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Hydrology, chemical quality, and characterization of w salinity in the Navajo aquifer in and near the Greater Ul ~ Aneth Oil Field, San Juan County, Utah By L.E. Spangler, D.L. Naftz, and Z.E. Peterman ABSTRACT Because of the concern of potentially increasing salinity in freshwater aquifers in the vicinity of the Greater Aneth Oil Field in south- eastern San Juan County, Utah, a hydrologic and geochemical study was done to determine the extent and concentrations of salinity in the fresh- water aquifers, to document changes in salinity throughout time, and to identify, if possible, the source and potential pathways of the saline water. The Navajo aquifer is the principal bedrock aquifer in the vicinity of the Greater Aneth Oil Field and includes the Entrada, Navajo, and Win- gate Sandstones. The Navajo aquifer is confined in this area, and wells typically discharge water at land surface. The Navajo aquifer generally ranges from 750 to 1,000 feet in thickness, with the top of the aquifer averaging 550 feet below land surface. The Navajo aquifer is recharged along the flanks of the Abajo Mountains, Sleeping Ute Mountain, and the Carrizo Mountains, to the north, east, and south of the study area, respectively. Potentiometric contours, increasing hydraulic head with depth, and gains in discharge indicate that water in the Navajo aquifer moves downgradi- ent from these recharge areas and discharges into the San Juan River. Water from the Navajo aquifer is discharged primarily from water wells, and dry holes and pre- viously producing oil wells that were plugged back to water-bearing formations. Measured discharge from flowing wells during the study ranged from less than I to as much as 150 gallons per minute. About 600 acre-feet of water discharged from the Navajo aquifer from flowing wells during 1992- 93. Water-level declines in some Navajo aquifer wells have been as much as 178 feet since the 1950s. Dissolved-solids concentrations in water from 56 wells in the Navajo aquifer ranged from 145 milligrams per liter (fresh) to as much as 17,300 milligrams per liter (very saline). Water from most wells shows less than lO-percent varia- tion in salinity with time; however, increases of greater than 50 percent have been documented for selected wells. Water with dissolved-solids con- centrations greater than 10,000 milligrams per liter may be less than 500 feet below land surface in the Aneth area. High levels of salinity in water from some water wells prior to the late 1950s indicate that saline water was present in the Navajo aquifer before development of the Greater Aneth Oil Field. Salinity in the Navajo aquifer may have been derived from upward movement of saline water from the upper Paleozoic aquifer or possibly from localized dissolution of evaporites that were present in the Navajo aquifer. Apparent increases and decreases in salinity of water from selected wells could be caused by pumping or well bore effects. Potential upward movement of water from the upper Paleozoic aquifer into the Navajo aquifer may take place in an area near Aneth where the hydraulic head in the upper Paleozoic aquifer exceeds that in the Navajo aquifer. Mixing model results indicate that the bro- mide- and iodide-to-chloride weight ratios charac- teristic of oil-field brine in the study area decrease with increasing chloride concentration in water from the Navajo aquifer. and generally follow the end-member mixing lines constructed with the mean non-oil-field brine and the upper Paleozoic aquifer end-member water compositions. Neither oil-field brine nor comingled injection water is the source of salinity to the Navajo aquifer.