Changes in the chemistry of lakes in the Adirondack region of New York following declines in acidic deposition

Long-term changes in the chemistry of wet deposition and lake water were investigated in the Adirondack region of New York. Marked decreases in concentrations of SO₄^{2 -} and H⁺ have occurred in wet deposition since the late 1970s. These decreases are consistent with long-term declines in emissions of SO₂ in the eastern US. Changes in wet NO₃^- deposition and NO_x emissions have been minor over the same interval. Virtually all Adirondack lakes have exhibited large decreases in concentrations of SO₄^{2 -}, which coincide with decreases in atmospheric S deposition. Since 1992, concentrations of NO₃^- have also decreased in many (27 of 48) Adirondack lakes. As atmospheric N deposition has not changed appreciably over this period (1992-2004), the mechanism contributing to this apparent increase in lake/watershed N retention is not evident. Decreases in concentrations of SO₄^{2 -} + NO₃^- have resulted in increases in acid neutralizing capacity (ANC; 37 of 48 lakes) and pH (31 of 48 lakes), and decreases in concentrations of inorganic monomeric Al, particularly in acid-sensitive lakes. Concentrations of dissolved organic C (DOC) have also increased in some (15 of 48) lakes coinciding with decreases in acidic deposition. Examination of changes in lake chemistry by hydrologic classes showed that drainage lakes in watersheds with thin deposits of glacial till and mounded seepage lakes have generally been the most responsive to decreases in acidic deposition.