Page 118 - Soil and water contamination, 2nd edition

Major dissolved phase constituents 105

gibbsite near a pH of 6. Above neutral pH, the predominant dissolved form of Al is the
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anion Al(OH) .
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In addition to the pH, the presence of other dissolved species may control the solubility
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of Al. In solutions with a concentration above about 0.10 mg l fluoride, the solubility of Al
increases (Hem, 1989). In solutions with a considerable amount of dissolved silica, the Al
solubility is substantially decreased due to the formation of clay-mineral species. Although
the solubility of Al at near-neutral pH is very low, in such solutions Al may be detected in
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concentrations of up to about 1 mg l . These Al concentrations can be attributed to the
presence of colloidal or particulate Al in either Al hydroxides or aluminosilicates (clay
minerals ). This Al is thus not in the dissolved phase.
The main anthropogenic source of aluminium in surface water is the discharge of acidic
mine water. These waters may contain several hundreds or even thousands milligrams of Al
per litre. If the pH of the stream water increases downstream from the point of discharge, the
Al is gradually removed from solution through the precipitation of gibbsite . Furthermore,
atmospheric deposition of acidifying compounds, which are largely anthropogenic origin,
may bring about low pH values in poorly buffered catchments. In the soils of these acidified
catchments, Al is mobilised into the soil solution and some Al enters the drainage network.
Acid deposition and precipitation may thus contribute significantly to the soil and water
pollution by aluminium.

5.9 CHLORIDE
Chlorine (Cl) is the second most abundant halogen in the Earth’s crust (after fluorine),
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although more than 75 percent of the total amount is present as dissolved chloride (Cl )
in the oceans. Chlorine can exist in various oxidation state s, but in soil and water it exists
almost entirely as the chloride ion (1- oxidation state ). Like Na, Cl is used to maintain
osmotic pressure in the cells and to balance the cell cationic charges. The tolerance of plants
to Cl can vary widely. In animals and humans, Cl is a substantial constituent of the digestive
acids in the stomach and, consequently, is important in digestion.
Igneous rock minerals in which chloride is an important constituent are not very
common. Chloride is often only present as an impurity. Sedimentary rocks , particularly the
evaporite halite (NaCl), are far more important as a source of Cl. In coastal regions, Cl may
be supplied by surface intrusion of estuarine water or seawater or by subsurface intrusion
of saline groundwater. Another important source of Cl in these areas is the input from the
atmosphere. Salt spray is formed along the shoreline when waves break; small salt particles
may be suspended as the small seawater droplets evaporate, leaving aerosols behind. These
sea salt aerosols are transported inland by wind and are deposited on the land surface either
by dry deposition or by precipitation (wet deposition). The Cl deposition declines sharply
with increasing distance from the coast. Near the coast, the Cl deposition rates may be up
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to 100 kg ha y , but 200 km further inland the rate will have decreased to about 20 kg
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ha y . In areas remote from the coast, for example in the Midwestern USA or central
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Australia, the atmospheric input is no more than about 1 kg ha y (Whitehead, 2000).
The major anthropogenic sources of Cl in soil are road salt and, more important, agricultural
fertilisers . Some fertilisers (especially potassium chloride (KCl) and sewage sludge ) contain
large amounts of Cl, but Cl is not usually added as an essential soil amendment, since
chloride deficiency in soils is very rare. In KCl fertiliser, Cl is the balancing anion for the
essential potassium; in sewage sludge and slurries, Cl is commonly present as a contaminant.
In surface water, the most important anthropogenic sources are leaching of the Cl of
anthropogenic origin from soils in the catchment , irrigation waters, and the discharge of
domestic, industrial, and mine wastewater.

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