The Hueco Bolson Aquifer contributes between 30% and 60% of the yearly drinking water supply for El Paso's population, depending on surface-water availability from the Rio Grande. The aquifer contains both freshwater and brackish water. We used microgravity and well-log data in a highly urbanized area of the northwestern Hueco Bolson Aquifer, where the majority of currently operating water wells are located, to demarcate subsurface faults that may control the locations of the freshwater and brackish water. Our results extend the length of some previous mapped faults, as well as identifying several new faults. Because of the large distance between individual wells, well-log correlations themselves do not resolve the individual faults; however, structural cross sections suggest that at least some faults inferred from the microgravity surveys are present between the wells. The depositional environments inferred from the gamma-log responses and their stacking patterns are consistent with braided stream, alluvial-fan, playa-margin, delta, and sheetflood deposits. Faults in the western portion of the study area, in a region where the East Franklin Mountains fault steps over 2 km to the west, appear to serve as conduits for upwelling of deeper brackish water, while in the eastern study area, faults appear to serve as barriers to the flow of brackish water into the shallower portion of the aquifer. The thickest region of freshwater correlates with the deepest portion of the basin as delineated by the gravity data. PREVIOUS STUDIES The Hueco Bolson is one of a series of north-trending, interconnected asymmetrical grabens that form the Rio Grande rift system (