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w Ul .0;,.. ,.." The del oxygen-I 8 (SI80) and del deuterium (SO) values of water samples from the Navajo aquifer north of the San Juan River are distinctly lighter than the S180 and So values of water sam- ples from the Navajo aquifer east and south of the San Juan River. This difference is possibly caused by geographic segregation of water in the aquifer and not by mixin" with oil-field brine. The del strontium-87 (S8 Sr) values of oil-field brine sam- ples are substantially smaller than the values of water samples from the Navajo aquifer, indicating that oil-field brine is not a source of salinity. The S87Sr values of water samples from the upper Pale- ozoic aquifer are similar to the mean isotopic com- position of the more saline water from the Navajo aquifer and indicate that the upper Paleozoic aqui- fer is a possible source of salinity. Hierarchical cluster analysis using chemical constituents in 43 water samples from the Navajo aquifer, 4 water samples from the upper Paleozoic aquifer, and 5 oil-field brine samples indicates that oil-field brine is not the salinity source to the Navajo aquifer and that water from the upper Paleozoic aquifer may be a source of salinity. INTRODUCTION During a study of ground water in bedrock aqui- fers in eastern San Juan County, Utah, Avery (1986, p. 36) noted that water from the Navajo and other sand- stones of Mesozoic age near Aneth, Utah (fig. I), had higher-than-expected salinity in comparison to water from the same formations in other parts of southern and southeastern Utah and adjacent areas in Colorado, Ari- zona, and New Mexico. Analyses of water from wells drilled by Texaco, Inc. during the late 1950s also docu- mented anomalously high levels of salinity in ground water in the vicinity of Montezuma Creek and Aneth, Utah (J.R. Barnes, consulting geologist, unpub. data, 1959, table 3, p. 18). In addition, changes in quality of water from selected wells throughout time have been reported by residents of the area. Results of analysis of water samples collected from 1989 to 1994 have indi- cated that most of the affected water wells are in and near the Greater Aneth Oil Field (fig. I). Brines asso- ciated with oil production in the Greater Aneth Oil Field are generally injected back into the producing oil formations for secondary recovery of oil and have been suggested as the potential source of the saline water in the Montezuma Creek area (Kimball, 1992, p. 89). 2 Brines generally contain very high concentra- tions of dissolved solids, including numerous trace- and major-inorganic constituents and trace-organic com- pounds that can adversely affect drinking-water sup- plies (Thurman, 1985; Surdam and MacGowan, 1987; and Shipley, 1991). Because of the concern of poten- tially increasing salinity in freshwater aquifers in the vicinity of the Greater Aneth Oil Field, the U.S. Geo- logical Survey in cooperation with the U.S. Environ- mental Protection Agency; Utah Division of Oil, Gas, and Mining; Bureau of Reclamation; Bureau of Land Management; Bureau of Indian Affairs; Navajo Envi- ronmental Protection Agency; and the Navajo Water Resources Management Department did a hydrologic and geochemical study to determine the extent and con- centrations of saline water in the freshwater aquifers, to document changes in salinity throughout time, and to identify, if possible, the source(s) and potential path- ways of the saline water. The saline source(s) leading to increasing con- centrations of dissolved solids in fresh ground-water systems is difficult to determine when more than one potential source exists; however, this type of informa- tion is needed by regulatory agencies to develop plans to prevent furure increases in saliniry and to remediate existing salinity problems. As non-saline ground water becomes more important for domestic use in areas with limited ground-water supplies, an understanding of the processes of salinization and establishment of base-line water quality becomes increasingly important Purpose and Scope This report describes the results of an investiga- tion to determine the extent and concentrations of saline water in the principal freshwater aquifers, docu- ment changes in salinity throughout time, and identify the source(s) and potential pathways of saline water in the vicinity of the Greater Aneth Oil Field in southeast- ern San Juan County, Utah. Objectives include deter- mination of the directions of ground-water movement in the principal aquifers in the sludy area; identification of conservative and nonconservative inorganic, organic, and isotopic geochemical constituents that can be used to define the source(s) and pathway(s) of move- ment of saline water in the principal freshwater aqui- fers; using geochemical data in end-member mixing models and pattern-recognition techniques to identify and quantify source(s) of saline water in the principal freshwarer aquifers; and determinarion of rhe saliniza-