The Groundwater Flow Patterns Associated with the Formation of the Truth or Consequences, New Mexico Geothermal Resource

Monday, October 5, 2015
Jeff Pepin1, Mark Austin Person1, Fred M Phillips1, Shari Kelley2, Stacy Timmons3, Lara Owens4, James C Witcher5 and Carl W Gable6, (1)New Mexico Institute of Mining and Technology, Socorro, NM, United States, (2)New Mexico Bureau of Geology and Mineral Resources, Socorro, NM, United States, (3)Aquifer Mapping Program, New Mexico Bureau of Geology and Mineral Resources, Socorro, NM, United States, (4)Ormat Technologies, Inc, Reno, NV, United States, (5)Witcher and Associates, Las Cruces, NM, United States, (6)Los Alamos National Laboratory, Los Alamos, NM, United States
Abstract:
We have investigated two of the most plausible regional groundwater circulation scenarios responsible for the formation of the Truth or Consequences, New Mexico hot springs (~ 41 °C) in the southern Rio Grande rift. The first scenario is that the geothermal anomaly is the result of lateral forced convection associated with a gently-dipping carbonate aquifer. The second scenario is that high permeability of crystalline basement rocks permits circulation of groundwater down to depths of 8 km prior to discharging in Truth or Consequences. We constructed a 2D hydrothermal model of the region using FEMOC to test these hypotheses. Model parameters were constrained by calibrating to measured temperatures, specific discharge rates and groundwater residence times. We collected 16 temperature profiles, 11 geochemistry samples and 6 carbon-14 samples within the study area. The geothermal waters are Na+/Cl- dominated and have apparent groundwater ages ranging from 5,500 to 11,500 years. Hot springs geochemistry is consistent with water/rock interaction in a silicate geothermal reservoir, rather than a carbonate system. Peclet-number analysis of temperature profiles suggests specific discharge rates beneath Truth or Consequences range from 2 to 4 m/year, while geothermometry indicates maximum reservoir temperatures are around 167 °C. Multiple permeability values were tested for the most hydrologically influential rock units. We were able to reasonably reproduce observed measurements using the permeable-basement scenario while assigning a uniform effective basement permeability of 10-12 m2. Implementing the Manning and Ingebritsen (1999) exponential decay crustal permeability relationship that is often applied in regional groundwater models was unable to reproduce our field measurements. The carbonate-aquifer scenario failed to match observations.

Our findings imply that this geothermal system formed as a result of deep groundwater circulation within permeable crystalline basement rocks. While it is likely that the crystalline basement is highly heterogeneous, modeling the crystalline basement as a uniform aquifer with a high effective permeability had more success than using a permeability decay function. Ongoing work is focused on refining our model using aquifer test results and magnetotellurics.

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