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

Abstract

Sulphate (SO ⁻²₄ ) 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 depth to identify catchments that produce SO ⁻²₄ pulses. Mean annual measured maximum SO ⁻²₄ concentrations were significantly greater in shallow till (<1 m depth) catchments containing wetlands than catchments covered with deeper tills (>1 m depth) containing wetlands or catchments with no wetlands. Average maximum SO ⁻²₄ concentrations in wetland catchments during years with dry summers were >20 mg/L in 19 of 20 catchments with average till depths of <1 m, whereas concentrations were <20 mg/L in 5 of 6 watersheds with average till depths of >1 m. Peaks in mean annual maximum SO ⁻²₄ concentrations from wetland catchments with shallow till occurred during summers with rain fall 150–200 mm less than potential evaporation estimates. There were no significant differences in mean average annual SO ⁻²₄ concentration among the different catchments during wet summers, with SO ⁻²₄ concentrations ranging from 6 to 13 mg/L. These observations suggest that a large portion of the temporal and spatial variation in SO ⁻²₄ chemistry and export can be predicted in headwater catchments of the Canadian Shield and perhaps in other landscapes where till depth influences upland-wetland hydrologic connections.