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

Abstract

Karst landscapes are unique in their complex hydrologic nature, containing unconventional pathways for flow and interconnections between surface and groundwater systems. An improved understanding of how karst heterogeneity impacts field-scale hydrology will support more accurate interpretation of hydrologic data. This study aimed to apply near-surface geophysics, specifically electrical resistivity tomography (ERT) and seismic refraction methods, 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. Replicated field-scale lysimeters studies are rare, having only been attempted on two farms in the UK (Armstrong and Garwood, 1991; Cannell et al., 1984; Harris et al, 1984, Haygarth and Jarvis, 1997; Haygarth et al., 1998; Preedy et al., 2001) and one study in the USA by Bosch et al. (2005). Each lysimeter is engineered to be hydrologically isolated from other lysimeters and from offsite drainage. The scale, replications and engineering of these lysimeters provide a unique framework for a hydrologic study, but true hydrologic isolation is questionable 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 established at the soil surface and the soil-bedrock interface in each plot and are rerouted to one of two main collection houses. High variability in storm responses among the plots supports the hypothesis that hydrologic isolation may be breached by interconnection with shallow karstic features in the weathered epikarst zone underlying the site. An extensive ERT survey was conducted consisting of 25 survey 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 outliers, which to reduced errors during inversion using RES2DINV/RES3DINV inversion software. Borehole and seismic refraction data were collected to constrain our ERT results and improve confidence in the resolution of our final model, bolstering our interpretations of subsurface structure and bedrock topography. The results of this study suggest that subsurface drainage in the epikarst influences subsurface flow at the site, complicating traditional topographic controls on hillslope hydrology. In addition, the methods used in this study reveal the effectiveness of paired near-surface geophysics in providing substantial subsurface information to improve our understanding of karst hydrology at a field scale without the need for invasive subsurface characterization.