An aqueous geochemical and hydrologic study of the springs and wells of the Sevilleta National Wildlife Refuge : evaluating hydrochemical pathways

The Rio Grande is a regionally important water source, but the smaller rift springs are also a vital resource for livestock and wildlife. Several springs are located on rift-bounding faults and exhibit a mixing of larger volume meteoric recharge with small volume, chemically potent "endogenic" fluids. It has been hypothesized that deep-seated faults within the rift provide conduits for the ascent of deeply derived fluids, possibly from the lithospheric/asthenospheric mantle, while others have proposed that upwelling sedimentary basin brines at interbasin constrictions represent a significant salinity input to the modern Rio Grande. This study (a) provides the first hydrochemical data on a comprehensive suite of springs and wells, and (b) tests and refines existing models for water quality in the rift using hydrochemistry (major, minor and trace elements, Cl/Br ratios, δ18O, δD, δ13C, and 87Sr/86Sr ratios) and geochemical modeling along a series of transects within the Sevilleta National Wildlife Refuge. In the rift, several potential flow systems can be envisioned: 1) exogenic fluids in shallow unconfined aquifers with recent meteoric recharge, which are characterized by low temperatures, CO2 values, and 87Sr/86Sr ratios, 2) mesogenic fluids, categorized as subregional basin fluids in Tertiary rift fill, 3) regional/intermediate waters residing in the confined aquifers of Paleozoic/Mesozoic sedimentary strata, 4) deep sedimentary basin brines, also in confined strata, and 5) endogenic waters, defined as deeply-circulating regional fluids that may have mantle derivation, source from faults, and are characterized by elevated temperatures, salinity, CO2 values, and 87Sr/86Sr ratios. Major ions indicate the interaction of five fluids with distinct hydrochemical facies: 1) Na-Cl, 2) mixed ion-HCO3, 3) Ca-SO4, 4) mixed cation/anion (corresponds with local precipitation chemistry), and 5) Na-mixed anion. δ18O and δD indicate mixing between brines and the Rio Grande, and δ13C values suggest a mixing of organic C and a mantle-derived C input in springs. Radiogenic 87Sr/86Sr ratios indicate mixing between endogenic fluids and meteorically-derived waters and principal component analysis indicates a common deeply-derived source in select waters. These tracers conclude that endogenic fluids are a volumetrically small but potent addition to middle Rio Grande rift springs, and may contribute to river salinization.National Science Foundation Hydrologic Sciences Program (NSF-EAR-0538304 and 0838575). National Science Foundation Ecology Division (NSF-DEB-0620482). New Mexico Water Resources Research Institute (W03010). New Mexico Geological Society. University of New Mexico Office of Graduate Studies. University of New Mexico Graduate and Professional Student Association. University of New Mexico Department of Earth and Planetary SciencesEarth & Planetary SciencesMastersUniversity of New Mexico. Dept. of Earth and Planetary SciencesCrossey, LauraKarlstrom, KarlDahm, Cliffor