The Adirondack region of New York has experienced among the most severe impacts of acidic deposition in North America, including acidification of soil and surface waters with effects on forest vegetation and aquatic biota. We report the results on long-term changes in atmospheric deposition, using data from the National Trends Network at Huntington Forest (NY20) and Total Deposition Measurement Model Fusion (TDep MMF) estimated for the Adirondack region, and lake chemistry from the Adirondack Long-term Monitoring (ALTM) program. The Adirondack region has experienced decreases in acid deposition over the past decades, including marked decreases in sulfate and nitrate deposition and minor decreases in ammonium. Current levels of sulfate deposition measured in the Adirondacks are comparable to values reported for remote regions relatively unimpacted by human emissions. Consistent with decreases in atmospheric sulfate and nitrate deposition have been significant decreases in concentrations of sulfate and nitrate in ALTM lakes. Of the 48 ALTM lakes not been treated by liming, 48 have shown significant decreases in concentrations of sulfate with a mean rate of change of -2.5 μeq/L-yr and 43 have experienced significant decreases in nitrate at a rate of - 0.4 μeq/L-yr since 1992. Long-term measurements of nitrate also reveal a marked decrease in the amplitude of seasonal variation in concentrations. Changes in these inputs have influenced the acid-base status of lakes. We report two metrics of trends in the acid-base chemistry of ALTM lakes: measured values of acid neutralizing capacity (ANC) and theoretical ANC calculated as the sum of base cations less the sum of strong acid anions. While both metrics reveal widespread long-term increases in ANC (44 lakes with significant increases), increases in theoretical values of ANC exceed measured values. This discrepancy appears due to marked increases in naturally occurring organic acids that have occurred over the measurement interval, as evidenced by significant increases in dissolved organic acid concentrations in 39 lakes at a mean rate of +9.6 μmol C/L-yr, which have altered the recovery of Adirondack lakes from acid deposition.

¹ Department of Civil and Environmental Engineering, Syracuse University, Syracuse, NY

² U.S. Environmental Protection Agency, Office of Air and Radiation, Washington, DC

* Corresponding Author: ctdrisco@syr.edu