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Beginning with the foundation date of Spanish Tucson in 1775 and continuing through the 1890s, shallow groundwater and related features, including a perched aquifer, stream-bed springs and a short perennial reach in the Santa Cruz River, progressively disappeared from the Downtown Tucson area. The 20th-Century city grew rapidly, and increasing groundwater withdrawals caused large declines in the regional aquifer of Tucson basin, leaving a second perched aquifer with non-aqueous contaminants beneath Downtown. Isotope data show that this fully perched aquifer retains residual water from the regional aquifer, plus recharge derived from a nearby small watercourse. The present regional aquifer beneath Downtown is locally layered and partly confined, and contains water 7-20 Ka years old originating at the northern and southeastern basin margins. [ABSTRACT FROM AUTHOR]

Sulfate (S and O) isotopes used in conjunction with sulfate concentration provide a tracer for ground water contributions to base flow. They are particularly useful in areas where rock sources of contrasting S isotope character are juxtaposed, where water chemistry or H and O isotopes fail to distinguish water sources, and in arid areas where rain water contributions to base flow are minimal. Sonoita Creek basin in southern Arizona, where evaporite and igneous sources of sulfur are commonly juxtaposed, serves as an example. Base flow in Sonoita Creek is a mixture of three ground water sources: A, basin ground water with sulfate resembling that from Permian evaporite; B, ground water from the Patagonia Mountains; and C, ground water associated with Temporal Gulch. B and C contain sulfate like that of acid rock drainage in the region but differ in sulfate content. Source A contributes 50% to 70%, with the remainder equally divided between B and C during the base flow seasons. The proportion of B generally increases downstream. The proportion of A is greatest under drought conditions. [ABSTRACT FROM AUTHOR]

Cenozoic evaporites (gypsum and anhydrite) in southwestern North America have wide ranges of δ34S (−30 to +22‰; most +4 to +10‰) and δ18OSO4 (+3 to +19‰). New data are presented for five basins in southern Arizona. The evaporites were deposited in playas or perennial saline lakes in closed basins of Oligocene or younger age. Very large accumulations in Picacho, Safford and Tucson Basins have isotope compositions plotting close to a linear δ34S-δ18OSO4 relationship corresponding to mixing of two sources of sulfur: (1) sulfate recycled from Permian marine gypsum and (2) sulfate from weathering of Laramide-age igneous rocks that include porphyry copper deposits. In the large evaporites, sulfate with δ34S > +10‰ is dominantly of Permian or Early Cretaceous marine origin, but has locally evolved to higher values as a result of bacterial sulfate reduction (BSR). Sulfate with δ34S < −10‰ formed following exposure of sulfides, possibly formed during supergene enrichment of a porphyry copper deposit by BSR, and have values of δ18OSO4 higher than those of local acid rock drainage because of participation of evaporated water in BSR. Accumulations of 30 to 100 km3 of gypsum in Picacho and Safford Basins are too large to explain as products of contemporaneous erosion of Permian and Laramide source materials, but may represent recycling of Late Cretaceous to Miocene lacustrine sulfate. [ABSTRACT FROM AUTHOR]