A Complicated Groundwater Flow System Supporting Ridge-and-Swale Wetlands in a Lake Michigan Strandplain
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WETLANDS AND CLIMATE CHANGE
A Complicated Groundwater Flow System Supporting Ridge-and-Swale Wetlands in a Lake Michigan Strandplain Douglas A. Wilcox 1 & Steve J. Baedke 2 & Todd A. Thompson 3 Received: 15 January 2020 / Accepted: 6 April 2020 # Society of Wetland Scientists 2020
Abstract Beach ridges and wetland swales formed in embayments along Great Lakes shorelines during Holocene lake-level changes. Vegetation differences among swales suggested influence from differing groundwater flow systems. We characterized the hydrology across 79 ridge/swale wetlands in the Manistique/Thompson embayments of Lake Michigan using chemical and physical methods. Cross-sections were built from geologic data, and nested piezometers were installed across three ridges/swales where upwelling was noted. Stainless steel piezometers driven in 30 swales were sampled and water analyzed for specific conductance, alkalinity, and major ions. Surface water from 11 swales was analyzed. Water dominated by Ca-Mg-HCO3 was prevalent across the strandplain, with specific conductance generally less than 100 μS/cm. Conductivity, Ca, Mg, and HCO3 in groundwater were greater at identified groundwater discharges; where an amalgamated beach ridge forms a surficial groundwater divide; and swales nearer Lake Michigan that likely receive greatly mineralized water from a deeper aquifer. Repositioning of the shoreline as the embayments filled over the past 4700 years, coupled with isostatic rebound and changes in lake water levels, altered head differentials and changed the sources of discharge from local, intermediate, and deep flow systems over time. Extant plant communities are consistent with the groundwater dependence of these wetlands. Keywords Groundwater hydrology . Geochemistry . Multiple flow systems . Ridge/swale wetlands . Lake Michigan
Introduction Wetland hydrology is widely recognized as the primary influence on wetland ecology and development (Hayashi and Rosenberry 2002), and information about hydrology is essential for understanding and quantifying wetland functions and processes. In the Laurentian Great Lakes, studies of the role of groundwater in selected wetlands in a range of physiographic settings were reported by Crowe and Shikaze (2004), but further information on wetland hydrology is needed. A Great Lakes wetland type not studied by Crowe and Shikaze forms in the swales between sandy, dune-capped beach ridges. In
* Douglas A. Wilcox [email protected] 1
Department of Environmental Science and Ecology, SUNY – The College at Brockport, Brockport, NY 14420, USA
2
Department of Geology and Environmental Science, James Madison University, Harrisonburg, VA 22807, USA
3
Indiana Geological and Water Survey, Bloomington, IN 47405, USA
general, these arcuate swales are believed to be recharged from the groundwater flow system because they are typically depressions in the overall landscape topography. However, as noted by Doss (1993) and Carlson Mazur et al. (2014), water budgets and flows in these wetlands are complicated, and
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