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Summary of ground-water conditions in the Almena Unit area
The Almena Unit is a proposed project in the U.S. Bureau of Reclamation's Missouri Basin development program. The area is now being investigated and if approved for construction, a dam would be built on Prairie Dog Creek a few miles west of Norton to form a storage reservoir. Water from this reservoir would be brought downstream through a series of canals to an irrigation district comprising much of the terrace area between Almena and the point where Prairie Dog Creek flows into Nebraska. At present, irrigation in this area is limited to five well plants, four of which are in Norton County, and to several surface-water pumping plants which have not been used for several years.
Like the main part of the area covered by this report, ground water in the Almena Unit area is derived ultimately from precipitation in or near the area in the form of rain or snow. Some of the water is carried away by surface drainage, some is evaporated into the atmosphere, and a part is transpired by vegetation. A small part percolates downward through the soil to join the body of ground water in the zone of saturation.
The rocks which immediately underlie the terrace surface along Prairie Dog Creek Valley consist of silt and sandy silt, from 30 to 40 feet in thickness, underlain by sand and gravel. This material is described in detail in the test-hole logs at the end of this report. These unconsolidated deposits fill the trough which Prairie Dog Creek has cut into the Cretaceous bedrock (Pl. 3). The deposits of sand and gravel have large interconnected openings which allow the storage of a considerable amount of water and permit it to move freely through the rocks. Plate 2 shows that the ground water moves generally toward the northeast, but also moves from the upland areas toward the valley. Within the valley, there is a general downstream movement of the water with subordinate movement toward the stream channel. Prairie Dog Creek gains water from the water table and it is therefore an effluent stream throughout this area. It is in equilibrium with respect to the water table for a distance of several miles in the vicinity of Long Island. The stream seems to gain water by seepage from the uplands south of the valley, and to lose water into the terrace deposits north of the stream. The ground-water trench seems to correspond, in part at least, with the position of the deep channel in the Cretaceous bedrock beneath the terrace deposits.
The average gradient of the water table in the terrace deposits in the Almena Unit is about 8 feet per mile. The gradient in eastern Norton County is about 7 1/2 feet per mile and increases to about 9 feet per mile near Long Island. Just southwest of the hairpin loop of Prairie Dog Creek 4 miles west of Woodruff the gradient decreases to only 7 feet per mile. After the stream resumes its eastward course the gradient steepens slightly so that it is about 8 feet per mile near Woodruff.
The water table fluctuates in response to additions to or withdrawals from the ground-water reservoir. In general, the water table declines during dry periods and rises during times of heavy precipitation. There is a lag between the time of the rainfall and the rise in the water table, representing the time required for water to percolate downward to the water table. Figure 7 illustrates the changes in the level of the water table due to variations in precipitation. The hydrographs for wells 1-20-13ad and 2-21-11aa show a rise of nearly 2 feet in the water table following the period of heavy rainfall in June 1947. The hydrographs also show a sharp decline in the water table during September 1947, when rainfall was only a fraction of normal.
The addition of water to the ground-water body is called recharge. The source of much of the recharge in the terrace area along Prairie Dog Valley is local precipitation. Part of the water which falls in the valley as rain or snow percolates down to the water table and moves downstream through the porous material that comprises the lower part of the terrace deposits. Recharge in the terrace areas is relatively high compared to the more dissected, steeply sloping areas bordering the valley because runoff from the flat terrace areas is small and the surficial materials are somewhat more permeable than the loess which blankets the upland areas. In addition to the recharge directly from precipitation, some recharge comes from small tributary streams whose channels lie above the water table. Many of these tributaries do not have definite channels across the terrace area, but empty their drainage onto the terrace surface where much of it seeps into the ground and recharges the water table.
In order that the ground-water reservoir shall remain in equilibrium, the amount of discharge in the area must be equal to the amount of recharge. The water table beneath the Almena terrace is neither rising steadily nor declining continually; therefore the ground-water system must be in equilibrium and the amount of discharge over a period of time must be equal to the recharge. During dry seasons, of course, the recharge may be considerably less than the discharge, but during periods of heavy precipitation recharge will be greater than discharge, so that over a long period of time the system remains in equilibrium.
The principal methods of discharge are evaporation from the ground-water reservoir, transpiration by plants, natural discharge into streams, and pumping from wells. Evaporation from the water table can occur only where the water table surface is within a few feet of the land surface. This condition occurs only along the flood plain of Prairie Dog Creek and its principal tributaries. The flood-plain areas are narrow and represent a very minor part of the valley area. The amount of ground water discharged by evaporation in Prairie Dog Creek Valley is relatively small.
A more important method of natural discharge is through transpiration by trees and other plants whose roots reach the water table. Transpiration probably accounts for a considerable amount of the discharge from the reservoir underlying the Almena terrace. The water table lies from 25 to 40 feet below the land surface, within easy reach of the roots of trees and plants such as alfalfa, which is a common crop in this area.
Another important method of discharge is by seepage from the ground-water reservoir into the permanent streams. Prairie Dog Creek is an effluent stream and gains water from the water-table throughout its course in Norton and Phillips counties. This type of discharge serves to keep the ground-water system in equilibrium. When the ground-water level is high the gradient toward the stream is greater, allowing more seepage into the stream. This lowers the water table and reduces the gradient and the amount of discharge.
Wells that pump water from the ground-water reservoir constitute an important means of discharge. Individual domestic and stock wells pump only small quantities of water, but the total volume pumped by them must be quite large. Wells such as irrigation and municipal supply wells which discharge large amount of water frequently lower the water table for a considerable area about the pumped wells. This lowering of the water table is called a cone of depression, and it may extend for some distance if the wells are not allowed to recover fully before being pumped again. The Almena city wells seem to have developed a cone of depression about that area (Pl. 2). After a period of prolonged pumping the irrigation wells also probably have cones of depression. Because these wells are pumped only a short time each year they have ample time to recover before the next season and the cone of depression soon disappears.
Wells for stock and domestic use in the Almena unit area are generally drilled or bored and cased with 5- or 6-inch galvanized casing, slotted opposite the water-bearing sand and gravel. A few are dug wells of large diameter, walled with stone. Most of the wells are equipped with a cylinder pump and windmill. Many are drilled only a few feet below the water table and no attempt is made to penetrate all the water-bearing material. The irrigation wells in the area, however, are constructed somewhat more scientifically. Sites for these wells are generally determined by test drilling or boring and they penetrate all the water-bearing gravel. Commonly they are constructed with either natural or artificial gravel walls, and they are usually equipped with turbine pumps powered with gasoline engines. All the irrigation wells in this area at the time of the field investigation are listed and described in Table 8.
Irrigation--As already stated, only five irrigation wells are now in use in the Almena Unit area. All these wells obtain water supplies from the gravel deposits underlying the Almena terrace. Test drilling in the valley in the vicinity of Long Island has revealed that in Phillips County the bedrock floor beneath the terrace is not flat but has a deep, narrow notch or channel which was cut by the stream in an earlier erosion cycle. This channel contains about three times as much coarse saturated material as occurs elsewhere beneath the terrace. Wells that penetrate this channel have much higher yields than those penetrating only the thinner gravel deposits. Well 1-19-8ad (Table 8, Pl. 10C) seemingly is drilled into this channel. This well penetrated 56 feet of saturated material and was reported to yield 700 gallons per minute when test pumped in 1948. Unfortunately there is no surface indication of this buried channel; it can be located only by test drilling.
Irrigation wells might be located in favorable places in the Crete deposits along the north side of the valley. Test hole 1-20-14ad penetrated 32 feet of these deposits, including nearly 20 feet of saturated sand and gravel. In general, the Crete deposits are not so likely to furnish large supplies of water as the terrace deposits because they are thinner, contain a smaller percentage of coarse material, and the water table is deeper beneath the surface so that the pumping lift is greater. In addition, the area underlain by Crete is not satisfactory as an irrigation area because it is so dissected that only small areas are flat enough to be irrigated, and it is traversed by numerous deep gullies which make the construction of irrigation ditches very difficult.
Several ground-water problems occur in connection with irrigation which are not encountered under other conditions. One is the problem of subsurface drainage of the irrigation water in excess of that used readily by plants. Poor subsurface drainage will cause the soil to become waterlogged and will drown out the vegetation. Throughout the terrace area in Prairie Dog Creek Valley subsurface drainage is good. The surficial deposits consist of sandy silt which seems to allow surface water to seep readily into the soil. After the water has moved through the silty layers it reaches a layer of sand and gravel, which is thick enough and permeable enough to provide good subsurface drainage. None of the irrigation plants now in operation has experienced drainage difficulties. However, should a large area, such as the irrigation district proposed in the Almena Unit plans, be irrigated, the water table might be raised high enough to saturate the near-surface silty material. This can be foreseen and corrective measures designed before serious damage occurs if an observation well program is begun and maintained to note any abnormal rises in the water table. In order for such a program to be a success, it is essential that normal ground-water conditions in the area be known. The determination of the normal ground-water conditions in the area of the Almena Unit has been one of the primary purposes of this investigation.
Another problem affecting irrigation is the chemical character of the water used for that purpose. Water that has a concentration of sodium ions equal to 50 percent or more of the total bases (percent sodium) may cause an alkaline condition when used to irrigate certain types of soil. Table 6 shows that the percentage of sodium in 15 samples of water analyzed ranges from 1 to 30. The average for nine samples of water from the terrace and Crete deposits is 10 percent.
Chloride is another chemical constituent which may render water unfit for irrigation use. Water that has a high concentration of chloride is too salty for irrigation. Boron is also one of the dissolved constituents which might also affect the use of water for irrigation. Although the samples of ground water analyzed from this area contain relatively large amounts of some constituents, they are well within the limits for irrigation use. Surface water is generally somewhat less mineralized than ground water, so that the chemical character of the water in this area should present no problems, unless prolonged irrigation should alter the character of the soil.
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Kansas Geological Survey, Geology
Placed on web July 18, 2008; originally published Dec. 1949.
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