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A gravimetric survey has proved its usefulness in the Tucson Basin in locating important structural features, their geometric shapes and extensions. Interpretation was made possible through the correlation of available geologic and hydrologic data from water well logs and water table contour maps. Geophysical logs from a recently drilled test well in North Tucson have been interpreted and have confirmed the existence of another promising aquifer, namely, the deformed gravel which underlies the upper basin-fill aquifer. In most cases, it is apparently separated by a thin aquiclude of clay which results in artesian condition in the lower aquifer. Gravity interpretation discloses the presence of buried channels that may be of importance to groundwater exploration. The buried high basement ridges or faulted blocks as interpreted from gravity data add more information to the understanding of the hydrologic behavior of the basin. Deep drilling of test wells, such as the one drilled recently on Orange Grove Road, will be a useful check to the structures interpreted from gravity data. Geophysical logs of bore holes are of utmost importance in correlation of lithologic units and structures in addition to the hydrologic interpretation that is possible from these logs. The Pantano beds as described here are not promising for new groundwater sources that may be used for domestic needs due to the very low permeability and the expected poor quality of the water. Still more information is necessary to determine clear answers to many problems related to the geology and hydrology of this basin.

Hydrogeochemical studies are generally qualitative in nature. The goal of this study is to investigate the possibility of quantitative interpretation of hydrogeochemistry by considering the chemical characteristics and the isotopic compositions of oxygen, hydrogen, and carbon of the water. This study examines ephemeral stream and well waters from Canada del Oro valley, southern Arizona. By chemical and isotopic considerations, this study finds that the change of chemical composition of the wash water was mainly due to water-rock interaction. The concentrations of dissolved constituents increase between 10 to 50% from upstream to downstream samples, while the evaporation loss of water is less than 3%. By chemical and isotopic considerations of the well waters, this study identifies three recharge waters in the CDO ground-water system. The chemical and water isotopic compositions of the well waters are results of mixing between these three recharge waters and subsequent dissolution of the aquifer. By thermodynamic consideration, albite, kaolinite, montmorillonite, and calcite are the main phases that influence the chemical characteristics of this ground-water system. Simulations with the computer program PHREEQE verifies the above conclusions. The mechanisms that influence the chemical and carbon isotopic compositions of the water are quite different in a system open to a CO2 gas reservoir than in a closed system. Deines, Langmuir, and Harmon (1974) derived a set of chemical-isotopic equations to calculate the carbon isotopic composition of water under open system condition. Wigley, Plummer, and Pearson (1978) formulated a mass transfer equation to calculate the change of carbon isotopic composition of natural water in closed system environment. This study implements these two type of equations as a subroutine--CSOTOP to the computer program PHREEQE. With this PHREEQE-CSOTOP package, the evolution of carbon chemical and isotopic composition of natural water can be conveniently modeled from open to closed system conditions. This study also uses this package to date water samples from the Tucson basin, and finds that choice of reaction path may cause a difference in carbon-14 age of up to a few thousand years. This study concludes that it is possible to rigorously interpret hydrogeochemistry in a quantitative way. With sufficient measurements to define the reaction path, followed by thermodynamic consideration, chemical-isotopic evaluation, and computer modeling, one should be able to achieve this goal.

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