A S-ISOTOPE APPROACH TO DETERMINE THE RELATIVE CONTRIBUTION OF REDOX PROCESSES TO NET SO4 EXPORT FROM UPLAND, AND WETLAND-DOMINATED CATCHMENTS

Abstract

Reoxidation of S stored in lowlands after summer droughts has been reported to be responsible for the excess SO4 export observed in many catchments in south central Ontario. Stable S isotopes can be used to identify the source of SO4 export in stream water, and are particularly well suited to evaluating zones of dissimilatory SO4 reduction (DSR) and the contribution of oxidation of reduced S species to stream SO4. The Plastic Lake-1 (PC1) stream drains an upland coniferous forest and then passes through a Sphagnum-dominated swamp before discharging to Plastic Lake. Measurements of SO4 fluxes and isotope ratios were used to determine the source of net SO4 export and the contribution of redox processes to S retention and export in the upland and wetland, respectively. Mass balance budgets for the years 1999/00 and 2000/01, which had comparatively wet summers, indicated that the upland part of the catchment consistently exported SO4 in excess of bulk deposition inputs. In contrast, mass budget calculations for the swamp indicated a net retention of 3 and 2 g S-SO4/m2 of wetland area, in 1999/00 and 2000/01 respectively. Higher δ34SO4 ratios and lower SO4 concentrations in the swamp outflow (average +8.6 ± 2.6‰; 1.5 ± 0.6 mg S-SO4/L) compared to the inflow draining the upland (+5.4 ± 0.7‰; 2.4 ± 0.3 mg S-SO4/L) indicated that DSR was at least partly responsible for net SO4 retention in the swamp. Isotope values in upland stream water (+5.7 ± 0.7‰) were only slightly higher than values in bulk deposition (average +5.1 ± 0.6‰) and soil leachate (+4.4 ± 0.4‰) over the 2-year period of study. Similar δ34SO4 values in upland stream water compared to deposition and soil leachate, despite substantial variations in water table height in the streambed (92 cm), suggest that reoxidation of reduced sulphides is not an important contributor to SO4 export from the upland. Rather, net SO4 export from the upland subcatchment is likely due to net release from upland soil, and slight differences in δ34SO4 between bulk deposition and soil leachate are consistent with SO4 release from organic S forms.