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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Oliver Ranch Well, Red Rock Canyon National Conservation Area

Primary Investigator: Mike Pavelko

Well Data

USGS Site ID
Local Name Altitude Uppermost
Opening
Lowermost
Opening
Primary Aquifer Transmissivity
(ft2/d)
360332115254501 Oliver Ranch #2 3480 81 101 CARBONATE ROCKS 3500

Aquifer Test

All Aquifer Test Files (zip)

Oliver Ranch

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

Site and Geology

The aquifer test occurred at the Oliver Ranch facility and future site of the Red Rock Desert Learning Center and Wild Horse and Burro Facility near the southern end of the Red Rock Canyon National Conservation Area (fig. 1). Based on the lithologic log from the OR-2 well, the alluvial aquifer in the vicinity of Oliver Ranch primarily comprises undifferentiated intervals of silt, sand, and gravel from 0 to 89 ft below land surface; below 89 ft, interbeds of siltstone and carbonate rock occur to a depth of at least 101 ft, the depth of OR-2. A highly cemented layer occurs between 52 to 54 ft below land surface that possibly acts as a confining unit between a relatively thin water-table aquifer above and a semi-confined aquifer below.

 

Location of Oliver Ranch Well #2 aquifer test, Red Rock Canyon National Recreation Area, Nevada
Figure 1. Location of Oliver Ranch Well #2 aquifer test, Red Rock Canyon National Recreation Area, Nevada.

 

The main observation well, Oliver Ranch Well #1 (OR-1), was drilled in 1975 to a depth of 90 ft, through alluvium. The well is screened from 55 to 90 feet below land surface. Before the aquifer test, OR-1 was sounded to a depth of about 80 ft, indicating that the well is silting up.

Measurements

One pumping well and three observation wells were used for the aquifer test (Table 1, fig. 1). Each well was instrumented with a pressure transducer and water-levels were measured at least once every half hour. Water levels were measured in OR 1 beginning on August 25, 2004, in OR-2 beginning on September 7, 2004, and in the Bonnie Springs Road (BS) and Wheeler Camp (WC) wells beginning on September 14, 2004. The water level in OR-2 before pumping was 49.6 feet below land surface. Water levels are still being monitored in all four wells, however, a pump in well OR 1 intermittently was pumped beginning on September 27, 2004, two days after the aquifer test ended.

 

Table 1. Well location and construction data for the OR-2 multiple-well aquifer test.
 Well location and construction data for the OR-2 multiple-well aquifer test

 

Water level change in observation wells for the Oliver Ranch Well #2 aquifer test, Red Rock Canyon Recreation Area, Nevada
Figure 2. Water level change in observation wells for the Oliver Ranch Well #2 aquifer test, Red Rock Canyon Recreation Area, Nevada.

 

Well OR-2 began pumping September 22, 2004, and discharged about 58 gpm for 69 hours. Water was discharged into a dry channel that was 300 ft away from well OR-2. Production rates were continuously measured using an in-line flow meter and intermittently using a timed-volume method. During the pumped period, small water-level fluctuations in OR 2 (fig. 2) likely were caused by diurnal heating and cooling of the discharge hose.

Drawdowns were estimated by subtracting the water level measured before pumping from subsequent water levels. Barometric, earth-tide, and regional effects were removed from measured water levels before drawdowns were estimated. Drawdowns were estimated only for the pumping phase of the test. Recovery data were not analyzed because well OR-1 was pumped before full recovery and because uncertainty of drawdown estimates increases while drawdowns decrease during recovery. There were no measurable drawdowns in the BS and WC wells.

Analysis

The transmissivity, specific yield, and specific storage of the alluvial aquifer were estimated with a numerical model. An analytical model using the Moench solution for unconfined aquifers (Barlow and Moench, 1999) yielded unreasonable property estimates, likely because the tested aquifer is semi-confined and has a leaky bottom. The numerical model was solved with MODFLOW (McDonald and Harbaugh, 1988, and Harbaugh and McDonald, 1996) and calibrated with MODOPTIM (Halford, 2006). All hydraulic properties were estimated by minimizing the weighted sum-of-squares differences between simulated and measured drawdowns.

Observations from well OR-2 were weighted less than observations from wells OR 1 and BS because of skin effects and noisy drawdowns. Simulated and measured drawdowns in well OR 2 were compared after 1.8 hours of pumping to minimize the effects of well construction (for example, wellbore storage) on the calibration. Maximum simulated drawdown in well OR-2 also was constrained to less than the maximum measured drawdown.

The aquifer system was simulated with a two-dimensional, radial model that incorporated wells OR-1, OR-2, and BS. Because the BS and WC wells are about the same distance from well OR-2, about 3,320 ft and 3,250 ft, respectively, and neither well had measurable drawdowns thus, both wells effectively were simulated in the radial model by including the data from the BS well. A highly cemented, 2-ft thick, zone just below the water table and a bedrock layer 12 feet from the bottom of well OR-2 were simulated explicitly (fig. 3). Hydraulic conductivity was assumed homogeneous and vertically anisotropic within each aquifer-system unit.

 

Radial cross-section of geology and wells at Oliver Ranch within 300 feet of OR-2
Figure 3. Radial cross-section of geology and wells at Oliver Ranch within 300 feet of OR-2.

 

The model domain was discretized into 116 layers and 69 columns. The model extended laterally 200,000 ft from well OR-2. The vertical extent was from 50 to 550 ft below land surface. Column widths were 0.2 ft near well OR-2 and increased in width by a factor of 1.2 to the edge of the model. Layer thicknesses were 0.5 ft throughout the alluvial section between 50 and 90 ft below land surface. The confining unit (cemented zone), from 52 to 54 feet below land surface, was simulated in the model with 4 0.5 ft layers. Layer thicknesses increased by a factor of 1.5 throughout the bedrock to the base of the model. All external boundaries were no-flow. Changes in the wetted thickness of the aquifer were not simulated because the maximum drawdown near the water table was small relative to the total thickness. The OR-2 aquifer test was simulated with a 3-day stress period.

The vertical-to-horizontal anisotropy of the aquifer system was not estimated. Vertical-to-horizontal anisotropy was correlated highly with hydraulic conductivity and could not be estimated. A value of 0.1 was assigned. The analysis suggests that the hydraulic conductivity of the alluvium must be an order of magnitude greater than the hydraulic conductivity of the bedrock. Initial simulations with a uniform hydraulic conductivity in the alluvium and bedrock caused simulated drawdowns in well OR-2 to be greater than 130 ft. The maximum observed drawdown in well OR-2 was less than 15 ft. A hydraulic conductivity of 1 ft/d from regional aquifer investigations was assigned to the bedrock.

The vertical distribution of simulated drawdowns at the end of the model is shown in figure 4. Measured drawdowns in wells OR-1, OR-2, and BS were compared with simulated drawdowns from the numerical model (fig. 5). Simulated drawdowns matched measured drawdowns with a root-mean-square (RMS) error of 0.09 ft. The RMS error was about 1 percent of the 7-ft range in drawdowns that were analyzed. The model indicates that approximately 99 percent of the water pumped from well OR-2 comes from alluvium, 13 percent of the alluvial water that flows into OR-2 originates in the bedrock, and 86 percent of the alluvial water that flows into OR-2 originates from above. Hydraulic-property estimates for the model were about 90 ft/d for the lateral hydraulic conductivity of alluvium, 2 x 10-6 ft-1 for specific storage, 0.005 for specific yield, and 0.7 ft/d for the lateral hydraulic conductivity of the confining unit. The hydraulic conductivity of the alluvium corresponds to a transmissivity of 3,500-ft²/d for the 40-ft thick aquifer. The hydraulic conductivity of the bedrock and vertical-to-lateral anisotropy of the aquifer system were not estimated by the model (Table 2).

 

Simulated drawdowns after three days of pumping well OR-2 for 55 gallons per minute
Figure 4. Simulated drawdowns after three days of pumping well OR-2 for 55 gallons per minute.

 

Table 2. Estimated hydraulic properties.
Estimated hydraulic properties

 

Simulated and measured drawdowns from numerical model in wells OR-1, OR-2, and BS.  Comparisons of OR-2 drawdowns did not begin until 0.075 days (1.8 hours) after pumpage began to minimize borehole storage and partial penetration effects.  Drawdown in well OR-2 was 13.13 feet at 0.075 days
Figure 5. Simulated and measured drawdowns from numerical model in wells OR-1, OR-2, and BS. Comparisons of OR-2 drawdowns did not begin until 0.075 days (1.8 hours) after pumpage began to minimize borehole storage and partial penetration effects. Drawdown in well OR-2 was 13.13 feet at 0.075 days.

 

The sensitivity of specific yield estimates to vertical anisotropy was tested with model runs in which vertical anisotropy was set to 0.05, 0.1, and 0.5. Specific yield estimates for those runs were 0.004, 0.005, and 0.002, respectively. Water-level declines from predictive 50-year models with the hydraulic properties estimated from these three runs and a pumping rate of 6 acre-feet/year are shown in figure 6.

 

Simulated drawdowns after 50 years of pumping 6 acre-feet per year
Figure 6. Simulated drawdowns after 50 years of pumping 6 acre-feet per year.

 

 

 

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