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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Powerline Schurz

Primary Investigator: Kip Allander

Well Data

USGS Site ID
Local Name Altitude Uppermost
Opening
Lowermost
Opening
Primary Aquifer Transmissivity
(ft2/d)
385342118473101 Powerline Irrigation Well 4082.93 136 375 ALLUVIAL FILL 10000

Aquifer Test

All Aquifer Test Files (zip)

Powerline Schurz

Aquifer Test (pdf)

Introduction

Walker Lake has been declining at a rate of about 1.6 feet per year since 1917 resulting in total dissolved solids concentrations increasing to the point where the lakes fishery and ecosystem health are being threatened. Uncertainties in the water budget in the Lower Walker River Basin led the U.S. Geological Survey in cooperation with the U.S. Bureau of Reclamation to undertake a study to revise the water budget of Walker Lake. The study was initiated in 2004 and a revised water budget is scheduled to be published in 2009.

Developing a better understanding of the ground-water system in the Lower Walker River Basin is a critical component of this study. To understand how ground-water interacts with Walker Lake it is necessary to understand ground-water levels and aquifer properties in the alluvial aquifers adjacent to Walker Lake. It is planned to conduct a series of aquifer tests in the alluvial aquifers of the Lower Walker River Basin. Planned tests are 1) slug tests of monitor wells installed as part of this study, 2) analysis of historic slug test data (1979) on monitor wells installed on the Hawthorne Army Ammunition Depot, 3) a multiple-well aquifer test on the Walker River Indian Reservation, and 4) a single well aquifer test on the Double Springs flowing well. This paper documents the methods, data, analysis, and results of the multiple-well aquifer test on the Walker River Indian Reservation.

This multiple-well aquifer test was performed in the Lower Walker River basin near Schurz, NV (Figure 1). This test was conducted on Walker River Indian Reservation tribal land with cooperation and consent from local officials and farmers. This test was designed to help us obtain estimates of properties of the local aquifer system. The local aquifer is an alluvial aquifer system with material originating from stream and lake alluvial processes. Pumping was initiated on April 27 and discontinued on April 29, 2008. Aquifer recovery was monitored until May 8, 2008.

 

Location of Powerline Irrigation well aquifer test in the Lower Walker River Basin, near Schurz, Nevada
Figure 1. Location of Powerline Irrigation well aquifer test in the Lower Walker River Basin, near Schurz, Nevada.

 

Site layout for Powerline Irrigation well aquifer test
Figure 2. Site layout for Powerline Irrigation well aquifer test.

 

Site

The location of wells used for this aquifer test was about 2,000 ft east of highway 95 approximately 4 mi S of Schurz, NV and 8 mi NNW of Walker Lake and about 2,000 ft west of Walker River (Figure 1 and Figure 2; Table 1). The pumping irrigation well (PIW) utilized for this test is steel-lined with a diameter of 16 inches. Total depth of this well is 375 feet and has multiple perforated intervals with the first interval at 136 to 298 ft and the second interval at 328 to 375 ft. Drawdown response in well OW2 was delayed which suggested that the annular space and screen of PIW was fouled to more than 200 ft below land surface.

The pump is an electric turbine pump and is used to irrigate a center pivot alfalfa field approximately 250 acres in size (0.5 mile diameter field). Water is discharged horizontally through a 10.8 inch diameter (34 inch circumference) pipe that lies above ground for 15 ft, and then is buried for nearly 0.25 mi between the well and the center pivot of the motorized irrigation device (Figure 3). The circular crop field had not been planted or irrigated during the previous growing season, but the irrigation system was in reasonable working condition.

Three monitor wells designated Observation Well #1 (OW1), Observation Well #2 (OW2), and Observation Well #3 (OW3) are horizontally located 47, 96, and 288 ft from PIW, respectively (Table 1; see Appendix A for field notes). These three wells are 2-inch diameter PVC monitor wells and are all inline on a bearing from PIW of about 20 degrees east of north (Figure 2). Drillers logs for the monitoring wells were not filed with the Nevada State Engineers office and any available information on their construction was not made available by the Walker River Paiute Tribe. Therefore important information such as depth of well, screened interval and lithologic information for these wells are unknown. Later in this write-up, information will be presented that will be helpful for guessing at the depths of the screened intervals. Although OW3 was intended to be used as a part of this multi-well aquifer test, it was obstructed (possibly with mud) leaving only about 1.6 ft of water in the well at the time of the test, so even though water-level measurements were made, data from this well are not used in this analysis.

 

Table 1. Well and site configuration information for multiple well aquifer test in the Lower Walker River Basin.
Well and site configuration information for multiple well aquifer test in the Lower Walker River Basin

 

Photograph of irrigation well and pump with center pivot irrigation device in the background. View is to the southwest on April 29th, 2008 by Lindsay R. Burt
Figure 3. Photograph of irrigation well and pump with center pivot irrigation device in the background. View is to the southwest on April 29th, 2008 by Lindsay R. Burt.

 

Procedures

Several weeks before the aquifer test was initiated, pressure transducers were installed in monitor wells OW1 and OW2 to observe the natural fluctuation in the water table due to atmospheric pressure changes as well as the current trend of the water table. The pressure transducers were calibrated prior to installation. Depth to water was measured in each of the wells using an electric tape prior to the start of the aquifer test. For one measurement, a steel tape was also used to confirm the electric tape’s calibration. Monitor wells OW1 and OW2 were sounded to estimate total depth of wells. An Instrumentation Northwest Inc. PS9105 pressure transducer (0-15 PSIG) was installed in OW1 to a depth of about 50 ft below the water surface and operated using a Campbell Scientific CR10X data logger. A Global Water pressure transducer (0-15 ft; internal logging) was installed in OW2 at a depth of about 10 ft below the water surface. Both transducers were programmed for automatic monitoring of water level in the wells during the aquifer test.

The length of the pressure transducer cables were calculated by adding the measured depth to water to the height of the water column measured by the pressure transducer. The cable lengths were recorded at the beginning and end of the test and periodically during the test to document possible changes in position of the pressure transducers in the wells.

The electric-motor driven center pivot irrigation system (Figure 3) was started prior to pumping. The center pivot system allows for the water discharged from the aquifer test to be distributed over the field and minimizes the potential for recharge of the pumped water, which is a concern in any aquifer test that discharges large volumes of water. Pumping was initiated at 17:11.25 hrs PDT on April 27, 2008. The discharge was measured using a ThermoPolysonics DCT 7088 ultrasonic flowmeter. The flowmeter was mounted to the exterior of the horizontal outlet pipe (Figure 4). Water levels in OW1 and OW2 were recorded at a high frequency at the outset of the test when drawdown was occurring rapidly. As water level changes became more gradual the loggers were programmed to take measurements less often to conserve memory. Pumping was terminated on April 29 09:30.50 PDT after nearly 40.3 hours (1.68 days) of continuous and relatively stabile pumping. Ideally, pumping would have continued longer but due to problems with the pump shaft lubrication system, pumping had to be terminated. Data from OW1 and OW2 were downloaded to a laptop PC daily during pumping, and the transducers were left in place after pumping was discontinued to record the recovery of the local water table.

Similarly to how pressure transducers were programmed during the pumping phase of the test, they were reprogrammed for the recovery phase of the test to record more frequently during initial recovery and then as time went on, the recording intervals became larger. However, a problem with the program with the pressure transducer for OW1 caused the rapid recording frequency to repeat during the recovery period, which caused the datalogger to overfill with data and overwrite all data during the first 6 days of the recovery period. Therefore the recovery data for OW1 was incomplete and not available for analysis.

 

Pumping rate was measured with an ultrasonic flow meter mounted to exterior of discharge pipe
Figure 4. Pumping rate was measured with an ultrasonic flow meter mounted to exterior of discharge pipe.

 

Analysis

Hydraulic properties of the aquifer were determined based on the extent and timing of drawdown, interpreted aquifer and lithologic thicknesses, the proximity of the monitor wells to the pumping well and the discharge rate of the pumping well. Drawdown observed in OW1 and OW2 are shown in Figure 5A and Figure 5B, respectively, and the data is provided in appendix B. The maximum observed drawdown in OW1 was 3.5 ft just before the irrigation pump was shut down after 40 hours of pumping.

An interesting effect was observed with OW1 immediately following starting and stopping of the pump, in which the head in the well responded oppositely to what was expected. Immediately following the start of pumping, head in the well increased about 0.2 feet after about 1.5 minutes of pumping and remained at that level for another two minutes before the more typical drawdown pattern began. This pattern repeated when pumping was terminated, but this time head in the well immediately dropped before the expected recovery pattern began. This occurrence is called the ‘Noordbergum effect’ and is typically a result of propagation of deformation-induced stresses caused by a pumping well which can proceed the propagation of hydraulic head signal when head is being measured in a layer that is separated from the pumping layer by a clay aquitard (Kim and Parizek, 1997). This suggests that OW1 is not completed within aquifer layer being pumped and further suggests that there is a clay aquitard with relatively low permeability and high specific storage between this well and the top-most aquifer layer being pumped by PIW.

Although drawdown data from OW3 is not being used to compute aquifer properties in this analysis, it is helpful to understand that it too experienced a more mild, but more prolonged ‘Noordbergum effect’. In response to pumping, head in OW3 initially rose by 0.13 feet over a 17 minute period and remained at that level for another 17 to 30 minutes before normal drawdown behavior began. Water level in OW3 dropped below the soft bottom of the well sometime between 24.6 and 39.3 hours after the start of pumping.

Even though OW2 is nearly twice the distance from PIW as OW1, it had a substantially greater drawdown than OW1 with about 9 ft by the time pumping was discontinued (Figure 5B). The ‘Noordbergum effect’ was not observed in OW2. This, in combination with the greater drawdown observed as compared with OW1, suggests that OW2 is completed at a deeper depth than OW1 and that there is not a semi-confining clay aquitard unit separating the completion depth of OW2 from the uppermost pumping interval of PIW.

Drawdown in PIW was occasionally measured during the first day of pumping but was discontinued the second day due to interference in the well that prevented proper measurements. The maximum drawdown in PIW was greater than 52.6 ft which was the observed drawdown after just over 1.5 hours of pumping. This observed drawdown in this well was greater than the drawdown reported by the driller in 1977, which was about 45 feet after pumping at 1500 gpm for 1.5 hours (and this is after pumping at even higher rates over the previous 6.5 hours; see drillers log in appendix A). Since the depth to water prior to starting the pump for this test and that reported by the driller in 1977 were nearly identical (around 36 feet below land surface), this indicates that the capacity of this well has decreased over time which could be caused by encrustation of the upper screened interval. Three hours after pumping was terminated, the irrigation well had recovered to within 22.9 feet of the water level observed prior to the start of pumping.

The flow data measured using the ultrasonic flowmeter was fairly noisy but provided an adequate estimate of discharge from PIW (Figure 6). The noise in the data is probably mainly due to the method of measurement rather than actual fluctuations in flow rates. Discharge from PIW was initially about 1600 gallons per minute (gpm) but quickly declined to about 1350 gpm as the drawdown in the well increased. The discharge from the well then gradually declined to about 1250 gpm near midday on 4/28/08. The flowmeter stopped recording flow data during the first evening of pumping due to low battery condition (battery was only good for about 8 hours of operation). Measurement of the pumping discharge rate was resumed on the morning of 4/28/08 upon arrival of field crew. Measurement of discharge was discontinued just before noon that same day so that the flowmeter could be used in another aquifer test across the valley (Double Springs aquifer test). Measurement of flow was attempted again later that day; however, the flowmeter became inoperable and was no longer functional. The discharge rate from PIW is assumed to have been adequately measured while the flowmeter instrument was still in operation. For the purposes of simplifying this aquifer test analysis, an average discharge rate measured using the flowmeter is assumed to adequately define the pumping rate for the entire period of the test. The pumping rate used for the aquifer test analysis is a constant 1300 gpm.

 

Hydrographs for (A) OW1 and (B) OW2

 

Hydrographs for (A) OW1 and (B) OW2
Figure 5. Hydrographs for (A) OW1 and (B) OW2.

 

Discharge from Powerline Irrigation well during aquifer test
Figure 6. Discharge from Powerline Irrigation well during aquifer test.

 

Lithology

The lithology described by the well drillers report for the irrigation well (appendix A) is assumed to be laterally continuous within and beyond the region of this aquifer test. The lithology is generally varying mixtures of sand, silt, clay, and gravel. The alternating sequences of the lithology logged by the driller was interpreted and generalized into 7 layers (Figure 7). The 7 layers consist of alternating beds of sand and gravel, and silt and clay starting at the water table at about 35 feet below land surface and extending down to the bottom of the well at about 375 feet below land surface. Within each of the four layers of sand and gravel there are stringers of silt and clay and likewise within each of the 3 layers of silt and clay are interbedded layers of sand and possibly gravel. This ultimately has the physical effect of decreasing vertical permeability in the sand and gravel layers as compared with horizontal permeability and likewise may contribute to a higher horizontal permeability within the silt and clay layers as compared with vertical permeability. The original drilled hole extended to a depth of 400 feet with the last 27 feet being in clay. For the purposes of this analysis, 375 feet is assumed to be the bottom of the aquifer. The total thickness of the aquifer is therefore interpreted to be about 340 feet thick.

Some ancillary data indicates that the uppermost silt and clay layer may be especially tight and gives clues to the completion depths of each of the observation wells. This evidence was the observation of the ‘Noordbergum effect’ in OW1 and OW3 but not in OW2 when pumping started. In order for ‘Noordbergum effect’ to be present there must be relatively tight clay between the layer being pumped and where head is being observed (Kim and Parizek, 1997). It appears evident that OW1 was completed within a tight clay layer, OW2 was completed within the upper sand and gravel unit being pumped, and OW3 may have been completed within the uppermost sand and gravel unit above the tight clay layer (Figure 7). This evidence arises from Kim and Parizek (1997) in which they compared the ‘Noordbergum effect’ for three observation wells within a three layer system; with the three layers being two sand aquifers separated by a clay aquitard and pumping from the bottom sand aquifer (the three observation wells were each completed at a radius of 33 feet from the pumping well but were completed in each of the three layers). The observation well completed within the pumping layer had no “Noordbergum effect’, the observation well completed within the clay aquitard had the most pronounced ‘Noordbergum effect’, and the observation well completed in the overlying aquifer had a much more subdued ‘Noordbergum effect’ which is analogous to the observed response from OW2, OW1, and OW3, respectively.

 

Generalized lithology in the region of the multiple well aquifer test and schematic of well completion characteristics
Figure 7. Generalized lithology in the region of the multiple well aquifer test and schematic of well completion characteristics.

 

Model

The aquifer properties of interest were estimated by simulating this aquifer test using a two-dimensional, radial-transient ground-water flow model that was constructed using MODOPTIM (Halford, 2006) and MODFLOW-96 (McDonald and Harbaugh, 1988; Harbaugh and McDonald, 1996). The PIW is at the center of this radial grid and has radius of 0.67 ft. Radial spacing starts at 0.67 ft, increases by 0.05 ft initially but is then increased by a constant multiplier until the total distance of 200,000 feet is achieved at column 71 (Appendix B). A radial grid extended 200,000 feet from PIW so simulated drawdowns were not affected by the distant boundary.

The model was vertically discretized into 169 2-foot thick layers representing 7 layers of alternating lithologic units (see Figure 7 for lithology) starting at the water table (-36 ft) and extending to the base of the aquifer (-374 ft). Actual breaks for the lithologic layers were assigned to the nearest 2-foot interval of the models vertical discretization. Pumping and recovery were simulated with two stress periods of 1.68 and 10 day duration. Pumping from PIW is the only stress simulated because the effects of recharge, evapotranspiration, and any other stress was removed by estimating drawdown. A constant pump rate of 250,000 ft3/day (1300 gpm) was distributed along the entire screen interval of PIW (Figure 7). Assumed encrustation of the annulus and screen in PIW was simulated between 136 and 250 ft below land surface as a low hydraulic conductivity annular zone.

The hydraulic properties of the aquifer system and well were defined with 8 parameters (Table 2). Only five parameters were estimated because drawdowns were not observed near the water table and wellbore storage in the pumping well. Hydraulic conductivity of the sand, silt, and clay units, specific storage of the clay units, and hydraulic conductivity of the encrusted well annulus were estimated with MODOPTIM (Halford, 2006). Specific yield, specific storage of the sand and gravel units, and vertical-to-horizontal anisotropy of the hydraulic conductivities were assigned reasonable values and not estimated (Table 2). Differences between simulated and measured drawdowns in wells OW1 and OW2 were minimized to estimate the hydraulic properties of the aquifer system and well (Figure 8).

 

Table 2. Parameters estimated using MODOPTIM, their description/definition and determinations.
Parameters estimated using MODOPTIM, their description/definition and determinations

 

Simulated and observed (A) drawdown for OW1 and OW2 and (B) recovery for OW2

 

Simulated and observed (A) drawdown for OW1 and OW2 and (B) recovery for OW2
Figure 8. Simulated and observed (A) drawdown for OW1 and OW2 and (B) recovery for OW2.

 

Aquifer Property Estimates

The aquifer property of most importance determined from this aquifer test was hydraulic conductivity. Horizontal hydraulic conductivities of the sand, silt, and clay units were 50, 0.8, and 0.0006 feet/day, respectively. Aquifer transmissivity was 10,000 feet2/day based on 200 ft of sand and gravel with an average hydraulic conductivity of 50 feet/day.

Often it is possible to determine storage properties for an aquifer from a multiple well test. However, due to uncertainties introduced by lack of knowledge on screened depths of observation wells, specific storage could only be estimated for the silt and clay layers and was only assumed for the sand and gravel layers. Specific storage in the silt and clay layers was estimated to be about 7x10-6. Specific storage in the sand and gravel layers was assumed to be about 2x10-6. Specific yield for the upper unconfined part of the aquifer also was not determined but was assumed to be 0.1, which is a typical value for basin and range alluvial aquifers.

 

 

 

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