APPLICATION OF GEOPHYSICS TO BETTER UNDERSTAND WETLAND HYDROGEOLOGY

Conducting field work in wetland settings poses challenges not encountered in typical hydrogeologic investigations in other environments. Vehicle, equipment, and even personnel access to the field site are often limited. Wetland settings often require alternative methods for interpreting subsurface geology and hydrology.

Geophysics can often provide those alternatives. We have applied ground conductivity measurements, electrical resistivity imaging, passive seismic measurements, and ground-penetrating radar during projects at several different wetlands in Wisconsin. The geophysical instruments are portable by one or two persons, even into difficult, wet, and hummocky terrain. These tools were used to determine depth to bedrock or lithic sediment and thickness of peat beneath the wetland areas. When possible, the geophysical estimates were confirmed by cores collected with a Russian peat corer, pushing a steel rod to refusal or during hand installation of a piezometer. In general, there is good agreement between the geophysical data, assuming a two-layer resistivity model of marl or clay sediment over lithic sediments or bedrock, and the push and coring methods. In some instances, we saw correlation between changes in vegetation as seen in air photos and changes in the subsurface as indicated by the geophysics.

These results can be incorporated into conceptual models of the wetlands to improve our understanding of these complex systems. We found electrical resistivity imaging and ground conductivity measurements were the most easily applied and interpreted. The end result is a better understanding of the hydrogeology of wetlands and improved management of these sensitive habitats.