ACID
SULFATE SOILS
by Johanna, for the KCEC
Newsletter (May 1999)
Since the dumping of construction waste as
land-fill at a Braeside industrial site, the issue of acid sulfate soils has
gained wider attention via the media. Toxic drainage waters emanating from such
soils have caused extensive kills of gilled organisms in coastal rivers,
restricted agricultural practices and severely corroded engineering
infrastructure. Concern about the effects of acid sulfate soils extends around
the globe, where the drainage of coastal wetlands has created serious problems
for farmers, particularly in the tropics.
Acid sulfate soils are common in marine and
estuarine environments, often as soft, dark grey to dark greenish-grey muds.
Although their age may vary from the present-day to the Tertiary geological
period (more than a million years ago), most acid sulfate soils were deposited
on the Australian coastline after the end of the last ice age (approx. 10,000
years ago). With the onset of global warming, rising sea levels mobilised
sediments that later accumulated in coastal embayments. Mangroves colonised and
stabilised these sediments, adding organic matter in the process. Breaking down
the organic matter in the absence of oxygen, bacteria reduced the iron sulfate
from seawater to iron pyrite or iron sulfide, concentrating these properties in
the top metre or more of the sediment.
When this pyritic layer is submerged and
protected from the atmosphere it is described as an innocuous "potential
acid sulfate soil" or PASS. However, when the land is drained or cleared
for agriculture or urban development, oxygen may reach the iron pyrite and
oxidise it to sulfuric acid. This in turn mobilises aluminium, present in
unlimited amounts in soil clay, which may be washed into coastal rivers and
estuaries after heavy rains flood the soil. The sudden influx of acid water
high in aluminium and iron, clogs the gills of fishes, crustaceans, oysters and
worms, causing dramatic mass kills (as in Northern NSW) and sub-lethal effects
such as epizootic ulcerative or "red spot" disease in fish. It has
also been suggested that acid waters may be responsible for the disappearance
of sea grasses in estuaries. Questions are beginning to be raised about the
impact of such waters on human health, particularly the ramifications of
ingesting acid, aluminium-rich water and swimming in contaminated water.
Acid waters are corrosive to engineering
structures, attacking both steel and concrete. In the vicinity of the Tweed
River, several millions of dollars worth of iron water-pipes have been replaced
because of corrosion from acid groundwaters.
Although the Environment Protection Authority
has moved to improve the management of acid sulfate soils, particularly in
regards to urban development, experts point out that avoidance of development
is the simplest and easiest solution. Land-use planning authorities must
recognize the problems associated with acid sulfate soils and integrate this
knowledge into planning strategies and schemes. Where avoidance is not
possible, the guiding principle should be that no acid discharge be generated
by the development. Regrettably, the State government has recently proposed
that waste acid sulfate soils be accepted at municipal land-fills.
REFERENCES:
Creagh, C. (1993) "Working together with
acid sulfate soils" in, Ecos, Iss. 77, Spring, pp. 25-29.
Environment Protection Authority (1998a) EPA
Information Bulletin - Acid Sulfate Soils, Pub. 620, August, EPA,
Melbourne.
Environment Protection Authority (1998b) Managing
Waste Acid Sulfate Soils - Draft Industrial Waste Management Policy and Draft
Policy Impact Statement, Pub. 630, December.
White, I. & Sammut, J. (1995) "Acid
Sulphate Coastal Soils" in, Trees and Natural Resources, Vol. 37,
Iss. 2, June, pp. 15-18.