Characterizing subsurface void spaces and water distribution and flow patterns in Cave Hill Karst using resistivity

Describing the distribution of groundwater is essential in understanding the evolution of geomorphologic features in karst topography. Electrical resistivity allows us to find a model of subsurface distribution of resistivity that enables the visual recognition of groundwater and void spaces. The purpose of this research is to implement electrical resistivity to describe the spatial relationship of groundwater and karstic features at Grand Caverns National Natural Landmark, Grottoes, Virginia. Two locations of interest, a karstic swale and sinkhole area, were identified for the deployment of electrical resistivity. Both, dipole-dipole and Schlumberger arrays were collected for each deployment. A total of ten deployments, consisting of 14, 28, and 56 -electrodes spaced 6.25 m apart, were conducted in and/or around the features at both locations. Collected arrays were merged and inverted using AGI EarthImager 2D-Inversion Software. Geologic cross-sections were created for each location with collected strike and dip data and field observations made throughout the park. These along with geospatial digital elevation model data were used to correlate inverted resistivity sections to surface features. The results indicate that bedding geometry and rock type are controlling the water flow patterns and type of karstic features on Cave Hill. Groundwater escapes the southwestern portion of the hill by flowing along shallowly dipped bedding planes with the trend of plunge. The formation of the swale is likely due to collapse of long and narrow conduits created from this flow pattern. The results also show that two steeply-dipping confining beds hinder the expulsion of surface and groundwater from the northwestern portion of Cave Hill. This water is collected in perched aquifers situated above the Caverns and bellow sinkholes that have formed along the confining beds. These aquifers likely feed water to the Caverns both from slow percolation and directly through conduits