LOCALIZED ANISOTROPIC TRANSMISSIVITY IN A KARST AQUIFER

Maps of cave passageways in the outcrop area of the uplifted Madison L imestone in the Black Hills, South Dakota, show that principal cavern development is oriented in the major direction of ground-water flow, r oughly radial to the Black Hills, Fracture-trace analysis and measurem nt of joints in the Wind Cave area show that these orientations coinci de with cave passageways. Aquifer testing at Rapid City indicates that a local principal transmissivity tenser 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 tha t a localized anisotropic permeability (principal direction of transmi ssivity) is developed by ground water flowing through fractures, disso lving 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 surf ace, stream-aquifer hydrographs, mapped cave passageways, aquifer test s, fracture traces, and measurements of joints in the field.