An evaluation of processes regulating spatial and temporal patterns in lake sulfate in the Adirondack region of New York

As a result of the Clean Air Act Amendments of 1970 and 1990, there have been significant decreases in sulfate (SO₄^2-) concentrations in surface waters across the northeastern United States. The 37 Direct/Delayed Response Program (DDRP) watersheds in the Adirondacks receive elevated levels of atmospheric S deposition and showed considerable variability in lake SO₄^2- concentrations. In response to decreases in atmospheric S deposition, these sites have generally exhibited relatively uniform decreases in surface water SO₄^2- concentrations. In this study, an integrated biogeochemical model (PnET-BGC) was used to simulate the response of lake SO₄^2- concentrations at these DDRP sites to recent changes in atmospheric S deposition. Using default parameters and algorithms, the model underpredicted lake SO₄ ^2- concentrations at sites with high SO₄ ^2- concentrations and overpredicted at sites with low SO₄^2- concentrations. Initial predictions of lake SO₄^2- were relatively uniform across the region. Initial model simulations also underpredicted decreases in lake SO ₄^2- concentrations from 1984 to 2001. We identified seven hypotheses that might explain the discrepancies between model predictions and the measured data. Model inputs, parameters, and algorithms were modified to help test these hypotheses and better understand factors that control spatial and temporal patterns in lake SO₄^2- in this acid-sensitive region.