Regional hydrodynamics of the proposed high-level nuclear-waste repository sites in the Texas Panhandle

Abstract

Bedded salt in the Palo Duro Basin area of the Texas Panhandle is being evaluated as possible host rock for a high-level nuclear-waste repository. The two potential sites are underlain by three major hydrostratigraphic units: a shallow freshwater aquifer system developed in the Tertiary Ogallala Formation and the Triassic Dockum Group, a thick shale and evaporite aquitard developed in an Upper Permian evaporite sequence, and a deep-basin brine aquifer system predominantly developed in the Wolfcamp Series and the Pennsylvanian System. Water level, shut-in pressure, and specific-gravity data from inventoried wells, drill-stem tests, and long-term pumping tests were used to construct potentiometric surfaces, potentiometric profiles, and pressure-depth diagrams. Water in the Ogallala and Dockum aquifers predominantly flows horizontally and discharges to springs and wells. Equipotential patterns on potentiometric profiles indicate that some water may be moving downward from the Ogallala into the Dockum across an intervening confining unit, as well as moving downward from the Dockum across the shale and evaporite aquitard and into the deep-basin aquifer system. This is corroborated by the slope of pressure-depth data which indicate the potential for regional downflow in the shale and evaporite aquitard. Equipotential patterns and pressure-depth data indicate predominantly horizontal flow in the deep-basin aquifer system which is influenced by permeability variations related to facies changes. Hydraulic gradients are flatter in the more permeable shelf, shelf margin, and fan-delta facies and steeper in the less permeable deep-basin facies.