Geohydrologic data were collected from Irwin Basin at Fort Irwin National Training Center in the Mojave Desert of southern California by the U.S. Geological Survey during 199296 to deter mine the quantity and quality of ground water available in this basin. In addition to data collected from existing wells and test holes, 17 monitoring sites were constructed in Irwin Basin to provide data on subsurface geology, ground-water levels, and ground-water quality. Eleven of these sites were multiple-well monitoring sites that were constructed to provide depth-dependent geohydrologic data in the aquifer system. The aquifer system of Irwin Basin, defined on the basis of hydrologic data collected from wells in Irwin Basin, consists of an upper and a lower aquifer. A 1994 water-table contour map shows that a cone of depression beneath Irwin Basin well field has developed as a result of ground-water development. Water-quality samples collected from Irwin Basin wells to determine potential sources of ground-water degradation indicate that water in three areas in the basin contains high nitrate and dissolved-solids concentrations. The stable isotopes of oxygen and hydrogen indicate that present-day precipitation is not a major source of recharge in this basin. Tritium and carbon-14 data indicate that most of the basin was recharged before 1953 and that this water may be more than 14,000 years old.