A. Henriksen et al., CRITICAL LOADS OF ACIDITY FOR SURFACE WATERS - CAN THE ANC(LIMIT) BE CONSIDERED VARIABLE, Water, air and soil pollution, 85(4), 1995, pp. 2419-2424
The critical load of acidity for surface waters is based on the concep
t that the inputs of acids to a catchment do not exceed the weathering
less a given amount of ANC. The Steady State Water Chemistry (SSWC) M
ethod is used to calculate critical loads, using present water chemist
ry. To ensure no damage to biological indicators such as fish species
a value for ANC(limit), of 20 mu eq/l has been used to date for calcul
ating critical loads. The SSWC-method is sensitive to the choice of th
e ANC(limit). In areas with little acid deposition the probability of
acid episodes leading to fish kills is small even if the ANC(limit) is
set to zero, while in areas with high acidic deposition fish kills ma
y occur at this value. Thus, the ANC(limit) can be a function of the a
cidifying deposition to the lake, nearing zero at low deposition and i
ncreasing to higher values at higher deposition. A formulation for suc
h an ANC(limit) has been worked out and we have tested the effect of t
he ANC(limit) as a linear function of the deposition, assuming ANC(lim
it) = 0 at zero deposition with a linear increase to 50 mu eq/l at a d
eposition of 200 meq.m(-2).yi(-1). For areas with high deposition the
effect of a variable ANC(limit) is small, while in areas with low depo
sition the effect is significant. For Norway the exceeded area decreas
es from 36 to 30% using a variable ANC(limit) instead of a fixed value
of 20 mu eq/l.