THE INFLUENCE OF THE EDWARDS-TRINITY AQUIFER ON THE PECOS RIVER: A STUDY OF DISCHARGE AND GEOCHEMISTRY

The Pecos River flows 1490 kilometers from its headwaters in New Mexico to its confluence with the Río Grande in West Texas near Amistad National Recreation Area and Amistad Reservoir. The area of interest for this project stretches 79 km downstream of Pandale, Texas and just upstream of Amistad Reservoir. A major contributor to the Pecos River in this reach is the Edwards-Trinity Plateau Aquifer (ET). The groundwater from the ET contributes to base flow and improves water quality in the river and Amistad Reservoir. Field data for the springs and the river were collected to evaluate the groundwater input.

The surface geology of the area consists primarily of flat-lying Cretaceous carbonate rocks of the Edwards Limestone Group. These include the Fort Terrett, Segovia, and Devils River limestone members. There is abundant evidence of travertine in the river channel and in mounds where springs emerge from rock walls near the river.

Discharge measurements during three campaigns showed an average increase of 34%. In this reach the springs have an average specific conductivity of 612 μS/cm. Other field data include average values for temperature (22.71 °C), pH (7.59), and dissolved oxygen (5.81 mg/L). In comparison, average values in the river include temperature (19.61 °C), specific conductivity (3,446 μS/cm), and dissolved oxygen (9.12 mg/L). The river value for specific conductivity declined from 4,827 to 3,514 μS/cm. This improvement in water quality is an indication of the groundwater input.

Hydrogen and oxygen stable isotopes for the spring sets ranged from -25.08 to -33.54 per mil and -3.76 to -6.36 per mil, respectively. Heavier δ¹⁸O waters that plotted below the Global Meteoric Water Line (GMWL) were more influenced by upstream sources, evaporated waters, and rock-water interaction, whereas lighter δ¹⁸O values above the GMWL suggested a greater contribution from localized groundwater. Concentrations were consistent with the calculated GMWL.

Tritium and noble gas results, along with molar ratio evaluations, provided evidence that residence time of the ET groundwater in this portion of the aquifer is short, less than approximately 30 years. However, the comprehensive results of the age dating were hindered by evidence of excess terregenic helium trapped in the surrounding formations.