HYDROLOGIC EVOLUTION OF GROUNDWATER-DEPENDENT ECOSYSTEMS IN COASTAL RIDGE AND SWALE WETLANDS OF THE UPPER GREAT LAKES

The evolution and persistence of wetlands, a type of groundwater-dependent ecosystem (GDE), are controlled or “driven” by recharge patterns, geomorphology, aquifer geometry and stratigraphy, and hydro-chemistry. Wetlands in ridge-and-swale-filled embayments around the Great Lakes are constantly changing, both spatially and temporally, due to changes in “drivers” such as climate change, changes to aquifer dimensions due to embayment filling, and glacial isostatic rebound (GIA). As a result, wetlands experience and potentially record a continuum of hydrologic conditions, ranging from wet to dry. Furthermore, during wet conditions, these wetlands can be dominated by either shallow, deep, or mixed groundwater sources. Consequently, proper understanding and long-term management of these GDEs needs to consider both the modern- and paleo-hydrology of the system.

The Manistique/Thompson strandplain southwest of Manistique, Michigan is an embayment that filled with about 98 beach ridges and intervening wetlands during the past 4,700 years. We have characterized the modern hydrology of the system both chemically (with bulk water chemistry and isotopes of oxygen and deuterium) and physically (with hydro-stratigraphic cross-sections built from well logs, cone penetrometer tests, and a series of shallow nested piezometers). Results show that these systems exist within a hydrologic continuum between local (shallow) and regional (deep) flow systems. These results are consistent with Toth’s (1963) and Winter’s (1998) models which demonstrate that shallow groundwater and water within wetlands can be sourced from physically and chemically distinct shallow, deep, or mixed groundwater zones largely dependent upon distance from modern shoreline, topography and subsurface geology.

We further demonstrate through the construction of simple conceptual- and groundwater-flow models that changes in “drivers” such as embayment shape, extent, GIA, and rainfall (due to changes in paleoclimate) will also cause the portion of the hydrologic continuum that controls individual coastal GDE ecosystems (wetlands) to evolve through time.