GEOLOGIC AND GEOCHEMICAL CHARACTERIZATION OF THERMAL SPRINGS ALONG THE RIO GRANDE IN BIG BEND NATIONAL PARK AND THE RIO GRANDE WILD AND SCENIC RIVER

Numerous thermal springs in the Trans-Pecos Texas region provide substantial base flow to the Rio Grande. While geological, geochemical, geophysical, and isotopic studies have been conducted on thermal springs in the southeastern portion of Big Bend National Park, thermal springs in the Lower Canyons of the Rio Grande Wild and Scenic River are not well-characterized. Thermal springs in the National Park issue from a transborder aquifer comprised of the Boquillas and Santa Elena limestones along north to northwest-trending faults or fractures. The spring waters are dominated by calcium, magnesium, bicarbonate, and sulfate, with temperatures ranging from 30 to 40 degrees Celsius. Chemical analyses suggest that the springs are meteoric in origin and range in age from 18,700 to 29,000 years. These thermal springs are believed to be recharged in the Sierra del Carmen Range and heated by a relatively normal geothermal gradient as ground water circulates to the base of the Cretaceous carbonate rocks. Impermeable rocks of the basal Glen Rose Formation may control the depth of circulation and force ground water to rise along faults and discharge at fractures where it provides base flow to the Rio Grande. In contrast, the Lower Canyons thermal springs are associated with folding in the Del Carmen and Glen Rose limestones. These springs issue at diffuse seeps as well as large-volume well-defined discharge points on both sides of the Rio Grande, with average temperatures of about 32 degrees Celsius. Field work on thermal springs in the Lower Canyons has focused on the collection of GIS coordinates, discharge measurements, and water quality and invertebrate samples, but more detailed studies are needed to better understand their origin, geologic controls, ground-water flow paths, and water-rock interactions at depth. Additional discharge measurements are also needed to quantify the contribution of all of the thermal springs in Big Bend National Park and the Rio Grande Wild and Scenic River to base flow in the Rio Grande. The results of field work conducted on the thermal spring complexes are synthesized here to aid in shaping future research and making informed management decisions about factors affecting water quantity and quality in the Rio Grande.