Geological Survey Water-Supply Paper 1475-F Ground Water at Grant Village Site Yellowstone National Park, Wyoming

Ground water in the Grant Village area originates from infiltration of precipitation through the pore spaces of the soil and underlying rocks to the zone of saturation (the zone in which water is under hydrostatic pressure). The upper surface of the zone of saturation, if not confined, is known as the water table. A saturated section of rock that will yield water to wells is known as an aquifer. If water in an aquifer is between less permeable beds, confined or artesian conditions exist and water in wells will rise above the level at which it is reached. Such conditions were found in test well 1. The piezometric surface of a confined aquifer is an imaginary surface that everywhere coincides with the static level of the water in the aquifer and, therefore, is the surface to which water in an aquifer will rise under its full pressure head.

MOVEMENT OF GROUND WATER

Ground water moves in the direction of the gradient of the water table or piezometric surface from points of recharge to points of discharge such as streams, springs, and lakes. The gradient of the water table or piezometric surface generally follows the slope of the topographic surface, but is not so irregular.

Ground water in the Grant Village area generally is moving from the topographically high area near the Continental Divide northeastward toward Yellowstone Lake. The hydraulic gradient in the area under investigation, as indicated by the altitudes of water levels in the test wells and auger holes, is about 90 feet per mile. This gradient, which is relatively steep compared to gradients in many other areas, suggests that the aquifer will not readily transmit large quantities of water to wells.

TEST-DRILLING PROGRAM

During the summer of 1959, six test holes were drilled by the percussion (cable-tool) method in the Grant Village area under a National Park Service contract with the Van Dyken Drilling Co., Bozeman, Mont. Initial plans provided for test drilling to begin in middle to late May, but adverse weather and deep snow in parts of the area required postponement of drilling until late June. The contractor began drilling test well 1 on June 23, 1959. Logs of the six test wells drilled are given in table 1.

TABLE 1.—Logs of test wells and auger holes, Grant Village area, Yellowstone National Park, Wyo.
[All material penetrated in the test wells and auger holes is of Quaternary age, with the possible exception of the welded tuff penetrated at 55 to 150 feet in test well 4, which may be of Pliocene age]

During July 7-12, 1959, 11 test holes were bored in the Grant Village area with a power auger. The locations of the 6 test wells and the 11 auger holes are shown in figure 31.

FIGURE 31.—Location of teat wells and auger holes, Grant Village area, Yellowstone National Park, Wyo. (click on image for an enlargement in a new window)

The test wells were drilled with bits of sufficient size that 6-inch I.D. (inside-diameter) casing could be installed in the holes. Casing left in place in wells was perforated opposite water-bearing strata by use of a casing perforator which cut vertical slots in the casing walls at selected intervals.

Samples of materials penetrated were taken at 5-foot intervals. The temperature of the cuttings or the water from different strata was measured, and samples of water were collected from the test wells for analysis. Brief bailing tests were made as needed to determine approximate yields of water-bearing strata. Water-level measurements were made as necessary to determine the depth to the water surface of the different, water-bearing strata reached.

TEST WELLS

Test well 1 was drilled about 100 yards east of the South Entrance road and about 0.6 mile south of the bridge over Thumb Creek. Water was first tapped at a depth of 92 feet, in a soft sandstone, under slight artesian head. The water rose in the hole to about 83 feet below the land surface. The temperature of the water at 92 feet was 120°F (fig. 30). As drilling progressed, the water temperature increased to 125°F at 100 feet and 142°F at 112.5 feet. At 200 feet, the total depth of the well, the temperature was 180°F

A brief bailing test made after completion of the hole indicated that the well was capable of producing water at a rate of about 40 gpm (gallons per minute). The temperature of the water was about 165 °F, which indicated a mixture of water from strata at various depths in the hole. Because of the high temperature of the water, the casing was removed and the hole was plugged and abandoned.

Test well 2 was drilled about 125 feet from the lakeshore about 0.1 mile south of the mouth of Thumb Creek. A very small quantity of cool water was found in a mixture of sand and clay at about 15 to 20 feet. The principal water-bearing material, a gravel containing medium- to coarse-grained sand and some silt, was reached at 60 feet.

The temperature of the water was 96° at a depth of 100 feet; at 118.5 feet, the bottom of the hole, it was about 100°F. As preliminary bailing tests indicated that the well would yield a moderate quantity of water, the casing was perforated from a depth of 70 to 118 feet.

Two aquifer tests, of 10- and 48-hour duration, were made at test well 2; results of the tests are discussed on pages 189-194. The water temperature initially was about 96°F but increased slightly during the tests. At the end of the 48-hour test, the temperature apparently had stabilized at about 100°F. The increase in temperature probably was the result of pumping action in the test well which removed some fine sand from the lower part of the aquifer; this allowed a proportionately greater amount of water to move into the well from the lower part of the aquifer in the later stages of the test. Vigorous development by earlier bailing had developed the upper part of the aquifer.

Because of the relatively large yield of test well 2, the casing was left in place. A grout seal was placed around the casing from the surface to a depth of about 10 feet, and a steel plate was welded to the top of the casing. The steel plate can be removed easily if the well is to be utilized for water supply.

Test well 3 was drilled about 100 feet from the lakeshore, 0.2 mile south of the mouth of Thumb Creek and 500 feet south of test well 2. A small quantity of water was found in angular silty sand and gravel in the interval between 18 and 37 feet. However, the material was so angular and so poorly sorted that it would yield very little water to the well. Angular silty sand and gravel was again penetrated from 55 to the bottom of the well, but this zone also was low in permeability. The well was completed at a total depth of 110 feet. A bailing test made at this depth indicated that the well would have a maximum sustained yield of about 5 gpm. The temperature of the water in the well rose from 76°F at 75 feet to 106°F at 110 feet. As water at greater depths probably would be too warm to drink, the well was plugged and abandoned.

Test well 4 was drilled about 1,000 feet south of the lakeshore, just south of the intersection of two unnamed intermittent streams and about 0.4 mile east of the South Entrance highway (fig. 31). The well was drilled through silty sand and gravel to a depth of 55 feet. A small quantity of water was obtained from the interval between 35 and 55 feet. At 55 feet a bed of welded tuff was reached, and the well was completed in the welded tuff at a depth of 150 feet. Temporary casing was installed to a depth of 58 feet below the land surface. A 20-minute bailing test obtained a yield of 2.6 gpm from the uncased section in the welded tuff at 58 to 150 feet. The temperature of the water in the well ranged from 58°F at 40 feet to 99°F at 150 feet. Because the yield of the well was small, the casing was pulled and the hole was plugged and abandoned.

Test well 5 was drilled about 100 feet from the lakeshore, about 400 feet east of a bay formed by the inlet of an intermittent stream, and about 0.6 mile east of the South Entrance highway (fig. 31). A moderate quantity of water was found in sand at a depth of 45 to 56 feet. A well screen was placed at the interval between 45 and 50 feet and a brief bailing test was made. The well yielded about 10 gpm of water at a temperature of 44°F. The screen was removed and the well was drilled and cased to 120 feet, at which depth the well was again tested by bailing. As the well yielded very little water at this depth, it was drilled and cased to a total depth of 200 feet. The casing was not perforated. The water temperature gradually increased from 44°F at 50 feet to 81°F at 200 feet. The well yielded very little water during the final bailing test, and therefore the casing was removed and the well was abandoned.

Test well 6 was drilled about 150 feet from the lakeshore, about 100 feet west of the inlet of an unnamed intermittent stream, about 0.6 mile south-southeast of test well 5, and about 1.0 mile east of the South Entrance highway (fig. 31). The well was drilled to a depth of 75 feet and was cased to a depth of 55 feet. The interval from 55 to 75 feet was bailed at 45 to 50 gpm for 15 minutes, but the yield diminished on further bailing. Therefore, the well was deepened and cased to a total depth of 120 feet. The casing was perforated from 77 to 111 feet, the well was developed by bailing and surging, and a 3-hour bailing test was made. The test indicated that the well should have a sustained yield of at least 15 gpm. The water temperature was about 50°F.

The casing for test well 6 was sealed in place with a cement grout and a steel plate was welded to the top of the casing. If the well is later used for water supply, the yield might be increased appreciably by perforating the upper part of the casing and further developing the well.

AUGER HOLES

The auger holes, which were 4 inches in diameter, ranged from 65 to 103 feet in depth. Some of the holes were bored to provide stratigraphic information as a guide in selecting favorable sites for the test wells; others were bored near test wells 2 and 3 and were equipped with temporary casing in order that they might be used for water-level observations during aquifer tests. Logs for the auger holes are given in table 1.

Auger hole 1, which was drilled near the lake just north of Thumb Creek, was cased with 1-1/4-inch pipe and completed as an observation well at a depth of 103 feet. The water temperature at the bottom of the hole was 105°F. The water level in the hole was about 32 feet below the land surface. The fine-grained character of the sediments that were penetrated indicated that only small quantities of water could be obtained at this site.

Auger hole 2 was bored about 100 yards east of the South Entrance road and about 1.0 mile south of the bridge over Thumb Creek, on a small valley flat adjacent to a small stream. The temperature of the water in the hole increased rapidly with depth (fig. 30); it increased from 90°F at 25 feet to 162°F at 40 feet, then declined to 110°F at 45 feet. The temperature then increased at a fairly uniform rate to 188°F at 62 feet, and to 202°F at 65 feet, the bottom of the hole, where the auger reached a well-cemented conglomerate which it could not penetrate. Steam under low pressure was emanating from the hole upon completion of drilling. Upon withdrawal of the auger the hole caved, thus shutting off the steam.

The abnormally high temperature gradient in the auger hole probably indicates the presence of a cooling mass of intrusive rock at shallow depth in the vicinity of the test hole, or the upward movement of superheated water or steam from a considerable depth through fractures in the rock. The abnormal temperature increase at a depth of 40 feet may be due to lateral movement of hot water through the obsidian sand which was penetrated at that depth.

Auger holes 3 through 8 were bored in the immediate vicinity of test wells 2 and 3. Auger hole 3 was drilled to a depth of 70 feet; the others were completed at 103 feet. Each of these holes was cased temporarily with 1- or 1-1/4-inch pipe having a screen attached to the bottom. Auger holes 3 through 6 were used as observation wells during the aquifer tests at test well 2. Auger holes 7 and 8 provided information regarding stratigraphic conditions near test well 3.

Auger holes 9 and 10 were completed at depths of 103 and 97 feet, respectively. Both holes penetrated a thick sequence of permeable sand, but much of the section was above the water table. This information was used in the selection of the site for test well 4.

Auger hole 11 was completed at a depth of 103 feet. Most of the material penetrated consisted of clay and silt of low permeability, indicating that little water could be obtained from a well in that locality.

AQUIFER TESTS

As a part of the test-drilling program, a series of aquifer tests by pumping and bailing were made to evaluate the feasibility of the establishment of a well, or series of wells, that would furnish a sufficient water supply at the proposed new development of Grant Village.

Two aquifer tests were made at test well 2. The first test, of 10 hours' duration, was made to ascertain whether the aquifer at that location would yield water for an appreciable period and to determine the most suitable pumping rate for the second test.

In the first of the aquifer tests, made on July 11, 1959, test well 2 was pumped at a nearly constant rate of 45 gpm. Changes in water levels were measured in observation wells A—6 and A—3, which were north of the pumped well, and observation wells A—4 and A—5 south of the pumped well, as shown in figure 31. The depths of the observation wells were about 103 feet, exception well A—3 which was about 70 feet deep. Well A—3 had 1-1/4-inch casing; the rest had 1-inch casing. The pumped well, test well 2, was about 118 feet deep and was cased to the bottom with 6-inch steel casing, which was perforated from 70 to 118 feet. Pumping was discontinued after a 10-hour period at which time the drawdown of the water levels in observation wells A—3, A—4, A—5, and A—6 were 4.99, 5.00, 3.55, and 10.05 feet, respectively. Drawdown readings were erratic in wells A—3 and A—4. The wells had been flushed out before the start of the test but the water levels had not yet reached a static position. The recovery of the water levels in all wells was measured for 10 hours.

A second aquifer test was made on July 28, 1959, at these same wells. Test well 2 was pumped at a rate of 40 gpm for the first 10 minutes and at a nearly uniform rate of 36 gpm for the balance of the 48-hour period. The drawdown of the water level was observed in the pumped well and in observation wells A—3, A—4, A—5, and A—6. The drawdowns in the observation wells at the end of the period were 8.86, 6.50, 3.33, and 6.60 feet, respectively; that in the pumped well was 71.4 feet. The rate at which the water level rose after pumping ceased was observed for about 15 hours. The water rose to within 0.52 foot of the static level in the pumped well and 0.28, 0.59, 0.33, and 0.57 foot, respectively, in observation wells A—3, A—4, A—5, and A—6. Normally, the observed period of recovery should be equivalent to the drawdown period, but in this test the measurements indicated that the recovery data were similar to the drawdown data, so the period of observation was shortened. The rate of discharge of test well 2 was measured by filling a 55-gallon barrel and timing the operation with a stopwatch. The water was discharged into Yellowstone Lake. Depths to water in the observation wells were measured manually with a steel tape; the water levels in the pumped wells were measured with an electric measuring device. All measurements were recorded to the nearest 0.01 foot. Graphs of the drawdown and recovery of the water level at each site during the aquifer test are shown in figure 32.

FIGURE 32.—Fluctation of water levels in observation wells and pumped well during aquifer test of July 28-31, 1959. (click on image for an enlargement in a new window)

A bailing test was made in test well 6 on August 27, 1959. The well was bailed for 3 hours at a nearly constant rate of 18 gpm, and the water level was measured periodically throughout the test. The drawdown of the water level was about 80 feet at the end of the test. After bailing ceased, recovery of the water level was measured for 4 hours and one measurement was made 8 hours 8 minutes after bailing ceased, at which time the water had risen to within 0.13 foot of the original level. Fluctuation of the water level during the test is shown in figure 33.

FIGURE 33.—Fluctuation of water level in test well 6 during hailing test of August 27, 1959.

ANALYSIS OF AQUIFER TESTS

Withdrawal of water from any permeable material causes the water level to decline near the point of withdrawal, and the shape of the water table or piezometric surface becomes an inverted cone whose apex is at the point of withdrawal. In some cases the shape of the cone is modified or elongated because of variations in the hydrologic characteristics and areal extent of the aquifer. The overall size, shape, and rate of growth of the cone of depression caused by pumping a well depend on the rate and duration of pumping, the coefficients of transmissibility and storage of the aquifer, the presence of hydrologic and geologic boundaries, and recharge to or leakage from the aquifer. The lowering of the water level at any point within the cone of depression is termed drawdown and depends on the above variables and the distance from the point of withdrawal.

The coefficient of transmissibility, as used by the Geological Survey, may be expressed as the number of gallons of water per day, at the prevailing water temperature, transmitted through a cross section of the aquifer 1 mile wide under a hydraulic gradient of 1 foot per mile. The coefficient of transmissibility is expressed in gallons per day per foot.

The coefficient of storage of an aquifer is the volume of water it releases from or takes into storage per unit surface area of the aquifer per unit change in the component of head normal to that surface. Under water-table conditions, this quantity is virtually equal to the specific yield, which is related to the quantity of water that a unit saturated volume of the aquifer will yield by gravity drainage. For artesian conditions, the storage coefficient is related to the unit expansion of the water and unit compression of the aquifer under a unit reduction of pressure. The storage coefficient is a ratio of volumes, and is dimensionless.

Theis (1935, p. 519-524) developed a method for determining the coefficients of transmissibility and storage by observation of the change in drawdown with time in a pumped well and in nearby wells. In developing his method, Theis assumed that the aquifer is homogeneous, isotropic, and of infinite areal extent, that the coefficient of transmissibility remains constant at all places and all times, and that water is released from storage instantaneously with a decline in head.

The aquifer at Grant Village does not fit the assumption of infinite areal extent, as the intersection of the aquifer with the lake forms a hydrologic boundary. This boundary limits the expansion of the cone of depression in the direction of the lake, and in turn the shape and extent of the cone in all directions are altered. Stallman (1952) developed a modification of Theis' method which could be used to analyze data from wells in the vicinity of a boundary to determine the coefficients of transmissibility and storage and the effective distance from the well to the boundary. Stallman's method was used in this report.

The analysis of the aquifer-test data gathered near test well 2 is summarized in the following table.

Summary of aquifer-test data, test well 2 and vicinity

This test indicates that the coefficient of transmissibility is fairly constant for at least a few hundred feet north of test well 2, and that it increases immediately to the south of the test well. However, information gained from auger hole 11, about 1,000 feet west of the test well (fig. 31), and test well 3, about 500 feet south of the well, indicates that the coefficient of transmissibility is very low in those areas, and that it changes markedly over relatively short distances.

The computed distance from the pumped well to the lake is only about 100 feet, whereas the measured distance is 125 feet. This difference probably indicates that the coefficient of transmissibility increases significantly toward the lake, and that water moves more freely through that section of the aquifer than it does in the surrounding area described by the cone of depression.

The test indicates that the drawdown in the well is influenced to a marked degree by the nearby lake, and that the water pumped was derived from two sources—ground-water flow diverted from the lake, and ground-water storage.

Theis (1941, p. 734-738) developed a method whereby the amount of ground-water flow diverted from a stream or lake to a nearby pumped well could be determined at any time if the coefficients of transmissibility and storage and the effective distance from the well to the body of water were known. This method was used to determine the quantity of ground-water flow diverted from the lake by the well during this test. The data indicate that after a few days of pumping more than 95 percent of the water pumped would be derived from flow previously directed toward the lake.

Other calculations, based on several assumptions, were made to determine the time required for water now in the lake to reach the well, if the well were pumped continuously at a rate of 40 gpm. These calculations indicate that the water would reach the well after a period of about 50 days, using the effective distance of separation of 100 feet determined from the aquifer test, or about 80 days, using the measured distance of 125 feet. These figures indicate the approximate range in time of pumping before the water in the well would cool appreciably.

Test well 2 could be used to supply 35 to 40 gpm of water at a temperature of 100°F and a drawdown of 70 to 80 feet. Any decrease in temperature would occur only after a few days to a few weeks of pumping, and pumping for several months might be required before appreciably cooler water could be obtained from the well.

An aquifer test made by bailing test well 6 indicated a coefficient of transmissibility at that site of about 50 gpd per foot. This is very low, but because of the proximity of the lake, the well seems to be capable of producing about 15 gpm at a drawdown of about 80 feet. This water has a temperature of about 45°F.