Lakes and Watercourses

industrial pH). The change in acidity since the pre-industrial era –
acidification – should essentially reflect human impact.
TABLE 17.
DEVIATION from reference value, alkalinity
Class Description Present alkalinity/reference value Corresponding
(alk/alk o for alk ≥ 0.01 meq/l)* difference in pH
1 Insignificant deviation > 0.75 ≤ 0.1
2 Moderate deviation 0.50 – 0.75 0.1 – 0.3
3 Large deviation 0.25 – 0.50 0.3 – 0.6
4 Very large deviation 0.10 – 0.25 0.6 – 1.0
5 Extremely large deviation ≤ 0.10 > 1.0
* Classification for alk < 0.01 meq/l, see modified alkalinity ratio below
The reference value (alk o ) is calculated as follows (a simplified and
modified version of the method described by Bernes, 1991):
alk o = alk + (1 - F) (SO 4 * - SO 4 * o )
SO 4 *
SO 4 * o
BC*
= SO 4 - 0.103 Cl
= 0.005 + 0.05 BC*
= Ca + Mg + Na + K - 1.111 Cl
F = 0.8 arctan (4.3 (alk + 0.2))
where
alk o = reference value
alk = present alkalinity
SO 4 * = present concentration of sulphate ions of nonmarine
origin
SO 4 * o = pre-industrial concentration of sulphate ions of
non-marine origin
BC* = present concentration of base cations of non-marine
origin
F = a measure of the proportion of anthropogenic sulphur
deposition neutralised by ionic exchange reactions in the
soil
All concentrations are expressed in meq/l, whereas F is expressed in
radians. If the water has no detectable alkalinity, acidity should be
measured instead. Acidity is expressed throughout as negative alkalinity.
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