Abstract
Temperate wetlands are generally net retainers of sulphate (SO~4~^β2^). However, recent research indicates that some wetlands may export SO~4~^β2^ during drought conditions. In temperate wetlands, the susceptibility to drought will depend on wetlandβstream and groundwaterβsurface water interactions. Here we present results of summer SO~4~^β2^ dynamics in a confined and unconfined stream within a temperate swamp.
At the confined transect, the water table levels dropped greatly (β55 cm) and groundwater flow patterns reversed in response to summer drought. Although high porewater SO~4~^β2^ concentrations (36Β·4 mg l^β1^) were measured, no downstream export of SO~4~^β2^ was observed. A SO~4~^β2^ mass balance analysis on this stream reach indicated near zero retention. At the unconfined transect, water flowed from the stream to the wetland throughout most of the summer, maintaining relatively high water table levels (β25 cm). Sulphate porewater concentrations were significantly lower (9Β·2 mg l^β1^) than in the confined transect. Downstream discharge decreased to zero within this reach during extreme drought conditions, resulting in 100% SO~4~^β2^ retention for a threeβweek period. Upon rewetting stream SO~4~^β2^ levels increased to 161Β·7 mg l^β1^ resulting in a large net export of SO~4~^β2^. This study demonstrates that while water table position was an important control on SO~4~^β2^ dynamics within the wetland, the degree of hydrologic connectivity between the wetland and the stream was the main control on episodic SO~4~^β2^ release following summer drought. Copyright Β© 2001 John Wiley & Sons, Ltd.