Air Quality Guidelines - World Health Organization Regional Office ...

indirect effects of acidifying compounds
Table 34. Critical load ranges of acidity used for the various combinations
of parent material and texture in terrestrial ecosystems
Guideline range of Parent material � Texture �
critical loads of acidity
(eq/ha per year)
< 250 acidic coarse
250–500 acidic coarse-medium
intermediate coarse
basic coarse
500–1000 acidic medium, medium-fine
intermediate coarse-medium, medium
basic coarse-medium
1000–1500 intermediate medium-fine
basic medium
> 1500 intermediate fine
basic medium-fine
a Acidic: sand (stone), gravel, granite, quartzine, gneiss (schist, shale, greywacke, glacial till).
Intermediate: gronodiorite, loess, fluvial and marine sediment (schist, shale, greywacke, glacial till).
Basic: gabbro, basalt, dolomite, volcanic deposits.
b Coarse: clay content < 18%.
Medium: clay content 18–35%.
Fine: clay content > 35%.
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Table 35 lists the ranges of critical loads in relation to combinations of base
cation concentration and runoff classes. For each critical load class, at least
50% of the critical load values computed on the basis of lake data from
Finland (1450 lakes), Norway and Sweden fall within the class boundaries,
given the ranges for present base cation concentrations and runoff. Only in
two cases did the boundaries for the base cation concentration classes overlap
between two critical load classes, when the class boundaries were set on
the basis of the 25 th and 75 th percentile base cation concentrations for given
runoff classes. For those cases the critical loads are determined more by
other factors than base cation levels and runoff, and the guideline value set is
therefore more uncertain than those without overlap.