The Ogallala Aquifer by Y2038O


									       The Ogallala Aquifer

               By: Lyal Miller

                  Spring 2010

               Hydrologic Setting

       Hydrologic and Geologic Properties

                Water Resources

       Contamination and Improvements



Figure 1: Outline of the Ogallala Aquifer (Kromm)
Hydrologic Settings
       The Ogallala Aquifer is located within the central plains of the United States of America.
It is the largest aquifer in the United States as well as one of the largest ones around the world.
The aquifer extends from the southern part of South Dakota to western Texas panhandle. It
then scatters across five other states (Colorado, New Mexico, Wyoming, Nebraska, South
Dakota, Texas, Oklahoma and Kansas) so it is 800 miles long (north and south) and almost 400
miles wide from east to west (Todd). The aquifer underlies about 174,000 square miles with
65% of it being underneath Nebraska, 12% under Texas, and 10% under Kansas. In 1990, it
was shown that the aquifer contained about 3.270 billion acre/feet of water, with 417 million
acre/feet being under Texas. The Ogallala is one of the nation’s leading producers in
agriculture, accounting for a fifth of the United States ($20 billion worth). Many studies of this
aquifer have been focusing on the panhandle of Texas and western Kansas where depletion of
the aquifer is at its greatest (Kromm).

                   Characteristics of the High Plains aquifer
            Unit            Total         CO    KS      NE      NM     OK      SD      TX     WY
                           174,05 14,90 30,50 63,65 9,45 7,35 4,75 35,45 8,00
underlain by       mi2
                             0      0     0     0    0    0    0     0    0
 Percent of
total aquifer                100          8.6   17.5   36.6     5.4    4.2     2.7    20.4     4.6
 Percent of
 each state Perce
                              --          14    38      83       8      11      7      13       8
underlain by nt
                    Ft       190          79    101     342      51    130 207        110     182
thickness in
 Volume of
 drainable       Million
  water in       acre- 3,250         120      320 2,130        50    110    60     390      70
 storage in        ft

Table 1: Characteristics of the Ogallala Aquifer (Dugan et al., 1994)

       Recharge in the aquifer mainly comes naturally, through rains, rivers, basins, reservoirs,
and a new one, playa lakes. Playa lakes are ephemeral lakes that are formed after a large rain
when rainwater runs from a field into a depression in the landscape. It was later found that
this water slowly but surely continues downward into the Ogallala contributing partially to the
recharge. Farmers dislike these playas or as they call them “mud holes” because they lose farm
ground and can cause messes. However these playas have become yet another key component
in the recharge of the Ogallala aquifer. (U.S. Dept. of Ag., 2008)
                   FIGURE 2: PLAYA LAKE IN MEADE COUNTY, KS (Flowers)

       With the vastness of the Ogallala aquifer, recharge is hard and minimal, especially with
such a semi-arid climate. The aquifer loses around and foot or more of water per year
compared to a meager .35-3.24 inches/acre/year recharge. Certain areas of Texas lose more
than that per year because of the arid climate. Lubbock, Texas’ average rainfall per year is 18
inches with almost 80% of that getting evaporated and never reaching the aquifer. (Todd)

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Hydrologic & Geologic Properties
       The Ogallala began forming 10-12 million years ago during the late Tertiary period.
The aquifer is composed mainly of unconsolidated, poorly sorted clay, silt, sand, and gravel
with the groundwater filling the pore spaces in between. Some of the sands that make up the
Ogallala Aquifer vary in color and texture, generally tan, yellow, or reddish brown, with
medium to coarse textured. Sand is usually poorly consolidated to unconsolidated with local
cementation by calcium carbonate and silica occurs. Gravel is also usually associated with
sand, as well as silt and clay. The predominate rock throughout the aquifer is Quartzite
although there is a high percentage of limestone in the southern area. (McReynolds, 2009)

       Saturated thickness of the Ogallala ranges mightily from a few feet to more than 525
feet. Some areas with the greatest thickness are located in the Nebraska or the northern part of
the aquifer. The southern part, between Lubbock and Midland, the zone ranges from less than
50 feet to 200 feet. Water below the surface can range from anywhere between 400 feet in the
northern part to 100 and 200 feet in throughout much of the southern part. At some places the
saturation zone can reach upwards to 1,000 feet in Nebraska.

                                                          State             Average specific yield, in
                                                      Colorado                        15.4
                                                      Kansas                          16.1
                                                      Nebraska                        15.2
                                                      New Mexico                      14.8
                                                      Oklahoma                        18.5
                                                      South Dakota                    9.2
                                                      Texas                           15.6
                                                      Wyoming                         7.6
                                                      Eight States                    15.1

Figure 3: Showing depth of saturation zone          Table 2. Average specific yield in the High
throughout the Ogallala aquifer                     Plains aquifer (McGuire et al., 2003)
(McReynolds, 2009) Click to enlarge

       Since the Ogallala aquifer is an unconfined aquifer, it has no relative storativity. But
instead has a specific yield throughout the aquifer. Water storage levels throughout each state
have also dropped except for a few. The change in the volume of the water in storage from
2000 has decreased of about 200 million acre-feet (table 3). This change comes from an
increase in Nebraska of about 4 million acre-feet, to a decrease in Texas of about 124 million
acre-feet. Majority of this decline comes from a 17-million-acre area with 25 feet or more of
water-level declines (McGuire et al., 2003).

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Water Resources
       Groundwater from the Ogallala Aquifer is devotedly used for irrigation unlike other
aquifers. Approximately 95% of the Ogallala is used for irrigation which in turn accounts for
65% of the nation’s total acreage irrigated land. On average farmers will pump up to 14
billion gallons of water/ day out of the aquifer. Nebraska uses about 46% for irrigation, Texas
30%, and Kansas rounds it up with 14% of the irrigation. Besides irrigation taking up the
majority of the aquifer, 2.2 million people also use the aquifer for municipal water supply
which pumps nearly 332 million gallons/day. Water usage of the aquifer was to believed at
first to be infinite and everlasting until monitoring wells were put in the 1940’s and noticed
considerable drops in water levels. Declines were raped throughout the 40’s and on into the
1970’s with levels dropping upwards to a foot per year at times. In some states, Kansas for
example, counties are creating their own water districts where they will have stricter
regulations on pumping water from the aquifer. These districts are around to help minimize/
lessen the depletion of the Ogallala aquifer (U.S. Geo. Survey, 2007).
Figure 4: Irrigated cropland, Morton County, Kansas. (McGuire et al., 2003)

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Contamination and Improvements
       With the Ogallala being such a vast aquifer, it can be hard for it to not be contaminated
in some areas. Studies have warned us that heavy use for irrigation and public supply can leak
down inactive irrigation wells and results in long-term gradual increases in contaminants such
as nitrate and dissolved solids (Dennehy et al., 2009). However recent studies have shown faint
detections of pesticides and nitrates. The percolation rates of contaminants from the surface to
the water table have yet to be established in areas where polluted water has been found
Figure 5: Ditch irrigation (Fitzsimmons, 1998)

       With the groundwater being depleted more and more each year from irrigation,
scientists and researchers are coming up with better more efficient ways to irrigate crops.
Texas alone has changed ways to prevent large wastes of water. Texans would pump water
straight from the ground into large irrigation ditches then to the fields they would be
irrigating. This alone would evaporate a lot of the water pumped from the ground itself
causing waste. Now instead of the ditches, there are more than 10,000 miles of underground
pipeline throughout Texas, eliminating wasted water. Another way landowners are helping to
reduce the waste of water is by using porous house, directly into the ground from the well that
allows the water immediate access into the roots/soil of the plant. Also eliminating evaporation
and a waste of good quality water (Marsh et al., 2003).

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        The Ogallala aquifer is one of the greatest aquifers in the world, with it being one of the
single most productive aquifers, without it, United States’ economy would change immensely.
It provides water for not only agriculture, but also industry and homes. Over-pumping has
become an issue in many parts of the aquifer with little or no recharge available to the same
areas. Monitoring of the Ogallala will help us better understand how it recharges and how to
change the depletion of the aquifer, so some day our children will be able to share its wealth.

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Dennehy, K., Gurdak, J., McMahon, B., & Qi, S. 2009. Water quality in the High Plains aquifer,
Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming,
1999-2004. U.S. Geological Survey circular 1337. 63p.
regional/govt-and-politics/article_f9a83f7a-5f6b-5ede-b666-db3b9ea593e7.html [retrieved
April 29, 2010]

Dugan, J., McGrath, T., & Zelt, R. 1994. Water-level Monitoring Study Resources and
Information. U.S. Geological Survey. [retrieved April 29,
Fitzsimmons, K. 1998. American Tilapia Association.
[retrieved April 29, 2010]

Flowers, T. Mud hole or Wetland?. N.R.C.S. [retrieved April 29, 2010]

Kromm, D. Ogallala Aquifer. Water encyclopedia.
Po/Ogallala-Aquifer.html [retrieved April 29, 2010]

Marsh, T., Peterson, J., & Williams, J. 2003. Conserving the Ogallala Aquifer: Efficiency, Equity,
and Moral Motives. Choices.
[retrieved April 29, 2010]
McGuire, V., Johnson, M., Schieffer, R., Stanton, J., Sebree, S., & Verstraeten, I. 2003. Water
Storage and Approaches to Groundwater Management, High Plains Aquifer, 2000. U.S.
Geological Survey. [retrieved April 29,

McReynolds, D. 2009. High Plains Underground Water Conservation District No.1. [retrieved April 29, 2010]

Todd, J. Depletion of the Ogallala Aquifer. Helium.
depletion-of-the-ogallala-aquifer [retrieved April 29, 2010]

U.S. Department of Agriculture. 2008. How will North America’s Largest Aquifer, the Ogallala
Aquifer, Fare?. Science Daily [retrieved April 29, 2010]

U.S. Geological Survey. 2007. High Plains Regional Groundwater (HPGW) Study. [retrieved April 29, 2010]

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