An Isotope Study of Groundwater in and around Saguaro National Park, Rincon Montain District, Tucson, Arizona

Eastoe, Christopher

A set of samples was collected by National Park Service Sonoran Desert Network and Saguaro National Park personnel from springs and tinajas in the part of the Rincon Mountains within the Saguaro National Park Rincon Mountain District (SAGU-RMD) in 2010. Stable isotopes of oxygen, hydrogen and sulfur were measured in these samples, along with tritium. To this data set have been added data for groundwater from Rincon Valley, Colossal Cave Mountain Park and the Agua Verde Hills (unpublished data of the Environmental Isotope Laboratory, The University of Arizona), the northeastern part of Tucson basin (Eastoe et al., 2004; Gu, 2005), and the Santa Catalina Mountains (Cunningham et al., 1998). The geology of the area has been described by Thorman and Drewes (1981). The study area encompasses the Rincon Mountains, the southeastern part of the Catalina metamorphic core complex. The core of the complex, in the lower plate of a regional detachment, consists of cataclastic granitic and gneissic rocks, including Eocene Wilderness Suite leucogranite and Proterozoic Oracle granite, and Proterozoic Pinal schist. All lower plate rocks are mylonitic to some degree. The upper plate of the detachment system consists of Paleozoic limestone, prominent near Colossal Cave and Vail, plus Proterozoic granite and Cenozoic basin-fill sediment in Rincon Valley and in the northeast corner of Tucson Basin. The Catalina detachment fault intersects the land surface near the base of Tanque Verde Ridge and at the limestone-granitoid contact near Colossal Cave. Outcrop of the fault in the western part of SAGU-RMD is a band of impermeable mylonite. Neogene high-angle normal faulting has deepened Tucson basin to the west and south of the study area, resulting in the deposition of as much as 3.6 km of basin-fill sediment, and has truncated the Catalina detachment fault in the subsurface. Lower-plate rocks host groundwater in rock fractures. Basin-fill sediments form a regional alluvial aquifer in Tucson basin. The Catalina detachment fault is thought to form an aquiclude between rock fractures in lower-plate rocks and basin-fill alluvium (see discussion below). A breach in the aquiclude localizes groundwater discharge at the surface at Agua Caliente spring, and appears to provide sources of mountain-block groundwater discharge to the basin-fill alluvium.