ESTIMATING GROUND WATER CONTRIBUTION FROM THE EDWARDS-TRINITY PLATEAU AQUIFER TO THE BIG BEND REACH OF THE RIO GRANDE, TEXAS

This study was made to quantify ground water contributions to flow of the Rio Grande in the Big Bend region. The project was a cooperative effort between National Park Service Water Resources Division (NPS WRD), Big Bend National Park (BIBE), the U.S. Geological Survey (USGS), and Sul Ross State University. This study will contribute to a larger effort to understand ground water systems that support base flows to tributaries of the Big Bend reach of the Rio Grande. The study area included the Rio Grande between Mariscal Canyon and Amistad Reservoir.

Ground water contributions to the Rio Grande from Cretaceous limestone aquifers sustain aquatic habitats during dry years, mitigate water quality impairment, and contribute significant water to Amistad Reservoir. Thermal springs occur along the Rio Grande from below Mariscal Canyon in BIBE to Langtry, TX just above AMIS. A first order approximation of the amount of ground water contributed by the springs can be found by examining International Boundary and Water Commission gage data for selected low flow periods. We examined these periods in 2001, 2002, 2003, and 2005. Net spring-induced flow increases for these periods ranged from 5.6 to 7.5 m³/s (196 to 266 ft³/s), yielding annual contributions of between 1.3 and 1.8 km³/yr (142,000 and 192,000 ac-ft/yr). An analysis of gage data (Saunders, 1987) from the period 1961 to 1985 for the same gages indicates that runoff and groundwater increases account for 23% of the mean annual flow in the reach. More quantitative gain loss studies in the form of seepage runs were conducted in the winters of 2006 and 2011. These data indicate that base flows increased by 7.95 and 9.17 m³/s (281 and 324 cfs) respectively. Extrapolating to annual contributions yields 0.25 and 0.28 km³/yr (203,444 and 234,576 ac-ft/yr). Ground water accounts for 75% (2006) and 70% (2011) of the base flow at Foster’s weir.

Ground water is generally better quality than surface water in the Rio Grande and therefore the addition of ground water improves the water quality in the river. During both seepage studies, estimates of total dissolved solids concentrations decreased downstream. In 2006, specific conductance measurements decreased from 3423 µS/cm at Solis to 1396 µS/cm at Foster’s weir. In 2011, specific conductance decreased from 3498 to 1505 µS/cm at the same locations.