<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> <P> The water - saturated thickness of the Ogallala Formation ranges from a few feet to more than 1,000 feet (300 m) and is generally greater in the Northern Plains . The depth of the water below the surface of the land ranges from almost 400 feet (120 m) in parts of the north to between 100 and 200 feet (30 and 61 m) throughout much of the south . Present - day recharge of the aquifer with fresh water occurs at an exceedingly slow rate, suggesting that much of the water in its pore spaces is paleowater, dating back to the most recent ice age and probably earlier . </P> <P> Groundwater within the Ogallala generally flows from west to east at an average rate of a foot per day . Hydraulic conductivity, or the ability for a fluid (water) to move through porous material, ranges from 25 to 300 feet (7.6 to 91.4 m) per day . Water quality within the Ogallala varies with the highest quality for drinking and irrigation in the northern region while the southern region had the poorest . Human and natural processes over the past 60 to 70 years, including irrigation density, climate, and nitrogen applications, have caused higher concentrations of contaminants including nitrates . Nitrate levels generally meet USGS water quality standards, but continue to gradually increase over time . This trend can impact the future groundwater sustainability for portions of the aquifer . </P>

How long did it take to accumulate all the water that is currently in the ogallala aquifer