INSIGHT INTO GROUNDWATER CONTRIBUTIONS TO WETLANDS USING AQUEOUS GEOCHEMICAL INDICATORS AND GROUNDWATER FLOW MODELING: THREE CASES STUDIES FROM RIDGE AND SWALE SEQUENCES IN THE UPPER GREAT LAKES

Surface-water and groundwater systems are viewed as representing end members of a hydrologic continuum that exists between local (shallow) flow systems and regional (deep) flow systems within many aquifers. In the Great Lakes, the extent to which these flow systems interact is a primary control on the hydrology, ecology, and permanence of coastal wetlands. The linkages between these flow systems, however, are generally not well understood. These coastal wetlands are dynamic systems that change both spatially and temporally due to: 1) natural and anthropogenic coastline modifications and shoreline behavior, 2) uplift of the aquifer due to rebound after ice sheet retreat, and 3) variations in precipitation due to climate change. We studied the interaction of these flow systems within beach-ridge complexes that contain wetlands at three sites in the upper Great Lakes (Negwegon State Park, MI; Manisitique, MI; Grand Traverse Bay on the Keweenaw Peninsula, MI) through an integrated approach of water sampling for aqueous chemical indicators and groundwater flow modeling.

Preliminary results show that a thorough analysis of aqueous chemical field parameters (specific conductance, temperature, pH, Eh, and alkalinity) and laboratory derived concentrations of major ions is capable of identifying portions of the aquifer that represent end members and intermediates of the hydrologic continuum. At two of the sites, Negwegon State Park and Manistique, MI, the spatial variability of geochemical indicators was extreme indicating a complicated distribution of deep, local, and intermediate flow systems. By examining the subsurface geology in these areas with well and cone-penetration testing logs and vibracores, we have found that the distribution of flow systems is controlled by the presence or absence of low conductivity zones (clay layers) and the depth to these layers. Groundwater flow modeling of these areas supports this conclusion.

The third site (Grand Traverse Bay on the Keweenaw Peninsula, MI) shows virtually no chemical variability between groundwater and surface water samples throughout the site. Waters are chemically similar to rainwater and represents a local flow system that persists throughout the entire study area. Groundwater flow modeling of this area also supports this conclusion.