ELECTRICAL RESISTIVITY TOMOGRAPHY (ERT) APPLIED TO AN INVESTIGATION OF RIDGETOP WETLAND HYDROGEOLOGY IN THE DANIEL BOONE NATIONAL FOREST, ROWAN COUNTY, KENTUCKY

At least 30 ridgetop wetlands supplied by groundwater have been identified in the Daniel Boone National Forest in Eastern Kentucky. These wetlands are critical for a variety of ecological functions such as being a habitat for endemic amphibians and providing drought support to forest vegetation. Previous investigations at Eastern Kentucky University have quantified the physical properties of the soil and determined the general stratigraphy of the area through coring. An ERT survey using a dipole-dipole array was conducted in a representative native wetland and its catchment under saturated conditions in order to look at the spatial extent of permeable and impermeable layers. Because these systems are extremely remote and sensitive, ERT offers a more complete view of these systems without heavy disturbance. Using the ERT, the resistivities gathered were used to identify what layers were present when paired with cores of the wetlands to determine stratigraphic layers, shapes, and thickness of them. The results show a high resistivity layer of >625 Ohm-m representing an organic layer that extends down to ~0.5 m. This is underlain by an intermediate resistivity layer below of ~200 Ohm-m that we interpret to represent a water-saturated zone of ~0.25 thickness. This groundwater layer rests above an ~1 m thick low resistivity, low hydraulic conductivity clay layer (~50 Ohm-m). At ~1.75 m total depth, the clay is underlain by a zone of increasing resistivity layer that we interpret to represent the sandstone bedrock. The results of our geophysical survey of the subsurface are consistent with the previous coring done in these wetlands. Knowing the resistivities of the layers can help to identify the 3-dimensional geometry of the subsurface allowing for further interpretations to be made about how these systems store and transmit water. Based on the geometry of this wetland, groundwater is thought to flow down the top of the ridge and feed lower lying wetlands surrounding it.