NEAR-SURFACE GEOPHYSICAL INVESTIGATION OF KARSTIC CONTROLS ON LOCALIZED HILLSLOPE HYDROLOGY

Karst landscapes are unique in their complex hydrologic nature, containing unconventional pathways for flow and interconnection between surface and groundwater components. These features may potentially influence measurements and interpretation of hydrologic data, driving a need for improved understanding of how karst heterogeneity impacts field-scale hydrology. This study aims to apply near-surface geophysical methods, namely electrical resistivity tomography and seismic refraction, to characterize karst bedrock structure and topography and to describe the role of these features in surface and subsurface hydrology. This study uses twelve replicated field-scale lysimeter plots established in a shallow hillslope. Each plot is engineered to be hydrologically isolated from one another and from the site’s exterior, but true hydrologic isolation is drawn into question due to karst development of the underlying Bellefonte formation dolomite.

Measurement and subsampling of surface runoff and shallow subsurface flows have been continuous since 2012. Flows enter pipe drains at the soil surface and soil-bedrock interface within each plot and are rerouted to collection houses. High variability in storm responses among the plots lend to the idea that hydrologic isolation may be breached by flow through shallow karstic features. An extensive ERT survey was conducted consisting of 25 lines configured as a rectangular grid. Each line was collected using Wenner and Dipole-Dipole arrays to compare and maximize resolution in both the vertical and horizontal directions. All data underwent reciprocal analysis to identify and remove outlier data to reduce error during inversion through RES2DINV/RES3DINV inversion software. Borehole and seismic refraction data were collected to improve confidence in the resolution of our ERT results and our interpretations of subsurface structure and bedrock topography. The results of this study suggest shallow karstic controls act in a secondary nature while parallelling topographic controls on hillslope hydrology. In addition, the methods used in this study reveal the effectiveness of paired near-surface geophysics to provide subsurface information extensive enough to improve understanding of karst hydrology at a field scale without subsurface invasion.