Atmospheric sulfur deposition 1950–1985 inferred from sulfate in groundwater

Authors:
  • WRW. D. Robertson
    W. D. Robertson
  • JCJ. A. Cherry
    J. A. Cherry
  • SSS. L. Schiff
    S. L. Schiff
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Abstract
Groundwater sulfate that occurs at concentrations of 6–27 mg/L in a shallow water table sand aquifer, at a forested site 100 km east of Sudbury, Ontario, is used to deduce the trend in atmospheric sulfur deposition between 1950 and 1985. The simple geologic nature of the aquifer made feasible this trend reconstruction. The age of groundwater that entered the aquifer during the past 60 years was determined with unusual accuracy (±3 years) from tritium analyses and flow modeling. Sulfur isotopes and mass balances indicate that the sulfate in the groundwater zone is of atmospheric origin and is not derived from solid phase material in the sediments. Evapotranspiration enriches atmospheric sulfate by a factor of about three as it enters the aquifer. Adsorption of sulfate is not significant. Groundwater sulfate indicates a decrease of 37% in sulfur deposition at the site between a maximum in the 1960s and lower values observed in the 1980s. We attributed most of the decrease ((bout 80%) to lower SO2 emissions from the Sudbury smelters. These emissions are now about one third of the 1960s levels. Further major decreases in sulfur deposition at this site can be achieved only if emissions from sources distant from Sudbury are lowered as well because Sudbury now contributes less than 25% of total sulfur deposition.

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