From Bureau of Economic Geology, The University of Texas at Austin (
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Bureau Seminar, April 06, 2007

Hydrochemical Variability and Detection of Cross-Formational Flow in the High Plains (Ogallala) and Plateau Aquifer Systems: Crockett and Glasscock Counties, Texas

Seay Nance

Bureau of Economic Geology, Jackson School of Geosciences, University of Texas at Austin.


Groundwater in the Early Cretaceous-age Fredericksburg Group (“Edwards”) aquifer ranges from sulfate-poor in southeastern Crockett County to relatively sulfate-rich in the northwest part of the county. Cross plots of sulfate and total dissolved solids suggest that two distinct populations of water types are present and show systematic geographic distributions. Each population is characterized by mixing between fresh recharge water with low dissolved-solid content and more saline sulfate-poor or sulfate-rich groundwater, respectively. End-member mixing models and strontium isotope data mutually support an interpretation of local flow of sulfate-rich groundwater from the Antlers sandstone aquifer (Early Cretaceous-age Trinity Group) into the overlying Edwards aquifer in Crockett County. A strong correlation exists between carbon-14 apparent ages and Mg/Ca values in the Edwards groundwater, which suggests that abundantly available Mg/Ca data might be used as a proxy for carbon-14 apparent age. However, tritium data indicate variable mixing between ancient and modern groundwater, and, therefore, carbon-14 ages are only “effective” ages. Proportions of ancient and modern waters in the mixtures are currently unknown.

Groundwater in Glasscock County is contained in both the Oligocene-age Ogallala sandstone and the Antlers sandstone. The Ogallala is restricted to the north half of the county, whereas the underlying Antlers is present throughout the county. Water in the Ogallala ranges from sulfate-poor with low total dissolved solids in the northeast part of the county to more saline, relatively more sulfate-rich in the northwest. Chloride/sulfate values also increase toward the west. Cross plots of chloride and total dissolved solids suggest that the main source of high salinity is oilfield brine. Cross plots of sulfate and total dissolved solids in Ogallala and Antlers water suggest that several end-member-mixing populations are present. Each population reflects mixing between fresh recharge water and several distinctly dissimilar more-saline waters. One of the Antlers mixing populations is chemically similar (in terms of SO4/Cl) to the most-saline Ogallala water. Geographic distribution of the several mixing populations indicates that a plume of brine-contaminated water is moving from the Ogallala into the underlying Antlers. Distribution of Antlers hydraulic-head values and distribution of Ogallala sulfate/TDS values suggest that some Antlers groundwater may be recycled up through the Ogallala, perhaps by irrigation pumpage.