The Pajarito Plateau is an important source of abundant potable groundwater for Los Alamos National Laboratory (LANL) and the communities of Los Alamos and White Rock. Geologic investigations were undertaken as part of a plateau-wide hydrogeological investigation to develop conceptual models of the groundwater system as a framework for numerical simulations of groundwater flow. The Pajarito Plateau is located in the western part of the Española basin where rocks of the Jemez and Cerros del Rio volcanic fields overlie and interfinger with Neogene basin-fill sedimentary rocks. The vadose zone is about 200 m (600 ft) thick beneath mesas on the east side of the plateau and more than 375 m (1245 ft) thick on the west side. Groundwater occurs as shallow groundwater in canyon-floor alluvium, moderately deep groundwater perched in bedrock units of the vadose zone, and groundwater associated with the regional saturated zone. The most productive rocks of the regional aquifer occur in a westward-thickening wedge of coarse-grained, Miocene and Pliocene volcaniclastic rocks derived from the Jemez volcanic field. Eastern aquifer rocks consist of fine-grained, somewhat less productive Miocene sedimentary deposits derived from highland sources to the east and north. Intermediate and mafic lavas interbedded with the Miocene and Pliocene sedimentary deposits are components of the regional aquifer locally. The hydrogeology of the Pajarito Plateau is probably typical of groundwater systems along the margins of the Rio Grande rift where arid to semiarid, sediment-filled basins receive most of their recharge from adjacent mountainous areas.