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