Nevada Water Science Center


Aquifer Tests

Contact Information

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

 

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

 

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Wells ER-20-7 and ER-20-8 main upper zone, Pahute Mesa, Nevada National Security Site

Primary Investigator: Keith Halford

Well Data

USGS Site ID
Local Name Altitude Uppermost
Opening
Lowermost
Opening
Primary Aquifer Transmissivity
(ft2/d)
371247116284502 ER-20-7 6208.9 2360 2875 VOLCANIC ROCKS 11000
371135116282601 ER-20-8 main 5848.3 2486 2912 VOLCANIC ROCKS 52000

 

Aquifer Test

All Aquifer Test Files (zip)

ER-20-7 and ER-20-8 main upper zone

Aquifer Test Report and Appendixes (zip) || ER-20-7 Groundwater levels (NWISweb) || ER-20-8 main Groundwater levels (NWISweb)

 

Introduction

During 2010, the U.S. Geological Survey (USGS) analyzed data from two multi-well aquifer tests (ER-20-8 #2 and ER-EC-11) on Pahute Mesa that were conducted in late-2009 and mid-2010 by Navarro-Intera, LLC (N-I), formerly known as Navarro Nevada Environmental Services (NNES). A multi-well aquifer-test package for these tests was completed and approved for release by the USGS (Halford and others, 2010).

The two recent multi-well aquifer tests on Pahute Mesa were conducted by N-I in late-2010 and mid-2011. The wells pumped during the aquifer tests were ER-20-7 and the upper zone of well ER-20-8 main (located at the ER-20-8 well cluster) (Figure 1). Well ER-20-7 produced water from welded tuff of the Topopah Spring aquifer and was pumped in September 2010. Well ER-20-8 main upper zone produced water from welded tuffs within the Tiva Canyon aquifer and was pumped in May and June 2011.During well development and aquifer testing, continuous water-level data from observation wells were collected by N-I at up to 12 well sites (Figure 1). Water-level responses to pumping were monitored in some hydrostratigraphic intervals that were similar to the pumped interval and others that were vertically separated from the pumped intervals. This was possible because most sites had multiple wells at different completion intervals. Water levels in some of the observation wells were monitored beginning in June 2009.

 

Figure 1. Location of well sites and geologic structures associated with multi-well aquifer tests at Pahute Mesa, Nevada National Security Site and vicinity, 2009-2011.

 

Estimates of drawdowns in observation wells for the multi-well aquifer tests at ER-20-7 and ER-20-8 main upper zone are documented in this aquifer-test package. Additionally, numerical models were constructed and documented here to simultaneously analyze the four aquifer tests completed from 2009 to mid-2011. The aquifers tests were analyzed simultaneously because they affect overlapping volumes of aquifer. By analyzing the tests together, a consistent set of hydraulic properties is estimated. These estimates will be revised as more tests occur beneath Pahute Mesa. The primary scope of this analysis is to estimate total transmissivity around each pumping well. Estimates of transmissivity and storage properties for the volcanic rocks at Pahute Mesa are needed to constrain hydraulic properties in groundwater flow and contaminant transport models at the Nevada National Security Site (NNSS).

Description of Well Network

The well sites monitored by N-I during the ER-20-7 (eight well sites) and ER-20-8 main upper zone (12 well sites) multi-well aquifer tests are located near the northwestern boundary of the NNSS (Figure 1). Table 1 provides location information for the 28 observation wells at the 12 well sites.

In addition to the N-I observation wells, the USGS has collected water-level data at multiple observation wells on Pahute Mesa (Figure 1). Two of these wells, ER-20-6 #3 and UE-20bh 1, were instrumented in 2008 and originally used to record water-level changes during aquifer testing at well U-20 WW (Fenelon and others, 2009). These wells currently are being used to monitor background water-level changes for the multi-well aquifer tests. In November 2009, data collection began in well PM-3-1 to monitor background water-level changes at Pahute Mesa. Well UE-20n 1 was originally instrumented in 2010 to record water-level changes during additional aquifer testing planned at well U-20 WW and is being used here as a background well. The background wells are assumed to be unaffected by well development and aquifer testing on Pahute Mesa. Well ER-20-2-1 was instrumented in 2011 to record water-level changes during aquifer testing planned at well ER-20-4 and is being used as an observation well.

Table 2 and Figure 2 provide detailed information on well construction for observation and pumping wells monitored during the multi-well aquifer tests. All wells in Table 2 were monitored by N-I, with the exception of ER-20-2-1. Construction information was obtained primarily from completion reports and written communications from the U.S. Department of Energy (DOE), NNES, N-I, Stoller-Navarro Joint Venture (SNJV), and USGS. Most of the observation and pumped wells (Table 2) were completed within the last ten years. The exceptions are ER-20-1, ER-20-2-1, ER-20-5 #1, and ER-20-5 #3, which were completed in the 1990´s.

 

Table 1. Site information for wells monitored during multi-well aquifer testing at Pahute Mesa, 2010-2011

Well name: Names are listed in alphabetical order. Bold part of name is well site as shown on Figure 1. Data from well names in italics were available for analysis of the ER-20-8 main (upper zone) aquifer test but not the ER-20-7 aquifer test.

Type of well: B, Background; O, Observation; P, Pumping

U.S. Geological Survey site identification number: Unique 15-digit number identifying well.

Latitude/Longitude: Latitude and longitude coordinates, referenced to North American Datum of 1927.

Land-surface altitude: Altitude, referenced to National Geodetic Vertical Datum of 1929.

Reference: DOE, U.S. Department of Energy; N-I, Navarro-Intera; SNJV, Stoller-Navarro Joint Venture; USGS, U.S. Geologicalurvey. Construction and hydrostratigraphic information from these references are presented in table 2.

 

Well Name Type of well U.S. Geological Survey site identification number Latitude (degrees, minutes, seconds) Longitude (degrees, minutes, seconds) Land- surface altitude (feet) Reference(s)
ER-20-1 O 371321116292301 37°13´20.8" 116°29´29.4" 6,180.9 DOE (1997a); USGS (2011)
ER-20-2-1 O 371246116240101 37°12´46.2" 116°24´01.1" 6,705.0 USGS (2011)
ER-20-4 deep O 371143116262503 37°11´43.4" 116°26´25.0" 5,736.1 DOE (2011a)
ER-20-4 shallow O 371143116262504 37°11´43.4" 116°26´25.0" 5,736.1 DOE (2011a)
ER-20-5-1 O 371312116283801 37°13´12.2" 116°28´37.8" 6,241.8 DOE (1997b)
ER-20-5-3 O 371311116283801 37°13´11.2" 116°28´37.9" 6,241.9 DOE (1997b)
ER-20-6-3 B 371533116251801 37°15´33.1" 116°25´17.5" 6466.0 DOE (1998)
ER-20-7 P, O 371247116284502 37°12´47.0" 116°28´44.8" 6,208.9 DOE (2010a)
ER-20-8 main P 371135116282601 37°11´35.1" 116°28´26.3" 5,848.3 N-I, (2010a); DOE (2011b)
ER-20-8 deep O 371135116282602 37°11´35.1" 116°28´26.3" 5,848.3 N-I, (2010a); DOE (2011b)
ER-20-8 intermediate O 371135116282603 37°11´35.1" 116°28´26.3" 5,848.3 N-I, (2010a); DOE (2011b)
ER-20-8 shallow O 371135116282604 37°11´35.1" 116°28´26.3" 5,848.3 N-I, (2010a); DOE (2011b)
ER-20-8-2 O 371135116282701 37°11´34.9" 116°28´26.9" 5,848.8 N-I, (2010b); DOE (2011b)
ER-EC-1 O 371223116314701 37°12´22.7" 116°31´47.1" 6,025.6 DOE (2000a); USGS (2011)
ER-EC-2A O 370852116340502 37°08´42.0" 116°34´02.6" 4,901.9 DOE (2002); USGS(2011)
ER-EC-6 deep O 371120116294803 37°11´19.6" 116°29´48.1" 5,604.4 DOE (2000b); SNJV (2009); USGS (2011)
ER-EC-6 intermediate O 371120116294804 37°11´19.6" 116°29´48.1" 5,604.4 DOE (2000b); SNJV (2009); USGS (2011)
ER-EC-6 shallow O 371120116294805 37°11´19.6" 116°29´48.1" 5,604.4 DOE (2000b); SNJV (2009); USGS (2011)
ER-EC-11 deep O 371151116294102 37°11´51.2" 116°29´41.1" 5,656.3 N-I (2010c); DOE (2010b)
ER-EC-11 lower intermediate O 371151116294103 37°11´51.2" 116°29´41.1" 5,656.3 N-I (2010c); DOE (2010b)
ER-EC-11 upper intermediate O 371151116294104 37°11´51.2" 116°29´41.1" 5,656.3 N-I (2010c); DOE (2010b)
ER-EC-12 deep O 371024116293102 37°10´23.8" 116°29´31.2" 5532.0 DOE (2011c)
ER-EC-12 intermediate O 371024116293103 37°10´23.8" 116°29´31.2" 5532.0 DOE (2011c)
ER-EC-12 shallow O 371024116293104 37°10´23.8" 116°29´31.2" 5532.0 DOE (2011c)
ER-EC-13 deep O 371010116325402 37°10´09.7" 116°32´53.9" 5175.1 DOE (2011d)
ER-EC-13 intermediate O 371010116325403 37°10´09.7" 116°32´53.9" 5175.1 DOE (2011d)
ER-EC-13 shallow O 371010116325404 37°10´09.7" 116°32´53.9" 5175.1 DOE (2011d)
ER-EC-15 deep O 371110116310502 37°11´10.1" 116°31´05.4" 5365.0 DOE (2011e)
ER-EC-15 intermediate O 371110116310503 37°11´10.1" 116°31´05.4" 5365.0 DOE (2011e)
ER-EC-15 shallow O 371110116310504 37°11´10.1" 116°31´05.4" 5365.0 DOE (2011e)
PM-3-1 (1919-2144 ft) B 371421116333703 37°14´20.7" 116°33´36.6" 5822.8 USGS (2011)
UE-20bh 1 B 371442116243301 37°14´41.9" 116°24´33.0" 6636.6 USGS (2011)
UE-20n 1
(2834 ft)
B 371425116251902 37°14´25.1" 116°25´19.0" 6460.7 USGS (2011)

 

Table 2. Well construction and hydrostratigraphic units open to observation and pumping wells monitored during multi-well aquifer testing at Pahute Mesa, 2010-2011

Well name: Names are listed in alphabetical order. Bold part of name is well site, shown on figure 1.

Drilled depth: Total depth, in feet below land surface, that the well was drilled.

Top and bottom of open casing: Depth, in feet below land surface, of the top and bottom of openings in well casing. The openings may be perforated or screened intervals. N/A, no open casing—open interval is uncased open hole.

Top and bottom of open annulus: Depth, in feet below land surface, of the top and bottom of open annulus. Open annulus includes: (1) the space between the well casing and borehole that is either empty or filled with sand and/or gravel; or (2) uncased open hole deeper than the well casing and shallower or equal to well depth.

Hydrostratigraphic units: Hydrostratigraphic units in contact with open casing or open annulus. Hydrostratigraphic units in bold type are the primary water-producing unit(s) for the well. FCCM, Fortymile Canyon composite unit; TMCM , Timber Mountain composite unit ; FCCU, Fluorspar Canyon confining unit; UPLFA, upper Paintbrush lava-flow aquifer; PBPCU, Post-Benham Paintbrush confining unit; BA, Benham aquifer; UPCU, Upper Paintbrush confining unit; SPA, Scrugham Peak aquifer; MPCU, Middle Paintbrush confining unit; TCA, Tiva Canyon aquifer; LPCU, Lower Paintbrush confining unit; TSA, Topopah Spring aquifer; CHZCM, Calico Hills zeolitic composite unit; CHCU, Calico Hills confining unit, CFCM, Crater Flat composite unit.

Water-level altitude: Altitude, in feet, referenced to the National Geodetic Vertical Datum of 1929. Water-level altitudes based on most recent water-level measurements by USGS unless otherwise noted with *. Water-level altitudes with * are recent measurements reported by Stoller-Navarro Joint Ventures, Navarro Nevada Environmental Services, or Navarro-Intera.

 

Well Name Drilled depth Top and bottom of open casing Top and bottom of open annulus Hydrostratigraphic units Water-level altitude;

measurement date
ER-20-1 2,065 N/A 1,940-2,065 TCA 4192.3 (5/18/2009)
ER-20-2-1 2,524 2,368-2,494 2,293-2,524 CHZCM 4431.5 (6/15/2011)
ER-20-4 deep 3,499 2,485-3,002 2,415-3,053 CHZCM,CFCU 4214.9 (6/21/2011)
ER-20-4 shallow 3,499 1,521-1,602 1,524-2,336 CHZCM 4215.2 (10/18/2010)
ER-20-5-1 2,823 2,315-2,374 2,249-2,655 TSA, CHZCM 4186.8 * (11/03/1995)
ER-20-5-3 4,294 3,430-3,882 3,348-3,954 CHZCM 4181.9 * (2/07/1996)
ER-20-7 2,936 2,360-2,875 2,292-2,924 LPCU, TSA, CHZCM 4188.9 * (9/09/2010)
ER-20-8 maina 3,442 2,486-2,912

3,127-3,298
2,440-2,940

3,070-3,442
MPCU, TCA,LPCU,

TSA, CHZCM
4181.3 * (8/22/2009)
ER-20-8 deep 3,442 3,141-3,302 3,070-3,440 LPCU, TSA, CHZCM 4181.8 * (05/09/2011)
ER-20-8

intermediate
3,442 2,498-2,909 2,440-2,940 MPCU, TCA,LPCU 4182.5 (03/07/2011)
ER-20-8 shallow 3,442 2,088-2,119 1,614-2,150 BA, UPCU, SPA 4181.4 * (05/09/2011)
ER-20-8-2 main 2,338 1,680-2,263 1,626-2,338 BA, UPCU, SPA, MPCU 4181.1 (06/28/2011)
ER-EC-1 5,000 2,298-2,821

3,348-3,760

4,448-4,750
2,258-2,863

3,286-3,776

4,399-4,895
BA,UPCU, TCA, LPCU, TSA, CHZCM, CFCM 4170.4 (5/19/2009)
ER-EC-2A 4,902 1,707-2,179

3,077-3,549

4,487-4,916
1,635-2,236

2,587-2,730

3,025-3,603

4,410-4,969
FCCM, TMCM 4147.9 (9/28/2010)
ER-EC-6 deep 5,000 3,437-3,811

4,420-4,904
3,392-3,820

4,369-5,000
TSA, CHZCM, CFCM 4178.2 (05/19/2009)
ER-EC-6

intermediate
5,000 2,194-2,507 2,138-2,510 UPCU, TCA 4180.7 (05/19/2009)
ER-EC-6 shallow 5,000 1,628-1,870 1,606-1,948 BA 4179.1 (5/19/2009)
ER-EC-11 deep 4,149 3,641-4,094 3,590-4,148 LPCU, TSA, CHZCM 4180.8* (06/17/2010)
ER-EC-11 lower intermediate 4,149 3,159-3,378 3,196-3,385 TCA 4179.2 (06/28/2011)
ER-EC-11 upper intermediate 4,149 2,678-2,991 1,662-3,024 FCCU, BA 4180.0* (06/16/2010)
ER-EC-12 deep 5,532 3,877-3,919 3,820-3,919 CHCU, CFCU 4175.1* (08/05/2010)
ER-EC-12

intermediate
5,532 3,240-3,722 3,188-3,770 TSA, CHCU 4167.75* (08/05/2010)
ER-EC-12 shallow 5,532 1,919-2,681 1,854-2,744 TCA, LPCU 4168.9 (06/28/2011)
ER-EC-13 deep 5,175 2,292-2,611 2,240-2,880 FCCM 4164.6* (11/05/2010)
ER-EC-13

intermediate
5,175 1,900-2,100 1,835-2,136 FCCM 4164.8 (06/29/2011)
ER-EC-13 shallow 5,175 1,014-1,094 1,089-1,541 FCCM 4164.5* (11/05/2010)
ER-EC-15 deep 3,254 2,800-3,120 2,752-3,189 LPCU, TSA, CHCU 4178.0 (03/23/2011)
ER-EC-15

intermediate
3,254 2,156-2,395 2,108-2,422 UPCU, TCA, LPCU 4176.3 (03/23/2011)
ER-EC-15 shallow 3,254 1,381-1,741 1,191-1,768 FCCU, UPLFA, PBPCU 4174.5 (06/29/2011)

 

aBridge plug installed at 3,005 ft below land surface.

 

 

 

 

 

 

 

Figure 2. Well construction and hydrostratigraphic units penetrated by wells monitored by N-I during multi-well aquifer testing at Pahute Mesa, 2010-2011. The first three sets of diagrams are grouped as shown on hydrostratigraphic sections in Figure 1.

 

Hydrogeology

The wells monitored during multi-well aquifer testing at Pahute Mesa are completed in Tertiary volcanic rocks. The volcanic rocks of Pahute Mesa are dominated by lavas and tuffs of rhyolitic composition (Laczniak and others, 1996). Geologic structures at Pahute Mesa include normal faults with surface exposure and buried structural zones and caldera margins.

Buried structural features affect the hydrostratigraphy encountered in wells at Pahute Mesa. The Northern Timber Mountain Moat Structural Zone (NTMMSZ) is a buried west-northwest trending fault zone (Figure 1). The NTMMSZ is a down-on-the-southwest fault zone, with rock displacements of more than 1,000 ft (U.S. Department of Energy, 2010a). The area that is bounded on the north by the NTMMSZ and on the south by the Timber Mountain caldera complex structural margin is referred to as “the Bench" (U.S. Department of Energy, 2009). South of the Bench is the Timber Mountain moat structural domain, a structural region that is the northwestern moat area of the Timber Mountain Caldera complex (U.S. Department of Energy, 2011d).

Oservation wells north of the Bench and west of the Boxcar fault (ER-20-1, ER-20-5 and ER-20-7) penetrate about 2,000 ft of unsaturated rock (Figures 2 and 3). The water table in wells north of the Bench area occurs in the Tiva Canyon aquifer (TCA) or Lower Paintbrush confining unit (LPCU). Major water-producing hydrostratigraphic units are the TCA and Topopah Spring aquifer (TSA), with some production from lava-flow aquifers in the Calico Hills zeolitic composite unit (CHZCM). Well ER-20-4 also is located north of the Bench, but is east of the Boxcar fault. ER-20-4 penetrates about 1,500 ft of unsaturated rock and produces water from a lava-flow aquifer within the CHZCM.

Observation wells on the Bench area include ER-20-8, ER-EC-1, ER-EC-6, ER-EC-11, ER-EC-12, and ER-EC-15 (Figures 2 and 3). These wells penetrate about 1,200 to 1,800 ft of unsaturated rock before encountering the water table in the Timber Mountain aquifer (TMA), Benham aquifer (BA), Fluorspar Canyon confining unit (FCCU), or Tannenbaum Hill composite unit (THCM). Wells on the Bench area were constructed to monitor five water-producing hydrostratigraphic units: the upper Paintbrush lava-flow aquifer (UPLFA), Benham aquifer (BA), Scrugham Peak aquifer (SPA), TCA, and TSA. The CHZCM and Crater Flat composite unit (CFCM) also supply water to observation wells on the Bench area (Table 2).

Observation wells south of the Bench (ER-EC-2A and ER-EC-13) penetrate about 750 to 1,000 ft of unsaturated rock (Figures 2 and 3). The water table in both wells south of the Bench area occurs in the Fortymile Canyon composite unit (FCCM). The two water-producing hydrostratigraphic units in wells south of the Bench area are lava-flow aquifers within the FCCM and welded-tuff aquifers of the Timber Mountain composite unit (TMCM).

The lithologies of major water-producing hydrostratigraphic units in the aquifer-test area are rhyolitic lava flows and welded ash-flow tuffs. The UPLFA, BA and SPA are lava-flow aquifers. These aquifers comprise rhyolitic lava flows with some intervals of vitrophyre, pumiceous lava, and flow breccias (U.S. Department of Energy, 2010b and 2011e). The TCA and TSA are welded-tuff aquifers. The ash-flow tuffs in these aquifers are partially to densely welded, with some nonwelded layers and local zeolitization. The FCCM, CHZCM, and CFCM are composite units of rhyolitic lava-flow aquifers and nonwelded tuff confining units with local to common zeolitization (Laczniak and others, 1996, p.11; U.S. Department of Energy, 1997b; 2000b). The CFCM also contains welded-tuff aquifers. The TMCM in the study area is a composite of welded-tuff aquifers and nonwelded tuff confining units (U.S. Department of Energy, 2002).

 

 

 

Figure 3. Hydrostratigraphic sections A-A´, B-B´, and C-C´, Pahute Mesa.

 

Data Collection

ER-20-7 Aquifer-Test Data

Aquifer-test analysis of pumping from ER-20-7 was limited to data collected from the pumping well and 16 observation wells from July through October, 2010 (Table 3). The horizontal distance between pumping and observation wells ranged from 0 to 14,990 ft. The observation wells were generally in azimuthal quadrants north, west, and south of the pumping well.

Data from the pumping and observation wells provided the drawdown, recovery, and discharge rates needed for analysis of the aquifer test. Water level and pumping rate were monitored in the pumping well and water levels were measured continuously in the observation wells using pressure transducers. The data used in the analysis can be found in appendix A (see Microsoft Excel spreadsheet, AppendixA_ER-20-7.xlsx). Data collected from three observation wells at site ER-EC-12 were not included in the analysis due to poor quality. In addition, data from well ER-EC-1 were not used in the analysis because of poor quality after 9/15/2010.

Well development and aquifer tests from pumping well ER-20-7 are summarized in Table 4 and Figure 4. The longest constant rate test lasted 73 hours and was conducted from 9/21/2010 at 13:40 to 9/24/2010 at 14:30. The discharge rate during this aquifer test was approximately 280 gallons per minute (gal/min) with a total withdrawal during this pumping period of about 1.2 million gallons. During well development, the data logger failed and discharge values were not recorded for a portion of September 14 and 15, 2010. A constant discharge rate for this period of missing data was calculated using in-line flowmeter values.

 

Table 3. Wells monitored during ER-20-7 multi-well aquifer test

Well name: Names listed in order of horizontal distance from pumping well ER-20-7.

Horizontal distance from pumping well: Horizontal distance, in feet, from ER-20-7.

Bearing relative to pumping well: True bearing, in degrees, from pumping well to observation well.

 

Well name Horizontal distance from pumping well, in feet Bearing relative to pumping well
ER-20-7 0 N/A
ER-20-5-3 2,520 13°
ER-20-5-1 2,620 12°
ER-20-1 4,970 314°
ER-EC-11 deep
ER-EC-11 lower intermediate

ER-EC-11 upper intermediate
7,250 219°
ER-20-8 deep
ER-20-8 intermediate

ER-20-8 shallow
7,440 168°
ER-EC-6 deep
ER-EC-6 intermediate

ER-EC-6 shallow
10,230 210°
ER-EC-1 14,920 261°
ER-EC-12 deep


ER-EC-12 intermediate
ER-EC-12 shallow
14,990 194°

 

Table 4. General pumping schedule from well ER-20-7 during multi-well aquifer testing at Pahute Mesa, September, 2010

Start date/time and End date/time: Start and end of pumping from N-I daily drilling reports.

Pumping duration: Approximate time, in minutes, that pump was turned on, rounded to 2 significant figures.

Discharge rate: Approximate discharge, to the nearest ten gallons per minute, of the pumping well between the start and end time. Value estimated from data collected from an in-line flowmeter.

Total discharge: Approximate discharge, to the nearest ten thousand gallons, of the pumping well between the start and end time. Value based upon data collected from in-line flowmeter.

 

Start date/time End date/time Aquifer-test description Pumping duration (minutes) Discharge rate (gallons per minute) Total discharge (gallons) Notes
9/14/2010 15:57 9/15/2010 11:30 Well development 1,170 300-328 1,122,000 (total discharge during well development and step drawdown testing) Step drawdown test completed on 9/15/2010 from 12:15 to 13:15.
9/15/2010 16:00 9/16/2010 09:05 Well development 1,025 280-300 1,122,000 (total discharge during well development and step drawdown testing) Step drawdown test completed on 9/16/2010 from 09:39 to 15:35.
9/16/2010 16:05 9/17/2010 15:00 Well development 1,380 245-290 1,122,000 (total discharge during well development and step drawdown testing)  
9/21/2010 13:40 9/24/2010 14:30 Constant rate 4,370 280 1,229,000  

 

 

Figure 4. Water levels and discharge from ER-20-7 during well development and aquifer testing, September-October, 2010.

 

ER-20-8 Main Upper Zone Aquifer-Test Data

Aquifer-test analysis of pumping from ER-20-8 main was limited to data collected from the pumping well and 27 observation wells from April 8 to July 12, 2011 (Table 5). During this period, a bridge plug was installed in ER-20-8 main to separate the well´s two open intervals (Figure 2). Pumping occurred in the open interval above the bridge plug, referred to as the “upper zone" of well ER-20-8 main. With the bridge plug installed, well ER-20-8 main has the same open interval as well ER-20-8 intermediate. The water producing unit in the upper zone of ER-20-8 main is the TCA.

The horizontal distance between pumping and observation wells ranged from about 0.5 to 32,000 ft (Table 5). The observation wells were located in three azimuthal quadrants around the pumping well ER-20-8 main upper zone. No observation wells were located to the southeast of the pumping well (Figure 1).

Data from the pumping and observation wells provided the drawdown, recovery, and discharge rates needed for analysis of the aquifer test. Water level and pumping rate were monitored in the pumping well and water levels were measured continuously in the observation wells. The data used in the analysis can be found in appendix B (see Microsoft Excel spreadsheet, AppendixB_ER-20-8_upper.xlsx).

Well development and aquifer tests of the ER-20-8 main upper zone are summarized in Table 6 and Figure 5. The longest constant-rate test lasted about 222 hours and was conducted from 6/18/2011 at 11:00 to 6/27/2011 at 16:45. The discharge rate during this aquifer test averaged 140 gal/min with a total groundwater withdrawal of about 1.9 million gallons.

 

Table 5. Wells monitored during ER-20-8 main upper zone aquifer test

Well name: Names listed in order of horizontal distance from pumping well ER-20-8 main upper zone.

Horizontal distance from pumping well: Horizontal distance, in feet, from ER-20-8 main upper zone.

Bearing relative to pumping well: True bearing, in degrees, from pumping well to observation well.

 

Well Name Horizontal distance from pumping well, in feet Bearing relative to pumping well
ER-20-8 deep
ER-20-8 intermediate
ER-20-8 shallow
0 N/A
ER-20-8-2 50 287°
ER-EC-11 deep
ER-EC-11 lower intermediate
ER-EC-11 upper intermediate
6,250 285°
ER-EC-6 deep
ER-EC-6 intermediate
ER-EC-6 shallow
6,790 257°
ER-20-7 7,440 348°
ER-EC-12 deep
ER-EC-12 intermediate
ER-EC-12 shallow
8,920 216°
ER-20-5-3 9,780 354°
ER-20-4 deep
ER-20-4 shallow
9,830 85°
ER-20-5-1 9,880 355°
ER-20-1 11,860 335°
ER-EC-15 deep
ER-EC-15 intermediate
ER-EC-15 shallow
13,090 259°
ER-EC-1 16,910 287°
ER-20-2-1 22,650 71°
ER-EC-13 deep
ER-EC-13 intermediate
ER-EC-13 shallow
23,270 248°
ER-EC-2A 32,330 237°

 

Table 6. General pumping schedule from well ER-20-8 during multi-well aquifer testing at Pahute Mesa, May-June, 2011

Start date/time and End date/time: Start and end of pumping from N-I daily drilling reports.

Pumping duration: Approximate time, in minutes, that pump was turned on, rounded to 2 significant figures.

Discharge rate: Approximate discharge, to the nearest ten gallons per minute, of the pumping well between the start and end time. Value estimated from data collected from an in-line flowmeter.

Total discharge: Approximate discharge, to the nearest thousand gallons, of the pumping well between the start and end time. Value based upon data collected from in-line flowmeter.

 

Start date/time End date/time Aquifer-test description Pumping duration (minutes) Discharge rate (gallons per minute) Total discharge (gallons) Notes
5/18/2011

11:00
5/18/2011

13:30
Pump function test 150 45-150 16,000 .
5/18/2011

15:40
5/19/2011

09:00
Well development 1040 150-140 151,000  
5/19/2011

10:00
5/19/2011

16:00
Step

drawdown test
360 60, 100, 140 36,000  
5/19/2011

16:00
5/20/2011

09:00
Well development 1020 140 143,000  
5/20/2011

10:00
5/20/2011

16:00
Step

drawdown test
360 60, 100, 140 36,000  
5/20/2011

16:00
5/21/2011

09:15
Well development 1035 140 145,000  
5/21/2011

10:15
5/21/2011

16:15
Step

drawdown test
360 60, 100, 140 36,000  
5/21/2011

16:15
5/22/2011

09:10
Well development 1015 140 142,000  
5/22/2011

10:30
5/22/2011

16:30
Step

drawdown test
360 140, 100, 60 36,000  
5/22/2011

16:30
5/23/2011

8:45
Well development 975 60 58,000  
5/23/2011

10:00
5/23/2011

12:05
Well development 125 140 18,000  
5/23/2011

14:45
5/24/2011

08:55
Well development 1090 60 65,000  
5/24/2011

08:55
5/24/11

12:45
Well development 230 140 32,000  
5/24/2011 12:45 5/26/11 15:15 Well development 3030 99 300,000 Per N-I, total discharge during well development and step drawdown testing was 1,222,000 gallons.
6/1/2011 09:30 6/3/2011 09:22 Constant rate test failures       Three attempts were made to start constant rate test. Pump stopped within an hour of turning on during each attempt. Pump pulled for replacement.
6/17/2011 14:15 6/17/2011 14:50 Pump function test 35 88-145    
6/18/2011 11:00 6/27/2011 16:45 Constant rate test 13,305 140 1,863,000 Per N-I, total discharge during the constant rate testing was 1,869,000 gallons.

 


Figure 5. Discharge from ER-20-8 main upper zone and water levels from ER-20-8 intermediate during well development and aquifer testing, May-June, 2011.

 

Water-Level Modeling & Drawdown Estimation

Drawdowns from pumping of wells ER-20-7 and ER-20-8 main upper zone were estimated by modeling water levels in the pumping and observation wells. Water-level modeling was necessary because environmental (naturally occurring) water-level fluctuations of more than 0.2 ft exceeded maximum drawdowns from pumping in most observation wells. Drawdowns were differentiated from environmental fluctuations by modeling synthetic water levels that simulated environmental water-level fluctuations and pumping effects. Environmental water-level fluctuations were simulated by summing individual time-series of barometric pressure, tidal potential, and background water levels (Halford, 2006). Pumping responses were simulated by superposition of Theis solutions.

Environmental water-level fluctuations were simulated with time series of barometric pressures, earth tides, and water levels from background wells PM-3-1, UE 20bh 1, UE-20n 1, and ER-20-6 #3 (Figure 1). These four background wells are assumed close enough to the observations wells to be affected by similar environmental fluctuations, yet distant enough to be unaffected by pumping from ER-20-7 and ER-20-8 main upper zone. Water levels from background wells were critical because they were affected by tidal potential–rock interaction, imperfect barometric coupling, and seasonal climatic trends. These effects also are assumed to be present in the observation wells.

Pumping responses from wells ER-20-7 and ER-20-8 main upper zone were modeled with a superposition Theis model, where multiple pumping periods were simulated by superimposing multiple Theis (1935) solutions. Superposition Theis models served as simple transform functions, where step-wise pumping records were translated into approximate water-level responses. Numerical experiments have confirmed that superposition Theis models closely approximate water-level responses through hydrogeologically complex aquifers. This approach will herein be referred to as the Theis transform model.

Synthetic water levels were fit to measured water levels by minimizing a sum-of-squares objective function (Halford, 2006). Amplitude and phase were adjusted in each time series that simulated environmental water-level fluctuations. Transmissivity and storage coefficient were adjusted in the Theis transform model. Estimated values of transmissivity and storage coefficient from the Theis transform model generally were not valid estimates of aquifer properties because the assumptions of the underlying Theis solution were significantly violated.

Fitting results are not degraded and spurious drawdowns are not estimated if extraneous time series are included. For example, environmental fluctuations occurring in a background well but not in the observation well are ignored during the fitting process. Extraneous time series, that is, those that do not correlate with fluctuations in the observation well, are eliminated functionally because amplitude estimates approach zero. Simulated pumping responses also can be eliminated functionally by Theis parameters becoming very large.

Environmental water-level fluctuations and pumping effects were modeled simultaneously because only a few weeks of water-level measurements prior to development and testing commonly were made. These are short periods relative to a combined well development, aquifer testing, and recovery period of about two months. Reliable estimation of drawdowns attributed to environmental water-level fluctuations without simulating pumping effects requires fitting synthetic water levels during a period prior to development. This fitting period should be more than 3 times greater than the drawdown estimation period (Halford, 2006).

Drawdown estimates were determined by summing the Theis transform models with the differences (residuals) between synthetic and measured water levels. Synthetic water levels matched measured water levels with root-mean-square errors between 0.002 and 0.016 ft in all wells, with the exception of water levels in ER-20-7 and ER-20-8 intermediate, where temperature changes and well-losses were significant. Pumping effects were considered definitive where the maximum drawdown estimate in a well exceeded 0.05 ft. This drawdown threshold was considered acceptable because it is large relative to the RMS fitting errors between synthetic and measured water levels.

ER-20-7 Aquifer Test

Water-level change resulting from the pumping of well ER-20-7 was approximated using simplified pumping steps. Seven pumping steps (Figure 6) were used to generate Theis transform models for the pumping and observation wells during water-level modeling. These seven pumping steps also were used in the numerical model simulation (see “Groundwater-Flow Models" section). Seven steps were sufficient to adequately calculate the pumping response in these wells.

 

Figure 6. Pumping from ER-20-7 during well development and aquifer testing, September 2010. Data were binned into seven pumping steps for use in the Theis transform models and numerical model.

Drawdowns were evaluated for the pumping well and 12 observation wells (Table 7). Drawdowns in well ER-EC-1 and the three observation wells at the ER-EC-12 well site were not evaluated due to poor data quality. The sum of the Theis transform model and the residuals that resulted from the fitting process was used to estimate the magnitude of the drawdown in each well. Estimated drawdowns were relatively large (0.15 ft or greater) in three wells, small (0.05 - 0.1 ft) in seven wells, and less than the drawdown-detection threshold (0.05 ft) in three wells (Table 7). The relative degree of certainty that the estimated drawdown is large enough to be distinguished from background noise in the data is provided in Table 7. A well with a low relative certainty indicates that drawdown is highly uncertain or was not detected in the well. If drawdown was detected, it is poorly constrained and probably has a magnitude that is equal to or less than the estimated drawdown. High relative certainty indicates high probability that drawdown was detected in the well.

 

Table 7. Estimated drawdowns in observation wells from pumping in well ER 20 7 during multi-well aquifer testing at Pahute Mesa, September, 2010

Estimated drawdown: Drawdown was estimated by matching measured water levels in the observation well to a synthetic curve of nonpumping (environmental) and pumping responses. The pumping response was estimated with a Theis transform model.

Relative certainty that drawdown detected: A relative scale indicating likelihood that estimated drawdown is large enough to be observed above background noise in data. High, very likely; Moderate, more probable than not; Low, drawdown could be or is zero.

 

Well name Estimated drawdown

(feet)
Relative certainty that drawdown detected
ER-20-1 0.08 Moderate
ER-20-5-1 0.15 High
ER-20-5-3 0.16 High
ER-20-7 3.3 High
ER-20-8 deep 0.10 Moderate
ER-20-8 intermediate <0.05 Low
ER-20-8 shallow <0.05 Low
ER-EC-6 deep <0.05 Low
ER-EC-6 intermediate 0.05 Low
ER-EC-6 shallow 0.06 Moderate
ER-EC-11 deep 0.08 Moderate
ER-EC-11 lower intermediate 0.10 Moderate
ER-EC-11 upper intermediate 0.09 Moderate

 

Three observation wells illustrating the range of estimated drawdowns during the aquifer test are shown in Figure 7. Using well ER-20-5-3 as an example, the drawdown response resulting from pumping of ER-20-7 is relatively large and clearly defined. Measured water levels from 8/10/2010 to 10/24/2010 were simultaneously fitted to time series of barometric pressure, earth tides, and water levels in background wells PM-3-1 and UE-20bh 1, and a Theis transform model. Very minor differences between the estimated drawdown and the Theis transform model show that most of the water-level changes in ER-20-5-3 have been explained (Figure 7). Smaller estimated drawdowns for the other two wells shown in Figure 7 are less certain. Plots of estimated drawdowns for all wells in Table 7 are shown in Figure 8. Worksheets showing fitting parameters, measured and synthetic water levels, and estimated drawdowns for all wells in Table 7 can be viewed in individual Excel files stored under the “WLM" directory in the ER-20-7 aquifer-test archive.

 

 

Figure 7. Examples of measured and synthetic water levels, Theis transform models, and estimated drawdown in three observation wells. Estimated drawdown is the summation of the Theis transform model and differences (residuals) between measured and synthetic water levels. The drawdowns resulted from pumping of well ER-20-7 during multi-well aquifer testing, September, 2010.

 

 

Figure 8. Estimated drawdowns for all observation wells monitored during the ER-20-7 multi-well aquifer test, September, 2010.

 

ER-20-8 main upper zone Aquifer Test

Water-level change resulting from the pumping of well ER-20-8 main upper zone was approximated using simplified pumping steps. Thirty-two pumping steps (Figure 9) were used to generate Theis transform models for the observation wells. These 32 steps were sufficient to adequately calculate the pumping response in all observation wells. The pumping history of well ER-20-8 main upper zone was further simplified to six pumping steps (Figure 9) for use in the numerical model simulation (see “Groundwater-Flow Models" section).

 

 

Figure 9. Pumping from ER-20-8 main upper zone during well development and aquifer testing, May-June, 2011. Data were binned into 32 and 6 pumping steps for use in the Theis transform models and numerical model, respectively.

Drawdowns were evaluated for the pumping well and each of the 27 observation wells that were monitored during the aquifer test (Table 8). The sum of the Theis transform model and the residuals that resulted from the fitting process was used to estimate the drawdown in each well. Estimated drawdowns were relatively large (0.15 ft or greater) in five wells, small (0.05 - 0.14 ft) in six wells, and less than the drawdown-detection threshold in 17 wells (Table 8). The relative degree of certainty that the estimated drawdown is large enough to be distinguished from background noise in the data also is provided in Table 8. A well with a low relative certainty indicates that drawdown is highly uncertain or was not detected in the well; if drawdown was detected, it is poorly constrained and probably has a magnitude that is equal to or less than the estimated drawdown. Moderate and high relative certainties indicate that drawdown likely was detected in the well.

 

Table 8. Estimated drawdowns in observation wells from pumping in well ER 20-8 main upper zone during multi-well aquifer testing at Pahute Mesa, May and June, 2011.

Estimated maximum drawdown: Maximum drawdown was estimated by matching measured water levels in the observation well to a synthetic curve of nonpumping and pumping responses. The pumping response was estimated with a superposition Theis model.

Relative certainty that drawdown occurred: A relative scale indicating likelihood that estimated drawdown is large enough to be observed above background noise in data. High, very likely; Moderate, more probable than not; Low, drawdown could be or is zero.

 

Well name Estimated maximum drawdown (feet) Relative certainty that drawdown occurred
ER-20-1 <0.05 Low
ER-20-2-1 <0.05 Low
ER-20-4 deep <0.05 Low
ER-20-4 shallow <0.05 Low
ER-20-5-1 <0.05 Low
ER-20-5-3 <0.05 Low
ER-20-7 0.06 Low
ER-20-8 deep 1.0 High
ER-20-8 intermediate 10 High
ER-20-8 shallow 0.8 High
ER-20-8 #2 main 0.16 High
ER-EC-1 <0.05 Low
ER-EC-2A <0.05 Low
ER-EC-6 deep 0.05 Moderate
ER-EC-6 intermediate 0.07 Moderate
ER-EC-6 shallow 0.08 Moderate
ER-EC-11 deep 0.12 Moderate
ER-EC-11 lower intermediate 0.14 Moderate
ER-EC-11 upper intermediate 0.15 Moderate
ER-EC-12 deep <0.05 Low
ER-EC-12 intermediate <0.05 Low
ER-EC-12 shallow <0.05 Low
ER-EC-13 deep <0.05 Low
ER-EC-13 intermediate <0.05 Low
ER-EC-13 shallow <0.05 Low
ER-EC-15 deep <0.05 Low
ER-EC-15 intermediate <0.05 Low
ER-EC-15 shallow <0.05 Low

 

Figure 10 shows three observation wells that illustrate different magnitudes of estimated drawdowns. Using well ER-EC-11 upper intermediate as an example, the drawdown response resulting from pumping of ER-20-8 main upper zone is large (relative to the RMS error) and clearly defined. Measured water levels from 4/20/2011 to 7/8/2011 were simultaneously fitted to time series of barometric pressure, earth tides, and water levels in background wells PM-3-1 and UE-20n 1, and two Theis transforms. The two Theis transforms were generated using the 32 pumping steps in ER-20-8 main upper zone, and different estimated values of transmissivity and storage coefficient. The summation of these two transforms and the residuals from the fitting process are the drawdown estimate. The good fit (small RMS error) between measured and synthetic water levels shows that most of the water-level changes in ER-EC-11 upper intermediate have been explained (Figure 10). The lower plot in Figure 10 (ER-EC-12 shallow) shows an example where most of the water-level changes have been explained and no drawdown was detected. Plots of estimated drawdowns for all wells in Table 8 are shown in Figure 11. Worksheets showing fitting parameters, measured and synthetic water levels, and estimated drawdowns for all wells in Table 8 can be viewed in individual Excel files stored under the “WLM" directory in the ER-20-8 upper aquifer-test archive.

Drawdowns in observation wells ER-EC-2A and ER-20-2-1 were estimated to be less than 0.05 ft (Table 8). These wells are more than 4 mi from the pumping well and their records are either short (ER-EC-2A) or interrupted (ER-20-2-1) (Figure 11). For these reasons, drawdown estimates from these wells were not used as observations in the numerical model.

Estimates of drawdowns in the three monitored intervals in ER-20-8 are uncertain because of frictional well loss in the pumping column and thermal contraction of the water columns during pumping. Both of these effects are positively correlated with drawdown. Water temperature was measured near the water table in all three well strings and showed cooling trends as the well was pumped. An attempt was made to estimate the contraction of the water columns in the well strings during pumping based on these shallow temperature measurements, but estimates are highly uncertain. The estimate of drawdown in well ER-20-8 shallow has the least uncertainty because thermal contraction is small due to the short length of the water column above the open interval. Uncertain estimates of temperature and well-loss effects in wells ER-20-8 deep and ER-20-8 intermediate accounted for one-half to three-quarters of the total measured decline in water level during pumping. Because the effects from well loss and varying water temperatures were large and correlated with aquifer drawdown, it was not possible to accurately estimate the drawdown in the TCA and TSA monitored by these two wells.

 

Figure 10. Examples of measured and synthetic water levels and drawdown estimates in three observation wells. Drawdown estimate is the summation of the Theis transform and differences between measured and synthetic water levels. Drawdowns resulted from pumping in well ER-20-8 main upper zone during multi-well aquifer testing, May-June, 2011.

 

 

 

 

 

Figure 11. Estimated drawdowns for observation wells monitored during the ER-20-8 main upper zone multi-well aquifer test, May-June, 2011.

 

Aquifer-Test Analysis

Numerical models were developed for each of the aquifer tests and used to obtain estimates of hydraulic properties for the volcanic rocks underlying Pahute Mesa. Numerical methods were used to estimate hydraulic properties because the groundwater beneath the study area flows through a complexly layered sequence of volcanic-rock aquifers and confining units that have been faulted into distinct structural blocks. The observation wells were vertically separated and distant from the pumping wells. Therefore, water-level responses in the observation wells could not be analyzed with simple analytical methods, such as the Theis solution (Theis, 1935), because simplifying assumptions of the methods were violated grossly.

Hydraulic properties of aquifers and confining units were estimated by interpreting pumping responses from multiple aquifer tests using a single, three-dimensional hydrogeologic framework and multiple groundwater-flow models. Multiple groundwater-flow models allowed grid refinement near each pumping well and different pumping schedules to match each aquifer test. Multiple groundwater-flow models also facilitate incorporating additional aquifer tests because each test is interpreted with an independent groundwater-flow model. This allows for each model to be developed and tested individually and assures that simulated drawdowns and sensitivities are computed and extracted correctly.

 

Hydrogeologic Framework Model

Aquifer-test results were interpreted with a single conceptual model because of project limitations. However, many conceptual models exist for distributing hydraulic properties beneath Pahute Mesa, including those where hydraulic properties of mapped faults differ from hydraulic properties of the HSUs. Interpretation of hydraulically unique fault structures was beyond the primary scope of estimating total transmissivity around each of the pumped wells. Because fault structures were not differentiated in the hydrogeologic framework model and in the groundwater flow models, the hydraulic properties of fault structures could not be estimated.

Hydraulic properties were distributed spatially with a single, three-dimensional hydrogeologic framework that was constructed from wellbore data, refined cross sections using data from newly drilled wells (Sigmund Drellack, written commun., National Security Technologies, LLC., 2011), and HSU picks from the Pahute Mesa Corrective Action Unit (CAU) framework model (Bechtel Nevada, 2002) (Figure 12). The hydrogeologic framework for this study used 11 simplified HSUs (Table 9). HSUs were simplified to create a more efficient and simple model and to focus on the major aquifers and confining units that are thought to be hydraulically important in the study area.

The TCVA, THLFA, THCM, TMA, and PVTA units (Table 9) are above or mostly above the water table. The joint Benham and Scrugham Peak aquifers (BA/SPA) simplified HSU incorporate the original BA and SPA units. A simplified Upper Paintbrush confining unit (sUPCU) combined the original UPCU and MPCU units. A simplified Calico Hills zeolitic composite unit (sCHZCM) aggregated the original CHZCM, CHCM, CHCU, and IA units. A simplified Crater Flat composite unit (sCFCM) was created and combined the original CFCM, CFCU, and BFCU units. A simplified pre-Crater Flat unit combined the BRA and PBRCM.

 

 

Figure 12. Simplified, three-dimensional, hydrogeologic framework of hydrostratigraphic units for distributing hydraulic properties. The Paintbrush units, which were subdivided into four units in the hydrogeologic framework, are shown here as one unit for illustrative purposes.

 

Table 9. Categorization of hydrostratigraphic units to simplified hydrostratigraphic units.

 

HSU name HSU abbreviation Simplified HSU abbreviation
Thirsty Canyon volcanic aquifer TCVA Not present
Timber Mountain composite unit TMCM TMCM
Tannenbaum Hill lava-flow aquifer THLFA sFCCU
Tannenbaum Hill composite unit THCM sFCCU
Timber Mountain aquifer TMA sFCCU
Fluorspar Canyon confining unit FCCU1 sFCCU
Windy Wash aquifer WWA sFCCU
Paintbrush vitric-tuff aquifer PVTA sFCCU
Benham aquifer BA BA/SPA
Scrugham Peak aquifer SPA BA/SPA
Upper Paintbrush confining unit UPCU sUPCU
Middle Paintbrush confining unit MPCU sUPCU
Tiva Canyon aquifer TCA TCA
Lower Paintbrush confining unit LPCU LPCU
Topopah Spring aquifer TSA TSA
Calico Hills vitric composite unit CHVCM sCHZCM
Calico Hills zeolitic composite unit CHZCM sCHZCM
Calico Hills confining unit CHCU sCHZCM
Inlet aquifer IA sCHZCM
Crater Flat composite unit CFCM sCFCM
Crater Flat confining unit CFCU sCFCM
Bullfrog confining unit BFCU sCFCM
Belted Range aquifer BRA pre-Crater Flat
Pre-Belted Range composite unit PBRCM pre-Crater Flat
Lower carbonate aquifer LCA LCA

 

1Comprises the majority of the saturated thickness of the sFCCU simplified HSU in the vicinity of the aquifer tests.

 

Displacements of HSUs along the Northern Timber Mountain Moat Structural Zone (NTMMSZ), Area 20 caldera structural margin, Thirsty Canyon lineament, M4 fault, ER-20-7 fault, ER-20-8 fault, West Boxcar fault, and West Greeley fault were simulated (Figure 1). Displacements along all other fault structures were considered minor and were not simulated explicitly.

Hydraulic conductivity was distributed throughout each of the simplified HSUs with pilot points, which are points in the model domain where hydraulic properties are estimated (RamaRao and others, 1995). Pilot points were assigned to simplified HSUs at 54 mapped locations (Figure 13). Less than 54 pilot points existed in any given HSU because pilot points were not defined where a HSU was absent. Hydraulic conductivity was distributed with a total of 431 pilot points. Hydraulic properties were interpolated from pilot points with kriging to node locations as defined for each groundwater-flow model (Doherty, 2008b). The spatial variability of log-hydraulic conductivity was defined with an isotropic, exponential variogram, where the specified range was 15,000 ft and no nugget was specified.

Mean hydraulic properties were assigned in each HSU beyond the pumping and observation wells by an ellipse of pilot points (Figure 13). Each ellipse was defined with 30 of the 54 pilot points in each HSU and a single hydraulic conductivity was estimated for the 30 pilot points. The hydraulic properties of the pilot points for an ellipse tended towards the geometric mean of the pilot points within the ellipse. This occurred because the single estimable pilot point in an ellipse primarily was affected by Tikhonov regularization observations that enforced preferred homogeneity in a HSU.

Hydraulic conductivities were not estimated independently at 211 of the 431 pilot points because drawdowns were below the detection threshold (0.05 ft) near these pilot points. These hydraulic conductivities were defined by pilot point values closer to the pumping wells that were sensitive to pumping effects. Drawdowns did not exceed 0.05 ft at distances of more than 3 miles from pumped wells or in the sCFCM.

Hydraulic-property estimates are affected by groundwater-flow model discretization, consequently, structural inconsistencies were avoided with a common vertical discretization in all groundwater-flow models. Geologic framework and groundwater-flow model layers were identical from 0 to 2,130 ft below the water table (Figure 14). Groundwater-flow model layers gradually thickened between 2,130 and 5,900 ft below the water table where vertical discretization was relatively coarse. Hydraulic properties were not interpolated vertically and assigned directly from simplified HSU to all layers with a corresponding HSU in the groundwater-flow model (Figure 14).

 

 

Figure 13. Location of pilot points that were simulated in the numerical models for the ER-20-8 #2, ER-EC-11, ER-20-7, and ER-20-8 main upper zone aquifer tests.

 

Groundwater-Flow Models

Drawdowns from each multiple-well aquifer test were interpreted with a three-dimensional MODFLOW model (Harbaugh and others, 2000), where each model was centered on the pumping well and rows in the model grid paralleled the NTMMSZ fault. Each model grid extended laterally about 200,000 ft away from the pumping well. All models were about 5,900 ft thick and extended vertically from the water table to 1,700 ft below sea level (Figure 14). Rows and columns in the grid were assigned widths of 10 ft at the pumped well. Columns and rows expanded successively by a factor of 1.25 away from the pumped well until a mid-point between well and faults or the model edge was reached. Rows and columns in each model differed, but ranged between 93 and 106 (Table 10). All external boundaries were specified no-flow boundaries. Changes in the wetted thickness of the aquifer system were not simulated because the maximum drawdown near the water table was small relative to the total thickness. Simulation periods were subdivided into stress periods that simulated simplified pumping schedules for each pumping well with either 6 or 9 stress periods (Table 10).

 

Table 10. Number of layers, rows, columns and stress periods, simulated period, and volume pumped for each MODFLOW model.

 

Pumping Well Layers Rows Columns Stress Periods Period Simulated Volume pumped,
in millions
of gallons
Begin End
ER-20-8 #2 main 19 104 94 9 11/28/2009 01/11/2010 1.9
ER-EC-11 19 106 96 6 04/30/2010 06/15/2010 5.6
ER-20-7 19 106 93 6 09/14/2010 10/04/2010 2.1
ER-20-8 main upper zone 19 104 94 6 05/18/2011 07/07/2010 3.1

 

All groundwater-flow models were discretized vertically into 19 layers that were defined with the water table and the discretization of the geologic framework (Figure 14). Layer 1 was 1-foot thick to better approximate drainage from the water table. Groundwater-flow model layers 2 to 14 were each 164-ft thick as in the geologic framework. Some simplified HSUs such as the sFCCU and BA/SPA occur in multiple groundwater-flow model layers (Figure 14). Other simplified HSUs such as the sUPCU and LPCU are locally absent because HSUs were less than 82-ft thick. A common vertical discretization was used to ensure consistency of simulated geometries of HSUs in all groundwater-flow models.

 

Figure 14. Vertical discretization and assignment of simplified hydrostratigraphic units to layers in numerical flow models.

 

Specific yield and specific storage also were distributed with pilot points but less pilot points were used than for assigning hydraulic conductivity. Specific yield was expected to range between 0.001 and 0.05, which are typical values for fractured rocks. Specific-storage initially was assigned as 2 x 10-6 1/ft and was allowed to range between 1 x 10-6 and 4 x 10-6 1/ft. These values are typical for most aquifer materials (U.S. Geological Survey, 2010). Vertical-to-horizontal anisotropy was assumed equal to 1 and was not estimated.

 

Parameter Estimation

Hydraulic-conductivity, specific yield, and specific storage distributions were estimated by minimizing a weighted composite, sum-of-squares objective function. These distributions were defined with 543 pilot points where 245 pilot points were adjusted with PEST (Doherty, 2008a). Differences between measured and simulated observations defined the goodness-of-fit or improvement of calibration. These differences, residuals, were weighted and summed in the objective function,


Although the sum-of-squares error serves as the objective function, root-mean-square (RMS) error was reported because RMS error was compared easily to measurements. Root-mean-square error is,

 

 

Measurement and regularization observations controlled model calibration. The models used 7,691 drawdowns as measurement observations. Estimated hydraulic conductivities were guided by regularization observations to preferred conditions in areas that were insensitive to measurement observations. This approach is Tikhonov regularization (Doherty, 2008a).

Measured drawdowns are the superposition Theis models plus differences between synthetic and measured water levels. For example, the bold, green lines in Figure 7 are the superposition Theis models. The small, black crosses in Figure 7 are the estimated drawdowns which are the unexplained noise plus the superposition Theis model.

The number of measurement observations was reduced by averaging drawdowns from a well every 6 hours. Averaging reduced the number of measurement observations from more than 84,000 to 9,455 and suppressed high-frequency noise (Table 11). Reliable drawdowns that were not affected by pumping losses, heating effects, or abridged records totaled 7,691 and were observed in 53 of 67 pumping-observation well pairs. These reliable observations were assigned weights of 1, except at the ER-EC-6 well cluster during the ER-EC-11 aquifer test. Weights of 0.5 were assigned to large measured drawdowns from the ER-EC-6 cluster to reduce sensitivity to these observations.

 

Table 11. Numbers of observation wells, original drawdown estimates, and averaged drawdown estimates during each aquifer test.

 

Pumping Well Number of
Observation Wells
Number of Drawdowns
Original 6-Hour Average Weighted greater than 0.5
ER-20-8 #2 main 14 6,604 1,595 1,415
ER-EC-11 main 13 6,507 2,013 1,968
ER-20-7 14 31,543 1,076 936
ER-20-8 intermediate 26 40,118 4,771 3,372
Total 27 84,772 9,455 7,691

 

Compromised observations in wells that were affected by pumping well head losses and heating effects were assigned small weights so that hydraulic-conductivity estimates were minimally affected. Measured drawdowns in well ER-20-8 #2 main during the ER-20-8 #2 aquifer test were uncertain because of the strong correlation between pumping well head losses and aquifer response. Assigned weights of 0.05 reflected the large uncertainty associated with these observations. Measured drawdowns in wells ER-EC-11 main and ER-EC-11 upper intermediate during the ER-EC-11 aquifer test were very uncertain because of pumping losses and heating effects. Assigned weights of 0.002 and 0.05, respectively, reflected the large uncertainty associated with these observations.

Tikhonov regularization limited hydraulic-conductivity estimates at pilot points to reasonable values (Doherty, 2003). Sharp differences between nearby values in similar simplified HSUs were penalized to ensure relatively continuous hydraulic-conductivity distributions. Unrealistic hydraulic-conductivity distributions were avoided by limiting the fit between measured and simulated observations (Fienen and others, 2009). This irreducible, weighted measurement error combined measurement and numerical model errors.

Regularization observations were equations that defined preferred relations between hydraulic-conductivity estimates. Regularization observations affected calibration most where the models were insensitive to measurement observations. Imposing preferred states, such as homogeneity, with regularization observations was preferable to rigid zones of assigned homogeneity. This was because hydraulic-conductivity estimates could differ where dictated by measured drawdowns.

Homogeneity within simplified HSUs was the primary preferred relation between pilot points. Minimum variance of hydraulic-conductivity estimates in all HSUs that were classified as confining units was an additional preferred relation. Similar preferred relations also were defined for HSUs that were classified as aquifers. More than 4,200 regularization observations constrained hydraulic-conductivity estimates with these preferred relations.

 

Goodness of Fit and Investigated Volumes

RMS was used to gauge the misfit between simulated and measured drawdowns. The RMS for the models was 0.04 ft, which was similar to the drawdown-detection threshold of 0.05 ft. Simulated and measured drawdowns agreed, with RMS errors less than 0.03 ft, in 46 of the 67 pumping-observation well pairs (Appendix C). Differences between simulated and measured drawdowns in well ER-EC-6 shallow during all four aquifer tests are shown in Figure 15. The fit between simulated and measured drawdowns was best in well ER-EC-6S during the ER-20-8 main upper zone aquifer test, a RMS of 0.005 ft. The fit between simulated and measured drawdowns was worst in well ER-EC-6D during the ER-EC-11 aquifer test, a RMS error of 0.10 ft.

Discretization errors from fixed vertical spacing of flow-model layers created relatively large RMS errors of 0.03 to 0.10 ft between simulated and measured drawdowns in the ER-20-8, ER-EC-6, and ER-EC-11 well clusters (Figure 15). Uniform 164-ft thick layers introduce significant errors where HSUs are less than 100 ft thick and well screens in nested wells are separated vertically by as little as 100 ft (Figure 2). Vertically deformed grids would be better than uniform grids where thin HSUs and relative well screen positions should be preserved.

The volume of aquifer that was investigated with these aquifer tests reasonably can be defined by the volume where simulated drawdowns exceeded 0.05 ft during the simulated periods (Figure 16). The investigated-volume threshold of 0.05 ft was supported by the drawdown-detection threshold of 0.05 ft and the overall RMS error of 0.04 ft.

 

Figure 15. Simulated and measured drawdowns in well ER-EC-6 shallow during the ER-20-8 #2, ER-EC-11, ER-20-7, and ER-20-8 main upper zone (ER-20-8I) aquifer tests.

 

Figure 16. Maximum simulated drawdowns that occurred at any time during all 4 aquifer tests and where contouring was limited to a range between 0.05 and 0.1 ft.

Hydraulic-Property Estimates

Total transmissivity estimates at each well site are integrated samples in this report, which differs from previous reported transmissivity estimates (Halford and others, 2010).Transmissivity was averaged horizontally in approximately 4,000-ft diameter circles around each well site in this report and summed vertically within selected groupings of HSUs (Figure 17). Transmissivity was averaged because of the additional variability in hydraulic conductivity that was introduced by additional pilot points. Transmissivity estimates were sampled previously from pilot points at each well of interest (Halford and others, 2010). This approach was adequate because pilot points were assigned almost exclusively at well sites. However, transmissivity estimates that are sampled directly from pilot points typically will be more extreme than integrated samples.

Transmissivities were estimated for selected groupings of HSUs around the three well sites where pumping occurred (Table 12). Three well sites exist rather than four because ER-20-8 #2 and ER-20-8 main upper zone both occur at well site ER-20-8. The BA/SPA, which comprises lava flows, is the most transmissive aquifer in the model area with a transmissivity estimate of 38,000 ft²/d around well site ER-20-8. High transmissivity of the BA/SPA around well site ER-20-8 may be caused by the presence of the SPA, which is limited primarily to the area east of ER-20-8 (Prothro and Drellack, 1997). High transmissivity also could result from the contact between BA and SPA HSUs or inferred fault structures.

Table 12. Estimated transmissivities of selected groupings of hydrostratigraphic units around mapped locations where wells were pumped.

[All values are in feet squared per day; na is Not Applicable; FCCU is the Fluorspar Canyon confining unit; BA/SPA is the Benham and Scrugham Peak aquifers; Paintbrush Tuffs includes the Upper Paintbrush confining unit (UPCU), Middle Paintbrush confining unit (MPCU), Tiva Canyon aquifer (TCA), lower Paintbrush confining unit (LPCU), and Topopah Spring aquifer (TSA); sCHZCM includes the Calico Hills zeolitic composite unit (CHZCM), Belted Range aquifer (BRA), and Inlet aquifer (IA)]

 

Mapped Location FCCU BA/SPA Paintbrush Tuffs sCHZCM TOTAL
ER-20-7 na na 9,600 1,800 11,000
ER-20-8 8,800 38,000 5,200 250 52,000
ER-EC-11 13,000 3,200 3,800 1,500 22,000

 

Transmissivity around well site ER-EC-11 totaled 22,000 ft²/d but the distribution of transmissivity among the HSUs likely is wrong (Table 12). Sixty percent of the transmissivity is attributed to the FCCU rather than the BA/SPA. The vertical distribution of tritium that was measured while drilling ER-EC-11 (U.S. Department of Energy, 2010b) and flow logs in ER-EC-1 and ER-EC-6 (Garcia and others, 2010) suggest strongly that most of the transmissivity should occur in the BA/SPA. The incorrectly distributed transmissivity likely resulted from discretization errors that were created by uniform 164-ft thick layers. The discretized sUPCU and LPCU HSUs were locally absent around ER-EC-11 in the flow models (Figure 14) which disagrees with mapped HSUs (Figure 3). Vertically deformed grids that preserve local well construction and relatively thin HSUs should correct this error when results from these four aquifer tests are reinterpreted with additional results from aquifer tests at ER-20-8D, ER-20-4, ER-EC-12, and ER-EC-13 during FY12.

Specific yield and specific storage estimates averaged 0.003 and 1.6 x 10-6 1/ft, respectively. Specific yield and specific storage estimates were constrained at 13 and 60 percent of the pilot points, respectively. Specific yield was constrained between 0.001 and 0.05. Specific-storage was constrained between 1 x 10-6 and 4 x 10-6 1/ft. The large number of constrained storage estimates likely resulted from compensating for known structural errors.

 

 

Figure 17. Hydraulic conductivity distributions in 4,000 ft diameter circles around selected well sites.

 

References Cited

Bechtel Nevada, 2002, A hydrostratigraphic model and alternatives for the groundwater flow and contaminant transport model of Corrective Action Units 101 and 102—Central and western Pahute Mesa, Nye County, Nevada: U.S. Department of Energy Report DOE/NV/11718—706, 383 p.

Doherty, J., 2008a, PEST: Model-Independent Parameter Estimation. Brisbane, Australia: Watermark Numerical Computing.

Doherty, J., 2008b, PEST Groundwater Data Utilities. Brisbane, Australia: Watermark Numerical Computing.

Doherty, J., and J.M. Johnston, 2003, Methodologies for calibration and predictive analysis of a watershed model: Journal of American Water Resources Association, v. 39, no. 2, p. 251-265.

Fenelon, J.M., Reiner, S.R., Laczniak, R.J., and Halford, K.J., 2009, Analysis of U-20 WW multiple-well aquifer test, Pahute Mesa, Nevada Test Site: U.S. Geological Survey Aquifer-Test Package, available at "Nevada Water Science Center Aquifer Tests" webpage, accessed September 19, 2011, at http://nevada.usgs.gov/water/aquifertests/index.htm

Fienen, M., Muffels, C., and Hunt, R., 2009, On constraining pilot point calibration with regularization in PEST, Ground Water, v. 47, no.6, p. 835–844.

Garcia, C.A., Halford, K.J., and Laczniak, R.J., 2010, Interpretation of flow logs from Nevada Test Site boreholes to estimate hydraulic conductivity using numerical simulations constrained by single-well aquifer tests: U.S. Geological Survey Scientific Investigations Report 2010–5004, 28 p.

Halford, K.J., 2006a, Documentation of a spreadsheet for time-series analysis and drawdown estimation: U.S. Geological Survey Scientific Investigations Report 2006-5024, 38 p.

Halford, K.J., Fenelon, J.M., and Reiner, S.R., 2010, Analysis of ER-20-8 #2 and ER-EC-11 multi-well aquifer tests, Pahute Mesa, Nevada National Security Site: U.S. Geological Survey Aquifer-Test Package, available at "Nevada Water Science Center Aquifer Tests" webpage, accessed September 19, 2011, at http://nevada.usgs.gov/water/aquifertests/index.htm

Harbaugh, A.W., Banta, E.R., Hill, M.C., and McDonald, M.G., 2000, MODFLOW-2000, the U.S. Geological Survey modular ground-water model -- User guide to modularization concepts and the Ground-Water Flow Process: U.S. Geological Survey Open-File Report 00-92, 121 p.

Laczniak, R.J., Cole, J.C., Sawyer, D.A., and Trudeau, D.A., 1996, Summary of hydrogeologic controls on ground-water flow at the Nevada Test Site: U.S. Geological Survey Water-Resources Investigations Report 96-4109, 59 p.

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Navarro-Intera, LLC, 2010c, Written communication prepared for U.S. Department of Energy-National Nuclear Security Administration Nevada Site Office, Subject: “Pahute Mesa ER-EC-11 Well Data Report, Preliminary Revision No.: 0" , April 2010.

Prothro, L.B. and Drellack, S.L., Jr., 1997, Nature and extent of lava-flow aquifers beneath Pahute Mesa, Nevada Test Site: U.S. Department of Energy Report DOE/NV/11718-156, 50 p.

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