<P> Large scale extraction for agricultural purposes started after World War II due partially to center pivot technology and to the adaptation of automotive engines for groundwater wells . Today about 27% of the irrigated land in the entire United States lies over the aquifer, which yields about 30% of the ground water used for irrigation in the United States . The aquifer is at risk for over-extraction and pollution . Since 1950, agricultural irrigation has reduced the saturated volume of the aquifer by an estimated 9% . Once depleted, the aquifer will take over 6,000 years to replenish naturally through rainfall . </P> <P> The aquifer system supplies drinking water to 82% of the 2.3 million people (1990 census) who live within the boundaries of the High Plains study area . </P> <P> The deposition of aquifer material dates back two to six million years, from the late Miocene to early Pliocene ages when the southern Rocky Mountains were still tectonically active . From the uplands to the west, rivers and streams cut channels in a generally west to east or southeast direction . Erosion of the Rockies provided alluvial and aeolian sediment that filled the ancient channels and eventually covered the entire area of the present - day aquifer, forming the water - bearing Ogallala Formation . In that respect, the process is similar to those currently prevailing in other modern rivers of the area, such as the Kansas River and its tributaries . The major differences are time and depth . </P> <P> The depth of the Ogallala varies with the shape of then - prevailing surface, being deepest where it fills ancient valleys and channels . The Ogallala Formation consists mostly of coarse sedimentary rocks in its deeper sections, which transition upward into finer - grained material . </P>

Where does the water in the ogallala aquifer come from
find me the text answering this question