The following discussion of the occurrence of ground water has been adapted from Meinzer (1923) and the reader is referred to his report for a more detailed discussion. A general discussion of the principles of ground-water occurrence with special reference to Kansas has been presented by Moore and others (1940).
The rocks that make up the crust of the earth generally are not solid, but have many openings, called voids or interstices, which may contain air, natural gas, oil, or water. The many different kinds of rocks differ greatly in the number, size, shape, and arrangement of their interstices; therefore, the occurrence of water in any region is determined by the geology of the region.
The interstices or voids in rocks range in size from microscopic openings to the huge caverns found in some limestones. The porosity of a rock is expressed quantitatively as the percentage of the total volume of the rock that is occupied by interstices or that is not occupied by solid rock material. Uncemented gravel deposits having a uniform grain size have a high porosity, whereas deposits made up of a mixture of sand, clay, and gravel may have a very low porosity because the smaller particles occupy the space between adjacent large particles. Relatively soluble rock such as limestone, though originally dense, may become cavernous as a result of the removal of part of its substance through the solvent action of percolating water. Hard, brittle rock may acquire large interstices through fracturing that results from shrinkage or deformation of the rocks or through other agencies.
The permeability of a rock is its capacity for transmitting water under pressure and is measured by the rate at which the rock will transmit water through a given cross section under a given difference of head per unit of distance. A rock containing many very small interstices may be porous, but not very permeable; whereas, a coarser-grained rock may have a low porosity but will generally be much more permeable. The specific yield of a rock or soil, with respect to water, is the ratio of (1) the volume of water it will yield by gravity after being saturated to (2) its own volume. This ratio is stated as a percentage. The specific retention of a rock is the ratio of (1) the volume of water it will retain against the pull of gravity after being saturated to (2) its own volume.
Figure 6--Diagram showing divisions of subsurface water. (From O.E. Meinzer)
The permeable rocks that lie below a certain level are generally saturated with water under hydrostatic pressure. These saturated rocks are said to be in the zone of saturation (Fig. 6). The zone of saturation ordinarily extends down to a depth much greater than is reached by modern drilling methods. The term ground water is used to designate that part of the subsurface water within the zone of saturation. The upper surface of the zone of saturation when not formed by an impermeable body is called the ground-water table, or simply, the water table. In most places there is only one zone of saturation, but in certain localities the water may be hindered in its downward course by an impermeable or nearly impermeable bed to such an extent that it forms an upper zone of saturation, or perched water body, that is not associated with the lower zone of saturation.
All rocks above the water table are in the zone of aeration, which ordinarily consists of three parts; the belt of soil water at the surface; the intermediate belt, and the capillary fringe.
The belt of soil water contains water held by molecular attraction; it lies just below the land surface, and extends down to the maximum depth to which evaporation and plant action are effective. The water in the belt of soil water is not available to wells, but is of the utmost importance to agriculture. Before any water can percolate downward to the water table through this belt, the amount of water present must exceed that which will be held by adhesion. The thickness of the belt of soil water is determined by the texture of the rock or soil, and by the character of the vegetation.
The space between the lower limit of the belt in which water can be withdrawn by plant action and the upper limit of the capillary fringe forms an intermediate belt that is thick where the depth to the water table is great and may be absent where the water table is at or near the surface. In this belt the interstices in the rocks contain some water held by molecular attraction but also may contain appreciable quantities of water that is moving downward from the belt of soil moisture to the water table.
The capillary fringe lies directly above the water table and is formed by water held above the zone of saturation by capillary force. The water in the capillary fringe is not available to wells, which must be deepened to the zone of saturation before water will enter them. The capillary fringe may be very thin in coarse-grained sediments, in which the capillary action is negligible, or it may be several feet thick in fine-grained sediments.
The water table has been defined as the upper surface of the zone of saturation. The zone of saturation is, in a sense, a groundwater reservoir just as a lake or river is a surface-water reservoir. The water table is not a static, level surface; generally it is a sloping surface having many irregularities and is constantly changing. There are many causes of irregularities and fluctuations in the water table.
Ground water moves in the direction of slope of the water table, but this movement is very slow because of the frictional resistance offered by the small interstices through which the water must pass. In an area where conditions are suitable for rapid recharge, water may percolate down to the water table faster than it can spread laterally, so a mound or ridge is formed in the water table. Conversely, if water is withdrawn from the the zone of saturation faster than it can flow in laterally, the water table is lowered locally, forming a cone or trough. The permeability of the water-bearing material has a significant effect upon the slope of the water table. For a given rate of flow the slope of the water table must be much steeper in a fine-grained deposit having a low permeability than in a coarse-grained, highly permeable deposit.
The configuration of the water table in Rawlins County is shown on Plate 1 by means of water-table contours. Each contour line connects points of equal altitude of the water table. The ground water moves in the direction of greatest slope of the water table; that is, at right angles to the water-table contours. The configuration of the water-table surface resembles the bedrock configuration (Fig. 7) and to a certain extent the topography.
Figure 7--Map showing location of test holes, and configuration of the top of the Pierre shale by means of contours.
The water table in Rawlins County has an average slope of about 14 feet to the mile. The general direction of movement of ground water in the county ranges from almost east to north-east. Near the valley walls of the lower reaches of Beaver Creek the water table has a very steep slope and the water moves toward Beaver Creek almost at right angles to the normal direction of movement in the county. In T. 1 S., R. 36 W., the water table is discontinuous and is not shown on Plate 1.
The amount of water in storage in the zone of saturation does not remain constant, but fluctuates with the precipitation and rate of withdrawal. Water is continually being discharged from the ground-water reservoir by several processes. The water table subsides in times of drought and rises during times of precipitation.
All ground water in the Tertiary and Quaternary deposits of Rawlins County originally fell as rain or snow within the county, or in the adjacent area to the west or south. Water reaches the zone of saturation in Rawlins County by direct recharge from local precipitation, by recharge from streams and ponds, and by subsurface movement from outside the area.
Recharge from Local Precipitation
The normal annual precipitation in Rawlins County is about 18.5 inches, but only a small part of this amount enters the zone of saturation as recharge to the ground-water reservoir. A large part of the precipitation is evaporated and transpired and a small part leaves the county as surface runoff.
The type of material above the water table, the depth to the water table, the slope of the land surface, and the weather all have an effect upon the amount of recharge the ground-water reservoir will receive from local precipitation.
Much of Rawlins County is mantled by thick massive silt deposits of relatively low permeability, and recharge is much less in these areas than in areas where sand and gravel are exposed at the surface. In general, the areas mantled by loess are fairly flat and generally have many undrained depressions in which water collects during periods of moderate to heavy precipitation. It is in these parts of the county that a large part of the precipitation is returned to the atmosphere as water vapor by the processes of evaporation and transpiration.
Sand and gravel of the Ogallala formation are relatively permeable, and areas in which this formation crops out would be favorable for recharge except that the slope of the surface is generally steep and runoff is greater.
The valleys of the major streams receive considerable recharge because the material above the water table consists principally of sand and gravel.
A large part of the precipitation falls during the period from May through September when the vegetation is growing, temperatures and wind velocities are high, and humidity is low; consequently, the rate of evaporation and transpiration is high. Runoff resulting from torrential rains is much greater than that resulting from gentle rains, and runoff is also greater during winter rains that fall when the ground is frozen and impervious to infiltration.
Recharge from Streams and Ponds
A stream whose surface is higher than the water table will contribute water to the zone of saturation and is said to be an influent stream. A stream whose surface is lower than the water table will receive water from the zone of saturation and is an effluent stream (Fig. 8).
Figure 8--Diagrammatic sections showing influent and effluent streams.
The stream surface of Beaver Creek is below the water table and receives water from the ground-water reservoir, except after heavy precipitation when the stream flow is increased and water enters the alluvium along the stream.
The western parts of North Beaver Creek and Little Beaver Creek in Rawlins County are ephemeral (intermittent) streams; during periods of stream flow, water percolates downward from the stream beds to the water table.
Middle Fork Sappa Creek and South Fork Sappa Creek are very nearly at the same level as the water table and are probably alternately influent and effluent because of fluctuations in the surface of the streams and water table. North Fork Sappa Creek is an intermittent stream and is influent during periods of flow.
Most ponds and undrained depressions in Rawlins County are in the relatively impervious silt of the Sanborn formation and probably do not appreciably recharge the ground-water reservoir.
Recharge from Outside Areas
The slope of the water table (Pl. 1), the thickness of saturated material (Fig. 9), and the geologic cross sections (Fig. 4 and 5) indicate that much water enters the county by subsurface movement from the area to the west. Some water enters Rawlins County by subsurface movement from the area to the south. Very little water enters the county by subsurface flow along the west side of T. 1 S., R. 36 W., where the water table is discontinuous or absent.
Figure 9--Map showing the thickness of saturated Tertiary and Quaternary deposits in Rawlins County.
Ground-water discharge is the removal, by any method, of water from the zone of saturation. Ground water is discharged in Rawlins County by transpiration and evaporation, by seepage into streams, by subsurface movement into adjacent areas, and by wells.
Discharge by Transpiration and Evaporation
Transpiration is the process by which water is taken into the root system of plants directly from the zone of saturation, or from the capillary fringe just above it, and discharged into the atmosphere. The depth from which plants will lift the ground water differs with plant species and types of soil. Ordinary grasses and field crops will not send their roots more than a few feet in the search for water, but alfalfa, some trees, and certain desert plants are known to draw water from much greater depths. The water table along most of the major valleys in Rawlins County is within easy reach of such deep-rooting plants, and much water is discharged from the zone of saturation in this way.
Discharge of ground water by evaporation can take place only where the water table is within a few feet of the surface and in Rawlins County is limited to areas along streams.
That considerable ground water is discharged in Rawlins County by transpiration and evaporation is illustrated by the significant rise in the water table in valley areas in the fall when vegetation becomes dormant and evaporation decreases. The rise in the water table may be several feet although there may be no recharge from precipitation.
Discharge by Springs and Seeps
Discharge of ground water in Rawlins County by springs is mostly in Beaver Creek valley and the small valleys tributary to Beaver Creek. Springs occur at the contact between the Pierre shale and the Ogallala formation. Water moving laterally at the base of the Ogallala formation on the top of the impermeable Pierre shale flows or seeps out at the surface where the top of the Pierre shale is exposed. Beaver Creek, which is an effluent stream, has many inconspicuous seeps along its entire course and receives considerable water from the ground-water reservoir.
Discharge to Areas Outside the County
The water-table contours on Plate 1 indicate that water leaves the county by subsurface flow along the east and north sides of the county. The map indicating thickness of saturated material (Fig. 9), however, shows very little saturated material along the north edge of the county, and only about one-third as much saturated material on the east side as there is on the west side of the county. The slope of the water table is about the same on the inflow and outflow sides of the county, and assuming that the permeability is uniform on the inflow and outflow sides of the county, much less water leaves the county by subsurface flow than enters the county by the same process. The water table in Rawlins County is in a state of approximate equilibrium, and as recharge from precipitation within the area is equal to or greater than the amount discharged by pumpage from wells, the reduction in the amount of the subsurface movement of ground water is probably due to transpiration and evaporation and discharge of ground water into streams.
Discharge by Wells
Most supplies of domestic, stock, municipal, railroad, and irrigation water in Rawlins County are derived from wells. Although wells are the most obvious method of ground-water discharge, the quantity withdrawn by this method is not large when compared with the amount discharged by other means. The total quantity of water pumped annually from wells is not accurately known, but is probably about equal to 0.03 inch over the entire area of the county.
When a well is at rest, the pressure of the water outside the well is equal to the pressure of the water inside the well. When water is removed from the well by pumping, the resulting drawdown or lowering of the water level produces a differential in head or pressure and water flows into the well. When water is being discharged from a well, the water table is lowered in an area around the well to form a depression in the water table that somewhat resembles an inverted cone. This depressed area is known as the cone of depression or the cone of influence. As the pumping rate of the well is increased, the drawdown becomes greater. When a well is first pumped, the water level will fall very rapidly; but as pumping is continued, the drawdown increases at a diminishing rate. When pumping is stopped, the recovery is rapid at first, but gradually tapers off and may continue for many hours or for several days.
The capacity of a well is the rate at which it will yield water continuously after the water stored in the well has been removed. The capacity depends upon how much the water level can be lowered, the thickness and permeability of the water-bearing bed, and the construction and condition of the well. The capacity of a well generally is expressed in gallons a minute.
The specific capacity of a well is the rate of yield per unit of drawdown and is expressed in gallons a minute per foot of drawdown. If a well yields 50 gallons of water per minute, and has a drawdown of 10 feet when pumped at that rate, the specific capacity of the well is 5 gallons a minute per foot of drawdown. In testing the specific capacity of a well, pumping is continued until the water level remains approximately stationary.
Kansas Geological Survey, Geology
Placed on web Nov. 17, 2008; originally published Dec. 1956.
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