5.1 Hydrogeologic Mapping

Hydrogeologic mapping is the identification of physical boundaries of aquifer systems and their hydrologic/hydraulic characteristics. An important distinction is that while hydrogeologic mapping incorporates conventional geologic mapping techniques—such as identifying and mapping the various stratigraphic or bedrock units present and their structural characteristics (geologic strike and dip, presence of faults and fractures, and so on)—it is focused on mapping the storage and transmitting properties of the rocks rather than strictly mapping the geologic stratigraphic units. In fact, in karst and many fractured rock aquifer systems, the aquifer “boundaries” may include multiple distinctive bedrock units or strata, and the aquifer may be defined physically as the interconnected networks of fractures and karst conduits that transmit water throughout a specific volume of rock. Hydrogeologic mapping is the first step in developing an understanding of the aquifer geometry and the locations where water enters and exits the aquifer. Compiling this information leads to the creation of a conceptual model of the aquifer—also called the hydrogeologic framework. This basic understanding is critically important for investigations and analysis of all types of aquifers.

Karst aquifers present unusual challenges as many of the rules for typical porous media do not apply. For example, groundwater flow in karstic watersheds and their subsurface basins may not be inferred from the surface topography. A first step is to identify and characterize the distribution of some of the surface and underground features of karst landscapes in the study area (Figure 40). Mapping the occurrence and distribution of surface karst features (springs, sinking streams, sinkholes) in relationship to the general topography, bedrock stratigraphy, and geological structure (bedrock dip, and visible fault and joint patterns) is critical for karst aquifer studies. Advances in technology have improved field mapping as internet, cell phone and geospatial technology allow real time collection of the location of karst features (Figure 41 and Figure 42).

Figure showing karst landscapes

Figure 40  Generalized features in many karst landscapes. Modified from Taylor and Greene (2008).

Photographs of geospatial technology

Figure 41  Modern internet and cellphone-based geospatial technology greatly help expedite and enhance collection of karst-hydrogeologic mapping data. a) Antonia Bottoms, a geologist at Kentucky Geological Survey, uses a portable tablet equipped with GIS (Geographic Systems Information) and geocaching software capabilities to verify sinkhole locations identified and delineated using processed LIDAR topographic data, and collect additional field data, such as b) geotagged photographs, needed to characterize their visible physical and hydrologic characteristics. c) On the LiDAR-based topographic map image, sinkhole depressions are delineated by blue polygons and potential swallets (open-throat sinkhole drains) are identified by black dots. The red polygon delineates the boundaries of a farm property which was the site of the field investigation. Photographs and graphics provided by Taylor (2021).

Map showing modern GIS technology

Figure 42  Use of modern GIS technology greatly facilitates the task of compiling, visualizing, and analyzing karst geospatial data needed for hydrogeologic mapping. Here GIS is used to visualize the relation between surface streams, watersheds and catchments (surface basins) of sinkholes as well as sinking streams (multicolored polygons) in the upper Lost River basin, south-central Indiana, USA. Dye-trace flow vectors (yellow lines) are added to help identify general directions of subsurface flow through conduits and point-to-point connections between sinkholes or sinking streams and mapped karst springs. Modified from Taylor and Nelson (2008).

Enhancing Mapping with Subsurface Data

Surficial karst-feature mapping needs to be enhanced with subsurface data including: water well construction and static water level measurements; groundwater withdrawal information; information from aquifer tests in the study area; and surface or borehole geophysics studies. These are the more conventional types of hydrogeologic methods used to investigate aquifers in many types of aquifer settings. They are useful and may provide critical information in the investigation of karst aquifers if the data are applied within the framework of a karst conceptual model (Taylor and Greene, 2008).

In karst terranes, an emphasis must be placed on delineation of karst basin boundaries; subsurface flow paths; contributions of water from localized recharge sources; and the geometric and hydraulic properties of conduits. The acquisition of these data requires a multidisciplinary study approach that includes combining conventional hydrogeological mapping methods with specialized investigation methods such as water-tracing tests and spring monitoring including the analysis of variations in spring discharge and water chemistry (White, 1993; Ford and Williams, 1989). By far, the most valuable single source of information is obtained using properly designed qualitative or quantitative water-tracing tests, usually conducted with non-toxic, fluorescent dyes. Applications of these tests are discussed in detail by Mull and others, 1988a; White, 1993; Goldscheider and Drew, 2007; and Taylor and Greene, 2008. Water tracing tests are discussed in Section 5.3.

The combination of water-level (potentiometric-surface) mapping and dye-tracing tests (Figure 43) has been extensively and successfully employed for decades to:

  • determine groundwater flow directions;
  • infer approximate locations of conduit-dominated flow paths;
  • identify or confirm hydrologic connections between specific sinkholes and other sources of recharge and karst springs;
  • identify and delineate karst basin boundaries; and,
  • investigate and characterize other physical hydrogeologic characteristics of the karst aquifer system.

Dye-tracing or water tracing tests conducted with other appropriate types of water tracing agents are the only reliable methods of confirming whether flow inferred by potentiometric or water-table contours indicate the groundwater flow direction, and of accurately assessing karst groundwater velocities and other important hydraulic parameters critical to wellhead protection as well as contaminant characterization and remediation efforts (Quinlan et al., 1995; Field and Nash, 1997; Ewers, 2006).

Maps showing Potentiometric and dye tracing tests

Figure 43  Potentiometric and dye tracing tests for delineation of karst spring boundaries. a) Example of combined use of potentiometric data and dye-tracing tests to delineate karst spring basin boundaries in Kentucky, USA. The “U”-shaped troughs or depressions in the contoured potentiometric surface often indicate the presence (and general but not precise location) of major karst conduits (Modified from Taylor and McCombs, 1998). b) Example demonstrating combined use of potentiometric data and dye tracing tests for hydrogeologic mapping in the Barton Springs karst basin, Edwards Aquifer system, Texas, USA. From Hunt and others (2005).

The hydrology of karst is dynamic and temporally variable, such that water levels, flow directions, groundwater velocities, basin boundaries, spring discharge locations and other downstream receptors of flow may change with changing hydrologic conditions. Thus, it is important to consider whether tracing tests and potentiometric mapping should be repeated under differing hydrologic conditions in order to accurately investigate and assess the range of hydrogeologic variation (Figure 44).

Map showing potentiometric surface

Figure 44  Examples of changes in configuration of the mapped potentiometric surface near monitoring well MW-7 at the southwest corner of a landfill which is outlined by a dashed line: a) under low flow conditions; and b) under high flow conditions with blue arrows indicating direction of groundwater flow inferred from the potentiometric surface with respect to MW-7. Showing a larger area surrounding the landfill, dye traced karst groundwater flow paths through probable conduits or enhanced fractures are indicated with red arrows: c) under low flow conditions; and d) under high flow conditions. Modified from McCann and Krothe (1992).

Enhancing Mapping with Geophysical Data

Geophysical studies can provide a wealth of information that is critical to gaining a better understanding of subsurface conditions needed for groundwater and environmental studies. Geophysical methods of investigation involve studying the earth by measuring its physical properties, with either passive methods such as, changes in gravity; electromagnetic field; or natural radiation; or active methods such as measuring the response of the earth to mechanical; electromagnetic; radioactive; or sound sources. Surface geophysical methods are applied from the earth’s surface or from the air. Borehole methods record and analyze measurements of physical properties made in wells or test holes. Probes that measure different properties are lowered into the borehole to collect continuous or point data that are graphically displayed as a geophysical log.

Successful application of a geophysical method requires an understanding of the general principal of the method, how the measuring tool works, the physical property that is measured, and what could be interpreted from the method. Table 4 summarizes this information and lists some studies that have applied surface geophysics to karst systems. Even in the most well-funded karst studies, there is unlikely to be enough drilling to characterize the hydraulic conductivity variations and the location of conduits. Using a combination of flow logging, stratigraphic correlation, and cross-borehole flow testing can be beneficial for smaller site studies, but it is too expensive to conduct enough of these to characterize a large area. Combining borehole measurements with surface geophysical soundings can provide the area-wide coverage needed to better characterize the subsurface.

The information in Table 4 is useful whether considering surface or borehole geophysical techniques. Exercise 18 invites the reader to learn about more about the terms used in Table 4.

Surface Geophysics Useful to Karst Aquifer Characterization

Some of the most effective surface geophysical tools commonly applied to karst for defining subsurface features are ground-penetrating radar and seismic methods (Cunningham and Aviantara, 2001; Cunningham et al., 2001; Kindinger et al., 1999, 2000, 2001; Kraemer et al., 2001; Steeples and Miller, 1987). Ground-penetrating radar is useful for shallower depths than seismic (Figure 45). Marine reflection seismic data are far easier to collect and interpret and can be run in rivers and lakes. This has been done throughout most of the waterways in Florida and used to define sinkholes (Figure 46).

Table 4  Surface geophysical tools and what they can detect (Modified from National Research Council, 2000).

Method How it works What property is measured What is detected Example Karst applications
Gravity Detects variations in the gravitational field of the earth caused by mass variation Density Depth, geometry, and density of local subsurface features; cavity detection Butler, 1984
Magnetic Detects variations in the earth’s magnetic field caused by local variations in magnetic properties of subsurface materials Magnetic properties Depth, geometry, and magnetic susceptibility of localized subsurface features Stanton and Schraeder, 2001
Smith et al., 2003, 2005
Gary et al., 2013
Seismic Sends vibrations (elastic waves) through the subsurface and analyzes changes in velocities (property dependent) and reflections or refractions as waves pass through heterogeneities Compressional, shear, and surface waves; seismic velocities; and elastic moduli Interface depths, layer velocities, geometry, or structure, elastic moduli, porosity Kindinger et al., 1999, 2000, 2001
Kraemer et al., 2001 Steeples and Miller, 1987
Electrical resistivity and electromagnetic (EM) Induction Detects natural or induced electrical current flow through subsurface materials; electrical properties controlled by material properties of the subsurface along with porosity and pore fluid compositions Electrical resistivity, magnetic susceptibility Depth, thickness, electrical resistivity, porosity, inferred fluid chemistry (for example, location of saltwater/freshwater interface) Fitterman and Deszcz-Pan, 1998
Stanton and Schraeder, 2001
Ground Penetrating Radar (GPR) Sends high-frequency radar waves through the subsurface; analysis similar to seismic reflection and refraction (velocities are property dependent) Dielectric permittivity, electrical resistivity, magnetic susceptibility EM wave speed, depths, thicknesses, geometry Cunningham and Aviantara, 2001
Cunningham et al., 2001.
Thermal (land based or remote) Qualitative using thermal images to spot thermal contrasts or quantitative using fiber optic methods or time series of temperature with various devices Temperature over area in an image or survey or time series temperature at a point or along a line. Often used for groundwater- surface water interactions in karst terrain Qualitative temperature contrast usually used to note where groundwater enters surface water. Quantitative time series of temperature changes and can calculate flow Anderson, 2005
Integrated interpretation of multiple methods Soil and rock type (lithology), structure and stratigraphy, porosity, permeability, fluid content Cunningham and Aviantara, 2001
Cunningham et al., 2001
Figure showing ground-penetrating radar profile

Figure 45  Ground-penetrating radar profile and interpretation from the Indian Mound site in Glades County. a) Radar profile showing parallel, oblique prograding reflections that are b) interpreted to be images of low angle, accretionary foreset beds. This geophysical technique profiles a unique view into the internal geometry of the subsurface, producing information that can be used for interpretation of depositional environments and hydraulic conductivity. A deep sand pit is located about 500 feet north of the profile, which suggests the entire profile is imaging a quartz sand lithology. From Cunningham and others (2001).

Figures showing interpretations of marine seismic profile

Figure 46  Line-drawn interpretations shown above their associated marine seismic profile explaining six types of features beneath lakes of northeastern Florida, USA. Note: Multiples are multiplicative events seen in seismic sections. These events have undergone more than one reflection. They are produced in the data gathering process when the signal does not take a direct path from the source to the geologic feature and back to the receiver on the surface. Multiple events occur in the example labeled “Type 1: Record Obscured” where a large area is indicated as “multiples”. From Kindinger and others (2001).

Ground-based magnetic surveys are often used to locate metallic objects such as abandoned wells and have been applied in karst aquifers over small areas (Stanton and Schraeder, 2001). Areal magnetic surveys combined with ground surveys, such as direct current resistivity have proven useful in identification of large features and lithologic changes near the surface over larger areas (Smith et al., 2003, 2005; Gary et al., 2013).

Temperature contrasts have been used to identify groundwater inflow to surface water (springs) using infrared cameras at the local level and areal infrared imaging for examination of temperature contrasts over larger areas (Anderson, 2005). Such surveys must be conducted at a time of year when there is a significant difference between surface-water and groundwater temperature. Additionally, electrical resistivity surveys have been conducted in marine environments from boats to find submarine groundwater discharge in many depositional environments including karst (Henderson et al., 2010). Airborne electromagnetic methods were used to map salinity in the shallow coastal Biscayne Aquifer (Fitterman and Deszcz Pan, 1998).


Introduction to Karst Aquifers Copyright © 2022 by Eve L. Kuniansky, Charles J. Taylor, and Frederick Paillet. All Rights Reserved.