The aquifer system of the Mesilla ground-water basin is the Quaternary flood-plain alluvium and the Quaternary and Tertiary Santa Fe Group. The hydraulic gradient in the shallow flood-plain alluvium within the Mesilla Valley is generally between 4 and 6 feet per mile. The hydraulic gradient in the Santa Fe Group ranges from 100 feet per mile in the northwestern part of the study area to less than 2 feet per mile in the southwestern part of the study area. Ground-water levels in nearby observation wells correspond to increases in river stage and indicate significant recharge to the aquifer at the Rio Grande. Water in storage within the Rio Grande flood-plain alluvium/Santa Fe Group aquifer system occurs under unconfined and semi- confined conditions. Horizontal permeability usually exceeds vertical permeability by several orders of magnitude. The thickness and extent of finer grained, less permeable material increase with depth and horizontally toward the southern end of the basin. Seasonal trends in the shallow water table generally correspond to recharge during the irrigation season. Freshwater zones are overlain by zones of slightly saline to saline water in the Mesilla Valley. Geohydrologic data indicate that the thickness of the freshwater zone is significantly less than previously estimated in the vicinity of the proposed West Mesa well field. Lithologic data, borehole-geophysical logs, water-quality data, and potentiometric contours of ground-water in the upper Santa Fe Group indicate a hydraulic connection between the Mesilla Valley and West Mesa.