SULFATE 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. S ulphate dynamics at the outlet stream reflected hydrological and bioge ochemical processes within the valley wetlands, which in turn were par tly 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 concen trations 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 wate r with anoxic peats, and therefore, SO4 reduction. Contrasts in till d epth 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 t hin 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 swam p in the catchment with deeper till was critical to maintaining satura ted surfaces and efficient SO4 retention during both dry and wet summe rs. A conceptual model of wetland SO4 retention and export, based on c atchment hydrogeology, is developed to generalize the SO4 dynamics of valley bottom wetlands at the landscape scale. (C) 1997 by John Wiley & Sons, Ltd.