![[logo] US EPA](http://www.epa.gov/epafiles/images/logo_epaseal.gif){kind=logo}
Driscoll, C.T., G.E. Likens, and M. Robbins Church. 1998. Recovery of surface waters in the northeastern U.S. from decreases in atmospheric deposition of sulfur. Water, Air, and Soil Pollution 105:318-329. Reprinted In: R.K. Wieder, M. Novák and J. Cerny, editors, Biogeochemical Investigations at the Watershed, Landscape, and Regional Scales, Kluwer Academic Publishers, Boston, pp. 319-329.
A simple mass flux model was developed to simulate the response of- SO42-concentrations in surface waters to past and anticipated future changes in atmospheric deposition of SO42- . Values of bulk (or wet) SO42- deposition and dry deposition of S determined from measured air concentrations and a deposition velocity were insufficient to balance watershed SO42- 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: 1) a significant underestimation of dry S deposition, kind/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 SO42- 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 SO42- Model simulations indicated that soil adsorption reactions significantly delayed the response of stream water to declines in SO42 -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.