The Santa Fe Group aquifer in the Middle Rio Grande Basin is the main source of municipal water for<br>the greater Albuquerque metropolitan area. One objective of U.S. Geological Survey studies is to improve the<br>hydrogeologic models of the Middle Rio Grande Basin, so as to help land managers plan and develop water<br>supplies. Airborne time domain electromagnetic (TEM) data near the town of Rio Rancho, northwest of<br>Albuquerque, New Mexico, provided a three-dimensional depiction of the electrical resistivity distribution that<br>was used to infer the extent of geologic units within the Santa Fe Group aquifer. The correlation between<br>resistivity and grain size was based on lithologic and induction resistivity logs, which showed that the bulk<br>average resistivity in the saturated zone correlate with grain size as follows: Coarse (40-70 ohm-m), Medium<br>(20-40 ohm-m), Fine (10-20 ohm-m). These resistivities were used to infer likely grain sizes from the TEM<br>inversions. From lithologic well data, axial river-channel deposits are primarily composed of coarse grain size<br>facies, fluvial sand deposits are primarily composed of medium grain size facies, and fluvial silt and clay deposits<br>are primarily composed of fine grain size facies. Significant changes in the TEM response occur at several<br>mapped faults and at faults inferred from a high-resolution airborne magnetic survey. Coarse-grained sediments<br>that exist at 1500 m above sea level appear to be less than 50 m thick below the water table (apparently<br>pinched-out at the 1450 m level). At the 1350 m level, the lateral extent of axial-channel sand deposits appears<br>to be much reduced, suggesting that they are pinching out. The resistivity model provides a framework for<br>forecasting hydrologic conditions in areas less explored by drilling. Our interpretation of grain size facies<br>distribution provides direct input to ground-water flow models that are critical to water management agencies.<br>Introduction<br>The Albuquerque-Santa Fe region is rapidly growing. The Santa Fe Group aquifer in the Middle Rio<br>Grande Basin


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