Mountain-front recharge to the Tucson basin from the Santa Catalina Mountains, Arizona

Ground-water flow from the northern margin of the Tucson basin to the regional aquifer in the center of the basin was studied. The area studied was the eastern half of the Santa Catalina Mountains and adjacent foothills in Pima County, Arizona. Low-permeability sediments of Pliocene age separate the gneissic mountain block from the regional aquifer. A flow-net analysis and ion-balance calculations indicated that recharge from the lowpermeability sediments to the aquifer was approximately 50 ac-ft/hr/mile of mountain front. This is about 1/20 of the recharge from local streams. Analyses of oxygen-18, deuterium, and major ions in the ground water revealed that the fracture-flow system in the gneiss is recharged at both high and low elevations and is not well mixed. Ground-water chemistry also suggested that in some areas, water is flowing up from the gneiss along faults.hydrology collectio