Effect of drought on hydrology and sulphate dynamics in a temperate swamp

Temperate wetlands are generally net retainers of sulphate (SO4-2). However, recent research indicates that some wetlands may export SO4-2 during drought conditions. In temperate wetlands, the susceptibility to drought will depend on wetland-stream and groundwater-surface water interactions. Here we present results of summer SO4-2 dynamics in a confined and unconfined stream within a temperate swamp. At the confined transect, the water table levels dropped greatly (-55 cm) and groundwater flow patterns reversed in response to summer drought. Although high porewater SO4-2 concentrations (36.4 mg l(-1)) were measured, no downstream export of SO4-2 was observed. A SO4-2 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 (-25 cm). Sulphate porewater concentrations were significantly lower (9-2 mg l(-1)) than in the confined transect. Downstream discharge decreased to zero within this reach during extreme drought conditions, resulting in 100% SO4-2 retention for a three-week period. Upon rewetting stream SO4-2 levels increased to 161.7 mg l(-1) resulting in a large net export Of SO4-2. This study demonstrates that while water table position was an important control on SO4-2 dynamics within the wetland, the degree of hydrologic connectivity between the wetland and the stream was the main control on episodic SO4-2 release following summer drought. Copyright (C) 2001 John Wiley & Sons, Ltd.