Seasonal and event variations in delta34S values of stream sulfate in a Vermont forested catchment: implications for sulfur sources and cycling.

Stable sulfur (S) isotope ratios can be used to identify the sources of sulfate contributing to streamwater. We collected weekly and high-flow stream samples for S isotopic analysis of sulfate through the entire water year 2003 plus the snowmelt period of 2004. The study area was the 41-ha forested W-9 catchment at Sleepers River Research Watershed, Vermont, a site known to produce sulfate from weathering of sulfide minerals in the bedrock. The delta(34)S values of streamwater sulfate followed an annual sinusoidal pattern ranging from about 6.5 per thousand in early spring to about 10 per thousand in early fall. During high-flow events, delta(34)S values typically decreased by 1 to 3 per thousand from the prevailing seasonal value. The isotopic evidence suggests that stream sulfate concentrations are controlled by: (1) an overall dominance of bedrock-derived sulfate (delta(34)S approximately 6-14 per thousand); (2) contributions of pedogenic sulfate (delta(34)S approximately 5-6 per thousand) during snowmelt and storms with progressively diminishing contributions during base flow recession; and (3) minor effects of dissimilatory bacterial sulfate reduction and subsequent reoxidation of sulfides. Bedrock should not be overlooked as a source of S in catchment sulfate budgets.