Multi-stable isotope tracing of elevated sulfate export from a forested headwater wetland following an induced flood pulse event

Flooding events following periods of drought can export large quantities of sulfate (SO₄^2-) from headwater wetlands to surface waters, however the source and mechanism of SO₄^2- release have rarely been studied.  Due to the projected increases in severity and frequency of summer droughts and episodic flooding events as a result of climate change, there is a need to better understand the nature of episodic pulses of sulfate from wetlands and their downstream impacts on water quality. In this study, we monitored the evolution of the concentration and isotopic composition of surface and groundwater SO₄^2- in Beverly Swamp, a peat marsh area in southern Ontario, Canada, during a controlled field-scale flooding event. The event was created by the rapid drawdown of the upstream located Valens Reservoir at the end of a drought period. Up to seven-fold increases in SO₄^2- concentrations, relative to the pre-flood background levels, were observed during the flooding of the marsh. Stable S and O isotope ratios were analysed in stream and groundwaters to investigate the sources of SO₄^2-.

Following the flooding event, SO₄^2- concentrations in the outflow from the marsh increased significantly, while δ³⁴S-SO₄^2- values decreased. The latter is interpreted as indicative of SO₄^2- generated by sulphide oxidation (Schiff et al. 2005). Sulphide is likely produced by dissimilatory SO₄^2- reduction occurring during wet conditions, with storage of the resulting sulfide minerals in the upper peat layers. During the dry summer, the sulfides are re-oxidised to SO₄^2- and flushed from the wetland during flooding. Stable ¹⁸O-H₂O isotope signatures identified water released from Valens Reservoir as the initial driver of the SO₄^2- export across the wetland, followed by groundwater seepage from the deeper peat layers. Acidity increased shortly after the SO₄^2- pulse, but quickly dropped down to background levels due to buffering capacity of the wetland.