Nevada Water Science Center


Aquifer Tests

Contact Information

Kip Allander
Groundwater Specialist
phone: (775) 887-7675
Email:

 

Mailing Address
USGS
Nevada Water Science Center
2730 N. Deer Run Rd.
Carson City, NV 89701

 

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Carson Valley

Primary Investigator: Doug Maurer

 

Well Data

USGS Site ID
Local Name Altitude Uppermost
Opening
Lowermost
Opening
Primary Aquifer Transmissivity
(ft2/d)
385926119451201 Airport 4700 150 450 ALLUVIAL FILL 7300

 

Aquifer Test

All Aquifer Test Files (zip)

Airport

Aquifer Test (pdf)

 

Introduction

Pumping from an isolated well south of the Carson Valley airport, Nevada (Figure 1) was analyzed as an aquifer test. Hydraulic conductivity and vertical anisotropy of alluvial fill from 0 to 400 ft below land surface were estimated. The aquifer test was 365 days in duration and started on January 1, 2005 and ended on December 31, 2005. Water supply and ground-water management models were constrained with results from this aquifer test.

 

Location of wells for aquifer test in Carson Valley, Nevada
Figure 1 - Location of wells for aquifer test in Carson Valley, Nevada.

Site

The aquifer test occurred southeast of the Minden airport in Carson Valley, Nevada (Figure 1). The alluvial fill was comprised of gravel, sand, silt, and clay intervals from 0 to 410 ft below land surface. Clay predominated in units more than 400 ft below land surface (Figure 2). This contact was the base of the aquifer.

 

Plan view and radial section of wells used in aquifer test
Figure 2 - Plan view and radial section of wells used in aquifer test.

 

Procedures

A production well and seven observation wells were used for the aquifer test (Table 1, Figure 1). Water levels and pumping rates were monitored continuously in the production well during 2005 (Figure 3). Water levels were measured periodically in observation wells and typically were measured when the AIRPUMP well was not pumping (Figure 4). Water levels in observation wells were measured under pumping conditions during the summer because pumping did not cease.

 

Table 1. Location and construction of wells used in Carson Valley airport, Nevada aquifer test
Location and construction of wells used in Carson Valley airport, Nevada aquifer test

 

Volumes of produced water were monitored with an in-line, totalizing flow meter at 10-minute intervals. Weekly discharges from the AIRPUMP well that ranged between 0 and 1,300 gpm (Figure 3) were analyzed.

Drawdowns had large uncertainties compared to average weekly pumping rates because instantaneous discharges ranged between 0 and 1,400 gpm (Figure 3). Drawdowns were estimated relative to water-levels measured in late December 2005.

Pumping changes during the Minden-airport aquifer test Pumping changes during the Minden-airport aquifer test
Figure 3 - Pumping changes during the Minden-airport aquifer test.

 

Water levels during 2005 that were analyzed in the Minden-airport aquifer test
Figure 4 - Water levels during 2005 that were analyzed in the Minden-airport aquifer test.

 

Analysis

The hydraulic properties of the geohydrologic column were estimated for 3 lithologies (Figure 2) by minimizing differences between simulated and measured drawdowns. Drawdowns were simulated with a two-dimensional, radial MODFLOW model (McDonald and Harbaugh 1988; Harbaugh and McDonald, 1996). Parameter estimation was performed by minimizing a weighted sum-of-squares objective function with MODOPTIM (Halford, 2006).

The production well and aquifer system were simulated with an axisymmetric, radial geometry in a single MODFLOW layer. Radial distance increased with increasing column indices and depth increased with increasing row indices. Hydraulic conductivities and storages of the ith column were multiplied by 2(pi)ri to simulate radial flow where ri was the distance from the outer edge of the first column to the center of the ith column.

The model extended from a production well to more than 200,000 ft away and from the water table to 280 ft below land surface. The model domain was discretized into a layer of 73 rows of 89 columns (Figure 4). Cell widths ranged from 0.2 ft adjacent to the production well to 25,000 ft in the farthest column. Vertical discretization was a uniform 1 m. All external boundaries were specified as 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. Weekly discharges from the AIRPUMP well were simulated with 49 stress periods. Initial heads were specified as 0.

The production well was simulated as a high conductivity zone with vertical conductances multiplied by 107. Water was removed from the uppermost node in a well and MODFLOW was allowed to apportion inflow to the well. Wellbore storage associated with the production well was not simulated and was addressed by not comparing the first 5 minutes of drawdowns during a test.

 

Radial cross-section about the AIRPUMP well that shows model grid
Figure 5. Radial cross-section about the AIRPUMP well that shows model grid.

 

Hydraulic Property Estimates

The hydraulic properties of the geohydrologic column were defined with seven parameters, the horizontal hydraulic conductivity of sand & gravel, silt, clays shallower than 100 ft below land surface, and clays deeper than 100 ft below land surface. A uniform vertical–to-horizontal anisotropy was assigned from the water table to the base of the aquifer. Specific storage and specific yield were assumed to be uniform throughout the geohydrologic column and was defined with a single parameter. Parameter estimation was limited to the horizontal hydraulic conductivities.

Simulated drawdowns matched measured drawdowns reasonably well (Figures 6, 7, and 8). The RMS error was 1.2 ft relative to measured drawdowns of more than 70 ft.

Transmissivity of the alluvial fill was 7,300 ft2/d. Hydraulic conductivity estimates ranged from 0.0004 to 30 ft/d (Table 2). Weekly averages of highly variable pumping rates masked many transient responses so specific storage, and specific yield could not be estimated. Vertical–to-horizontal anisotropy was not estimated either because it is usually correlated with specific yield. Reasonable vertical–to-horizontal anisotropy, specific storage, and specific yield values for alluvial sediments were assigned (Table 2).

 

Table 2. Parameter estimates and relative sensitivities of the hydraulic properties that were estimated and assigned in the Carson Valley airport model.

Parameter estimates and relative sensitivities of the hydraulic properties that were estimated and assigned in the Carson Valley airport model

 

Measured and simulated drawdowns the AIRPUMP, AIR SHALLOW, AIR MID, and AIR DEEP wells during 2005
Figure 6 - Measured and simulated drawdowns the AIRPUMP, AIR SHALLOW, AIR MID, and AIR DEEP wells during 2005.

 

Measured and simulated drawdowns in the EXP2 SHALLOW, EXP2 MID, and EXP2 DEEP wells during 2005
Figure 7 - Measured and simulated drawdowns in the EXP2 SHALLOW, EXP2 MID, and EXP2 DEEP wells during 2005.

 

Measured and simulated drawdowns in the USGS-AG well during 2005
Figure 8 - Measured and simulated drawdowns in the USGS-AG well during 2005.

 

REFERENCES

Halford, K.J., 2006, MODOPTIM: a general optimization program for ground–water flow model calibration and groundwater management with MODFLOW, U.S. Geological Survey Scientific Investigation Report 2006-5009.

Harbaugh, A.W. and McDonald, M.G., 1996, Programmer's documentation for MODFLOW–96, an update to the U.S. Geological Survey modular finite difference ground-water flow model: U.S. Geological Survey Open-File Report 96–486, 220 p.

Maurer, D.K. and Welch, A.H., 2001, Hydrogeology and geochemistry of the Fallon Basalt and adjacent aquifers, and potential sources of basalt recharge, in Churchill County, Nevada, U.S. Geological Survey Water-Resources Investigations Report 2001–4130, 72 p.

Welch, A.H., Maurer, D.K., Lico, M.S., and McCormack, J.K., 2005, Characterization of surface-water quality in the S–Line Canal and potential geochemical reactions from storage of surface water in the Basalt aquifer near Fallon, Nevada, U.S. Geological Survey Scientific Investigation Report 2005–5102, 52 p.

 


 

 

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