Water Quality in Basin-Fill Aquifers of the Rio Grande Rift, Northern New Mexico

In northern New Mexico, the Rio Grande rift is a distinct physiographic feature that forms a chain of broad alluvial valleys, including the San Luis and Española valleys, through which the Rio Grande flows. The valleys are underlain by a series of offset, asymmetric structural basins that contain the region’s major aquifers. These basin-fill aquifers represent hydrogeologic settings with unique groundwater-flow and water-quality characteristics. In this area, rift valleys are bounded by large mountain uplifts (the Sangre de Cristo, Jemez, and Tusas Mountains) and have deep alluvial basins filled with hundreds to thousands of meters of alluvium and volcanic rocks and sediments. Large, complex fault systems and buried horst-graben structures partition aquifers and can create hydrologic discontinuities and foci for upward flow of warm (or hot) mineralized groundwater that mixes in the shallow meteoric system. Up-flow is particularly common where groundwater flows east-west across north-trending rift structures. In this setting, water quality characteristics can vary widely and unexpectedly. Examples from the Santa Fe and Peñasco embayments (southern Española basin) and the Taos area (southern San Luis basin) demonstrate water quality degradation in shallow aquifers associated with deeply circulating groundwater and up-flow of fluids along faults. Groundwater discharge temperature, stable isotopes (¹⁸O/¹⁶O and ²H/H), ion concentration (Na, Cl, SO₄, TDS), and trace element chemistry (Li-to-B ratio) are used to distinguish shallow and deep sources. Elevated concentrations of undesirable constituents (As, F, and U) occur in the vicinity of major faults—particularly horst-bounding faults—and tend to increase with depth or concentrate in specific depth horizons. As constraints on surface and shallow water sources force broader exploration and greater drill depths, naturally impaired water quality will become an important consideration during groundwater development in rift-basin aquifers.