Acid sulfate soils are naturally occurring soils, sediments
or organic substrates (e.g. peat) that are formed under waterlogged conditions.
These soils contain iron sulfide minerals (predominantly as the mineral pyrite)
or their oxidation products. In an undisturbed state below the water table,
acid sulfate soils are benign. However if the soils are drained, excavated or
exposed to air by a lowering of the water table, the sulfides react with oxygen
to form sulfuric acid.
Release of this sulfuric acid from the soil can in turn
release iron, aluminium, and other heavy metals (particularly arsenic) within
the soil. Once mobilized in this way, the acid and metals can create a variety
of adverse impacts: killing vegetation, seeping into and acidifying groundwater
and water bodies, killing fish and other aquatic organisms, and degrading
concrete and steel structures to the point of failure.
Acid sulfate soil formation
Polders with acid sulfate soils in Guinea Bissau along a
sea-arm amidst mangroves
The soils and sediments most prone to becoming acid sulfate
soils formed within the last 10,000 years, after the last major sea level rise.
When the sea level rose and inundated the land, sulfate in the seawater mixed
with land sediments containing iron oxides and organic matter. Under these
anaerobic conditions, lithotrophic bacteria such as Desulfovibrio desulfuricans
obtain oxygen for respiration though the reduction of sulfate ions in sea or
groundwater, producing hydrogen sulfide. This in turn reacts with dissolved
ferrous iron, forming very fine grained and highly reactive framboid crystals
of iron sulfides such as (pyrite). Up to a point, warmer temperatures are more
favourable conditions for these bacteria, creating a greater potential for
formation of iron sulfides. Tropical waterlogged environments, such as mangrove
swamps or estuaries, may contain higher levels of pyrite than those formed in
more temperate climates.
The pyrite is stable until exposed to air, at which point
the pyrite rapidly oxidises and produces sulfuric acid. The impacts of acid
sulfate soil leachate may persist over a long time, and/or peak seasonally
(after dry periods with the first rains). In some areas of Australia, acid
sulfate soils that drained 100 years ago are still releasing acid.
Chemical reaction
When drained, pyrite (FeS2) containing soils (also called
cat-clays) may become extremely acidic (pH < 4) due to the oxidation of
pyrite into sulfuric acid (H2SO4). In its simplest form, this chemical reaction
is as follows:
2 FeS2 + 9 O2 + 4
H2O → 8 H+ + 4 SO42− + 2 Fe(OH)3 (solid)
The product Fe(OH)3, iron(III) hydroxide (orange),
precipitates as a solid, insoluble mineral by which the alkalinity component is
immobilized, while the acidity remains active in the sulfuric acid. The process
of acidification is accompanied by the formation of high amounts of aluminium
(Al3+, released from clay minerals under influence of the acidity), which are
harmful to vegetation. Other products of the chemical reaction are:
Hydrogen sulfide
(H2S), a smelly gas
Sulfur (S), a
yellow solid
Iron(II) sulfide
(FeS), a black/gray/blue solid
Hematite (Fe2O3),
a red solid
Goethite (FeO.OH),
a brown mineral
Schwertmannite a
brown mineral
Iron sulfate compounds
(e.g. jarosite)
H-Clay (hydrogen
clay, with a large fraction of adsorbed H+ ions, a stable mineral, but poor in
nutrients)
The iron can be present in bivalent and trivalent forms
(Fe2+, the ferrous ion, and Fe3+, the ferric ion respectively). The ferrous
form is soluble, whereas the ferric form is not. The more oxidized the soil
becomes, the more the ferric forms dominate. Acid sulfate soils exhibit an
array of colors ranging from black, brown, blue-gray, red, orange and yellow.
The hydrogen clay can be improved by admitting sea water: the magnesium (Mg)
and sodium (Na) in the sea water replaces the adsorbed hydrogen.
Geographical distribution
Acid sulfate soils are widespread around coastal regions,
and are also locally associated with freshwater wetlands and saline
sulfate-rich groundwater in some agricultural areas. In Australia, coastal acid
sulfate soils occupy an estimated 80,000 km2, underlying coastal estuaries and
floodplains near where the majority of the Australian population lives. Acid
sulfate soil disturbance is often associated with dredging, excavation
dewatering activities during canal, housing and marina developments.
Acid sulfate soils that have not been disturbed are called
potential acid sulfate soils (PASS). Acid sulfate soils that have been
disturbed are called actual acid sulfate soils (AASS).
Impact of acid sulfate soil
Disturbing potential acid sulfate soils can have a
destructive effect on plant and fish life, and on aquatic ecosystems. Flushing
of acidic leachate to groundwater and surface waters can cause a number of
impacts, including:
Ecological damage
to aquatic and riparian ecosystems through fish kills, increased fish disease
outbreaks, dominance of acid-tolerant species, precipitation of iron, etc.
Effects on
estuarine fisheries and aquaculture projects (increased disease, loss of
spawning area, etc.).
Contamination of
groundwater with arsenic, aluminium and other heavy metals.
Reduction in
agricultural productivity through metal contamination of soils (predominantly
by aluminium).
Damage to
infrastructure through the corrosion of concrete and steel pipes, bridges and
other sub-surface assets.
Agricultural impacts
Sea water is admitted to a bunded polder on acid sulfate
soil for soil improvement and weed control, Guinea Bissau
Potentially acid sulfate soils (also called cat-clays) are
often not cultivated or, if they are, planted under rice, so that the soil can
be kept wet preventing oxidation. Subsurface drainage of these soils is
normally not advisable.
When cultivated, acid sulfate soils cannot be kept wet
continuously because of climatic dry spells and shortages of irrigation water,
surface drainage may help to remove the acidic and toxic chemicals (formed in
the dry spells) during rainy periods. In the long run surface drainage can help
to reclaim acid sulfate soils. The indigenous population of Guinea Bissau has
thus managed to develop the soils, but it has taken them many years of careful
management and toil.
In an article on cautious land drainage, the author
describes the successful application of subsurface drainage in acid sulfate
soils in coastal polders of Kerala state, India.
Also in the Sunderbans, West Bengal, India, acid sulfate
soils have been taken in agricultural use.
A study in South Kalimantan, Indonesia, in a perhumid
climate, has shown that the acid sulfate soils with a widely spaced subsurface
drainage system have yielded promising results for the cultivation of upland
(sic!) rice, peanut and soybean. The local population, of old, had already
settled in this area and were able to produce a variety of crops (including
tree fruits), using hand-dug drains running from the river into the land until
reaching the back swamps. The crop yields were modest, but provided enough
income to make a decent living.
Reclaimed acid sulfate soils have a well-developed soil
structure; they are well permeable, but infertile due to the leaching that has
occurred.
In the second half of the 20th century, in many parts of the
world, waterlogged and potentially acid sulfate soils have been drained
aggressively to make them productive for agriculture. The results were
disastrous. The soils are unproductive, the lands look barren and the water is
very clear, devoid of silt and life. The soils can be colorful, though.
Construction
When brickwork is persistently wet, as in foundations,
retaining walls, parapets and chimneys, sulfates in bricks and mortar may in
time crystallise and expand and cause mortar and renderings to disintegrate. To
minimise this effect specialised brickwork with low sulfate levels should be
used. Acid sulfates that are located within the subsoil strata has the same
effects on the foundations of a building. Adequate protection can exist using a
polythene sheeting to encase the foundations or using a sulfate resistant
Portland cement. To identify the pH level of the ground a soil investigation
must take place.
Acid sulfate soil restoration
By raising the water table, after damage has been inflicted
due to over-intensive drainage, the soils can be restored. The following table
gives an example.
Drainage and yield of Malaysian oil palm on acid sulfate
soils (after Toh Peng Yin and Poon Yew Chin, 1982)
Yield in tons of fresh fruit per ha:
Year 60 61 62
63 64 65 66 67
68 69 70 71
Yield 17 14 15
12 8 2 4 8
14 19 18 19
Drainage depth and intensity were increased in 1962. The
water table was raised again in 1966 to counter negative effects.
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