Geoscience Reference
In-Depth Information
acid, increasing the concentration of hydrogen ions
in solution. The normal pH of rainfall is some-
where between 5 and 6 but may drop as low as 4,
particularly if there is industrial air pollution nearby.
For example, Zhao and Sun (1986) report a pH
value of 4.02 in Guiyang city, China, during 1982.
Acidic substances may also be absorbed easily as
water passes through a soil column. A particular
example of this is water derived from peat, which
will absorb organic substances. These form organic
acids, giving peat-derived water a brown tinge and
a low pH value. At the other end of the spectrum
rivers that drain carbonate-rich rocks (e.g. limestone
and chalk), have a higher pH due to the dissolved
bicarbonate ions.
The pH value of rivers is important for the
aquatic fauna living within them. The acidity of a
river is an important control for the amount of
dissolved ions present, particularly metal species.
The more acidic a river is the more metallic ions will
be held in solution. For fish it is often the level of
dissolved aluminium that is critical for their survival
in low pH waters. The aluminium is derived from
the breakdown of alumino-silicate minerals in
clay, a process that is enhanced by acidic water.
Water with a pH between 6 and 9 is unlikely to be
harmful to fish. Once it drops below 6 it becomes
harmful for breeding, and salmonid species (e.g.
trout and salmon) cannot survive at a pH lower
than 4. Equally a pH higher than 10 is toxic to
most fish species (Alabaster and Lloyd, 1980). Table
7.3 summarises the effect of decreasing pH (i.e.
increasing acidity) on aquatic ecology.
Mention needs to be made of the confusing
terminology regarding
alkalinity
. Alkalinity is a
measure of the capacity to absorb hydrogen ions
without a change in pH (Viessman and Hammer,
1998). This is influenced by the concentration of
hydroxide, bicarbonate or carbonate ions. In water-
quality analysis the term 'alkalinity' is used almost
exclusively to refer to the concentration of bicarbon-
ate (HCO
3-
) ions because this is the most variable of
the three. The bicarbonate ions are derived from
the percolation of water through calcareous rocks
(e.g. limestones or chalk). It is important to know
their concentration for the buffering of pH and for
issues of water hardness. The buffering capacity of
soils, and water derived from soils, is an important
concept in water quality. The buffering capacity of
a solution is the ability to absorb acid without
changing the pH. This is achieved through a high
base cation load or high bicarbonate load. This is
why soil derived from limestone and chalk has fewer
problems coping with acid rain.
Dissolved oxygen
Dissolved oxygen is vital to any aquatic fauna that
use gills to breath. Salmonid species of fish require
dissolved oxygen contents greater than 5 mg/l,
Table 7.3
Effect of increasing acidity on aquatic ecology
Effect on organisms or process
pH value
Mayflies disappear
6.5
Phytoplankton species decline - green filamentous periphyton appears
6
Molluscs disappear
5.5-6.0
Waterfowl breeding declines
5.5
Bacterial decomposition slows/fungal decomposition appears
5
Salmonid reproduction fails - aluminium toxicity increases
5
Most amphibia disappear
5
Caddis flies, stone flies and Megaloptera (dobsonflies, alderflies, etc.) disappear
4.5-5.0
Beetles, dragonflies and damselflies disappear
4.5
Most adult fish harmed
4.5
Source
: Dodds (2002), adapted from Jeffries and Mills (1990
)
