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FRYAR, ALAN E., and WILLLAM F. MULLICAN III, Bureau of Economic Geology, The University of Texas at Austin, Austin, TX

ABSTRACT: Geochemical Characterization of Groundwater in the Vicinity of the Pantex Plant Near Amarillo, Texas

Groundwater samples from the perched and Ogallala aquifers in the vicinity of the Pantex Plant have been collected and analyzed for ionic and isotopic constituents. These data are needed to resolve three questions important not only for contaminant remediation on site, but also generally for water use on the southern High Plains. (1) How does the chemistry of groundwater evolve with flow from the land surface to the Ogallala aquifer? (2) How rapidly is the Ogallala aquifer recharged and from what sources? (3) How extensive are perched aquifers in the region, in particular the contaminated perched aquifer underlying Playa 1 at the Pantex Plant?

Away from the center of this perched aquifer, solute concentrations decrease and the composition evolves toward a mixed-cation bicarbonate facies observed regionally within the Ogallala aquifer. The evolving water chemistry may reflect reaction with pedogenic carbonates and adsorption of cations on clay minerals. Wells of less than 100m deep contain bomb tritium, indicating water less than 40 yr old. However, tritiated Ogallala wells also exhibit C14 levels as low as 20.8% modern carbon (equivalent to an uncorrected age of 13,000 yr), which suggests mixing of recent recharge with older water. Values of O18 are slightly enriched relative to the meteoric water line, indicating minimal evaporation, consistent with Nativ's finding of focused recharge through playas. Boron and nitrogen iso opes may allow us to quantify the volumetric contribution of playa waters to the Ogallala aquifer and assess the continuity of perched aquifers underlying playas.

AAPG Search and Discovery Article #90991©1993 AAPG Mid-Continent Section Meeting, Amarillo, Texas, October 10-12, 1993.