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The Ogallala, or High Plains, Aquifer is a porous body of complex sediments and sedimentary rock formations that conducts groundwater and yields significant quantities of water to wells and springs. The principal sediments and rocks of the aquifer range in age from 33 million years old to sediments being deposited today, but the majority is less than 12 million years old. Much of the aquifer is composed of the Ogallala Group or Formation. The dominant sediments in the Ogallala and the other hydrogeologic units in the aquifer are riverand wind-deposited sands.
The aquifer underlies about 174,000 square miles of the High Plains. The water-saturated part of the aquifer varies in thickness and is more than 1,000 feet thick in places. Both the thickest and the most extensive areas are in Nebraska. The water from the aquifer is being pumped by nearly 200,000 irrigation wells, most of them installed since the 1940s. Installation rates have varied, with the highest rates generally occurring during dry years. In places, particularly in the southern region, pumping has lowered water levels as much as 200 feet.