A Complicated Groundwater Flow System Supporting Ridge-and-Swale Wetlands in a Lake Michigan Strandplain

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.