A SALINIZATION STUDY WITHIN THE SAN ACACIA REGION, SEVILLETA NATIONAL WILDLIFE REFUGE, NEW MEXICO

Water management and the scarcity of water are major concerns in the American Southwest, and both quality and quantity of ground and surface waters are major issues in the Rio Grande rift corridor, NM. High salinity and elevated trace element concentrations tend to impair water quality; hence identifying sources of these contaminants remains an important ongoing challenge. Geochemical studies show a marked jump in salinity near San Acacia, at the southern end of the Albuquerque Basin. Alternate models contend that: A) deep-seated faults within the rift provide conduits for the ascent of deeply-derived saline fluids, and B) upwelling of sedimentary basin brines takes place at interbasin constrictions. The purpose of this study is to test and refine existing models for Rio Grande salinization using hydrochemical data from rift springs combined with geologic data. We used aqueous geochemical techniques (field parameters, major and trace elements, Cl/Br ratios, δ¹⁸O and δD) and geochemical modeling to identify salinity components in the middle Rio Grande basin (MRGB). An integrated study of spring geochemistry with factors related to poor water quality will allow for an improved comprehension of natural contaminants in the Rio Grande hydrochemical system.

Our results show that the Rio Salado Box (RSB) and San Acacia (SA) springs are both major salinity inputs. SA contains the highest salinity concentrations of all Sevilleta NWR waters, and it is observed to influence the nearby canals. This spring and its evaporative pools appear to be the cause of the jump in Rio Grande salinity in this region. Major ion, stable isotope, and trace element analyses suggest that SA is chemically similar to the RSB waters and that the SA brine pool has evolved from evaporative concentration. RSB has been established as having deeply-derived fluids sources based on He isotopic data. We conclude that rift-bounding and intra-rift basement-penetrating faults can provide “fast paths” for the ascent of Cl-rich fluids. These deep fluids are potentially influenced by fluids from the Socorro Magma Body, and by deep basin brines, and both salinity sources contribute to salinization of the Rio Grande through fault-conduits at San Acacia that degrade water quality in the Rio Grande and its aquifers.