Sensitivity of Critical Load Estimates for Surface Waters to Model Selection and Regionalization Schemes

A critical load is the amount of atmospheric pollutant that can be deposited on a sensitive ecosystem without causing measurable, longterm degradation in ecosystem form or function. We compare several methods for making critical load estimates of SO4 deposition to lakes; these methods are among those identified by participants in the United Nations-Economic Commission for Europe (UNECE) Convention on Long-Range Transboundary Air Pollution. The models were originally developed to address surface water acidification issues. Critical loads are computed here as the levels of deposition required to reduce surface water acid neutralizing capacities (ANC) from present values to either 25 mueq L-1 or 0 mueq L-1. These end points were selected to provide a common basis for comparing critical load values obtained from the various models, and are not intended to provide definitive critical load estimates. Using the amount of deposition required to decrease steady-state surface water ANC values from 25 to 0 mueq L-1, about 9 +/- 2 kgSO4 ha-1 yr-1 in the northeastern USA as a basis for comparison, we find that models make significantly different projections regarding ecosystem sensitivites to deposition. For lakes with critical load values in the range of current deposition in the Northeast (i.e., 18-56 kg SO4 ha-1 yr-1), model results differ by as much as 82 kg SO4 ha-1 yr-1. Differences in results are observed not only between models, but within models when slightly different approaches are used for estimating certain parameters. The interpretation of model results depends strongly on how systems are grouped regionally. While the results suggest that reductions in deposition from current levels would reduce adverse effects in sensitive lakes, the magnitude and timing of the reductions necessary and the determination of appropriate population end points are decisions that will ultimately be made by risk managers rather than risk assessors.