7 Summary

The term karst refers to a landscape or terrane underlain by soluble bedrock and characterized by unique hydrogeologic features, such as caves, sinkholes, sinking streams, and springs, that have been created mostly by the process of dissolution. Most karst is formed by near-surface percolation and circulation of fresh water and is an integral part of the active meteoric water cycle and local-to-regional scale hydrologic systems. The process of karst formation involving dissolution and re-precipitation of carbonate minerals accompanied by mass transfer of CO2 makes karst an important, though often under recognized, component of the natural global carbon cycle.

Karst aquifers are globally distributed and provide unique freshwater resources that are vitally important to human life, aquatic and terrestrial ecosystems, as well as local-to-global-scale hydrologic and biogeochemical systems. Karst aquifers typically possess extremely heterogeneous and highly complex internal structure, presenting many unique hydrogeological characteristics, such as the interconnected networks of subsurface conduits that are arguably a karst aquifer’s most distinctive feature. In many karst areas, the surface and groundwater regimes are highly interconnected and function as a singular dynamic hydrologic unit. Subsurface groundwater drainage divides do not always coincide with topographic divides or groundwater divides defined by water table contours and directions of groundwater flow commonly changes with changing hydrologic conditions.

Karst aquifers can have three types of porosity.

  • Small pores less than 1cm, in which flow is slow and is rarely turbulent (called rock matrix porosity).
  • Pores greater than 1cm and less than 0.1m, called macro porosity and often the result of biological activity, in which flow is fast and the onset of turbulence occurs at relatively small Reynolds numbers similar to typical porous media.
  • And, the most unique porosity, large conduits, often greater than 1 m, created by dissolution along fractures and bedding planes with fast, often turbulent, flow at large Reynolds numbers similar to pipe or open-channel flow.

For simplicity two terms are generally used for describing flow; matrix (slow flow) and conduit (fast flow). Within a karst aquifer system, the matrix and conduits exchange water. High-frequency monitoring of spring discharge and water quality along with analysis techniques such as hydrograph separation and hysteresis plot analysis, help to: 1) identify the relative proportions of water contributed by matrix and conduit flow under varying hydrologic conditions; and, 2) develop better understanding of the temporal variability of recharge, storage, and through-flow within an individual karst aquifer.

Karst aquifers present many challenges to investigating their character because they do not conform to ideal Darcian flow. As a consequence, traditional methods of hydrogeologic investigation that rely on data collected from water wells may not provide sufficient data for proper interpretation of basic aquifer hydraulic properties and may lead to erroneous interpretations if not collected and analyzed with a proper conceptual understanding of karst aquifer structure and flow systems. Conventional methods of aquifer characterization based on well hydraulic tests are nevertheless useful, and often vital, for determining important aquifer properties—especially those related to assessing groundwater availability, effects of water withdrawals, and well sustainability. However, tracer tests, often conducted with fluorescent dyes, are required to positively identify groundwater flow directions, determine flow velocities and residence times, and to map groundwater basin and aquifer boundaries. Surface geophysical methods combined with borehole geophysics and structural mapping is useful, and sometimes critical, in delineation of conduit or preferential flow layers within the karst aquifer. More advanced hydraulic testing involving flow-meter logging under both ambient flow and pumping conditions is extremely beneficial along with hydraulic testing involving multiple wells when combined with numerical analysis.

Recent advances in computing technology and development of numerical groundwater modeling codes, have revolutionized the investigation, characterization, and understanding of karst aquifer behavior, and the management and protection of karst water resources. Modeling is now routinely used to synthesize karst aquifer data, analyze, and understand field observations and measurements, and simulate or forecast karst aquifer behavior under different natural conditions and human-induced changes or stresses. Karst aquifer modeling is often essential to understand, predict, and assess effects of climate change, water withdrawals, groundwater contamination, and effectiveness of natural attenuation or engineered contaminant remediation measures. Different mathematical modeling approaches can be applied depending on the model’s intended purpose, the data available, and the types of hydrogeologic complexity present, especially the presence of multiple types of porosity and permeability components, including conduits, in the karst aquifer. These include fitting models, lumped parameter models, distributed parameter models including both single and dual continuums, hybrid models that link continuum models with discrete models, and discrete fracture or conduit models.

The topics presented in this book introduce karst aquifers and foster better understanding of their unique characteristics, as well as the need for careful consideration of these characteristics when planning and implementing hydrogeological investigations. Although the topic of karst is extremely broad, the focus of this book is intentionally narrow, in order to address groundwater flow in karst aquifers and associated investigation methods.

Karst aquifers are not the mysterious hydrogeological entities that they are sometimes portrayed to be. They are complex aquifers that require careful consideration regarding the selection and use of hydrogeological investigative techniques for data collection and analysis. It is more difficult, time-consuming, and expensive, to collect and interpret data for proper characterization, utilization, and protection of karst aquifers compared to aquifers of other rock types.

This book highlights the methods that have been most useful in karst aquifer studies conducted over many decades. The audience for this book is upper-level undergraduate science and engineering students, and one goal is to assist them in the selection of a major for a more focused advanced degree. The numerous references provide a head start on a literature review for advanced degree research. We strive to provide information that is 1) useful to students and researchers who are planning investigations of karst, and 2) stimulates interest and excitement in readers to learn more about karst aquifers and karst hydrogeology.


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