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Ground Water, continued
Ground-water Recharge
Recharge to the ground-water reservoir at any particular place may occur in several ways, but the ultimate source of all ground water of quality suitable for use is precipitation in an area or in nearby areas. Of the total precipitation, a part runs off directly across the land surface to streams, a part is dissipated by evaporation from the land surface or from vegetation, a part is transpired by plants from the zone of soil moisture, and a part percolates downward to the zone of saturation. In addition to the direct infiltration of precipitation, the ground-water reservoir may be recharged locally by the infiltration of water from stream channels above the water table or by subsurface inflow from nearby areas. Under favorable conditions recharge may be effected artificially by such means as injection through wells, infiltration from recharge pits, or from water spreading, or by inducing the infiltration of river water into the reservoir by lowering the water table below the level of an adjacent stream.
Recharge from Precipitation
Precipitation is the source of most of the ground-water recharge in Sedgwick County, but the quantity of recharge at a particular site is strongly affected by local conditions. The amount and distribution of precipitation, the depth to the water table below the land surface. the slope of the land surface, the physical character and composition of the soil and subsoil, the condition and moisture content of the soil and subsoil, the type and density of vegetation, and the presence of soil cracks, roots, animal burrows, or other openings in the soil have a marked effect on recharge.
In the Arkansas Valley, conditions for recharge are very favorable and reasonably uniform, although local variations do exist. In areas of concentrated pumping, such as the Wichita well field, recharge conditions have probably changed in response to the changed conditions in the ground-water reservoir. Williams and Lohman (1949) estimated the recharge in the well-field area to be about 20 percent of the precipitation, or an average rate of about 320 acre-feet (1 acre-foot equals 325,850 gallons) per year per square mile. Although the amount of recharge varies from year to year in response to precipitation, later work in the area (Stramel, 1956) has shown these estimates to be of the right order of magnitude for the effective recharge in the well-field area. Table 4 is a tabulation of the quantity of recharge that has taken place for selected periods since the development of the well field. These computations were obtained from pumpage records and maps of water-level change. Specific yield was assumed to be 20 percent for the dewatered sediments. Similar recharge conditions would probably apply to much of the Arkansas Valley under similar conditions of development. In much of the Arkansas Valley within the County withdrawals of water by pumping are not large, and the water table is within a few feet of the land surface. In such areas there is little or no recharge. In areas where bedrock is at the surface, conditions for recharge from precipitation are much less favorable, but recharge can occur through fractures and solution openings developed by weathering of these rocks. Where loess overlies bedrock or permeable sand and gravel, the loess retards the downward percolation of precipitation but it is believed to be more receptive to infiltration than the bedrock. The porosity of the loess is very high, and because it is very fine grained its permeability is low. The loess probably retains most of the infiltrated water within the soil zone and transmits water to the zone of saturation only during excessively wet periods.
Table 4—Summary of recharge in the Wichita Well Field. Data from files of the Water Department, Wichita, Kansas.
| Period from August 30, 1940 to ... | Area affected by pumping, acres | Net reduction in storage, acre-feet | Quantity of water pumped, acre-feet | Quantity of water derived from recharge, acre-feet | Percent of total water pumped derived from recharge |
|---|---|---|---|---|---|
| January 1, 1944 | 32,300 | 33,900 | 51,900 | 17,700 | 34 |
| January 1, 1948 | 34,700 | 49,800 | 135,800 | 86,000 | 63 |
| January 1, 1952 | 32.400 | 50,200 | 230,000 | 179,800 | 79 |
| January 1, 1955 | 66,000 | 111,000 | 330,300 | 219,000 | 67 |
| January 1, 1958 | 47,400 | 115,000 | 409,000 | 294,000 | 72 |
| January 1, 1960 | 57,000 | 107,000 | 462,000 | 355,000 | 77 |
Recharge from Streams
The Arkansas, Little Arkansas, and Ninnescah rivers and the lower reaches of their larger tributaries are all gaining or effluent streams within Sedgwick County and derive a part of their flow from the discharge of ground water. The Arkansas River above Wichita is nearly in equilibrium with the ground water reservoir, neither receiving nor contributing water to it. The gradient of the river in this reach is nearly the same as and in the same direction as the slope of the water table.
During periods of flood when the streams run bankfull or spill onto their flood plains, some of the surface water enters the ground water reservoir. When the streams return to normal stages, the ground water stored in the alluvium adjacent to the streams is returned to them, and ground-water levels return to a normal stage.
Pumping from the Wichita well field has reduced the water-table gradient toward the Little Arkansas River but has not lowered water levels sufficiently to cause the river water to recharge the ground-water reservoir.
The upper reaches of small tributary streams in the County lie above the water table generally and may contribute some recharge to the underlying rocks. Parts of the courses of these streams are near the level of the uplands and carry water only after periods of precipitation, so that the opportunity for recharge is of short duration, and the quantity of recharge is probably insignificant.
Recharge from Subsurface Inflow
Ground water moving into an area in the direction of the water-table gradient from an adjacent area is commonly referred to as "inflow." The water-table contours shown on Plate I indicate that some ground water derived from precipitation on adjacent areas moves into Sedgwick County. The principal areas where inflow occurs is along the north half of the west county line. Some minor contributions occur adjacent to the North and South Fork Ninnescah rivers where they enter the County. The water-table gradient seems to parallel the north county line toward the Wichita well field and probably very little inflow takes place in this area. The water-table contours along the southern edge of the County show ground water to be flowing from the County in this area, and it is believed that the inflow and outflow are very nearly the same and that the net effect of inflow from adjacent areas is negligible. No attempt was made to compute the net quantity of inflow, but compared to recharge from precipitation, it is probably insignificant.
Artificial Recharge
Artificial recharge, whereby water is added to the zone of saturation by other than natural means, is one method of storing surplus surface water for later use and alleviating the adverse effects on the ground-water reservoir of concentrated pumping. The principal methods of artificially recharging a ground-water reservoir are water spreading or flooding, recharge through pits and other excavations, injection of water through wells, and induced infiltration of river water by pumping. Each method has advantages and disadvantages dependent on local conditions of climate, surface and subsurface geology, and the quality of the water available for recharging. With other conditions being favorable for artificial recharge, the two prime requirements for successfully recharging would be a water supply suitable in quantity and quality and at the right place for the purpose, and storage space in the ground-water reservoir adequate for conducting the added water away from the recharge site.
Artificial recharge is not intentionally practiced in the Arkansas Valley although geologic conditions are favorable in much of the area for some form of artificial recharge. The ground-water reservoir is nearly full in most of the area where artificial recharge might be practiced, and little storage space is available in the reservoir for additional water with the exception of the Wichita well field. Pilot studies of several methods of artificial recharge have been conducted by G. J. Stramel, hydrologist of the Wichita Water Department, to evaluate the feasibility of some form of artificial recharge in the city well field in Harvey and Sedgwick counties. The data collected during these studies are in the files of the Wichita Water Department and will not be presented here, but a brief summary of these studies and the pertinent results follows, as they are believed applicable to much of the Arkansas Valley in Sedgwick County.
The feasibility of recharging surface water to the ground-water reservoir through a well was studied, using an idle water-supply well in the SW SW SW sec. 26, T 24 S, R 2 W, in Harvey County. The water supply was from a small intermittent creek passing near the well. A conduit containing a trash screen and gravel filter was installed between the creek and the well so that water would flow into the well by gravity when available in the creek. The quantity of flow into the well was measured by a recording gage and the water level around the well was measured in nearby observation wells. The intake rate showed a steady decline although several million gallons of water could be injected before the well became badly plugged. The well could be cleaned by pumping, and investigation of the discharged water showed a high content of clay-size particles in the initial discharge from the well, indicating that the gravel filter was not satisfactorily removing the smaller particles of suspended sediment of the creek water. It was concluded that to operate the recharging well satisfactorily for a reasonably long period, the water injected would have to be of a biological, physical, and chemical quality that would have permitted direct use of the water.
A recharge pit was constructed near Kisiwa Creek in T 24 S, R 2 W, Harvey County, to utilize the intermittent flows of that creek as a source of water to study the feasibility of using untreated surface flows as a recharge source. The pit was excavated to a depth that exposed the sand and gravel of the aquifer, and an inlet structure suitable for measuring inflow to the pit was provided. The configuration of the water table under and near the pit was measured in observation wells. The initial outflow from the pit was quite rapid but the bottom and sides of the pit plunged within a short time. The bulk of the plugging material proved to be particle, of clay size that could not be easily removed by gravity settling alone but would require flocculation for removal. It was concluded that for successful operation of this recharge pit some pre-treatment of the water to remove most of the suspended sediment would be necessary.
Two experiments to determine the feasibility of inducing water from the Little Arkansas River to recharge the underlying aquifer were conducted at a site in SW sec. 23, T 25 S, R 1 W in Sedgwick County. Geologic conditions are more favorable for this type of recharge along the Arkansas River, but the quality of the river water is unsatisfactory for municipal supply most of the time.
The first experiment involved the installation of a horizontal well screen in the stream channel with a vertical riser pipe at one end for installation of pumping equipment. The well screen was 50 feet in length, 16 inches in diameter, and was buried about 8 feet below the bottom of the river bed. The excavation was back-filled with coarse sand and gravel as a filter medium. Attempts to develop the well screen by conventional methods encountered serious difficulties, and the yield of the well, about 400 gallons per minute, was disappointing. Inability to adequately develop the well screen and probably silting of the stream bed over the screen indicated that this type of installation was not satisfactory under the conditions at this well site.
The second experiment involved installation of a vertical well adjacent to the stream channel to induce the flow of water from the river into it. The well, about 75 feet from the stream channel, was 16 inches in diameter with a 3-inch gravel pack surrounding a 20 foot length of screen. The well extended 34 feet below the river bed. Observation wells were installed on both sides of the river to determine the configuration of the water table around the well. The well was pumped continuously for 32 days at an average rate of about 800 gallons per minute. The pumping test demonstrated that significant infiltration of river water would take place, but that even after 32 days of pumping not all of the water was being derived from the river.
No economic feasibility studies were made in conjunction with the artificial recharge studies, but of the three methods investigated, induced infiltration of river water is believed to be the most practical at the present time.
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Kansas Geological Survey, Geology
Placed on web Nov. 1998; originally published Dec. 1965.
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