The role of a perched aquifer in contaminant transport at a proposed Superfund site on the southern high plains of Texas

Hydrogeologic and hydrochemical investigations of a perched aquifer above the Ogallala aquifer are being conducted in the region of the US Pantex Plant, a proposed Superfund site, located approximately 20 mi northeast of Amarillo, Texas. Since the early 1950's, a small playa basin located on the Pantex Plant has been used as a wastewater discharge pond with daily discharge rates ranging from 400,000 to 1,000,000 gallons. Measured depths to the perched aquifer range from 210--300 ft below land surface, whereas depth to the regional Ogallala aquifer represents a ground-water mound, with the peak of the mound underlying the small playa basin. Solute concentrations tend to decrease outward from the center of the ground-water mound. Down the hydrologic gradient the composition of perched ground water appears to evolve toward that of the regional Ogallala aquifer. Tritium concentrations in the perched aquifer reached as high as 8.75 TU, indicating water younger than 40 yr. old. Modeling of the perched aquifer is focusing on how much, if any, discharge to the small playa basin has enhanced recharge to the perched aquifer and increased the vertical and lateral extent of the perched aquifer. Hydrogeologic and hydrochemical data illustrate how the perched aquifer is divertingmore » the downward vertical migration of potential contaminants laterally away from the small playa basin. Accurate delineation of rates and flow directions and understanding the hydrochemical evolution of ground water in the perched aquifer are critical to any successful remediation effort.« less