HYDROGEOLOGIC INVESTIGATION OF SPRINGS ALONG THE LOWER PECOS RIVER IN WEST TEXAS

The Pecos River originates in New Mexico and flows south into Texas to its confluence with the Rio Grande. The river is managed throughout its course with numerous diversions and has very high salinity in the reach where it first flows into Texas. As the Pecos crosses the Edwards Plateau, numerous springs emerge from the Edwards-Trinity Plateau Aquifer (ET).

The study area starts in the vicinity of the Pandale Crossing, 104 km upriver from the confluence with the Rio Grande. Between here and the Weir Dam near Langtry, TX (24.5 upriver from the confluence), two gain loss surveys have been conducted (November, 2010 and April, 2012). In this reach, several spring complexes that provide a fresh water inflow to the river have been delineated emerging from the Fort Terrett, Segovia and Devils River limestone members of the Edwards group..

The seepage run results illustrate the basic contribution of the ET aquifer to the base flows of the Pecos. Data from the 2010 trip indicate an increase from 3.54 to 5.21 cubic meters per second (cms; a maximum difference of 1.67 cms) between Pandale and the Everett spring group after which the discharge remained relatively constant. Data from the 2012 trip indicate an increase from 2.21 to 4.12 cms (with a maximum difference of 1.91 cms) in the same reach, after which the discharge declined to 3.56 cms. These discharge increases resulted in a concurrent decline in specific conductivity in the river (Woodburn and others, 2011). The spring chemistry varies from a no dominant ion type to a calcium bicarbonate type (~700 uS/cm and ~400 uS/cm, respectively).

Two continuously monitored gages are located in the vicinity of this reach. The USGS Brotherton gage is located in the upstream part of the reach and the IBWC gage near Langtry is located at the downstream end. Time and distance corrected discharge values for these two gages (corrected to the position and time when the study measurements were made) indicate an overall increase of 1.25 cms for the 2010 time frame and an overall increase of 1.24 cms for the 2012 time frame. These discharge numbers are slightly lower than the field measured values most likely due to losing reaches not included in the field estimates.

A conservative estimate of total ET spring contribution in this reach is 1.24 cms, which translates to 3.9E-2 cubic kilometers per year (~32000 acre-feet per year).