Aqueous Geochemistry of the Springs and Wells of the Sevilleta National Wildlife Refuge: Utilizing Natural Tracers to Identify Hydrochemical Flowpaths

Abstract

The Rio Grande is well studied as a regionally important surface-water source, but the small, poorly characterized springs that surface within the Rio Grande rift are also a vital water source. Several of these springs have water chemistries that suggest a mixing of larger volume meteoric recharge with small volume, deeply sourced fluids. It has been hypothesized that deep-seated faults within the rift provide conduits for the ascent of deeply derived fluids, while others have proposed that upwelling sedimentary basin brines represent a significant salinity input to the modern river. This study has developed the first hydrochemical data on a comprehensive suite of springs and wells in the Sevilleta National Wildlife Refuge (NWR), and we continue to test and refine existing models for water quality in the rift using hydrochemistry (major and trace elements, Cl/Br, δ 18 O, δD, δ 13 C, δ 34 S, 3 H, δ 234 U, and 87 Sr/ 86 Sr), microbial characterization and geochemical modeling along a series of transects within the rift. This suite of geochemical tracers is being used to analyze the geochemistry of 26 surface samples and 13 wells in and near the Sevilleta NWR. Our goal is to apply hydrochemical and microbiological analyses of these springs and groundwaters to a rift-wide model for subsurface flow paths. Results from major ions, trace elements, stable isotopes of H and O, and 87 Sr/ 86 Sr indicate the interaction of five distinct hydrochemical facies, several of which suggest that deeply derived fluids are mixing in this system. Continued analyses of major ions for temporal variation, and the addition of δ 234 U, δ 13 C, δ 18 O and δD analyses, will allow for a high-resolution hydrochemical image of the sources of these waters and their impact on the water quality of the Rio Grande.