A 2-COMPONENT MIXING MODEL FOR PREDICTING REGIONAL EPISODIC ACIDIFICATION OF SURFACE WATERS DURING SPRING SNOWMELT PERIODS

A two-component mixing model of acid neutralizing capacity (ANC) is pr oposed for explaining two observed features related to the episodic ac idification of surface waters during snowmelt periods: (1) maximum epi sodic declines in ANC are largest in high ANC systems and increase lin early with antecedent ANC and (2) relative depressions in ANC attribut able to increases in nitric acid concentrations are larger in low ANC systems, while relative depressions in ANC attributable to dilution of base cations are larger in high ANC systems, Conceptually, the model represents the physical mixing of two hydrochemical end-members within a surface water environment, although the physical sources of water i n the model are undefined. The model is shown to explain 55-72% of the total variation of these characteristics among various surface water systems within the Catskill and Adirondack mountain regions of New Yor k. In addition, the model also explains 11-47% of the relative depress ion in ANC attributable to natural organic acidity in surface waters i n these regions, The model is subsequently linked to an empirical equi librium acidification model for predicting the long-term episodic acid ification response of Adirondack lakes during snowmelt periods, Model predictions suggest that percentage decreases in sulfuric acid concent rations of the magnitude mandated by the 1990 Clean Air Act Amendments (40%) will not restore to positive values the ANC of ah Adirondack la kes which are currently acidic (ANC < 0) during spring snowmelt period s. Long-term increases in nitric acid concentrations may counterbalanc e the expected increases in ANC attributable to reductions in sulfur d eposition.