THE INFLUENCE OF SPRINGS ON DISCHARGE AND RIVER WATER CHEMISTRY IN THE LOWER CANYONS, RIO GRANDE WILD AND SCENIC RIVER, TEXAS

The Lower Canyons (LC) reach of the Rio Grande Wild and Scenic River defines the U.S.-Mexico international border downstream from Big Bend National Park (BBNP) between La Linda and Dryden, Texas. Numerous springs issue from a trans-boundary aquifer in the area. We have initiated structural and geochemical studies of the springs to elucidate recharge areas, groundwater flow paths to the springs, and the influence that the springs have on water quality and quantity in the Rio Grande. On the Texas side, the Cretaceous-hosted Edwards-Trinity Plateau aquifer is extensive. On the southern side, two aquifers in the Mexican State of Coahuila have been delineated (Cerro Colorado-La Partida and Serrania Del Burro).

International Boundary and Water Commission (IBWC) gage data indicate that base flows progressively increases by as much as 60% due to spring inflow in the LC. Independent discharge measurements made above and below several spring groups indicate that ground water from Cretaceous aquifers account for most of this increase.

Ground water input is also responsible for dilution of total dissolved solids (TDS) in river waters. Water quality for the river reach above BBNP fails to meet EPA standards for TDS. The Rio Grande segment that contains the LC springs meets standards because TDS diminishes downstream as lower TDS spring inputs contribute to river discharge and to the dilution of high TDS water.

Specific conductivity (SC) for the springs averages 734 uS/cm whereas the river samples adjacent to the springs average 2348 uS/cm. The SC in the river declines from a maximum of 2510 uS/cm at river mile 716 to 2150 uS/cm at river mile 761. Seven spring samples collected in March of 2009 averaged 461 mg/L TDS (706 uS/cm). Major element variations reveal two groups of springs, one with an average of 533 mg/L TDS which plots on a Piper diagram as a generic “no dominant ion” type of water and a second set with an average of 282 mg/L TDS that plots as a calcium-bicarbonate type water. Oxygen and hydrogen isotope analyses indicate that the springs cluster in two groups on a local meteoric water line (LMW) that is shifted to lower d¹⁸O by 0.5 ‰. Differences between, and variability within, the clusters may reflect differences in recharge elevation and water-rock reaction effects. River waters show evaporative enrichment and are distinctly different from spring waters.