HYDROGEOLOGIC FRAMEWORK OF SPRINGS IN THE LOWER CANYONS OF THE RIO GRANDE, RIO GRANDE WILD AND SCENIC RIVER, TEXAS

The Lower Canyons reach of the Rio Grande Wild and Scenic River is downstream from Big Bend National Park between La Linda and Dryden, Texas. Numerous springs in this reach issue from the Cretaceous Edwards-Trinity Plateau (ET) aquifer. International Boundary and Water Commission (IBWC) gage data from the gages at Johnson Ranch and Foster’s Weir indicate an increase in base flow of about 60%. Data from a winter 2006 USGS/NPS seepage run indicate an annual increase of 0.87 km³/year (94,482 acre feet/year (AFY)) from Taylor’s Farm to Dryden Crossing. 2011 seepage data for the same reach indicate an annual increase of 0.95 km³/yr (102,084 AFY). Discharge into the Rio Grande from these springs is important to many stakeholders, including the ecosystem, agriculture, industry, municipal supply, and tourism.

Major element variations reveal two groups of springs: one set with an average of 533 mg/L TDS is classified as a no dominant ion (NDI) water and a second with an average of 282 mg/L TDS that is classified as Ca-HCO₃ type water. The same two hydrogeochemical facies appear when the waters are classified using isotopic analysis of hydrogen and oxygen. With respect to the NDI waters, the Ca-HCO₃ are enriched with deuterium and oxygen-18. GIS analysis of the water chemistry reveals that the NDI waters are located south and west of a major northwest trending structure represented by the Bullis Fold and the Ca-HCO₃ waters are located north and east of this structure. Geochemical analysis of ground water reveals a similar patter with NDI water originating from wells southwest of the Bullis Fold and Ca-HCO₃ water found in wells northeast of the fold. Variations in the NDI waters could be the result of a longer residence time and possible interaction with Paleozoic sedimentary rocks in the Marathon Basin, located ~30 km to the northwest, beyond the mapped extent of the ET. Potentiometric maps generated of the study area reveal a southeast flow path and suggest potential communication with the Marathon aquifer. Alternatively, the Ca-HCO₃ waters may be associated with recent meteoric recharge directly into the ET.