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tion process(es). Results of the study will enable the Navajo Nation as well as the principal operators in the Greater Aneth Oil Field and vicinity to beller under- stand the hydrology and water quality of the principal aquifers so that ground-water resources can be properly W managed and adequately protected. U1 Water-quality, discharge, water-level, and well- >l::>. record data were obtained from u.s. Geological Survey ~ data bases. Well data also were obtained from the Navajo Nation Water Resources Management Depart- ment; the Utah Division of Oil, Gas, and Mining; the Bureau of Land Management; and oil companies oper- ating in the study area. Additional water-quality, dis- charge, water-level, and well-record data were collected, measured, and obtained from 1989 to 1994. A data base consisting of 52 water samples was used in multivariate statistical analysis. The most recent con- centration or value was used when a site was repre- sented by multiple analyses throughout time. Non-oil- field brine data were compiled from sites northeast of the study area near Paradox Valley, Colorado (Rosen- bauer and others, 1992, p. 276). PreviClUS InvestigatiClnS The hydrology and geology of Mesozoic-age for- mations in San Juan County, Utah, and parts of sur- rounding states have been studied on a regional to local scale by numerous investigators. Waring and Knechtel (1935) presented the first major overview of the general geology and ground-water conditions of southeastern Utah, including the valleys of the San Juan River at Bluff, Utah, and McElmo Creek. A section of the report is devoted to a discussion of artesian conditions. Goode (1958) completed one of the earliest studies of the rela- tion between freshwater aquifers and oil-bearing for- mations in the Aneth area and suggested that contamination of freshwater zones could take place by surface disposal of salt water or during drilling, produc- ing, and abandonment of oil wells. Barnes (un pub. data, 1959,36 p.) did a study for Texaco, Inc. to evaluate available sources of water for flooding the oil reservoirs to enhance secondary recovery. This study included pumping tests of the alluvial aquifer along the San Juan River and evaluation of the quality of water from Tex- aco artesian wells completed in the Entrada, Navajo, and Wingate Sandstones. Cooley and others (1969) studied the regional hydrogeology of the Navajo and Hopi Indian Reserva- tions in parts of Arizona, New Mexico, and Utah. This 4 investigation was initiated at the request of the Bureau of Indian Affairs to assess the ground-water resources of reservation land to determine the feasibility of devel- oping ground-water supplies. Sumsion (1975) prepared a report for the U.S. Public Health Service to determine the occurrence and chemical quality of ground water in the Montezuma Creek/Anerh area. The study con- cluded that potable water supplies could be obtained from aquifers in the Morrison Formation and from allu- vium along the San Juan River. Whitfield and others (1983), in a study for the U.S. Department of Energy, estimated water budgets for the ground-water system in the Blanding, Utah, area, about 20 mi north of the Aneth srudy area, ro assess the relations between the freshwater aquifers and the salt strata at depth and the suitability of the salt beds for storage of radioactive wastes. Avery (1986) studied the movement and chemi- cal quality of ground water and the hydrologic proper- ties of the principal aquifers in eastern San Juan County. Results of the study indicated that waler from some wells completed in the Navajo Sandstone in the Aneth area had shown increasing concentrations of dis- solved solids since oil-field development and that the possible source of salinity was the Cutler Formation or Hermosa Group (Paradox Formation). Thomas (1989) used data from previous investigations to develop a digital computer model to simulate ground-water movement in the Four Corners area. The model was used to improve the ground-water budget estimate and to beller understand vertical flow between the principal aquifers. Howells (1990) mapped the depth to the base of moderately saline (greater than 3,000 mg/L dis- solved-solids concentration) ground water in San Juan County and showed that water with dissolved-solids concentrations greater than 10,000 mglL may be less than 500 fr below land surface, in the Morrison Forma- tion, in some areas in the Greater Aneth Oil Field. Free- they and Cordy (1991) provided an analysis of the hydrogeology of Mesozoic-age formations in the Upper Colorado River Basin, inclUding southeastern Utah, as part of the Regional Aquifer Systems Analysis Program (RASA). The RASA report focused on the horizontal movement of ground water between recharge and discharge areas, vertical movement of water between aquifers, and water quality. Kimball (1992) used hydrochemical characteristics and isotopes to investigate the source of saline water in Mesozoic- age formations. According to Ihis srudy, which was based on data collected during 1983-84, chemical sim- ilarities of water in the Navajo aquifer and in the middle