| The Pecos River flows 1490 kilometers (926 miles) from its headwaters in New Mexico to its confluence with the Rio Grande in West Texas near Amistad Reservoir. The river is dammed in three locations and is known for its problems with salinity from various sources.;The area of interest of this project stretches from Pandale, Texas (river mile 64.5), south of Interstate 10 downriver to the Weir Dam (river mile 15.2), just northwest of Highway 90. 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 along this reach 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. Three sets of near-vertical joints exist in the area with approximate strikes of N7°E, N20°E, and N38°W. One fault was measured at mile 29.2 with a strike of N58°E and a dip of 75° SE.;International Boundary and Water Commission and United States Geological Survey gage data on the Lower Pecos River provide evidence that base flow increases the discharge by ~75%. The ET aquifer is responsible for diluting the saline river water and allowing for a sustainable ecosystem. Understanding the origins and mechanisms for surrounding groundwater will help to determine its characteristics and influence on the river.;Discharge was measured during three campaigns and increased the flow by 34% on average due to contributions from spring input. In this reach, the majority of the springs are relatively dilute, with an average specific conductivity of 612.71 &mgr;S/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 for the river include temperature (19.61 °C), specific conductivity (3,445.71 &mgr;S/cm), and dissolved oxygen (9.12 mg/L). The river value for specific conductivity declined from 4,827 &mgr;S/cm at river mile 64.57 to 3,514 &mgr;S/cm at river mile 15.2. The decline is an indication of the groundwater input and its tendency to improve water quality.;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. Waters with greater contribution from upstream sources contained higher delta18O values. Concentrations were consistent with the calculated global meteoric water line. Samples were also analyzed for tritium and noble gases, but because of excess air and terregenic helium, the results were inconsistent. |
