The response of lake water in the Adirondack region of New York to changes in acidic deposition

The Adirondack region of New York is characterized by a large number of lakes with low values of acid neutralizing capacity (ANC; ~25% of lakes have summer ANC values <0 μeq/l), which are sensitive to atmospheric deposition of strong acids. Time-series analysis showed that concentrations of sulfate (SO₄^2-), nitrate (NO₃^-), ammonium (NH₄⁺) and basic cations have decreased in precipitation, resulting in increases in pH. A relatively uniform rate of decline in SO₄^2- concentrations in lakes across the region (1.91±0.27 μeq/l yr) suggests that this change was due to decreases in atmospheric deposition. Despite the marked declines in concentrations of SO₄^2- in Adirondack lakes, there has been no systematic increase in pH and ANC. The limited response of lake water ANC and pH to decreases in atmospheric deposition of SO₄^2- may be attributed to a combination of factors, including: (1) depletion of exchangeable pools of basic cations in soil, (2) additional inputs of SO₄^2- to watersheds in the form of unmeasured dry deposition and/or an internal supply of sulfur (S) from mineralization of soil organic S pools or weathering of S minerals, (3) elevated leaching losses of NO₃^-, and/or (4) pH buffering associated with elevated concentrations of aluminum (Al) and/or naturally organic acids.