Critical loads of acidity for surface waters

Abstract

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 μeq/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 ueq/l at a deposition of 200 meq.m⁻².yr⁻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 μeq/l.