Evaluation of halite dissolution at a radioactive waste disposal site, Andrews County, Texas

Waste Control Specialists (WCS) has been granted permits to dispose of radioactive waste at their surface facility in western Andrews County, Texas, USA. The facility is located over Permian-age, halite-bearing formations, and the possibility of dissolution and its effects on the long-term performance of the disposal site must be considered. We compare data from the WCS site and vicinity to three conceptual hydrologic models of dissolution processes (shallow, deep, and stratabound). Geophysical logs yield stratigraphic and lithofacies data from the underlying halite-bearing units, the Permian Rustler and Salado Formations. The halite units are continuous, and thickness and lithofacies vary by depositional process, with no discernible postdepositional dissolution. Modest structural trends in these units differ from local changes in evaporite thickness. Therefore, deformation is not related to thickness differences. Local thickness changes in the middle of the Salado Formation indicate depositional variations. Core obtained from upper evaporite-bearing formations at the WCS site show normal depositional features and no brecciation that would be associated with postdepositional dissolution. Deeply buried bedded halite behaves as a ductile material, and the effective stress is close to zero, so that the pressure in pore fluids approaches lithostatic. Hydraulic gradients are outward from halite toward over lying and underlying rocks, eliminating access for low-salinity fluids. Formation fluids at depth are commonly saline and slow moving, further limiting the dissolution process. The hydrologic systems at the site limit the potential for future dissolution.