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.

Digital modeling of karst aquifers has had poor success for several reasons. Aquifer-wide details of karst hydraulics are poorly known. Nearly all numerical groundwater models are designed only for laminar flow, and even when turbulent-flow conduit modules are added, it is very difficult to predict well yields, flow directions, flow velocities and contaminant transport because of the local heterogeneity of karst aquifers. Nevertheless, there are several benefits from attempting to design digital karst models. By comparing the output from idealized models with actual field observations, the behavior of real aquifers can be better understood. Models of idealized conduit systems, designed to investigate theoretical hydraulic behavior, can provide insight into how these systems function in real aquifers. Most importantly, in attempting to design a digital model, one is forced to examine the detailed hydraulics of karst aquifers and to recognize the kinds of field data that are necessary to understand them. The promise of karst modeling is not in prediction, but in encouraging modelers to expand their understanding of karst aquifer behavior.

Abstract Any single geologic setting may include a variety of geochemical environments, each capable of producing a different type of carbonate solution porosity Also, many types of porosity can form in more than one geologic setting. Thus, the interpretation of solution porosity is best approached by first delineating the geochemical processes necessary to form the observed pattern of porosity, and then using these insights to assess the broader geologic context. Throughout most of any carbonate formation the solution process is highly selective, and only those openings of maximum groundwater flow are enlarged, while surrounding openings undergo little or no enlargement. Pervasive macroscopic porosity, in which nearly all initial openings are enlarged by solution, is formed by: (1) meteoric water with high discharge and/or low flow distance, (2) mixing of waters of disparate chemistry, (3) oxidation of hydrogen sulfide, or (4) production of acids by redox reactions involving carbon compounds in reducing environments. Areally extensive solution porosity within a narrow stratigraphic range usually indicates solution or reduction of sulfates. Cavernous solution porosity is negligible where aggressive infiltration is lacking, in deep zones where groundwater chemistry is uniform, and in low-flow areas of diagenetically mature carbonate rocks far from sources of groundwater recharge.

Abstract Karst landscapes are the foremost examples of ground-water erosion on this planet. The sculpturing and removal of bedrock is predominantly by solution, aided in some cases by soil piping and collapse. Karst landforms develop best in limestones and dolomites, gypsum, and salt. Carbonate rocks crop out over approximately 10 p.rcent of the earth’s land area and are found in most nations and all climatic regions. It is estimated that 25 p.rcent of the world's population depends on fresh water in karst aquifers. Karst rocks and their contained minerals, oil, and gas are of considerable importance to the extractive industries, while caves and other karst features have been of great cultural significance. Karst aquifers are the mavericks of hydrogeology. Closed depressions input a recharge that is intermediate between the classic ideas of infiltration and surface runoff. Integrated conduit systems act as short circuits for the ground-water flow system. Conduits are gross heterogeneities in the permeability distribution, and flow within them does not obey Darcy’s Law. The purpose of this chapter is to outline some of the geomorphic features of karst areas and to indicate their relationship to karstic aquifers. Recent English language books on karst studies include Sweeting (1972), Ford and Cullingford (1976), Bögli (1980), Milanović (1981), Jennings (1985), James and Choquette (1988), and White (1988).

Abstract The Turnhole Spring groundwater basin covers more than 85 mi2 (220 km2) in parts of Edmundson and Hart Counties, Kentucky, approximately 30 miles (50 km) north of Bowling Green, Kentucky and 10 miles (16 km) west of Interstate 65 at Cave City, Kentucky (Fig. 1).