A GEOCHEMICAL STUDY OF THE IGNEOUS AQUIFERS OF WEST TEXAS

The igneous aquifers of west Texas are essential to both the natural and economic health of the region; however flow and chemical evolution within these aquifers is still not well understood. The purpose of this study was to improve the understanding of the geochemistry of these waters. Field water quality parameters were measured and water samples were collected from 49 springs and wells in Brewster, Jeff Davis, and Presidio counties between May 2009 and November 2009. Field parameters measured included acid neutralization capacity, pH, and specific conductivity. Collected samples were analyzed for major and trace ion composition, and oxygen (δ¹⁸O), hydrogen (δD), and strontium (⁸⁷Sr/⁸⁶Sr) isotope ratios. These results were then compared to data from other regional aquifers and values from other igneous systems.

Most of the waters sampled were calcium-bicarbonate dominant. Sodium-bicarbonate waters were found for the Marfa city well, and calcium sulfate waters were found in the Chisos Mountains of Big Bend National Park. Major and trace ion composition was generally consistent with expected values for water flowing through igneous rock. Oxygen δ¹⁸O values ranged from -3.54‰ to -8.62‰. When deuterium (δD) was plotted versus oxygen (δ¹⁸O) the lighter oxygen isotope ratios plot below the global meteoric water line in an evaporative trend. Oxygen δ¹⁸O values from Big Bend Ranch State Park in the south of Presidio County vary from -5.29‰ to -7.79‰ with corresponding δD values remaining relatively constant from -46.00‰ to -49.02‰. This suggests that rock-water interaction may be altering the oxygen isotope ratio of this groundwater.

Strontium isotope ratios were found to be below that of modern seawater (0.709) for all locations sampled. The ⁸⁷Sr/⁸⁶Sr ratios ranged from 0.7056 in Big Bend Ranch State Park in southern Brewster County to 0.7080 at the Chihuahuan Desert Research Institute in Fort Davis. Trends in both water chemistry and isotope ratio can be seen in the data based on the geology and geographic region. The data can be used to better assess the rock-water interactions and water flow relations in the region.