Lakes and Watercourses

Acidity / acidification
Introduction
The acidity of water is significant to aquatic organisms because it affects a
number of important biotic and abiotic processes. Indirectly, acidity is also
important to aquatic organisms because it governs the chemical form in
which metals occur. Dissolved aluminium is particularly important, since
this may occur in toxic form at high concentrations under acid conditions.
Most waters have a buffering capacity, ie, they are able to neutralise
the input of acidic substances. Buffering capacity is principally determined
by hydrocarbonate; only when this is nearly exhausted can water
become severely acidified. Alkalinity is used here as a measure of
buffering capacity. The lower the alkalinity, the greater the effect of acidic
input on the acidity.
An alternative measure of buffering capacity is ANC (acid neutralising
capacity), which, in addition to hydrocarbonate, also includes organic
anions. The difference between ANC and alkalinity is fairly small in clear
waters, but in brown (humic) waters, ANC may be substantially higher
than alkalinity. ANC has become more widely used internationally for
acidification assessments in recent years, although alkalinity has a simpler
and clearer correlation to the acidity of water. When alkalinity approaches
zero pH falls most rapidly, regardless of the ANC level at that point.
The water’s natural content of organic anions may have an appreciable
effect on its acidity and sensitivity to acidification. However, the fact
that alkalinity rather than ANC has been chosen as the measure of
buffering capacity does not mean that this natural effect is assumed to be
non-existent, nor that it is confused with anthropogenic impact. The
calculation below showing how present buffering capacity differs from
that during the pre-industrial era, solely refers to the change caused by
the sulphur deposition of recent years, regardless of the original acidity of
the water. In practice, this change in buffering capacity will be equally
great, whether it is measured as the difference in alkalinity or the
difference in ANC. However, the estimated correlation between present
and pre-industrial alkalinity is easier to use than the equivalent ANC
correlation as the basis for an assessment of whether the pH of the water
has been affected by acid deposition.
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