The Delaware Basin study area includes four major aquifers: Capitan aquifer, Rustler Formation, Santa Rosa Sandstone (Dockum Group), and aquifers in the Cenozoic alluvium. Water from the Capitan aquifer is used for domestic and irrigation purposes in Eddy County, New Mexico, and for irrigation and industrial purposes in Texas. Available analyses indicate that dissolved solids concentrations range from 303 to 31,700 mg/L, chloride concentrations range from 16 to 16,689 mg/L, and fluoride concentrations range from 0.5 to 3.0 mg/L. The Rustler Formation contains water that generally is not suitable for domestic use because of its salinity. Chloride concentrations range from 15 to 210,000 mg/L, and dissolved solids concentrations range from 286 to 325,800 mg/L. Fluoride concentrations range from 0.5 to 11.4 mg/L. Water from this aquifer is used for irrigation and stock watering where it is of suitable quality. The Santa Rosa Sandstone is the principal source of groundwater in the western third of Lea County and in the eastern part of Eddy County. In parts of Texas, the Santa Rosa Sandstone and the Cenozoic alluvium are hydraulically connected and are called the Allurosa aquifer. The Santa Rosa Sandstone-Allurosa aquifer is the source of municipal supply for the cities of Barstow, Pecos, Monahans, and Kermit, Texas. Water quality is variable. For those analyses where the Santa Rosa Sandstone is a distinct entity, chloride concentrations range from 10 to 4,800 mg/L, dissolved solids concentrations range from 205 to 2,990 mg/L, and fluoride concentrations range from 0.4 to 5.0 mg/L. Water from the Cenozoic alluvium is used extensively for public water supplies, irrigation, industry, livestock watering, and rural-domestic supply throughout the Delaware Basin. The quality of water in the Cenozoic alluvium is variable. Chloride concentrations range from 5 to 7,400 mg/L, dissolved solids concentrations range from 188 to 15,000 mg/L, and fluoride concentrations range from 0.3 to 10 mg/L. The Cenozoic alluvium is hydraulically connected to Cretaceous units in parts of Reeves and Pecos Counties, Texas; in these areas, the units are considered as one aquifer, the Pecos aquifer. (Author 's abstract)