The chemical response of surface waters and organic soils across the northeastern United States, following reduced inputs of acidic deposition: 1984-2001

I studied the changes in lake water chemistry and Oa-horizon base status between 1984 and 2001 at 130 stratified random sites across the northeastern U.S. to evaluate the population-level effects of decreases in acidic deposition. Surface-water SO42- concentrations decreased across the region at a median rate of -1.53 mueq L-1yr -1. Calcium concentrations also decreased, with a median rate of -1.73 mueq L-1yr-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 mueq L-1yr -1. There were small increases in pH in all sub-regions except Central New England and Maine, where the changes were not statistically significant. Median NO3- trends were not significant, except in the Adirondacks, where NO3- concentrations increased at a rate of +0.53 mueq L-1yr-1. I observed a region-wide decrease in the concentration of total Al, especially in ponds with low ANC values (ANC < 25 mueq L-1) in the Adirondack sub-region. 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 U.S. will depend on further controls on S and N emissions.; Over the study period, median base saturation in Oa-horizon soils decreased from 56.2% in 1984 to 33.0% in 2001. Effective cation exchange capacity, normalized to soil carbon concentration, showed no significant change between 1984 and 2001. The change in base saturation was the result of almost equivalent changes in exchangeable calcium (CaN) and exchangeable aluminum (Al N). The median CaN declined by more than 50%, from 23.5 to 10.6 cmolc/kgC, while median AlN more than doubled, from 8.8 to 21.3 cmolc/kgC. I observed the greatest change in soil acid-base properties in the montane regions of Central New England and Maine, where base saturation decreased by more than 50% and median soil pHs (0.01M CaCl2) decreased from 3.19 to 2.97. The acidification of Oa-horizon soils between 1984 and 2001 occurred despite substantial reductions in atmospheric acidic deposition. These results may help to explain the surprisingly slow rate of recovery of surface waters.