The High Plains Aquifer system
The High Plains aquifer system is the primary source for ground water in the High Plains region, most notably for agricultural use. This aquifer covers a large area (174,000 sq. miles) and spans vertically over eight states (Figure 1). The name was derived from the High Plains Physiographic Province, an area of relatively flat topography that progresses eastward from the Rocky Mountains (Figure 2) (USGS, 1997). The Ogallala aquifer is the leading geologic formation of the system, although two smaller formations are present (Brule and Arikaree). Collectively, over 170,000 wells are tapped into this natural resource which contributes to irrigating some 500,000 sq. kilometers of farmland, making it the largest irrigation-sustained cropland in the world. For example, agriculture from South Dakota to Texas is has been supported solely by irrigation from the High Plains aquifer for nearly a century. The High Plains aquifer is an open system mainly developed in shallow sands and gravels. Much of the water originally entered the aquifer during a wetter climate during the last ice age (High Plains, 2000). Probably the most critical issue facing this aquifer is the overuse caused by the increasing irrigational practices of modern-day agricultural techniques.
Figure 1; Extent of the High Plains Aquifer system
Picture from USGS, 1997 ; Modified by: Tyler Ringler
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Figure 2; Physiographic Province that gave name to the High Plains Aquifer system
Picture from USGS, 1997 ; Modified by: Tyler Ringler
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The High Plains Aquifer system
Hydrologic setting
Recharge of the enormous aquifer is attained through precipitation on a large-scale, and more by streams and canals locally. A huge drop in the holding capacities and generalized water levels throughout the region can be seen in the last several decades due to increased irrigation (Figure 3). Some of the runoff through irrigation actually proves to be a moderate source of the aquifer’s own recharges as well (USGS, 1997). The dry arid climate in the west high plains coupled with the higher precipitated eastern region allows for moderate recharge, but over usage is still driving down the overall volumes of water being retained. Between the years of 1950 and 1980, for example, their was a 300% increase in groundwater pumped from the High Plains aquifer system (High Plains, 2000). Average recharge rates for Nebraska and Kansas for the aquifer is around 0.05 and 0.6 inches per year. Most of the recharge occurs in areas where sand dunes or other highly permeable material makes up the land surface (USGS, 1997).
Figure 3; Recharge process of the High Plains Aquifer system.
Picture from Google Image search ; Modified by: Tyler Ringler
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Hydrologic and Geologic Properties
Ground water quality in the High Plains aquifer system is dependant upon several factors including chemical composition and solubility of aquifer materials. Also, ground water samples near a recharge area typically show less amounts of dissolved solids present due to dilution and a shorter residence time. Inversely, ground water near discharge regions show higher dissolved solids due to a longer residence time (USGS, 1997). The dissolved solid count for most of Kansas and Nebraska fall short of the 500 milligrams per liter standard set by the Environmental Protection Agency (Figure 4). One of the biggest players in the dissolved particle content around the Kansas-Nebraska region of the aquifer is sodium. High contents of sodium can negatively affect soil tillability and permeability. The highest contents can be seen in south-central Kansas (Figure 5) where the High Plains aquifer overlies Permian bedrock that contains saline water derived from partial dissolution of salt beds (USGS, 1997).
Figure 4; Dissolved solid content for Kansas and Nebraska regions of aquifer
Picture from USGS, 1997
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Figure 5; Dissolved sodium ion content for Kansas and Nebraska regions of aquifer
Picture from USGS, 1997
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The High Plains aquifer system is unconfined, and composed of several Quaternary and Tertiary age geologic units. The oldest formation included in the aquifer system is the Brule formation of Oligocene age. Figure 6 shows the prominent geologic rock formations involved in the aquifer system, along with the name, age, and thickness of each. The Brule is mainly found under western Nebraska and is made up heavy siltstone beds with channel deposits of sandstone (USGS, 1997). The Arikaree formation has thick beds of volcanic ash, siltstone, claystone, and marl which stretch from eastern Nebraska (1,000 ft. thick) to western Nebraska in
