Localized Anisotropic Transmissivity in a Karst Aquifer

Abstract

AbstractMaps 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.