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

Building on the discovery of excess O-17 in atmospheric sulfate by Lee et a l. (2001), we have carried out a case study to determine whether O-17 might provide a new tool for quantifying the impact of atmospheric deposition on surface-water sulfate loads. In Rocky Mountain alpine regions, excess O-17 was found to be characteristic of atmospheric sulfate deposited in snow. E xcess O-17 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 th at (1) despite abundant outcrop and sparse soil in these areas, significant sulfate may be taken up and released by soil microbes before being exporte d in streams, and (2) surface waters can carry multiple non-atmospheric sul fate types, some possibly anthropogenic. Measurements of O-17 may prove ver y useful in studies of sulfate behavior in a variety of surficial environme nts.