Episodic sulphate export from wetlands in acidified headwater catchments: prediction at the landscape scale

Sulphate (SO4-2) concentrations in 34 intensively measured Canadian Shield streams near the Dorset Research Centre, central Ontario, were used to test a hydrogeologic model that uses simple measures of wetland area and till d epth to identify catchments that produce SO4-2 pulses. Mean annual measured maximum SO4-2 concentrations were significantly greater in shallow till (< 1 m depth) catchments containing wetlands than catchments covered with deep er tills (>1 m depth) containing wetlands or catchments with no wetlands. A verage maximum SO4-2 concentrations in wetland catchments during years with dry summers were >20 mg/L in 19 of 20 catchments with average till depths of tl m, whereas concentrations were <20 mg/L in 5 of 6 watersheds with ave rage till depths of >1 m. Peaks in mean annual maximum SO4-2 concentrations from wetland catchments with shallow till occurred during summers with rai n fall 150-200 mm less than potential evaporation estimates. There were no significant differences in mean average annual SO4-2 concentration among th e different catchments during wet summers, with SO4-2 concentrations rangin g from 6 to 13 mg/L. These observations suggest that a large portion of the temporal and spatial variation in SO42- chemistry and export can be predic ted in headwater catchments of the Canadian Shield and perhaps in other lan dscapes where till depth influences upland-wetland hydrologic connections.