People are complaining about "dumping in the marine park" but do they
realise what's being dumped?
It's dredge spoil which is just seabed
material. It has to be tested for contaminants before being dumped at
sea.
What are the alternatives? - Land dumping which could lead to acid
sulfate soils.
Dumping the Dredged Spoil from the Expansion at Abbot Point on land has been brought up time and time again from the anti coal activists. One of the main reasons why it is not a good idea is because once the dredged material is exposed to oxygen it becomes Acid Sulfate!! Let's explain exactly how this process works and why it is better to replace what comes from the ocean back to the ocean!!
What are acid sulfate soils?
Acid sulfate soil is the common name for soils that contain metal
sulfides. In an undisturbed and waterlogged state, these soils may pose
no or low risk. However, when disturbed or exposed to oxygen, acid
sulfate soils undergo a chemical reaction known as oxidation. Oxidation
produces sulfuric acid which has led to these soils being called acid
sulfate soils.
How and where are acid sulfate soils formed?
Acid sulfate soils are formed by bacterial activity in waterlogged conditions when there is no or little available oxygen.
Naturally occurring bacteria convert sulfate (dissolved salt) from
seawater, groundwater or surface water into sulfide (another type of
compound that contains sulfur). This sulfide reacts with metals
especially iron in the soil sediments or water column, to produce metal
sulfides (the main components of acid sulfate soils). In order to
convert the sulfate into sulfide, the bacteria also need a source of
energy provided by organic material such as decaying vegetation.
Diagram Figure 2.1 from National Guidance for the
Management of Acid Sulfate Soils in Inland Aquatic Ecosystems —
Environment Protection and Heritage Council and Natural Resource
Management Ministerial Council (2011)
Acid sulfate soils occur in inland and coastal environments.
Coastal
Supratidal iron-staining, Cairns QLD
Photo: B. Powell
Acid sulfate soils occur naturally around Australia's coastline.
These soils were formed and deposited in areas that are, or once were,
coastal environments.
Coastal acid sulfate soils are commonly found in mangrove forests,
saltmarsh, floodplains, and salt- and freshwater wetlands.
Why are acid sulfate soils an issue?
Jarosite is a field indicator that iron sulfides in acid sulfate soils are oxidising and forming sulfuric acid
Photo: B. Powell
When acid sulfate soils are disturbed or exposed and react with
oxygen, they produce sulfuric acid which may be accompanied by certain
hazards. Metals may be released from sediments and become bioavailable
in the environment, oxygen may be removed from the water column and
gases such as hydrogen sulfide, sulfur dioxide and methane may be
released.
What makes acid sulfate soils an issue?
The majority of Australia's population is concentrated along the
coastline and there may be significant development pressure on this
land. Urban development and other land use changes such as marinas,
canal estates, underground pipes and swimming pools, may result in the
disturbance or exposure of acid sulfate soils.
Dredging operations may disturb or expose metal sulfides with serious
effects if dredging and dredge spoil disposal is not suitably managed.
What may be affected by acid sulfate soils?
Acid sulfate soils may affect the following key environmental values or uses:
- aquatic ecosystems
- primary industries
- recreation and aesthetics
- drinking water
- industrial water, and
- cultural and spiritual values
Source: Department of the Environment
So, is it best to replace what has naturally come from the ocean on land?? Or, back to where it originally came from?
The technique that will be used for the dredging at Abbot Point will ensure that none of the dredged material will be exposed to oxygen. It is sucked up from the seabed directly into the dredger where it is kept wet at all times and then released from the bottom of the dredger straight back into the ocean. The majority of the spoil will sink straight back to the seabed. Back to an area of similar seabed floor.!!
It therefore is not wise to put it on land. Yes, it can be treated. But, that is timely, costly and a large area is needed to facilitate this. Most ports have not got the added area where this can take place!! As the Caley Valley Wetlands are close by to the Abbot Point Terminal, it is also wise to keep the spoil away from such a significantly beautiful area.
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