. Soil microbes convert SO42- to hydrogen sulfide (H2S) that then converts to FeS2. Soils that have one or more layers of FeS2, or other metal sulfides, are called acid sulfate soils.”

Scene three

Scene three shows how a upper part of the pyrite layer is exposed to oxygen when the watertable drops. Exposing pyrite to oxygen produces sulfuric acid. The colour change to red represents an actual change in soil colour typically seen in the natural environment, as the production of sulfuric acid releases iron (think of the colour of rust) into the soil. The accompanying text reads “As the watertable drops, the soil drains of water and oxygen enters. When pyrite FeS2 is exposed to oxygen, sulfuric acid (H2SO4) is produced. This can cause extreme acidity and can occur rapidly.”

Scene four

Scene four shows an excavator digging a hole from the surface soil into the pyrite layer. The pile of pyritic soil is then exposed to oxygen as the water drains out, producing sulfuric acid and the pile of soil turns red. The accompanying text reads “Typically, acid sulfate soils can be disturbed by excavating. When the excavated soil is dumped in a pile, the water drains out, oxygen enters the soil and oxidation begins the production of sulfuric acid (H2SO4).”

Scene five

The excavated site from Scene Four now shows how the water table lowers to equilibrate the movement of water from the soil into the drain. The pyrite exposed in situ then turns red to represent the production of sulfuric acid. The accompanying text reads “Installation of drains can lower the watertable, exposing pyrite (FeS2) to oxygen in situ, thus producing sulfuric acid (H2SO4).”

Scene six

The watertable in the soil is connected by hydrologic processes to the level of water in nearby waterways. If the level of water in the waterway, in this example an estuary, is lower, the watertable also drops. Consequently, pyrite is exposed to oxygen, and sulfuric acid is produced as shown by the pyrite layer turning red. The water of the estuary also becomes highly acidic as the sulfuric acid moves into the waterway. Typically, waterways contaminated by sulfuric acid become more green/aqua in colour and fish kills often result. The accompanying text reads “Waterways are connected to the watertable in the soil. By lowering the level of water in waterways (e.g. in an estuary), the watertable drops, exposing the pyrite (FeS2) to oxygen and producing sulfuric acid (H2SO4).”

Scene seven

Scene seven shows that the watertable in the soil can also be lowered in by the pumping of groundwater, and also through the use of deep-rooted vegetation. The accompanying text reads “The watertable can also be lowered by pumping groundwater or by planting deep-rooted vegetation.”

Scene eight

A building on the surface of the soil causes the soil directly underneath to subside with the weight of the infrastructure. This soil must be distributed elsewhere so is shown as uplifted soil at the edges of the building. Consequently, the uplifted soil is exposed to oxygen as it sits above the stationary watertable, and sulfuric acid is produced as shown by the soil turning red. The accompanying text reads “The construction of buildings, bridges or other infrastructure can cause the soil to subside and subsequently cause soil at the edges to uplift. The uplifted soil is exposed to oxygen and sulfuric acid (H2SO4) is produced.”

Scene nine

The final scene simply zooms out to show the entire coastal landscape first depicted and summarises the potential impacts of exposing acid sulfate soil to oxygen, producing sulfuric acid, and therefore highly acidic soils. The accompanying text reads “Unmanaged acid sulfate soils can have significant detrimental impacts on biodiversity, infrastructure, human health, recreation and leisure, water quality and fisheries, and agricultural productivity.”

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