Nevada Water Science Center


Aquifer Tests

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Phil Gardner
Groundwater Specialist
Phone: (775) 887-7664
Email:pgardner@usgs.gov

 

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Nevada Test Site, UE-20f

Primary Investigator: Robert Graves

Well Data

USGS Site ID
Local Name Altitude Uppermost
Opening
Lowermost
Opening
Primary Aquifer Transmissivity
(ft2/d)
371617116291701 UE-20f 6116.3 4456 13686 VOLCANIC ROCKS 30

 

Aquifer Tests

All Aquifer Test Files (zip)

UE-20f

Aquifer Test (pdf) || Groundwater levels (NWISweb)

Introduction

Numerous aquifer tests have been conducted in and around the Nevada Test Site. Many of these tests have been completed in a fractured rock medium. Methods used to analyze these aquifer tests have included the Theis and Cooper-Jacob solutions. Although both methods are used to estimate aquifer characteristics in fracture media, the results may be qualified because both methods were developed for porous rock media. Recently, GeoTrans Inc., working in cooperation with the U.S. Department of Energy (DOE), evaluated time/drawdown data collected in wells drilled for DOE in the Oasis Valley area (ER-EC wells, completed in fractured volcanic rock) using a fractured-rock, double-porosity model (Moench, 1984). Based on this evaluation, it was thought that analyzing aquifer-test results from these wells with a dual-porosity solution would yield a better transmissivity estimate in these wells. Subsequently, individuals from GeoTrans Inc. identified approximately 62 wells in the vicinity of the Nevada Test Site with aquifer test data that could potentially be reevaluated with a fractured-rock, double-porosity model. Transmissivity estimates from these aquifer tests will support ground-water flow models being developed for DOE.

The U.S. Geological Survey (USGS) proposed to DOE to work in cooperation with GeoTrans Inc. to review these aquifer tests for the availability of aquifer-test data that might be suitable for reevaluation. Well UE-20f was one of the wells selected by the USGS for reevaluation. Transmissivity in well UE-20f has been estimated to be 130 ft2/d by Blankennagel and Weir (1973, p. B12, table 3), from an aquifer test conducted on August 9-11, 1964. The aquifer-test data from this test were reanalyzed using the Cooper-Jacob solution (Cooper and Jacob, 1946) and Moench's dual-porosity spherical-shaped block and slab-shaped block solutions (Moench, 1984). Transmissivity estimates from each solution were compared.

Test Description

Well UE-20f is located in Area 20 of the Nevada Test Site (fig. 1). On August 9, 1964, at 8:20 am (Pacific Daylight Savings Time, PDT) the USGS began a single-well aquifer test on well UE-20f which lasted approximately 47 hours (pump off at 7:00 am, PDT, on August 11, 1964) (Blankennagel and Weir, 1965, p. 21). Average discharge during the test was 100 gallons per minute.

Blankennagel and Weir, (1965, p. 22, footnote a/) reported that prior to the aquifer test, well UE-20f was pumped on August 6-7, 1964, for approximately 885 minutes. The static water level in the hole before pumping was 1,834.45 feet below land surface. The water level on August 9, 1964, at 08:00 am was 1,867.20 feet below the measuring point. The pump was started at 08:10 am but stopped after 7 minutes. Pumping was resumed at 08:20 am. The static water level used to determine drawdown after the pump was restarted at 08:20 am was 1,879 feet below land surface (Blankennagel and Weir, 1965, p. 21, table 8). No adjustments to the drawdown data due to recovery, barometric, tidal, or temperature effects were made.

On page 5, Blankennagel and Weir (1965) reported that:

"Water levels were measured with a deep-well electrical line capable of detecting relative changes in water level as small as 0.02 foot. The static-level measurements have not been corrected to a steel tape secondary standard and should not be used for water-level contouring.

A Reda submersible pump was used in the test on hole UE-20f. A positive displacement check value was placed immediately above the pump. Discharge measurements were made using Sparling water meters. In most tests the meter accuracy was checked with a 44 gallon oil drum or a 10,000 gallon tank."

Test Site

Well UE-20f is located at 37° 16' 17" N.; 116° 29' 16" W., in Area 19 of the Nevada Test Site (fig. 1).

 

Location of well UE-20f on the Nevada Test Site
Figure 1. Location of well UE-20f on the Nevada Test Site.

 

Construction

Well UE-20f is one of 19 exploratory wells drilled in the Pahute Mesa to collect data for the evaluation of the subsurface geologic and hydrologic environment (Blankennagel and Weir, 1973, p. B1-B2). Well UE-20f was drilled to a depth of 13,686 feet below land surface. The well is completed with 13 3/8-inch outside diameter casing from land surface to 755 feet below land surface and with 9 5/8-inch outside diameter casing from land surface to 4,456 feet below land surface. The well is open hole from 4,456 to 4,530 feet below land surface with a hole diameter of 12 2/8-inch; from 4,530 to 13,670 feet below land surface with a hole diameter of 8 6/8-inch; and from 13,670 to 13,686 feet with a hole diameter of 6 1/8-inch. The major production zones were reported to be in Rhyolite from 4,570 to 4,680 feet below land surface (Blankennagel and Weir, 1965, p. 21). Blankennagel and Weir (1973, p. B12, table 3) report that the most permeable zone in well UE-20f was cased off, however, they do not report an approximate aquifer thickness. The saturated thickness of aquifer tested is about 9,230 feet.

 

Construction of well UE-20f
Figure 2. Construction of well UE-20f.

 

Hydrogeologic Characteristics

Orkild and Jenkins, (1978, p. 69 - 70) present a detailed description of rock type and stratigraphic units for well UE-20f (table 1).

 

Table 1. Rock type in well UE-20f from 0 to 13,686 feet below land surface (adapted from Orkild and Jenkins (1978, p. 69 - 70).
Rock type in well UE-20f from 0 to 13,686 feet below land surface (adapted from Orkild and Jenkins (1978, p. 69 - 70)

 

Cooper-Jacob Analysis

The Cooper-Jacob method (Cooper and Jacob, 1946), commonly referred to as the straight-line method, is a simplification of the Theis (1935) solution for flow to a fully penetrating well in a confined aquifer. Using the Cooper-Jacob method, a transmissivity was estimated to be 33 ft2/d by fitting a straight line to late-time drawdown data (fig. 3). Lohman (1979, p. 22) states that the Cooper-Jacob method is only valid when the well function of u is less than or equal to 0.01 (u = r2 S/4 T t, where r = distance to observation well, S = aquifer storage, T = aquifer transmissivity and t = time of pumpage). Assuming an r of 1 foot and S of 0.001, the criteria of a value of u less than or equal to 0.01 was met after the first second of pumping.

 

Measured and straight-line approximation drawdowns for August 9 – 11, 1964, aquifer test conducted at well UE-20f
Figure 3. Measured and straight-line approximation drawdowns for August 9-11, 1964, aquifer test conducted at well UE-20f.

 

Conclusions

Transmissivity could only be reliably estimated around well UE-20f with the Cooper-Jacob solution. Estimate of transmissivity determined for this report using the Cooper-Jacob solution was 30 ft2/d. Constraining the Moench solution with reasonable values of S, Ss, K', Ss', and Sf gave poor and unreasonable matches to the drawdown data. Figure 4 shows the best match using the Moench solution. The Root Mean Square error for this match was 44 feet. The best estimate of transmissivity for the available data in well UE-20f is considered to be 33 ft2/d. To explain the poor match of the Moench solution to the drawdown data there could have been an error recording the zero time water level, and drawdown could be greater than actually recorded. An offset in the drawdown record would not affect transmissivity estimates with the Cooper-Jacob solution.

 

Measured and case (2) simulated drawdowns for August 9 – 11, 1964, aquifer test conducted at well UE-20f
Figure 4. Measured and case (2) simulated drawdowns for August 9-11, 1964, aquifer test conducted at well UE-20f.

 

 

 

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