CLIMATIC CONTROLS ON THE POREWATER CHEMISTRY OF MID-CONTINENTAL WETLANDS: DON SIEGEL’S LAST DISSERTATION

In the early 1990’s, Don Siegel and colleagues observed a transition from drought to a period of heavy rain in the Glacial Lake Agassiz Peatlands (GLAP) of northern Minnesota. Water table mounds that formed under the crests of large raised bogs during this time created vertical hydraulic head gradients that drove meteoric recharge and labile organic carbon compounds down through the active porespaces of nearly 4 m of peat in a large bog-fen complex (Siegel et al., 1995). Around the same time, his colleague and academic older brother, Tom Winter, observed that the transition from record drought to deluge at a prairie wetland complex in east-central North Dakota (The Cottonwood Lake Study Area - CLSA) expanded the inundated area of a well-studied closed-basin prairie wetland (P1) to record extent where it has remained to present (Winter and Rosenberry, 1998).

As Don Siegel’s last Ph.D. student, I was given the unique opportunity to study how the hydrogeochemistry of these two very different, neighboring wetland systems have responded to the shift of mid-continental North American climate to wetter conditions in the latter part of the 20^th century. We used coupled geochemical and geophysical methods to investigate these two systems. At the GLAP, we compared peat porewater stable isotopes to major mineral solutes along a 6 km transect through a 120 km² raised bog-fen complex to show that while porewaters across the entire peatland have been flushed with modern meteoric recharge of low mineral solute content, areas affected by paleo-hydrogeologic discharge have retained groundwater-derived solutes and circumneutral pH due to dual domain mass transfer from the peat matrix. At the CLSA, we used geoelectrical methods to detect a lens of drought-derived saline (TDS > 10 g L^-1) groundwater in the shallow (2 – 4 m depth) mineral sediments beneath the center of P1 that developed during paleodroughts in a process we defined as “drought-induced recharge.” Drought-derived saline groundwater has the potential to discharge back into the wetland pond and increase surface water salinity during periods of wet climate. Both of these studies illustrate the diverse ways that subsurface storage and release of mineral solutes in response to wet and dry conditions help wetlands to modulate their geochemical environments in response to changes in climate.