Editing Ogallala Aquifer (section)

==General characteristics==
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 processes|aeolian]] sediment that filled the ancient channels and eventually covered the entire area of the present-day aquifer, forming the water-bearing Ogallala Formation.<ref>Gustavson, T. C, Winkler, D. A. (1988). Depositional facies of the Miocene-Pliocene Ogallala Formation, northwestern Texas and eastern New Mexico. Geology, 16(3), 203–206. Retrieved from: http://geology.gsapubs.org/content/16/3/203.short</ref><ref>Diffendal, R. F. (1984). Comments on the geologic history of the Ogallala Formation in the southern panhandle of Nebraska. Papers in Natural Resources. Paper 116. Retrieved from http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1117&context=natrespapers</ref> 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.

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. 

The water-saturated thickness of the Ogallala Formation ranges from a few feet to more than 1,000&nbsp;feet. Its deepest part is 1200 ft. (300&nbsp;m) and is generally greater in the Northern Plains.<ref>[http://www.hpwd.com/the_ogallala.asp High Plains Underground Water Conservation District #1 (Texas)] {{webarchive|url=https://web.archive.org/web/20070514235824/http://www.hpwd.com/the_ogallala.asp |date=2007-05-14 }} retrieved April 9, 2007.</ref> The depth of the water below the surface of the land ranges from almost {{convert|400|ft}} in parts of the north to between {{convert|100|and|200|ft|m|0}} 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 [[porosity|pore]] spaces is [[paleowater]], dating back to the [[last glacial period|most recent ice age]] and probably earlier. An impervious geological layer between the aquifer and surface of the land, combined with an arid climate, prevents much recharge from occurring.<ref>Thompson, J., et al. (2018). ''Legal Control of Water Resources: Cases and Materials'', 6th ed. West Academic Publishing. p. 13. ISBN 978-1-68328-983-8.</ref>

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 {{convert|25|to|300|ft|m}} per day.<ref>Gutentag, E. D, Heimes, F. J, Krothe, N. C, Luckey, R. R, Weeks, J. B. (1984). "Geohydrology of the High Plains Aquifer in parts of Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming". U.S. Geological Survey Professional Paper 1400-B. Retrieved from http://pubs.usgs.gov/pp/1400b/report.pdf</ref> 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.<ref name="Gurdak">Gurdak, J. J, McMahon, P. B, Dennehy, K, Qi, S. L. (2009). "Water quality in the High Plains Aquifer, Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming". ''National Water-Quality Assessment Program, USGS Circular 1337''. Retrieved from http://pubs.usgs.gov/circ/1337/pdf/C1337.pdf</ref> 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.<ref name="Gurdak" /> This trend can impact the future groundwater sustainability for portions of the aquifer.