The base of moderately saline ground water (water that contains from 3,000 to 10,000 milligrams per liter of dissolved solids) was delineated for San Juan County, Utah, based on water-quality data and on formation-water resistivities determined from geophysical well logs using the resistivity-porosity, spontaneous-potential, and resistivity-ratio methods. These data and the contour map developed from them show that a thick layer of very saline to briny ground water (water that contains more than 10,000 milligrams per liter of dissolved solids) underlies the eastern two-thirds of San Juan County. The upper surface of this layer is affected by the geologic structure of the area, but it may be modified locally by recharge mounds of less saline water and by vertical leakage of water through transmissive faults and fractures. The highest altitude of the base of moderately saline water is west of the Abajo Mountains where it is more than 6,500 feet above sea level. The lowest altitude is in the western part of the county and is below sea level: depressions in the base of moderately saline water in recharge areas in the La Sal and Abajo Mountains also may be that low. The base of moderately saline water commonly is in the Permian Cutler Formation or the Pennsylvanian Honaker Trail Formation of the Hermosa Group, but locally may be as high stratigraphically as the Triassic (?) and Jurassic Navajo Sandstone north of the Abajo Mountains and in the Jurassic Morrison Formation south of the mountains.
