Use of stable sulfur isotopes to identify sources of sulfate in Rocky Mountain snowpacks

Stable sulfur isotope ratios and major ions in bulk snowpack samples were monitored at a network of 52 high-elevation sites along and near the Continental Divide from 1993 to 1999. This information was collected to better define atmospheric deposition to remote areas of the Rocky Mountains and to help identify the major source regions of sulfate in winter deposition. Average annual delta S-34 values at individual sites ranged from + 4.0 to + 8.2 parts per thousand and standard deviations ranged from 0.4 to 1.6 parts per thousand. The chemical composition of all samples was extremely dilute and slightly acidic; average sulfate concentrations ranged from 2.4 to 12.2 mu eq 1(-1) and pH ranged From 4.82 to 5.70. The range of delta S-34 values measured in this study indicated that snowpack sulfur in the Rocky Mountains is primarily derived from anthropogenic sources. A nearly linear relation between delta S-34 and latitude was observed for sites in New Mexico, Colorado, and southern Wyoming, which indicates that snowpack sulfate in the southern part of the network was derived from two isotopically distinct source regions. Because the major point sources of SO2 in the region are coal-fired powerplants, this pattern may reflect variations in the isotopic composition of coals burned by the plants. The geographic pattern in delta S-34 for sites farther to the north in Wyoming and Montana was much less distinct, perhaps reflecting the paucity of major point sources of SO2 in the northern part of the network. Published by Elsevier Science Ltd.