Modern scientific study of karst phenomena came into being during the 90 years before the Geological Society of America was founded in 1888. It began with broad acceptance of the uniformitarian principle (1800s), basic understanding of processes of carbonate and sulfate rock dissolution and precipitation (1820s), and the equations of Hagen, Poiseuille, and Darcy for groundwater flow in porous, fractured, and soluble media (1840–1856). The Dalmation descriptive name “karst” (meaning “stony ground”), adopted by regional surveyors and travelers, came into general use in the 1850s also. The first U.S. Geological Survey (USGS) report on hydrogeology by Chamberlin in 1885 was one of many early texts that stressed the importance of conduit flow in limestone areas. The 50 years following 1888 were dominated by studies in the “classical” karst region of western Slovenia, including definition of the principal types of surface landforms and proposals for their development within cycles of erosion, two sharply contrasted models for storage and flow in limestone aquifers, and promotion of a theory that accessible caves formed chiefly in the vadose zone. Following publication of a USGS report on the major springs in the nation in 1927, American scientists entered the debates in force, proposing that caves should develop primarily below the water table, along it, or create it; they also emphasized the importance of soil CO 2 in boosting rates of solution in carbonate rocks. Russian investigators established the principles of mixing corrosion. The pace of development throughout karst studies accelerated after the Second World War. In the later 1940s and 1950s, the formative studies of solution kinetics began, while improvements in methods of measuring solute concentrations set the stage for global rate models to be developed in succeeding decades. Spatial quantitative analysis came to dominate study of surface landforms, particularly sinkhole distribution patterns. The confusion that had arisen regarding the development of meteoric water (epigene) caves was resolved with a general model emphasizing the controlling roles of lithology and geologic structure: Increasingly, it was recognized that these two variables also explained many of the differences observed between karst aquifers and landform assemblages in different geographical areas. Opening of China to western scholars after 1980 gave access to the astonishing karst lands in the south of that country.

In the late 1950s to early 1960s, there was a paradigm shift in the study of caves and karst. Instead of a science of speleology, the focus changed to using caves and their contents to provide information of much wider geological interest. Cave and karst science has also borrowed heavily from other sciences. One technique that was borrowed was the restructuring of carbonate chemistry to define saturation index, CO 2 partial pressure, and other parameters that are widely used to describe karst waters. Equilibrium chemistry was followed by chemical kinetics, which proved to be the key to understanding the development of conduit systems. Karst hydrology was advanced by recognizing the importance of the karst drainage basin in surface water–groundwater interactions and the hydrodynamics of conduit flow. Much new interpretation was made possible with data provided by greatly improved tracer techniques. Karst hydrology has moved from qualitative descriptions to computer models that take account of matrix, fracture, and conduit permeability. Sediment and contaminant transport as well as new understanding of sinkhole collapses and other land-use hazards have become part of the hydrogeologic framework of karst. All aspects of cave and karst science have been revolutionized by the development of accurate dating methods for speleothems and for clastic sediments in caves. Following from the dating techniques, one of the most important developments has been the use of speleothems as paleoclimate archives.

The earliest known comprehensive karst map of the entire United States was published in 1969, based on compilations of William E. Davies of the U.S. Geological Survey (USGS). Various versions of essentially the same map have been published since. The USGS published new digital maps and databases in 2014 depicting the extent of known karst, potential karst, and pseudokarst areas of the United States, including Puerto Rico and the U.S. Virgin Islands. These maps are based primarily on the extent of potentially karstic soluble rock types, and rocks with physical properties conducive to the formation of pseudokarst features. These data were compiled and refined from multiple sources at various spatial resolutions, mostly as digital data supplied by state geological surveys. The database includes polygons delineating areas with potential for karst tagged with attributes intended to facilitate classification of karst regions. Approximately 18% of the surface of the 50 United States is underlain by significantly soluble bedrock. In the eastern United States, the extent of outcrop of soluble rocks provides a good first approximation of the distribution of karst and potential karst areas. In the arid western states, the extent of soluble rock outcrop tends to overestimate the extent of regions that might be considered as karst under current climatic conditions, but the new data set encompasses those regions nonetheless. This database will be revised as needed, and the present map will be updated as new information is incorporated.

The scientific study of Texas caves and karst has passed two distinct periods and is on the verge of crossing into a third. The first period dates from 1849 to 1982. At that time, the word “karst” was unknown to most geologists in the state. Caves were occasionally mentioned in geological reports until 1948, but they were rarely studied. In the 1960s, cave explorers-turned-scientists began investigating Texas caves, but their work was often seen as lightweight science because caves were usually considered geologic curiosities of little importance. However, extensive exploration and documentation of caves, plus some respected cave research outside of the geosciences, led to the second period of research, dating from 1983 to the present. This was a transitional period where karst was increasingly recognized and karst research methods were pioneered, although often by people lacking sufficient understanding of karst to be fully accurate or effective. During this period, the expertise of karst scientists gained respect despite the persistence of some old prejudices, and caves began to reappear with greater frequency in technical reports. Many of these changes were prompted by the listing of several karst invertebrates as endangered species, which required detailed hydrogeological cave and karst research, in addition to biological research, across much of the Edwards Aquifer region. The third period of karst research has nearly arrived. It will be recognized when most geoscientists understand karst, that it requires specialized training and experience like other geological disciplines, and when research of karst areas routinely uses karst-appropriate tools, theories, and consideration for the many degrees of permeability that occur. The future of cave and karst research in Texas will see important technological advances and a focus on hydrological and biological karst resource management as the combined impacts of population growth and climate change increasingly affect the state.

Cave type and morphology are controlled by hydrogeological and geological factors; therefore, by inverse analogy, cave type and morphology could be used to determine the hydrogeological and geological conditions under which the caves developed. Euclidian metrics have traditionally been used to quantify and compare cave morphologies, even though caves have irregular and complex shapes. Caves have been shown to possess characteristics that identify them as fractals within certain ranges, so the use of Euclidean-based metrics alone to define and characterize them may be a limitation in morphometric analyses. Other factors that limit full morphometric analyses of caves include focus on two-dimensional cave data, as these are typically what are available, and exploration bias, as cave exploration and documentation are limited to spaces that are humanly passable or of immediate interest to the explorer, epitomizing the subjective nature of anthropogenic-based measurements. This research involves a proof-of-concept study that uses fractal indices as a means for identifying and classifying cave morphology and distinguishing genetic cave types. The fractal indices used are fractal dimension, which quantifies the complexity of a pattern, and lacunarity, which quantifies the texture of a pattern. Three-dimensional cave survey data were used to generate cave models that were converted to cave pattern image files and analyzed with image-processing software. Fractal indices were calculated for digital patterns of a limited subset of known cave types that included tafoni, littoral caves, stream caves, flank margin caves, and continental hypogene caves. The quantitative morphological distinctions in cave patterns as identified by fractal dimension and lacunarity proved to be statistically significant within the subset of cave types analyzed for this study. Similarities in geological and/or hydrogeological processes or overprinting by such processes can skew fractal indices, so geological and hydrogeological context is critical when interpreting fractal indices. The results of this study demonstrate that cave morphometry as defined by fractal indices can be used to augment the identification of cave type, which provides insight into the geological and hydrogeological controls on development of the cave type and its cavernous porosity and permeability.

Cave development in the Madison aquifer of the Black Hills has taken place in several stages. Mississippian carbonates first underwent eogenetic (early diagenetic) reactions with interbedded sulfates to form breccias and solution voids. Later subaerial exposure allowed oxygenated meteoric water to replace sulfates with calcite and to form karst and small caves. All were later buried by ~2 km of Pennsylvanian–Cretaceous strata. Groundwater flow and speleogenesis in the Madison aquifer were renewed by erosional exposure during Laramide uplift. Post-Laramide speleogenesis enlarged paleokarst voids. Most interpretations of this process in the Black Hills invoke rising thermal water, but they fail to account for the cave patterns. Few passages extend downdip below the present water table or updip to outcrops. None reaches the base of the Madison Limestone, and few reach the top. Major caves underlie a thin cover of basal Pennsylvanian–Permian Minnelusa Formation (interbedded quartzarenite and carbonates). Water infiltrating through the Minnelusa Formation dissolves carbonates in a nearly closed system, producing low p CO 2 , while recharge directly into Madison outcrops has a much higher p CO 2 . Both are at or near calcite saturation when they enter caves, but their mixture is undersaturated. The caves reveal four phases of calcite deposition: eogenetic ferroan calcite (Mississippian replacement of sulfates); white scalenohedra in paleovoids deposited during deep post-Mississippian burial; palisade crusts formed during blockage of springs by Oligocene–Miocene continental sediments; and laminated crusts from late Pleistocene water-table fluctuations. The caves reveal more than 300 m.y. of geologic history and a close relationship to regional geologic events.

Calcite spar (crystals >1 cm in diameter) are common in limestone and dolostone terrains. In the Guadalupe Mountains, New Mexico and west Texas, calcite spar is abundant and lines small geode-like caves. Determining the depth and timing of formation of these large scalenohedral calcite crystals is critical in linking the growth of spar with landscape evolution. In this study, we show that large euhedral calcite crystals precipitate deep in the phreatic zone (400–800 m) in these small geode-like caves (spar caves), and we propose both are the result of properties of supercritical CO 2 at that depth. U-Pb dating of spar crystals shows that they formed primarily between 36 and 28 Ma. The 87 Sr/ 86 Sr values of the euhedral calcite spar show that the spar has a significantly higher 87 Sr/ 86 Sr (0.710–0.716) than the host Permian limestone (0.706–0.709). This indicates the spar formed from waters that are mixed with, or formed entirely from, a source other than the surrounding bedrock aquifer, and this is consistent with hypogene speleogenesis at significant depth. In addition, we conducted highly precise measurements of the variation in nonradiogenic isotopes of strontium, 88 Sr/ 86 Sr, expressed as δ 88 Sr, the variation of which has previously been shown to depend on temperature of precipitation. Our preliminary δ 88 Sr results from the spar calcite are consistent with formation at 50–70 °C. Our first U-Pb results show that the spar was precipitated during the beginning of Basin and Range tectonism in a late Eocene to early Oligocene episode, which was coeval with two major magmatic periods at 36–33 Ma and 32–28 Ma. A novel speleogenetic process that includes both the dissolution of the spar caves and precipitation of the spar by the same speleogenetic event is proposed and supports the formation of the spar at 400–800 m depth, where the transition from supercritical to subcritical CO 2 drives both dissolution of limestone during the main speleogenetic event and precipitation of calcite at the terminal phase of speleogenesis. We suggest that CO 2 is derived from contemporaneous igneous activity. This proposed model suggests that calcite spar can be used for reconstruction of landscape evolution.

Karst landscapes provide unique challenges and opportunities to studies of processes within the critical zone, which spans from the top of the canopy to the base of active groundwater circulation. Dimensional analysis using the characteristic length scales and time scales of karst processes enables development of an initial framework for quantification of the rates and distribution of these processes throughout the critical zone. In particular, dimensional analysis provides a useful tool with which to identify the relative importance of different processes and to test the assumptions behind models of critical zone function. I briefly review prior use of dimensional analysis to understand various aspects of the karst critical zone and then introduce simple models for CO 2 transport within fractures and conduits of the vadose zone. Dimensional analysis of these models suggests that advection through karst fractures within both the gas and liquid phases will strongly impact vertical CO 2 profiles in the vadose zone in a variety of settings. Implications of this finding for karst critical zone development and future data campaigns are discussed.

Karst systems continually evolve in response to complex hydrological and geochemical processes. A factor not previously considered is the role played by wavy free-surface fluid films in the geochemical erosion of microfractures. Films seeping down nearly vertical walls naturally evolve into wavy films, with some waves growing into solitons. Solitons that continue to grow eventually contact the opposite fracture wall, developing into capillary droplets that persist. The combination of capillary droplets surrounded by free-surface films creates a dissolve-and-sweep mechanism for soluble rocks such as limestone, dolomite, and gypsum. While films participate in calcite transfer from the matrix by diffusive processes, the pressure gradient imposed by a capillary droplet can extract pore solution from within the matrix, chemically leaching the matrix to a deeper depth than film flow. This chapter presents experimental evidence of the formation of droplets and provides a first-order analysis of their solute transfer and transport potential.

The results of seven case studies by the author, colleagues, and graduate students performed at contaminated sites indicate that monitoring wells and piezometers installed in karstic carbonate aquifers often yield unreliable data. These devices more often gave misleading than useful information concerning aquifer properties, groundwater flow, and contaminant movement. These findings are in accord with the highly anisotropic and heterogeneous nature of these aquifers. The following cautions are provided when monitoring wells are to be used in karstic carbonate regions. (1) Monitoring wells may be unreliable in detecting contaminant releases. (2) A monitoring well that detects a contaminant is unlikely to provide valid data regarding the quantity of the release or the velocity and direction of the contaminant movement. (3) Water levels measured in wells often give erroneous indications of groundwater flow direction. (4) Well water levels and chemical parameters taken at random or traditional quarterly calendar intervals give little insight into the fluctuations that may actually occur in the well. (5) Head fluctuations in wells in response to nearby pumping or injection do not necessarily indicate flow connections. (6) Traditional well tests in carbonate aquifers typically do not sense the most important elements of the permeability structure. (7) Virtually every well in a carbonate aquifer is influenced by a unique suite of permeability and recharge elements. In spite of their manifest shortcomings in carbonate aquifers, monitoring wells are specified by law in virtually every case where contaminants may be or have been released. Unfortunately, these wells are usually placed using criteria appropriate for granular aquifers. Alternative and more appropriate means of aquifer assessment and monitoring in these aquifers are available, including wells augmented with tracer investigations and the use of springs and other access points to the conduit elements of the porosity system.

Geologic structure often controls the location of recharge points, flow paths, velocities, and discharge locations in karst regions such as Morell Cave and its springshed, Bluff City, Tennessee. This study explores groundwater recharge points, velocities, and discharge locations within the Morrell springshed and its associated cave. Two dye tracing experiments were conducted in the spring and fall of 2012 to identify recharge sources, delineate the springshed, and to interpret structural controls for groundwater flow. The experiments confirmed that allogenic recharge from the northern slopes of Holston Mountain enters the karst system through swallets and flows to the northwest following dominant joint trends that transect local folds. When the groundwater reaches Morell Cave, the flow is redirected northeast and parallels a shallow thrust fault, along which Morell Cave has developed, before resurging at Morell Spring. Using a joint-path flow model, groundwater velocities ranged from 0.04–0.007 m/s, which is consistent with typical groundwater velocities in karst systems.

Weekly water samples collected in the spring and summer of 2012 demonstrate the dynamic geochemistry within the epikarst of an alpine karst aquifer in Timpanogos Cave National Monument in the Wasatch Mountains near Salt Lake City, Utah, USA. The results of chemical analysis of water from four cave pools, supplemented with concurrent samples from the American Fork River, suggest three modes of recharge: (1) diffuse recharge through the permeable matrix of the carbonate rock, (2) rapid recharge through open fractures in the epikarst, and (3) rapid recharge via piston flow through fractures occluded with colluvium. Water levels in the cave pools recharged by diffuse flow were very stable during the study period. Elevated dissolved solids characterized the geochemistry, including solutes associated with hydrothermal activity in this region (e.g., SO 4 2− and F − ). Isotopes of sulfur and carbon, along with cation-anion ratios suggest that sulfide oxidation may play some role in modern dissolution of the carbonate bedrock. In situ geochemical reactions influence the concentration of some solutes (e.g., HCO 3 − , Ca 2+ , F − ) and may cause a shift in the isotopes of dissolved inorganic carbon. Water levels in the cave pools characterized by rapid recharge, in comparison, were highly variable. When the flow path was direct, the geochemistry of the pool was strongly influenced by the timing and rate of recharge. During times of limited recharge, the geochemistry of these pools evolved toward the values of pools dominated by diffuse flow. On the other hand, when the flow path was impeded by colluvium, recharge was stored, and the geochemical signal was homogenized. In both cases, the source of recharge may be from elevations substantially above the cave pool.