Use of ¹⁷O/¹⁶O to trace atmospherically-deposited sulfate in surface waters: A case study in alpine watersheds in the Rocky Mountains

Building on the discovery of excess ¹⁷O in atmospheric sulfate by Lee et al. (2001), we have carried out a case study to determine whether ¹⁷O might provide a new tool for quantifying the impact of atmospheric deposition on surface-water sulfate loads. In Rocky Mountain alpine regions, excess ¹⁷O was found to be characteristics of atmospheric sulfate deposited in snow. Excess ¹⁷O was also evident in stream sulfate in one of two high-elevation watersheds where analyses were made. Isotope mass balance calculations gave surprizingly low atmospheric contributions to stream sulfate suggesting that (1) despite abundant outcrop and sparse soil in these areas, significant sulfate may be taken up and released by soil microbes before being exported in streams, and (2) surface waters can carry multiple non-atmospheric sulfate types, some possibly anthropogenic. Measurements of ¹⁷O may prove very useful in studies of sulfate behavior in a variety of surficial environments.