Sulphate mass balances of Precambrian Shield wetlands; the influence of catchment hydrogeology

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Abstract
The sulphate budgets of two valley-bottom conifer swamps, located in headwater catchments with contrasting till depth, typical of the southern Canadian Shield were determined from June 1990 to August 1992. Temporal variations in the retention of SO42− in both swamps were related to the magnitude of upland runoff and water level in the wetlands. Contrasts in catchment till depth and nature of groundwater flow resulted in differences in SO42− retention between years and swamps. Transient groundwater inputs entering the swamp in the thin-till catchment resulted in a large water-table drawdown during dry summers, export of SO42− the following fall, and low annual SO42− retention. Continuous discharge of groundwater into the swamp in the moderate-till catchment maintained saturated surfaces and efficient SO42− retention during both years with dry and wet summers. Greater SO42− retention in this swamp may also be contributed by the greater interaction of groundwater, rich in SO42−, with deeper peat. Data from these two sites indicate that within the shallow till – rock ridge physiographic region of the Canadian Shield, small variations in catchment till depth can result in a large range in temporal and spatial variations in SO42− retention in valley wetlands.