AQUEOUS GEOCHEMISTRY AS AN INDICATOR OF SUBSURFACE GEOLOGY AND HYDROLOGY OF A BEACH-RIDGE/WETLAND COMPLEX IN NEGWEGON STATE PARK, MI

Nearly all surface-water features interact with groundwater by either acting as recharge or discharge areas of an aquifer. The extent to which groundwater and surface water interact is usually established on the basis of hydraulic head measurements given from vertically nested wells. However, physical measurements of head are often inaccurate and therefore may not allow detailed knowledge of interactions between these two sources of water. Head measurements can be even more inaccurate in wetlands due, for instance, to slow response in piezometers installed in low-permeability wetland sediments. In addition to hydrologic head differences between wetlands and groundwater systems, chemical variability also exists between these systems. For instance, wetland systems can be influenced by rainwater, evaporation, and biological activities. Groundwater chemistries, on the other hand, are usually dominated by geochemical interactions with aquifer materials and the residence times of water. As a result, aqueous chemistry may be a useful tool in determining the prevalent processes controlling the hydrology of these complex systems.

We collected samples from 21 wetlands and 23 wells that were installed approximately 5-8 feet into an aquifer from a ridge and swale system in the Negwegon State Park, near Alpena, MI. Samples were distributed along a transect originating near Lake Huron and extending approximately 800 meters inland (westward). Water chemistry field parameters of alkalinity, specific conductance, pH, temperature and Eh, along with laboratory-derived concentrations of major ions have been analyzed. Additionally, 80 vibracores taken from the beach ridge sequence and a ground penetrating radar line have been collected to define the subsurface geology.

Preliminary results suggest that both the chemical field parameters and major ion analyses are capable of identifying groundwater, surface water, and mixing of waters in the study area. Variability in water chemistry from wells and wetlands also appear to be correlated with a subsurface clay layer that is identified in both vibracores and GPR lines.