AN INTEGRATED GEOCHEMICAL AND STRUCTURAL STUDY OF THE SEVILLETA NATIONAL WILDLIFE REFUGE: GEOCHEMISTRY AND SALINITY SOURCES OF WATERS SPANNING THE RIO GRANDE RIFT, NEW MEXICO

Springs and wells are a vital resource on the Sevilleta National Wildlife Refuge (SNWR) located at the Albuquerque basin terminus. 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 Rio Grande.

The goal is to test and refine existing models for water quality in the rift using major and trace elements, δ¹⁸O, δD, δ¹³C, ³H, ⁸⁷Sr/⁸⁶Sr ratios and geochemical modeling. Several potential flow systems are modeled: 1) exogenic fluids in shallow unconfined aquifers with low temperatures, CO₂ concentrations and ⁸⁷Sr/⁸⁶Sr ratios, 2) mesogenic fluids, which are subregional fluids in Tertiary rift fill, 3) regional waters, which reside in the confined aquifers of Paleozoic/Mesozoic sedimentary strata, 4) sedimentary basin brines, also in confined aquifers, and 5) endogenic waters, defined as those derived from deep sources via faults; these consist of a mixture of mantle and crustal derived fluids and are characterized by elevated temperature, salinity, CO₂ values, and ⁸⁷Sr/⁸⁶Sr ratios.

Major ions indicate the interaction of these fluids with distinct hydrochemical facies, including high-chloride waters that are identified along deeply penetrating rift bounding and intrarift faults. 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 crustally-derived endogenic fluids and other waters. Principal component analysis indicates a common deeply-derived source in select waters. Hydrochemical data indicate that water quality is dependent on the proportional mixing between fluid sources and that endogenic fluids, although volumetrically minor, can be potent and cause degradation of water quality in middle Rio Grande rift springs. These findings have implications for management of SNWR water resources and models for salinization of the Rio Grande.