The purposes of this investigation were to: define the hydraulic character and subsurface distribution of the major aquifers and aquitards; identify and describe the principal areas of recharge to and discharge from the major aquifers; and determine the rate and the direction of ground-water movement within the major aquifers and aquitards. Of these objectives, the third was of prime importance for an evaluation of the rate of movement of various radionuclides from the vicinity of an underground nuclear detonation. The accuracy of the velocity estimates, however, rested heavily upon the other two study objectives. The Nevada Test Site occupies a small part of two ground-water basins - the Ash Meadows and the Oasis Valley-Fortymile Canyon basins. Consequently, the objectives are discussed for a region several times the size of the test site.