GEOCHEMICAL STUDIES OF HOT SPRINGS, GROUND WATER, AND STREAM WATER IN BIG BEND NATIONAL PARK, TEXAS

In the mountainous desert of Big Bend National Park (BBNP), Texas, water quality and water supply are persistent issues, and thus, periodic monitoring of a full suite of major, minor, trace, and isotope constituents is warranted so that sites can be revisited and reanalyzed to examine future trends related to climate change, drought, and other events. Water samples were collected from a broad range of geologic and geographic settings in BBNP, including hot springs near Rio Grande Village, non-thermal springs and well-waters throughout the Park, Rio Grande River waters, and other streams. Goals of this study include determining water composition, and the age, flow path, and host lithology of aquifers.

Hot spring waters have temperatures up to 40.1oC and are enriched in major elements (Na, K, Ca, Mg, SO4, and Cl) and some trace elements (As, F, Li, Rb, Sr, Tl, U, and W) relative to non-thermal springs. Potentially toxic trace metals in thermal water samples have the following concentration ranges: As (4.0-9.1 µg/L), Tl (0.13-0.68 µg/L), W (to 7.0 µg/L), and U (7.0-8.4 µg/L)] that are below drinking water maximum contaminant levels (MCLs) established by the U.S. Environmental Protection Agency. In Rio Grande Village, As concentrations approach the MCL of 10 µg/L in some water samples. Concentrations of Hg in collected well and spring water vary from 1.3-10 ng/L, and are all well below the 2,000 ng/L MCL drinking water standard. Oxygen and hydrogen isotope studies indicate that all water samples are strictly meteoric. Thermal and spring water samples are shifted from the Global Meteoric Water Line to slightly higher δ¹⁸O and δD values due to water-rock reactions in the subsurface and/or minor evaporation at the surface. Potential water quality problems in spring water relate to F, NO3, and As concentrations, but the As concentration of 16 µg/L in a sample of highly evaporated surface pond water is the only one that exceeds the As MCL.

Important aquifers may include confined sand layers within the Cretaceous Aguja Formation and/or younger rhyolitic sands and boulders near fractured contacts with Tertiary rhyolitic intrusive rocks. Relatively high concentrations of F thatreflect subsurface water reactions with rhyolites that contain an average of 1100 mg/kg F. Current water-rock interaction and helium-tritium studies are designed to refine these interpretations.