Critical loads of acidity for surface waters - Can the ANC(limit) be considered variable?

The critical load of acidity for surface waters is based on the concept 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) Method is used to calculate critical loads, using present water chemistry. 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 calculating critical loads. The SSWC-method is sensitive to the choice of the 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 may occur at this value. Thus, the ANC(limit) can be a function of the acidifying deposition to the lake, nearing zero at low deposition and increasing to higher values at higher deposition. A formulation for such an ANC(limit) has been worked out and we have tested the effect of the ANC(limit) as a linear function of the deposition, assuming ANC(limit) = 0 at zero deposition with a linear increase to 50 mu eq/l at a deposition 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 deposition the effect is significant. For Norway the exceeded area decreases from 36 to 30% using a variable ANC(limit) instead of a fixed value of 20 mu eq/l.