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

The Pecos River flows 1490 kilometers (926 miles) from its headwaters in New Mexico to its confluence with the Rio Grande in western 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, TX (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. We collected field data for the springs and the river along this reach 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 N7E, N20E and N38W. One fault was measured at mile 29.2 with a strike of N58E and a dip of 75 SE.

In this reach, the bulk of the springs are relatively dilute with an average specific conductivity of 669 µS/cm³ (+/- 193). One spring has a specific conductivity of ~2100 µS/cm³. Other field data include average values (and one standard deviation) for temperature (22.64 °C, 0.51), pH (7.5, 0.46), and dissolved oxygen (5.89 mg/L, 1.43).

In comparison, average values for the river include temperature (16.59 °C, 2.92), specific conductivity of (3528 µS/cm³, 499), and dissolved oxygen (9.53 mg/L, 0.75). The river value for specific conductivity declined from 4152 µS/cm³ at river mile 62.2 to 3514 µS/cm³ at river mile 16.2. The decline is an indication of the groundwater input and its tendency to improve water quality.

Discharge was measured at six locations. The discharge increased from 3.54 cubic meters per second (cms; 125 cubic feet per second (cfs)) at mile 64.6 to a maximum of 5.21 cms (184 cfs) at mile 33.2 after which is remained fairly constant. The increase in discharge is also due to the groundwater contribution.