The Hueco Bolson serves as one of the primary groundwater sources for the El Paso-Ciudad Juárez metropolitan region of over 2.5 million residents. The bolson lies at the point where the strike of the southern Rio Grande rift changes from north-south to northwest-southeast, likely due to its interaction with preexisting Mesozoic and Paleozoic structures. The region is tectonically active with recent (<750,000 years) movement along basin-bounding faults and low-level (M<4) seismicity. Over the past five years, we have used a combination of microgravity and water well logs to image the complex structure of this basin within an urbanized environment. Our results indicate the East Franklin Mountains fault, the main boundary fault on the west side of the bolson, extends as much as 30 km south of the end of its mapped surficial trace. Two intrabasin faults that have been mapped at the surface in the less developed portions of the city can be traced into central El Paso and Ciudad Juárez. These faults appear to control the boundary between fresh and saline water within the aquifer system beneath El Paso. Gravity modeling also suggests at least two additional concealed intrabasin faults are located beneath the metropolitan area.

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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 (