Historical Trends in Atmospheric Sulfur Deposition and Methods for Assessing Long-Term Trends in Surface Water Chemistry

Abstract

This chapter presents historical sulfur emission and deposition trends for regions in the United States and describes methods for assessing changes in water chemistry based on current spatial patterns, ion ratios and empirical models, and paleolimnological approaches. Reconstruction of sulfur deposition trends shows that current deposition to case study regions ranges from a factor of about 1 (Upper Midwest) to a factor of 10 (Catskills) above natural background. Deposition in the Northeast was high during the 1920s, 1940s, and 1960s, and has declined significantly since 1970. Sulfur deposition in the Southeast was low before the 1950s, but has increased significantly since then. Change in surface water chemistry can be assessed using simple empirical models, ion ratios, and analysis of current spatial patterns of chemistry. Many assumptions are implicit in these methods, so results should be interpreted carefully. Paleolimnological reconstructions of chemistry and biota from lake sediment records provide more direct evidence of past change than other approaches. Quantitative analyses of diatom and chrysophyte assemblages can be used to reconstruct past lakewater pH with a mean standard error of about ± 0.25 pH units.