Air-pressurized slug tests offer an efficient means of estimating the transmissivity (T) and storativity (S) of aquifers. Air-pressurized slug tests are conducted by pressurizing the air in the casing above the column of water in a well, monitoring the declining water level and then releasing the air pressure and monitoring the rising water level. The equipment needed to conduct an airpressurized slug test is easily constructed and assembled at the top of the well. The only equipment in contact with the water is a down-hole sensor to monitor water levels. During the pressurized part of the test, small changes in the applied air pressure result in water-level fluctuations, making it difficult to estimate T and S from the declining water-level data. However, if the applied air pressure is maintained until a new equilibrium-water level is achieved and then the air pressure in the well is released instantaneously, the slug test solution of Cooper and others (1967) can be used to estimate T and S from the rising water-level data. In lowpermeability formations, it may take an extended period of time to achieve the new equilibrium-water level for the applied air pressure. The total time to conduct the test can be reduced, however, if the pressurized part of the test is terminated prior to achieving the new equilibrium-water level. This is referred to as a prematurely terminated air-pressurized slug test. Type curves generated from the solution of Shapiro and Greene (1995) can be used to estimate T and S from the rising water-level data from prematurely terminated air-pressurized slug tests. The Fortran code AIRSLUG, included in this document, is used to generate the type curves from the solution of Shapiro and Greene (1995). A detailed discussion of the equipment and procedures for conducting air-pressurized slug tests is presented along with discussions of data preparation, use of the Fortran code AIRSLUG, and method of matching the data and the type curves to estimate T and S. Field examples are presented to demonstrate the applicability of air-pressurized slug tests in estimating T and S.

Air‐pressurized slug tests offer a means of estimating formation transmissivity and storativity without extensive downhole equipment and in situations where contact with formation fluids may pose a health concern. An air‐pressurized slug test, as discussed in this paper, consists of applying a constant pressure to the column of air in a well, monitoring the declining water level, and then releasing the air pressure and monitoring the recovering water level. If the maximum declining (or new equilibrium) water level is achieved for a constant applied air pressure, the slug‐test solution of Cooper et al. (1967) can be used to interpret the water‐level recovery data and estimate the formation properties. In low‐permeability terranes, the time required to achieve the equilibrium water level during the pressurized part of the test may be too long for practical purposes, and it may be necessary to terminate the applied air pressure prior to establishing a new equilibrium. To analyze data from such tests, a solution to the boundary‐value problem for the declining and recovering water level during an air‐pressurized slug test is developed for an arbitrary time‐dependent air pressure applied to the well. For the special case of applying a constant air pressure and then reducing it instantaneously to atmospheric pressure at a prescribed time, the general solution reduces to the superposition of the solution of Cooper et al. (1967) at two displaced times. Type curves are generated to estimate formation transmissivity and storativity from the recovering water level associated with prematurely terminated air‐pressurized slug tests. The application of the type curves is illustrated in tests conducted in wells completed in the Minnelusa and Madison aquifers near Rapid City and Spearfish, South Dakota.

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Stable isotopes of hydrogen and oxygen were used to trace the sources of recharge from sinking streams to wells and springs several kilometers downgradient in the karst Madison aquifer near Rapid City, South Dakota. Temporal sampling of streamflow above the swallets identified a distinct isotopic signature that was used to define the spatial dimensions of recharge to the aquifer. When more than one sinking stream was determined to be recharging a well or spring, the proportions were approximated using a two‐component mixing model. From the isotopic analysis, it is possible to link sinking stream recharge to individual wells or springs in the Rapid City area and illustrate there is significant lateral movement of ground water across surface drainage basins. These results emphasize that well‐head protection strategies developed for carbonate aquifers that provide industrial and municipal water supplies need to consider lateral movement of ground‐water flow from adjacent surface drainage basins.

Maps of cave passageways in the outcrop area of the uplifted Madison Limestone in the Black Hills, South Dakota, show that principal cavern development is oriented in the major direction of ground‐water flow, roughly radial to the Black Hills. Fracture‐trace analysis and measuremnt of joints in the Wind Cave area show that these orientations coincide with cave passageways. Aquifer testing at Rapid City indicates that a local principal transmissivity tensor is oriented in the direction of cave development and along the strikes of bedding‐plane fractures. This indicates that much of the permeability of the Madison aquifer is modern karst (post‐Laramide‐Orogeny). From the above, we conclude that a localized anisotropic permeability (principal direction of transmissivity) is developed by ground water flowing through fractures, dissolving the rock, and producing dissolution‐enhanced conduits along the direction of ground‐water flow. This localized principal direction of transmissivity can be deduced from analysis of the potentiometric surface, stream‐aquifer hydrographs, mapped cave passageways, aquifer tests, fracture traces, and measurements of joints in the field.