Integration of electrical resistivity imaging and ground penetrating radar to investigate solution features in the Biscayne Aquifer Albert Yeboah-Forson a,c,⇑ , Xavier Comas b , Dean Whitman a a Department of Earth and Environment, Florida International University, 11200 SW 8th St, Miami, FL 33199, USA b Department of Geosciences, Florida Atlantic University, 3200 College Ave., Davie, FL 33314, USA c Chemical and Physical Science Department, Missouri Sothern State University, 3950 E Newman Road, Joplin, MO 64801, USA article info Article history: Received 18 October 2013 Received in revised form 15 April 2014 Accepted 18 April 2014 Available online 1 May 2014 This manuscript was handled by Corrado Corradini, Editor-in-Chief Keywords: Anisotropy Biscayne Aquifer Karst Electrical resistivity imaging Ground penetrating radar Miami Limestone summary The limestone composing the Biscayne Aquifer in southeast Florida is characterized by cavities and solu- tion features that are difficult to detect and quantify accurately because of their heterogeneous spatial distribution. Such heterogeneities have been shown by previous studies to exert a strong influence in the direction of groundwater flow. In this study we use an integrated array of geophysical methods to detect the lateral extent and distribution of solution features as indicative of anisotropy in the Biscayne Aquifer. Geophysical methods included azimuthal resistivity measurements, electrical resistivity imaging (ERI) and ground penetrating radar (GPR) and were constrained with direct borehole information from nearby wells. The geophysical measurements suggest the presence of a zone of low electrical resistivity (from ERI) and low electromagnetic wave velocity (from GPR) below the water table at depths of 4–9 m that corresponds to the depth of solution conduits seen in digital borehole images. Azimuthal electrical measurements at the site reported coefficients of electrical anisotropy as high as 1.36 suggesting the presence of an area of high porosity (most likely comprising different types of porosity) oriented in the E–W direction. This study shows how integrated geophysical methods can help detect the presence of areas of enhanced porosity which may influence the direction of groundwater flow in a complex aniso- tropic and heterogeneous karst system like the Biscayne Aquifer. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction The Biscayne Aquifer of South Florida is a surficial Pleistocene karst system characterized by dissolution features such as cavities, touching-vugs, conduits and solution holes that contribute directly to groundwater flow dynamics (Fish and Stewart, 1991; Cressler, 1993; Cunningham et al., 2009). Flow in the Miami Limestone (uppermost unit of the Biscayne Aquifer) and the underlying Fort Thompson Formation is not uniform but localized through second- ary porosity and permeability caused by these solution features that are highly heterogeneous not only in terms of size but also in their spatial distribution (Fish, 1988; Fish and Stewart, 1991). Solution features of different scales may interconnect to develop areas of enhanced porosity that could result in localized anisotropy, thus complicating studies related to groundwater flow modeling. For example, Geyer et al. (2010) showed that drainage in karst aquifers at the regional scale may be controlled by localized anisotropy. Well connected vugs can form major conduits resulting in preferential flow of groundwater, especially in carbonate rocks of young eogenetic karst aquifers such as the Biscayne Aquifer (Cunningham, 2004; Ginés and Ginés, 2007; Renken et al., 2008). For all these reasons, investigation of solution features both in terms of spatial distribution and anisotropic properties may pro- vide information about the lateral and vertical variability in groundwater movement and thus may help current modeling efforts to predict groundwater flow dynamics and contaminant transport in the Biscayne Aquifer. Although several studies over the last decade have investigated the presence of solution features across the Biscayne Aquifer in Miami-Dade County (Cunningham, 2004; Manda and Gross, 2006; Renken et al., 2008) there is a missing link between the petrophysical properties and the hydraulic properties of the aqui- fer particularly due to the limited number of exploration studies beyond point sample methods that provide better spatial resolu- tion. Furthermore, little is known about the anisotropy of the Biscayne Aquifer and very few studies have tried to characterize http://dx.doi.org/10.1016/j.jhydrol.2014.04.045 0022-1694/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author at: Chemical and Physical Science Department, Missouri Sothern State University, 3950 E Newman Road, Joplin, MO 64801, USA. Tel.: +1 417 625 9619; fax: +1 417 625 3169. E-mail addresses: yeboahforson-a@mssu.edu (A. Yeboah-Forson), xcomas@fau. edu (X. Comas), whitmand@fiu.edu (D. Whitman). Journal of Hydrology 515 (2014) 129–138 Contents lists available at ScienceDirect Journal of Hydrology journal homepage: www.elsevier.com/locate/jhydrol