Sulphate dynamics in relation to groundwater - Surface water interactions in headwater wetlands of the southern Canadian Shield

The spatial and temporal distribution of sulphate (SO4) concentrations in peat pore water and the outlet streams of two forested swamps was related to variations in the magnitude of upland runoff, wetland water levels and flow path. The swamps were located in headwater catchments with contrasting till depths typical of the southern Canadian Shield. Inputs of SO4 from shallow hillslope tills and streams showed little seasonal variation in either source or concentration in both swamps. Sulphate dynamics at the outlet stream reflected hydrological and biogeochemical processes within the valley wetlands, which in turn were partly controlled by catchment hydrogeology. During high runoff, maximum water table elevations and peak surface flow in the swamps resulted in upland inputs largely bypassing anoxic peat. Consequently, SO4 concentrations of 8-10 mg/l at the swamp outlets were similar to stream and groundwater inputs. During periods of low flow, concentrations of SO4 at the swamp outlets declined to less than 3 mg/l. At this time lower water table elevations resulted in increased interaction of input water with anoxic peats, and therefore, SO4 reduction. Contrasts in till depth and the nature of groundwater flow between catchments resulted in differences in SO4 dynamics between years and swamps. In dry summers the absence of groundwater inputs to the swamp in the catchment with thin till resulted in a large water table drawdown and re-oxidation of accumulated S, which contributed to maximum SO4 concentrations (up to 35 mg/l) during storm runoff. Continuous groundwater input to the swamp in the catchment with deeper till was critical to maintaining saturated surfaces and efficient SO4 retention during both dry and wet summers. A conceptual model of wetland SO4 retention and export, based on catchment hydrogeology, is developed to generalize the SO4 dynamics of valley bottom wetlands at the landscape scale. (C) 1997 by John Wiley & Sons, Ltd.