The High Plains aquifer includes an area of about 177,000 square miles, with 74 percent of the area in Kansas, Nebraska, and Texas, and 26 percent of the area in Colorado, New Mexico, Oklahoma, South Dakota, and Wyoming. The High Plains aquifer consists of one or more hydraulically connected geologic units of late Tertiary or Quaternary age. The late Tertiary rocks consist of the Brule Formation, Arikaree Group, and Ogallala Formation. The Quaternary rocks consist of alluvial, dune-sand, and valley-fill deposits. The configuration of the water table indicates that the High Plains aquifer is continuous throughout its extent, ground water generally flows west to east, and areas overlain by dune sand are recharge areas. Discharge from the aquifer is primarily to wells and streams. (USGS).