In this study, we have used precision gravity surveys to investigate the subsurface structure of groundwater basins in the El Paso, Texas region. This study focused on finding smaller faults within the Mesilla bolson (basin), which are believed to be a major control on ground water flow. The Mesilla bolson serves as one of two major aquifers for the city/county of El Paso. A previous 2-D gravity modeling study of data collected along three east-west trending lines had suggested that the eastern boundary of the Mesilla bolson is controlled by a series of smaller faults, rather than a single large one. Our study uses 3-D modeling techniques to more accurately locate these faults. Additional gravity data were also collected along three north-south trending lines. In our gravity modeling approach, topography and near surface geology were included in the model to better determine the detailed structure lying below the surface. The results of this study show that the locations of some of the previously identified faults that likely control groundwater recharge from the surrounding mountains as well as the flow of saline water in the southernmost bolson need to be modified.