Chemical recovery of surface waters across the northeastern United States from reduced inputs of acidic deposition: 1984-2001

Changes in lake water chemistry between 1984 and 2001 at 130 stratified random sites across the northeastern United States were studied to evaluate the population-level effects of decreases in acidic deposition. Surface-water SO₄^2- concentrations decreased across the region at a median rate of -1.53 μequiv L^-1 year^-1. Calcium concentrations also decreased, with a median rate of -1.73 μequiv L ^-1 year^-1. This decrease in Ca²⁺ retarded the recovery of surface water acid neutralizing capacity (Gran ANC), which increased at a median rate of 0.66 μequiv L^-1 year^-1. There were small increases in pH in all subregions except central New England and Maine, where the changes were not statistically significant. Median NO^{3 -} trends were not significant except in the Adirondacks, where NO₃^- concentrations increased at a rate of 0.53 μequiv L^-1 year^-1. A regionwide decrease in the concentration of total Al, especially in ponds with low ANC values (ANC < 25 μequiv L ^-1), was observed in the Adirondack subregion. These changes in Al were consistent with the general pattern of increasing pH and ANC. Despite the general pattern of chemical recovery, many ponds remain chronically acidic or are susceptible to episodic acidification. The continued chemical and biological recovery at sites in the northeastern United States will depend on further controls on S and N emissions.