Recovery of surface waters in the northeastern U.S. from decreases in atmospheric deposition of sulfur

A simple mass flux model was developed to simulate the response of SO₄^2- concentrations in surface waters to past and anticipated future changes in atmospheric deposition of SO₄^2-. Values of bulk (or wet) SO₄^2- deposition and dry deposition of S determined from measured air concentrations and a deposition velocity were insufficient to balance watershed SO₄^2- export at the Hubbard Brook Experimental Forest, NH and for a regional survey of watersheds in the northeastern U.S. We propose two explanations for the unmeasured S source: I) a significant underestimation of dry S deposition, and/or 2) internal watershed S sources, such as weathering and/or mineralization of soil organic S. Model simulations based on these two mechanisms agreed closely with measured stream SO₄^2- concentrations at Hubbard Brook. Close agreement between measured and model predicted results precluded identification of which of the two mechanisms controlled long-term trends in stream SO₄^2-. Model simulations indicated that soil adsorption reactions significantly delayed the response of stream water to declines in SO₄^2- inputs since 1970, but could not explain the discrepancy in watershed S budgets. Extrapolation of model predictions into the future demonstrates that uncertainty in the source of the S imbalance in watersheds has important implications for assessments of the recovery of surface water acid neutralizing capacity in response to anticipated future reductions in SO₂ emissions.