Geochemical studies of discharge water from a uranium acid-leach process
— Patrick A. Longmire and Douglas G. Brookins

Sohio Western Mining L-Bar tailings site occupies approximately 65 ha (160 acres) of Quaternary alluvial deposits resting unconformably on the Cretaceous Mancos Shale in the Laguna-Paguate uranium district, Cibola County, New Mexico. The site is located 4.8 km (3 mi) east of the village of Seboyeta, approximately 64 km (40 mi) northwest of Albuquerque. The Mount Taylor volcanic complex borders the site to the north and west. The mill tailings result from a sulfuric-acid leach processing of uranium ore and have been accumulating since 1976. The tailings are piled to a height of approximately 5 m and are contained to the west by a tailings dam, including a compacted starter dam, and to the northeast by a saddle dam constructed from local soils and clay materials. The tailings are characterized by low pH, high concentrations of soluble salts, a high moisture content, and interstitial fluids of high ionic strength. Chemically, toxic and radioactive elements occur in anomalous concentrations with nonuniform distribution.

The interstitial fluids of the tailings are potential sources of mobile, toxic, and radioactive species, and contain a pH less than one. This fluid has come in contact with the locally unconfined first Tres Hermanos aquifer, a fine- to medium-grained, well-sorted orthoquartzite-subarkose (Moench and Schlee, 1967), which acts as a recharge boundary near the base of the tailings. Ground water in the aquifer is dominately a sodium, calcium, sulfate-bicarbonate rich, alkaline pH water. Interstitial calcite occurs as a precipitate, constituting up to 3 percent by volume, in response to major element chemistry of the ground water. Contact of highly acidic tailings fluids and the moderately alkaline ground water in the first Tres Hermanos aquifer produces an interface boundary resulting in neutralization and precipitation of various toxic and radio-logical species.

The objective of this paper is to summarize some results of the investigation of contaminant behavior and mobility within the localized hydrologic environment near the L-Bar tailings pile. The investigation has resulted from an extensive review of Sohio's ground-water discharge plan by NMEID (New Mexico Environmental Improvement Division) staff.