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Paleozoic aquifer (Paradox Formation) suggested that t..;l saline water in the Montezuma Creek drainage area in CJt the Greater Aneth Oil Field originated from reinjection ~ of oil-field brine. ..;~ Acknowledgments The authors would like to acknowledge the help of all those who cooperaled wilh or contribuled to lhis study. The authors are particularly grateful for the assis- tance of both field and office personnel of operators in the Greater Aneth Oil Field and vicinity with regard to locating wells, obtaining well logs, collecting water samples, and providing expertise and resources. These include Texaco Exploration and Production, Inc.; Mobil Exploration and Producing, U.S., Inc.; Phillips Petroleum, Inc.; U.s. Oil and Gas, Inc.; Meridian Oil, Inc.; Von Engineering Company; Raymond T. Duncan, Inc.; and Elkhorn Operating Company. Personnel from these companies are too numerous to list, but without their help, much data would not have been available. Personnel from the Bureau of Land Management and the Utah Division of Oil, Gas, and Mining also helped obtain well-log information. Many people in various Navajo Nation agencies provided input to this study, particularly Michael Johnson of the Navajo Nation Water Resources Management Department, who helped locate well logs and other data from the Navajo water-well data base. Appreciation is extended to the Navajo Tribal Utility Authority for efforts to obtain water samples and to Montezuma Well Service for dis- posal of saline water. In the U.S. Geological Survey, the authors would like to thank personnel from the field office in Moab, Utah, particularly Michael Hawkins, for efforts in the collection of field data, and Shannon Mahan, Kiyoto Futa, and April Walker in Denver, Col- orado, for strontium isotope measurements. Finally, appreciation is extended to the members of the Aneth Technical Committee for support, input, and guidance throughout the study. DESCRIPTION OF STUDY AREA The study area includes about 1,200 mi2 in the southeastern corner of San Juan County, Utah, and the extreme southwestern corner of Montezuma County, Colorado (fig. I). The area is part of the Four Corners region where the borders of Colorado, New Mexico, Arizona, and Ulah meet at a common point. Most of the area is part of the Navajo Indian Reservation. Part of the study area in Colorado is in the Ute Mountain Indian Reservation. The Greater Aneth Oil Field cov- ers about 125 mi2 in the central part of the study area and is surrounded by numerous smaller oil and gas fields (fig. I). The largest communities are Montezuma Creek and Aneth, on the Navajo Indian Reservation, and Bluff, on the western edge of the study area. The combined population of these communities is about 1,000 people. The remainder of the study area is sparsely inhabited. The numbering system for geohydrologic-data sites used in Utah is shown in figure 2. This system locates sites in the study area by township, range, and section. Corresponding map numbers for each of the sites are used in tables within the report. Geographic Setting The area in the vicinity of the Greater Aneth Oil Field is part of the Colorado Plateau Physiographic Province (Stokes, 1988) and consists of dissected mesas and badland topography that have formed in response to downcutting by the San Juan River and its tributaries. Terraces along much of the San Juan River indicate former levels of the river. The San Juan River flows toward the northwest and west across the middle of the study area and discharges into Lake Powell about 65 mi west of the study area. McElmo Creek is the only entirely perennial tributary to the San Juan River in the study area, flowing generally toward the southwest before discharging into the San Juan River at Aneth (fig. I). Most drainages north and south of the San Juan River contain ephemeral streams. The lower reach of Montezuma Creek contains the largest ephemeral stream in the study area. Altitude in the region ranges from about 4,400 ft along the San Juan River to as much as 11,300 ft in the Abajo Mountains, about 35 mi north of the study area. Maximum altitudes in the Sleeping Ute Mountains, about 5 mi east of the study area, in Colorado, and in the Carrizo Mountains, about 10 mi south of the study area, in Arizona, are between 9,500 and 10,000 ft. Broad, flat-topped mesas are present north and south of the San Juan River (fig. I). Average relief from mesa tops to intervening valleys is generally 400 to 600 ft. Normal annual precipitation in the study area ranges from about 7 in. south of the San Juan River to lOin. north of the San Juan River (Avery, 1986, fig. 6, p. ] 1). Potential average annual evaporation through- out the area substantially exceeds annual precipitation 5