Effect of declining lake base cation concentration on freshwater critical load calculations

Steady-state critical load models have been extensively used as the scientific underpinning for air pollution control policies in Europe and are currently being applied to other parts of the world. An important assumption of steady-state models is that critical load estimates do not change through time (or time scale of interest). The most commonly used model for estimating freshwater critical loads is the steady-state water chemistry (SSWC) model. In this study we examined changes in SSWC critical load estimates for 29 lakes in south-central Ontario using data collected 13 years apart (1985-1998), during which time bulk sulfate (SO4(2-)) deposition decreased by 35%. In lakes with the lowest base cation concentrations (<120 microequiv L(-1) Ca2+), the decrease in SO4(2-) concentration was accompanied by an approximately equivalent decrease in base cation concentration, resulting in only a minimal increase in Acid Neutralizing Capacity (ANC) during the 13-year period (median increase 1.6 microequiv L(-1)), and the median critical load for acidity (CL(A)) estimated by the SSWC model decreased by 14.6%. These changes may have been brought about by declining base cation concentrations owing to continued soil acidification in the region. In contrast, in lakes with higher base cation concentrations (>150 microequiv L(-1) Ca2+), the relative decline in base cation concentration was not as great, resulting in a larger increase in ANC (median increase 13.1 microequiv L(-1)) and an increase in the estimated CL(A) (median 5.1%). Lakes with moderate base cation concentrations (120-150 microequiv L(-1) Ca2+) exhibited an intermediate response; the median ANC increased by 8.8 microequiv L(-1) and the estimated CL-(A) decreased by 2.2%. In central Ontario, SSWC critical load estimates based on data taken only 13 years apart change quite dramatically due to changing lake base cation concentrations, and the response appears to depend on the base status of the lakes. The changing values obtained from the SSWC model have important consequences for policy decisions regarding acceptable levels of acid deposition. The application of dynamic models that take into account changes in lake/soil chemistry appears more appropriate for estimating acceptable levels of acid deposition in the region.