Mountain Block Recharge and Fracture Flow as Revealed by Stable Isotopes, Southern Arizona

Mountain springs discharging at various elevations in the crystalline-rock ranges of southern Arizona provide information on the relative importance of summer and winter recharge, and about the continuity (or non-continuity) of fracture flow within each mountain block.  Winter and summer lapse rates (-0.9 and –2.1 per mil/1000 m respectively, for δ¹⁸O) are well established for Tucson basin.     Groundwater from the top 500 m of each range, where recharge altitude is known, has stable O, H isotope signatures reflecting recharge seasonality.  Winter recharge predominates in the Santa Catalina and Tucson Mountains, while examples of predominant summer recharge are found in the Rincon and Huachuca Mountains.   Groundwater discharging from springs more than 500 m below the range crests has a broader range of δ¹⁸O (compared with data for higher springs) in the Santa Catalina and Rincon Mountains, indicating discontinuous fracture flow, and local recharge from water courses draining high altitudes.  In the Huachuca Mountains, a narrowing of the δ¹⁸O range from 500 to 750 m below the range crest (compared to data for higher springs) suggests continuity of fracture flow.   Differences among mountain ranges reflect local geology and its effect on topography.