Lakes and Watercourses

The concentration of base cations in limed lakes must be determined
on the basis of reliable readings taken prior to liming or by calculating the
calcium concentration from the magnesium content of the water and the
Ca/Mg ratio in nearby, similar but unlimed waters. The latter alternative
presupposes that limestone containing no magnesium (ie, limestone
containing little dolomite) has been used for liming.
If present alkalinity is less than 0.01 meq/l, the alkalinity ratio derived
above cannot be used as a basis for classifying deviation. Instead, a
modified alkalinity ratio can be estimated as shown below.
alk < –0.01 –0.01< alk < 0.01 alk ≥ 0.01
alk o > 0.01 0.25 (–0.01 /alk) (0.01/alk o ) (0.01/alk o ) 2 –100 (0.01 – alk) alk/alk o
–0,01< alk o < 0.01 (–0.01 /alk) 2 –100 (alk o + 0.01)
2 –100 (alk o – alk)
alk o < –0.01
alk o /alk
The modified alkalinity ratio can be used instead of the normal
alkalinity ratio to determine the deviation class in the deviation table.
Example: Present alkalinity is measured (by acidity analysis) at -0.02
meq/l. Pre-industrial alkalinity is estimated at 0.02 meq/l. The modified
alkalinity ratio (using the expression at top left above) will then be 0.25 (-
0.01/-0.02)(0.01/0.02) = 0.25 · 0.5 · 0.5 = 0.06. It may be seen from the
deviation table that this represents deviation class 5.
0.40
Non-marine sulphate concentration (meq/l)
0.35
0.30
0.25
0.20
0.15
0.10
Extremely severe acidification
Very severe acidification
Severe
Moderate
acidification
acidification
Figure 2. Diagram
showing deviation class
(acidification) based on
recorded alkalinity and
concentration of nonmarine
sulphate.
0.05
Insignificant
acidification
0
–0.20 –0.15 –0.10 –0.05 0 0.05 0.10 0.15 0.20
Alkalinity (meq/l)
39