Rapid population growth and declining annual recharge to aquifers in the upper Santa Cruz basin area of southern Arizona, has increased the demand for additional groundwater resources. This demand is predicted to escalate in the future due to higher temperatures, longer droughts, less aquifer recharge, and decreased river and stream base flow. We conducted geologic studies to help evaluate and better understand groundwater resources in the basin. Results of these studies are presented in this report, which summarizes the basin geologic framework and hydrogeology, and presents a 3-D hydrogeologic model for the Rio Rico and Nogales 7.5’ quadrangles. Three major hydrogeologic units are displayed in the 3-D model; a lower basement confining unit, consisting of Jurassic, Cretaceous, and Tertiary (Paleocene and Oligocene) rocks; a middle unit composed entirely of the Miocene Nogales Formation; and an upper unit consisting of late Miocene to Holocene surficial deposits. The Nogales Formation and the Late Miocene to Holocene sediments are the main aquifers in the Santa Cruz basin. The 3-D model integrates the hydrogeologic units and structural features to define the geometry, structure, and thickness of the aquifer system that provides water to Nogales and surrounding communities of southernmost Arizona. The report includes an EarthVision 3D Viewer, consisting of software enabling viewing data interactively in three-dimensional space, in order to help understand the internal complexities of the basin geometry, structure, stratigraphy, and hydrology. The 3-D model is a synthesis of geologic data from geologic maps, cross sections, and lithologic descriptions and interpretations, and geophysical data including gravity, magnetic data, and airborne electromagnetic data.