Application of a sulfur biogeochemical model with isotopes to Sleepers River, Vermont, USA.
Selvendiran, Pranesh*,1, Driscoll, Charles1, Mitchell, Myron2, Shanley, James 3, 1 Syracuse University, Syracuse, New York, USA2 State University of New York, Syracuse, New York, USA3 U.S Geological Survey, Montpelier, Vermont, USA
ABSTRACT- An integrated biogeochemical model, PnET-BGC, was applied to investigate sulfur (S) dynamics at Sleepers River Watershed (SLR), Vermont, USA. In addition to quantify watershed S budgets, the model was applied to perform S isotope simulations. The PnET-BGC model was also modified to include wetland S transformation processes to account for the presence of wetlands within the SLR watershed. Finally, the model estimated S budgets at SLR, where there is a strong evidence of S weathering, was compared with previous estimates at the HBEF (Gbondo-Tugbawa et al.,2001), where atmospheric deposition dominates S cycling. Based on the average net release of S during the period 1992- 2000, 11 kg S ha-1yr-1 was assumed to be provided from weathering release. However, in addition to the weathering input, a dry-to-wet deposition ratio of 0.44 was required to explain the stream water export of S. Model predictions indicate that wetlands may retain up to 13% of the net drainage loss of S (17.8 kg S ha-1 yr-1). Isotopic simulations indicate that during organic S mineralization there was a small positive 34S shift of +0.3‰. Bacterial dissimilatory S reduction (BDSR) in wetlands resulted in enrichment in the 34S of residual SO42- up to +5.5‰. The influence of BDSR on stream water 34S may increase with percent wetland coverage in the watershed. Comparison of model estimated S budgets showed distinct differences in S cycling at SLR and the HBEF. Weathering release and atmospheric deposition of S were the dominant sources of stream water SO42- at SLR, whereas, at the HBEF atmospheric deposition is the predominant source of stream water SO42-. This phenomenon was corroborated by del34S measurements in stream water at these sites. For SLR the shift in stream water 34S occured between atmospheric and weathering end member during high flow and base flow respectively, but closely represented atmospheric end member at the HBEF. At both SLR and HBEF, the model predicted that biotic cycling (i.e., mineralization and plant uptake) are the largest flux of S dynamics and were almost equal to one another.
Key words: Sleepers River Watershed (Vermont, USA), Sulfur isotopes, Dry to wet S deposition ratio, Sulfur weathering
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