The High Plains Aquifer System

By: Tyler Ringler

Introduction to Hydrogeology, Spring 2006



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Introduction
The High Plains Aquifer system

The High Plains Aquifer system
Hydrologic setting
Hydrologic and Geologic Properties
Water Resource
Ground Water Contamination


    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 thinning sections. The Ogallala aquifer is the primary constituent of the system and is present throughout most of the entire High Plains area. The Ogallala consists of unconsolidated gravel, sand, silt, and clay. Locally, it also includes caliche, which is a hard deposit of calcium carbonate that precipitated when part of the ground water that moved through the formation evaporated (USGS, 1997). In Kansas, the High Plains aquifer system can be separated into slightly smaller divisions for easier management of water quality and control. One of these sub-formations, the “Equus beds” aquifer, is located on the eastern extension of the High Plains system including several counties (Reno, Harvey) in Kansas and contains much younger sediments. Another smaller formation associated with this deposit of younger sediments includes the “Great Bend Prairie aquifer” in Kiowa, Pratt, and other Kansas counties (Figure 7). These sediments were deposited during the Pleistocene Epoch during the most recent Ice Age (KGS, 2001).

    Figure 6; Diagram of the underlying constituents of the High Plains aquifer

    Picture from USGS, 1997 ; Modified by: Tyler Ringler

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    Figure 7; Locality of younger Equus beds in Kansas

    Picture from USGS, 1997 ; Modified by: Tyler Ringler

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    Transmissivity of the aquifer systems vary regionally. Many of the northern regions of the aquifer system covered by loosely consolidated sediments with high porosity can see higher rates. Storativity in the aquifer is high mostly due to the enormous availability of water. Pumping rates of over 1,000 gallons/minute are not rare in some places (KGS, 2001). Saturated thickness in the aquifer system was measured to be 90 feet for Kansas, but ranges up to 1,000 ft in north-central Nebraska were not uncommon. Figure 8 and Figure 9 compares the High Plains aquifer system’s predevelopment saturated thickness to that of present showing considerable drops in areas of the Ogallala in western Kansas (USGS, 1997). This once again shows the huge variance in hydrologic aspects that can be seen in a large aquifer system like the High Plains.

    Figure 8; Aquifer depletion over time

    Picture from USGS, 1997 ; Modified by: Tyler Ringler

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    Figure 9; Drop in storativity in the High Plains Aquifer

    Picture from USGS, 1997 ; Modified by: Tyler Ringler

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    Water Resource

    As previously stated, the High Plains aquifer system is primarily used for irrigational purposes. A high period of over stress in the aquifer caused considerable drops in ground water levels in large localities. Significant drops (60%) were seen in extremely dry areas of western Kansas before successful ground water management took control. However, with the new technology of center-pivot irrigational techniques, farmers are now capable of covering much larger areas of land with minimal water usage (Figure 10). Economically, recent studies have suggested the annual income generated due to irrigation in southwestern Kansas alone is around $188 million, respectively (KGS, 2001).

    Figure 10; Center-pivot irrigational practices

    Picture from Kansas, 2001 ; Modified by: Tyler Ringler

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    The estimated life of this aquifer system is dependant on many things. Without doubt, this system could support small level wells in relatively low populated areas well into the future (Figure 11). The price of irrigational practices (cost to pump), as well as the market prices of the crops being irrigated is a significant factor (KGS, 2001). Water management also plays a huge part in the life expectancy of this aquifer system. Each state that the system underlies has differing management practices, but they all basically serve the same purpose. In Kansas, sections of the aquifer are divided amongst counties and grouped into five Rural Water Districts (Figure 12). These responsibilities are held by the Division of Water Resources of the Kansas Department of Agriculture, as well as setting regulations related to the quantity of water used. Individual water issues are decided within these districts (KGS, 2001). Other agencies in Kansas that aid in overlooking our states most precious natural resource include the Kansas Water Office, the Kansas Geological Survey, the Kansas Water Authority, as well as the Kansas Department of Health and Environment.

    Figure 11; Life expectancy of the High Plains Aquifer

    Picture from KGS, 2001 ; Modified by: Tyler Ringler

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    Figure 12; Kansas's Rural water districts associated with the High Plains aquifer

    Picture from KGS, 2001

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    Ground Water Contamination

    The general shallow depth of the High Plains aquifer makes it extremely susceptible to run-off contamination. The over-application of various herbicides and pesticides become diluted into run-off rain water and eventually percolate into the underlying aquifer unit. Significant increases in concentrations of sodium, alkalinity, nitrate, and triazine (a herbicide) have been found in water that underlies small areas of irrigated cropland in Nebraska and Kansas (USGS, 1997). In a recent study of 132 wells in Nebraska in 1984-1985, 43 had dangerous amounts of atrazine. Also to note was an abrupt increase in 2, 4-D (2,4-Dichlorophen-oxyacetic acid) was found in water that underlies rangeland in a small area around Great Bend, Kansas (USGS, 1997.) In events of regional contaminant spills, the Environmental Protection Agency oversees remediation responsibilities, depending on the size and composition of the spill.

    References:

    The High Plains Aquifer Information Network. World Wide Web homepage, www.hiplain.org [Retrieved 22, April 2006].

    USGS. United States Geological Survey. Ground Water Atlas of the United States; Kansas, Missouri, and Nebraska. World Wide Web, http://capp.water.usgs.gov/gwa/ch_d/D-text2.html [Retrieved 22, April 2006].

    Groundwater and Wetlands. The McGraw-Hill Companies, World Wide Web, www.mhhe.com/earthsci/geology/mcconnell/demo/hpaq.htm [Retrieved 22, April 2006].

    Kansas Geological Survey, Public Information. World Wide Web, http://www.kgs.ku.edu/Publications/pic18/pic18_1.html [Retrieved 26, April 2006].