Rocky Mountain Section - 64th Annual Meeting (9–11 May 2012)

Paper No. 3
Presentation Time: 11:00 AM


JOHNSON, Peggy S. and BAUER, Paul W., New Mexico Bureau of Geology, 801 Leroy Place, Socorro, NM 87801,

The Taos Plateau is a high-elevation, basalt-capped plain along the western margin of the Rio Grande rift that lies between the Rio Grande and the Tusas Mountains, and overlaps the New Mexico-Colorado border. Pliocene Servilleta basalts, together with volcanic domes and flows, form the major aquifer. A water-table surface defines: 1) regional west-to-east groundwater flow from the Tusas Mountains to the Rio Grande; 2) a groundwater divide aligned with the Rio San Antonio in southern Colorado; and 3) local recharge mounds superimposed on regional flow. Seepage data from a 19-mile reach of the Rio Grande on the eastern margin of the plateau quantify river gains of over 81 cfs (~59,000 afy) from more than 60 springs and seeps.

Chemical, thermal and isotopic data from wells and springs define three sources of inflow or recharge to the Taos Plateau aquifer. Groundwater underflow originating in the Tusas Mountains is distinguished by depleted 2H, a positive 18O thermal shift, high SiO2, warm temperatures (>19 ºC), and 14C apparent ages of 5500 to 8000 RCYBP. Mixing with precipitation recharge on the central plateau yields cooler (<16 ºC), low TDS, Ca-Na-Mg-HCO3 groundwater with a 14C apparent age of 3500 to 5000 RCYBP, measureable 3H, and CFC-12 ages of 30‑45 years. Warm, high TDS, Na-HCO3-Cl-SO4 water near the Red River and Cerro de la Olla fault zones appears to originate from a deep mineralized source.

Groundwater from the Taos Plateau discharges to the west wall of the Rio Grande 33 miles downstream of the Colorado border in a 1.5-mile reach at the Red River fault zone. Discharge to gorge springs from aquifers east of the Rio Grande is controlled, in part, by a network of faults that includes the Cerro de la Olla‑Ute Mountain, Gorge, and Red River fault zones. Mixing models estimate that approximately 20% of water discharging on the west wall of the Rio Grande at the Red River fault originates as deep flow from the Taos Graben to the east; 80% flows from the Taos Plateau.