Introduction Purpose and scope Well-numbering system Description of the study area Geologic framework Mineralogy Hydrogeology Data compilation 11 Geochemistry Generalized chemical characteristics Hydrochemical processes Hydrochemical evolution Secondary mineral controls Basalt-water mass balance Summary and conclusions 61 Selected references 8 O Oxygen-18/oxygen-16 ratio, in per mil V-SMOW

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The Mogollon Highlands, 4,855 square miles of rugged, mountainous terrain at the southern edge of the Colorado Plateau in central Arizona, is characterized by a bedrock-dominated hydrologic system that results in an incompletely integrated regional groundwater system, flashy streamflow, and various local water-bearing zones that are sensitive to drought. Increased demand on the water resources of the area as a result of recreational activities and population growth have made necessary an increased understanding of the hydrogeology of the region. The U.S. Geological Survey conducted a study of the geology and hydrology of the region in cooperation with the Arizona Department of Water Resources under the auspices of the Arizona Rural Watershed Initiative, a program launched in 1998 to assist rural areas in dealing with water-resources issues. The study involved the analysis of geologic maps, surface-water and groundwater flow, and water and rock chemical data and spatial relationships to characterize the hydrogeologic framework. 2 Hydrology of the Mogollon Highlands, Central Arizona Local, generally shallow aquifers of variable productivity occur in plateau and mesa-capping basalts in the sedimentary rocks of the Schnebly Hill and Supai Formations, in fractured zones of the Proterozoic Payson granite, and in the alluvium of the lower Tonto Creek Basin. Where time series data exist, such water-bearing zones are shown to be sensitive to short-term climatic fluctuations, in particular, the drought which began in the mid-1990s and continued during the course of this study. A regional water budget for the C and limestone aquifers was developed from precipitation, spring, and streamflow data. Of an estimated 1,730,000 acre-feet of precipitation that falls on the Mogollon Rim annually, about 8 percent is estimated to recharge the regional aquifers. About 40 percent of recharge to the limestone aquifer is estimated to be leakage from the overlying C aquifer.

Introduction Water quality for irrigation Methods Acknowledgments Geohydrologic setting Geology Basalt mineralogy and physical characteristics Hydrology 10 Basalt-ground water interaction Secondary alteration Alteration sequence Smectite and clinoptilolite composition 19 Relative phase solubility Chemical evolution of ground water Mineral-water equilibria Geochemical model of sodium enrichment