5 Methods of Karst Characterization

Characterization of the unique hydrogeologic features in karst aquifers require more data to achieve an understanding similar to what can be attained for granular and fractured aquifer systems, and the nature of the required data makes it more difficult to obtain (Teutsch and Sauter, 1991; Kiraly, 2003). Groundwater professionals working in karst need to anticipate the presence of heterogeneities and non-Darcian conditions that may limit analysis by conventional hydrogeologic methods. An understanding of open-channel flow and pipe flow is necessary when describing flow in karst conduits. Field investigations must provide the data that is necessary to properly conceptualize the recharge, storage, and throughflow components of the karst aquifer system.

Karst aquifer studies need to have greater focus on the identification of hydrologic boundaries and preferential subsurface flow paths created by the integrated network of conduits and solution-enhanced fractures. Acquisition of these data typically requires a multidisciplinary study approach that includes using more specialized investigation methods such as water-tracing tests and the analysis of variations in spring discharge and water chemistry (White, 1993; Ford and Williams, 2007). Multiple hydraulic, hydrologic, and geophysical investigative methods have been successfully developed and employed to probe the subsurface karst environment. However, many of these technologies and methods have not obtained widespread usage due to technical challenges, cost, and labor requirements. Some technologies and methods are only capable of evaluating part of the karst environment and must be used in combination with other complementary methods to provide a complete picture of the aquifer.

Applicability of a karst-aquifer investigation method is dependent on 1) the volume of the aquifer sampled or tested and 2) the proportion of conduit-dominated flow in the volume of the aquifer sampled or tested. Additionally, the selection of investigative methods requires a good understanding of the question that is to be answered or problem to be solved.

Basic geologic mapping and understanding of the depositional environment of the carbonate or evaporite rock type is applicable to all scales of investigation. Readers who want to know more about carbonate geology and karst geomorphology may want to read Folk (1981), Scoffin (1987), White (1988) and Ford and Williams (2007).

At a local scale (small study areas less than 1 km2), karst-aquifer investigation methods rely on the use of single wells and application of borehole tests at single wells for sampling or measuring small volumes of the aquifer. Surface geophysics may also prove useful. Unless a well penetrates one or more conduits, data obtained by these methods are influenced by the diffuse-flow component (non-conduit permeability) of the aquifer. If data can be obtained at multiple wells, sometimes geophysics along with reliable lithologic and geologic information can be used for understanding which local units transmit and store water at the basin and regional scales.

At the basin scale (~20 to 3,000 km2), conduit-flow may be the most significant component of flow in a karst aquifer. A karst basin is usually controlled by the number, distribution, and interconnection of conduits (White, 1988). Methods of well testing that are applicable at the local scale may not be capable of characterizing the karst aquifer’s properties at the basin scale. Basins are typically drained by spring(s), which integrate flow from individual conduits along with water contributed from diffuse-flow components of the aquifer. Therefore, a spring is the most appropriate natural sampling point for basin scale investigation. Water-tracing tests, spring hydrographs and spring chemograph analyses are the most useful techniques for investigating the hydrologic and hydraulic behavior of the karst aquifer at the basin scale. Quantitative tracer tests and chemograph analyses are some of the better tools for understanding the character of a karst aquifer within the diffuse-versus-conduit-flow continuum.

At the regional scale, (> 25,000 km2) karst aquifers consist of multiple groundwater basins. Water-level (or potentiometric surface) mapping, water-tracing tests with natural or artificial tracers, and water-quality or geochemical sampling, field mapping of faults and joints, and geophysical methods (borehole and surface) are all appropriate methods of investigation and are used to understand the hydrogeologic framework of the karst aquifer.

The selection and use of a karst groundwater investigative method and the proper interpretation of the resulting data requires careful consideration of the appropriate scale for applying each method (Table 3).

Table 3  Applicability of research techniques to investigation of karst aquifers. Spaced dashes indicate greater difficulty in application due to effects of heterogeneity.

Scale of Applicability

Local (Site)BasinRegional

< 1 km2> 25,000 km2
Hydrogeologic Mapping (surface geology, potentiometric surfaces, location of karst features, well inventories, and compilation of all existing data sets and reports on the site) ‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑
Surface Geophysics (seismic, gravity, ground-penetrating radar, electromagnetics) ‑‑‑‑‑‑‑‑‑‑‑‑‑‑
Single Borehole Geophysics (including flowmeter tests, tomography, packer tests, etcetera) ‑‑‑‑‑‑‑‑‑‑‑‑‑‑
Multiple borehole geophysical logs and surface geophysics (combined with stratigraphic and lithologic information) ‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑
Airborne Geophysics (electromagnetics, aerial infrared photography) ‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑
Well hydraulic (aquifer) tests ‑‑‑‑‑‑‑‑‑‑‑‑‑‑
Qualitative and Quantitative Water-Tracing Tests (artificial tracers-dyes, solutes, or microspheres) ‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑
Natural Tracers (isotopes, naturally occurring dissolved solutes) ‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑
Well Hydrograph/Chemograph Analysis ‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑
Spring Hydrograph/Chemograph Analysis ‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑
Mathematical Modeling (Distributed Parameter, Lumped Parameter, and Fitting Models) ‑ ‑ ‑ ‑ ‑ ‑ ‑ ‑ ‑ ‑ ‑ ‑ ‑ ‑ ‑ ‑ ‑ ‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑

In general, multiple methods of investigation provide a better understanding of groundwater flow in karst aquifers. Different mathematical models are used at all scales of study: for hydrograph and chemograph analyses; or to approximate the entire physical flow system with a groundwater flow simulation. Thus, it is best to have multiple investigators or advisors with multi-disciplinary backgrounds for investigations of karst aquifers. This introduction describes some of the more useful technologies and approaches to the investigation of karst aquifers. Application of these methods improve our ability to manage the effect of human activities in karst areas and manage water resources.

Exercise 17 invites readers to ask themselves, what is a hydrograph or chemograph?


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