1 Introduction

The term “karst” is often interchangeably used in areas underlain by soluble bedrocks (carbonates and evaporites) to describe:

• a landscape, known as karst terrain, characterized by distinctive surface features such as sinkholes, caves, sinking streams, and springs, whose formation is attributable in part to the process of chemical dissolution, with examples shown in Figure 1;
• a hydrogeologic setting, called a karst terrane, in which the surface and subsurface hydrology is largely influenced by the presence of underlying karstified bedrocks and karst aquifers; and,
• a groundwater flow system, known as a karst aquifer, in which water storage and movement occurs mainly through subsurface openings created or modified by dissolution, including unique voids known as “solution conduits”.

Figure 1 – Karst terrain varies depending on the physiographic and geohydrologic setting as can be seen in both the cover collage and these photos. The photographic images on the cover of this book and this figure present only a few of the many appearances of karst terrain. a) Tufa rimmed lakes at Plitvice Lakes World Heritage karst, Croatia¹. b) Aqua culture village in drowned karst in the South China Sea, Vietnam². c) Satellite view of Wakulla Springs, Florida, USA³. d) Outcrop of the middle portion of the Glen Rose limestone in Canyon Gorge, Texas, USA⁴. e) Alapaha River, river sink, Jennings Bluff-Avoca Tracts, Suwannee River Water Management District Lands, Hamilton County, Florida, USA⁵.

The term karst comes from the Slovenian word “Kras” because the Slovenian Kras region has characteristic karst rocks. This region is in southwestern Slovenia and extends across the border into northeastern Italy. Karst occurs throughout the globe (Figure 2) in a variety of physiographic areas: on islands and in coastal regions, in prominent mountain ranges and massifs, and throughout interior continental regions. In many, perhaps most locations, karst’s presence is easily recognizable by the occurrence of caves, sinkholes, disappearing or losing streams, and the distinctive grooved, scalloped, and fluted erosional features that develop on exposed soluble bedrock surfaces that are called karren. In other locations however, the surficial expression of karst is subtle and less easily recognized.

Figure 2 – Outcrop of carbonate and evaporite rocks forming karst terrains and aquifers around the world indicates the unconfined parts of karst aquifers not the total extent as many of these formations dip underground with confined extents often greater than unconfined extents (created from the World Karst Aquifer Map spatial data set Chen and others (2017) on the Mollweide map projection WGS-84 Datum and Spheroid).

Many geological, topographical, climatological, and hydrological factors influence the formation and physical manifestations of karst; not all karst develops in the same way, or exhibits the same types of surface and subsurface features. However, where karst occurs, its development and physical manifestation is influenced by a combination of groundwater flow and the hydrogeological characteristics and geochemical evolution of the soluble rocks. In areas underlain by limestone rocks, for example, the formation and characteristics of karst are influenced by the original depositional environment, lithologic and stratigraphic variability, diagenetic processes, post-depositional structural deformation, geomorphological evolution of the landscape, as well as variability and changes in precipitation and geochemical weathering processes over both short and long geological time scales.

The focus of this book is on karst aquifers. Karst aquifers serve as vital water resources for a large population, providing fresh drinking water to an estimated 10 percent of the world’s population (Stevanović, 2018) and accounts for up to 25 percent of all groundwater withdrawals (Ford and Williams, 2007). Thus, karst aquifers are important components of local and regional hydrologic and hydrogeologic systems. In karst areas, groundwater and surface water are highly interconnected, with karst aquifers and springs serving as headwaters and major tributaries to surface streams and rivers.

Karst aquifers help to sustain important biological and bio-geochemical systems in, for example, the hyporheic zone of surface steams and in subterranean aquatic ecosystems, including habitats for threatened and endangered cave species. In recent years, concerns about effects of global climate change have increased and carbon cycling in karst areas and aquifers has become a topic of greater scientific interest. Hydrogeochemical processes associated with karst areas involve reactions that both generate and sequester carbon and the effects of karstification in areas underlain by carbonates may be one of the larger variables that remain relatively unaccounted for in global carbon budgets and climate-change models (Fong et al., 2013; Cao et al., 2012, 2018).

Flow in karst aquifers is unique (Worthington et al., 2017). Aquifers are comprised of either granular material in which water moves through small pores, and/or fractured material where water moves through cracks and interconnected fractures. Some aquifers have both pores and fractures. Pores and fractures occur in karst aquifers, but in some karst aquifers so much rock has dissolved that the openings form large and interconnected conduits. In these cases, water moves not only through cracks and pores, but also through conduits of large aperture (greater than 0.1 m) or submerged caverns where subsurface flow is similar to flow in open channels and pipes. In some places, subsurface conduits occur in insoluble rocks such as lava tubes that form in volcanic rocks when molten lava solidifies and is called pseudokarst. Where conduits are interconnected, locations of water inflow and outflow are more localized than in granular and fractured aquifers, and flow moves more rapidly. At times the flow is turbulent.

In contrast to porous media flow and small fracture flow, conduit flow can have very high velocities and provide rapid transit of water from its entry into the subsurface to its discharge location. In some locales, conduits may exist, but their entry and/or exit points may be clogged such that the rate of groundwater movement is similar to aquifers composed of granular rocks, such as sandstone or alluvium. The potential for conduit flow in karst aquifers leads investigators to supplement standard aquifer investigation methods with mapping of surficial karst features, dye tracing, and more elaborate monitoring of the volume of groundwater discharge and water chemistry at springs.

The complexities presented by karst aquifers are among the most fascinating and challenging in hydrogeologic sciences. For example, karst aquifers present extreme forms of heterogeneity, including multi-scale porosity and permeability structures and groundwater flow dominated by networks of solutional openings that are fed by a combination of surface runoff and water leaking from the rock matrix. Groundwater flow in karst conduits can occur under high velocities (greater than 10 to 100 meters per day [m/d]) under both laminar and turbulent conditions. Hydrologic, hydraulic, and geochemical (for example, water-quality) characteristics in karst aquifers are typically among the most spatially and temporally variable in hydrogeology. These and other notable hydrogeologic complexities contribute to a widespread perception that groundwater resources provided by karst aquifers are difficult to develop, sustain, and protect. The likelihood of this perception being true increases where karst data collection and interpretation, as well as understanding of karst hydrogeology concepts, are insufficient to enable effective characterization of the aquifer and resource-management decisions.

This book reviews and highlights important basic hydrogeological characteristics of karst aquifers and summarizes investigative methods useful in their study and characterization. One key aspect to the investigation of karst aquifers is the realization that traditional methods of groundwater investigation developed and used successfully to characterize granular and many fractured-rock aquifers are often less successful, or must be modified and interpreted differently, when applied to karst. For example, mapping of water levels—a fundamental practice used to determine groundwater flow directions and rates in most aquifers—is often problematic in karst for reasons that will be discussed later. In addition, the hydrogeological complexities presented by karst generally require a multidisciplinary investigative approach that incorporates the use of several highly-specialized techniques such as water-tracing tests, spring-discharge monitoring, and various borehole and surface geophysical methods that often are not taught in traditional groundwater course curricula. Many of these techniques are the subject of additional books planned for the Groundwater Project.

The vast range of specialized topics related to karst are beyond the scope of this introductory book. After reading this introductory book, readers may want to consult other publications that provide overviews of karst hydrogeological concepts and investigative techniques, such as Goldscheider and Drew (2007), Taylor and Greene (2008), and Taylor and Doctor (2017). Textbooks authored by White (2019, 2016, 1993, 1999, 1988), Ford and Williams (2007), Milanovic (1988) are highly recommended resources for anyone interested in areas related to karst hydrogeology and water resources.

Readers who are not familiar with groundwater flow are encouraged to read other introductory books of the Groundwater Project including the overview book “Groundwater in Our Water Cycle” and the basics presented in “Hydrogeologic Properties of Earth Materials and Principles of Groundwater Flow”.

¹Photo by Kuniansky, 2008a.
²Photo by Kuniansky, 2012.
³Public domain image clipped from Google maps (Kuniansky, 2020).
⁴Photo by Morris, 2017.
⁵Photo by Allan Cressler taken November 25, 2010, used with permission.