Hydrologic control of sulfate mobility in a forested watershed

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Abstract
Five years of atmospheric wet sulfate deposition and stream sulfate export data were evaluated to determine the fate and mobility of sulfate in a forested watershed in central Pennsylvania. Sulfate concentrations in stream water were inversely related to sulfate concentrations in precipitation. Sulfate export from this watershed exceeded wet deposition by approximately 20% in 4 out of the 5 years studied, indicating an additional source of sulfate, possibly dry deposition. The average sulfate export from this watershed during February through May was 69.8% of the total annual export. The fact that sulfate export during this 4‐month period could not result from wet deposition during this period suggests that atmospherically deposited sulfate is temporarily stored on the watershed and that the release of this stored sulfate is controlled strongly by the hydrologic responsiveness of the watershed. Analysis of both monthly and storm flow sulfate export/input data supports these conclusions and suggests that the release of stored sulfate can be described by the variable source area concept. The results also suggest that the sulfate adsorption capacity of the soils on this watershed has been satisfied and that a steady state condition exists between atmospheric sulfate inputs and export. Because hydrologic conditions appear to significantly control sulfate export, multiple years of record are required when using the watershed mass balance technique to estimate dry deposition or determine the status of sulfate adsorption on a basin. It is recommended that at least 3–5 years of input/export data be used when using this technique.