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.
aqueous geochemistry, Rio Grande Rift, Sevilleta
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