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–20–4 main, ER–20–7, ER–20–8 main upper zone, ER–20–8 main lower zone, ER–20–8 #2 main, ER–20–11, ER–EC–11 main, ER–EC–12 main upper zone, ER–EC–12 main lower zone, ER–EC–13 main upper zone, ER–EC–13 main lower zone, ER–EC–14 main upper zone, ER–EC–14 main lower zone, ER–EC–15 main upper zone, ER–EC–15 main intermediate zone, and ER–EC–15 main lower zone

Primary Investigator: Tracie Jackson

Well Data

USGS Site ID
Local Name Altitude Primary Aquifer Transmissivity
(ft2/d)
371143116262503 ER-20-4 main 5736 VOLCANIC ROCKS 1700
371247116284502 ER-20-7 6208.9 VOLCANIC ROCKS 18000
371135116282601 ER-20-8 main upper zone 5848.3 VOLCANIC ROCKS 140000
371135116282601 ER-20-8 main lower zone 5848.3 VOLCANIC ROCKS 140000
371135116282701 ER-20-8 #2 main 5848.8 VOLCANIC ROCKS 110000
371146116290301 ER-20-11 5834 VOLCANIC ROCKS 4000
371151116294101 ER-EC-11 main 5656.3 VOLCANIC ROCKS 25000
371024116293101 ER-EC-12 5532 VOLCANIC ROCKS 50
371024116293101 ER-EC-12 5532 VOLCANIC ROCKS < 1
371010116325401 ER-EC-13 5175 VOLCANIC ROCKS 5000
371010116325401 ER-EC-13 5175 VOLCANIC ROCKS 5000
370825116302401 ER-EC-14 5185.9 VOLCANIC ROCKS 1000
370825116302401 ER-EC-14 5185.9 VOLCANIC ROCKS 1000
371110116310501 ER-EC-15 5365 VOLCANIC ROCKS 1200
371110116310501 ER-EC-15 5365 VOLCANIC ROCKS < 10
371110116310501 ER-EC-15 5365 VOLCANIC ROCKS 40

 

Aquifer Test

Aquifer Test Report (2MB)

Appendix A–Well Construction(1.5MB)

Appendix B–Hydrographs (7.7MB)

Appendix C–NNSS–PM Aquifer Test Models: MODFLOW and PEST (289MB)

Appendix D–Supporting Memos (1GB)

 

ER–20–4 main, ER–20–7, ER–20–8 main upper zone, ER–20–8 main lower zone, ER–20–8 #2 main, ER–20–11, ER–EC–11 main, ER–EC–12 main upper zone, ER–EC–12 main lower zone, ER–EC–13 main upper zone, ER–EC–13 main lower zone, ER–EC–14 main upper zone, ER–EC–14 main lower zone, ER–EC–15 main upper zone, ER–EC–15 main intermediate zone, and ER–EC–15 main lower zone Groundwater Levels (NWISweb)

ER–20–4 main || ER–20–7 || ER–20–8 || ER–20–8 #2 main || ER–20–11 || ER–EC–11 main || ER–EC–12 || ER–EC–13 || ER–EC–14 || ER–EC–15

 

This memorandum documents the simultaneous interpretation of 16 multiple–well aquifer tests to estimate hydraulic properties on Pahute Mesa, Nevada National Security Site (NNSS). Multiple–well aquifer tests were conducted by Navarro–Intera, LLC (N–I) between November 2009 and May 2014 (Table 1). A cumulative volume of 63 million gallons was pumped during these aquifer tests so that drawdowns could be observed in a network of 34 wells. Water levels in these observation wells have been measured continuously by N–I and the U.S. Geological Survey (USGS). The primary purpose 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 NNSS. This work expands on a previous investigation where 8 of the 16 multiple–well aquifer tests were interpreted simultaneously (Halford, Fenelon, and Reiner, 2012).

The 16 multiple–well aquifer tests used pumping wells that contained a main casing with either single or multiple completions. In pumping wells with multiple completions in the main casing, packers were used to separate completions so that distinct intervals could be pumped as individual aquifer tests. Aquifer tests in multiple completion wells are reported herein with the designation of main upper zone, main intermediate zone, or main lower zone. Many pumping wells also contained piezometers completed in the annulus alongside the main completion zone or in shallower or deeper zones within the borehole. Piezometers in wells with multiple completions are reported herein as observation wells with the designation of shallow, intermediate, or deep. During an aquifer test, water levels in the pumping well and in a network of observation wells were monitored with pressure transducers (Figure 1; Appendix A). Distances between pumping and observation wells range from less than one foot to a few miles.

 

Map of Pahute Mesa

Figure 1. Location of well sites and geologic structures associated with multiple-well aquifer tests at Pahute Mesa, Nevada National Security Site (NNSS) and vicinity, 2009-2014.

 

Table 1. Pumping periods and volumes pumped during each aquifer test.

Table of Pumping Periods

 

Drawdowns from pumping wells were used to obtain hydraulic property estimates across a well network comprising pumping, observation, and background wells (Figure 1; Table 2). Drawdowns were estimated from measured water levels because water–level measurements are affected by environmental water–level fluctuations in addition to pumping signals (Garcia and others, 2013). Drawdown estimates were differentiated from environmental fluctuations with analytical water–level models (Halford, Garcia, and Reiner, 2012). Drawdowns from aquifer tests at well sites ER–20–11, ER–EC–14, and ER–EC–15 were estimated during 2014 (Garcia and others, 2014; Jackson and Halford, 2014). More than 200 drawdown time series were estimated for pumping and observation well pairs that are separated by horizontal distances between 0 and 23,000 ft. These drawdown estimates were documented previously in nine supporting memoranda (See Appendix D) (Table 1). Well construction (Table 2), water–level collection, and discharge measurements also were documented in these supporting memoranda.

Hydraulic properties were estimated with numerical groundwater flow models by fitting simulated drawdowns to drawdowns estimated from measured water levels. Separate numerical models characterizing the 16 multiple–well aquifer tests were created and analyzed simultaneously because of the effects of drawdown interference in aquifers affected by multiple aquifer tests. Simultaneous interpretation of all tests assured that a consistent set of hydraulic properties were being estimated. Generally, for multiple completion pumping wells, aquifer tests from different open intervals were analyzed with a single model because the second test typically occurred within a month of the first test and water levels in distant observation wells simultaneously were affected by both aquifer tests. Site ER–EC–13 was an exception because the system recovered during the 7 months between the end of the ER–EC–13 main upper zone aquifer test and ER–EC–13 main lower zone aquifer test. In total, 11 numerical models were created to simulate drawdowns from the 16 aquifer tests.

A typical “aquifer test”, as simulated in the numerical model, consisted of about 10 days of intermittent pumping to develop the well and perform step–drawdown tests, followed by about 10 days of continuous pumping at a constant–rate. Pumping periods were shorter in low–productivity wells where pumping could not be sustained (ER–EC–12 main lower zone, and ER–EC–15 main intermediate and lower zones) or in contaminated wells with limited capacity for storage of discharge water (ER–20–7). Pumping periods and total volumes pumped for each of the aquifer tests are summarized in Table 1.

 

Table 2. Well location and construction data for analyzed wells during multiple-well aquifer testing at Pahute Mesa, Nevada National Security Site.

Table of Pumping Periods

 

Hydrogeology

The wells monitored during multiple–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, some with surface exposure, and buried structural zones and caldera margins (Figure 1).

Structural features offset 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) that displaces rocks by 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, 2011a).

Observation wells north of the Bench and west of the Boxcar fault (Figure 1) penetrate about 2,000 ft of unsaturated rock. Major water–producing hydrostratigraphic units (HSUs) are the Tiva Canyon aquifer (TCA) and Topopah Spring aquifer (TSA), with some production from lava–flow aquifers in the Calico Hills zeolitic composite unit (CHZCM) (Appendix A). North of the Bench and east of the Boxcar fault, well ER–20–4 penetrates about 1,500 ft of unsaturated rock and produces water from a lava–flow aquifer within the CHZCM.

Observation wells in the Bench (Figure 1) penetrate about 1,200 to 1,800 ft of unsaturated rock. Wells in the Bench were constructed to monitor five water–producing HSUs: 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 (Appendix A).

Observation wells south of the Bench (Figure 1) penetrate about 750 to 1,000 ft of unsaturated rock. The three water–producing HSUs in wells in this area are lava–flow aquifers within the Fortymile Canyon composite unit (FCCM) and welded–tuff aquifers of the Timber Mountain composite unit (TMCM) and Timber Mountain aquifer (TMA).

The lithologies of major water–producing HSUs in the aquifer–test area are rhyolitic lava flows (UPLFA, BA and SPA) and welded ash–flow tuffs (TCA, TSA, and TMA) (U.S. Department of Energy, 2010b and 2011b). The FCCM, CHZCM, and CFCM are composite units of rhyolitic lava–flow aquifers and non–welded tuff confining units with local to common zeolitization (Laczniak and others, 1996, p.11; U.S. Department of Energy, 1997; 2000). The CFCM also contains welded–tuff aquifers. The TMCM in the study area is a composite of welded–tuff aquifers and non–welded tuff confining units (U.S. Department of Energy, 2002a).

 

Aquifer-Test Analysis

Aquifer–test results were analyzed with numerical models to estimate hydraulic properties for the volcanic rocks underlying Pahute Mesa. Numerical methods were used 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. Observation wells are often 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.

Hydraulic properties of aquifers and confining units were estimated by interpreting drawdowns 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 specific to each aquifer test. Multiple groundwater–flow models also facilitate independent aquifer test assessments and provide assurance that simulated drawdowns and sensitivities are computed and extracted correctly.

Hydrogeologic Framework Model

A single conceptual model of the hydrogeologic framework was used to interpret aquifer–test results. Many conceptual models exist for distributing hydraulic properties beneath Pahute Mesa, including those where hydraulic properties of mapped faults and structural zones 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 either the hydrogeologic framework model or 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, National Security Technologies, LLC., written commun., 2011), and HSU picks from the Pahute Mesa Corrective Action Unit (CAU) framework model (Bechtel Nevada, 2002) (Figure 2). The hydrogeologic framework was discretized vertically into 51 layers between 1,700 ft below sea level and 6,500 ft above sea level, where each layer was about 164 ft thick. The hydrogeologic framework for this study used 15 modified HSUs (Table 3). Existing HSUs were modified so that observed hydraulic responses could be adequately replicated with the groundwater–flow models.

Hydrogeologic framework

Figure 2. Three–dimensional, hydrogeologic framework of hydrostratigraphic units for distributing hydraulic properties. The modified Fluorspar Canyon confining units (mFCCU), Paintbrush units, and Timber Mountain composite units (TMCM) were subdivided into six units, four units, and five units, respectively, in the hydrogeologic framework. The mFCCU, Paintbrush units, and TMCM are each shown here as one unit for illustrative purposes. Hydrostratigraphic unit abbreviations are described in Table 3.

 

Table 3. Existing and modified hydrostratigraphic units.

Table 3

 

Hydrostratigraphic units in the bench area (Figure 1) were modified by grouping certain HSUs based on either hydraulically similar properties (e.g., BA/SPA) or the presence of multiple thin HSUs where their hydraulic properties cannot be differentiated in the groundwater–flow model (e.g., HSUs in the mCFCM). The THLFA, THCM, TMA, FCCU, WWA, and PVTA were undifferentiated to form one modified HSU, denoted mFCCU, where the FCCU comprises the majority of the saturated thickness (Table 3). The joint Benham and Scrugham Peak aquifers (BA/SPA) modified HSU incorporates the BA and SPA units. A modified Upper Paintbrush confining unit (mUPCU) combined the UPCU and MPCU units. A modified Calico Hills zeolitic composite unit (mCHZCM) aggregated the CHVCM, CHZCM, CHCU, and IA units. A modified Crater Flat composite unit (mCFCM) combined the CFCM, CFCU, BFCU, BRA, PBRCM, and LCA units.

The mCHZCM was differentiated further north of the NTMMSZ because of observed drawdowns in well ER–20–4 shallow during the ER–20–4 main aquifer test (Halford, Garcia, and Reiner, 2012). The mCHZCM was divided into two HSUs at an altitude of about 3,600 ft so that hydraulic conductivity could differ vertically between wells ER–20–4 shallow and ER–20–4 main. The modified upper and lower HSUs were mCHZCMu and mCHZCMl, respectively. Hydraulic conductivity of mCHZCMu was expected to be less than the hydraulic conductivity of the mCHZCMl because the upper HSU primarily is bedded tuff near site ER–20–4, whereas the mCHZCMI is stony rhyolite lava. The mCHZCMu laterally extends east of the West Boxcar fault and north of the NTMMSZ (Figure 1).

The TMCM was differentiated into 5 HSUs south of the Bench area based on observed drawdowns during aquifer tests at the ER–EC–13 site (Halford and Reiner, 2013). The TMCM was differentiated into two lava–flow aquifers that intersect the upper and lower screens in wells ER–EC–13 main and ER–EC–14 main. These two lava–flow aquifers are overlain, separated, and underlain by ash–flow tuff confining units (U.S. Department of Energy, 2011a). The ash–flow tuffs adjacent to the lava–flow aquifers are non–welded and zeolitized, and similar units at Pahute Mesa typically are characterized as confining units (Laczniak and others, 1996, p.11; U.S. Department of Energy, 1997).

Hydrostratigraphic unit displacements along the NTMMSZ, Timber Mountain caldera complex structural margin, Thirsty Canyon lineament, 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.

 

Estimating Hydraulic Properties with Pilot Points

Hydraulic conductivity was distributed throughout each of the modified HSUs with pilot points. Pilot points are locations in the model domain where hydraulic properties are estimated (RamaRao and others, 1995). Pilot points were assigned to modified HSUs at 182 mapped locations (Figure 3), with a denser spacing of pilot points specified around the pumped wells (Figures 3 and 4). Modified HSUs were spatially discontinuous in the model domain, causing modified HSU extents to be locally absent within parts of the mapped pilot point area. Therefore, less than 182 pilot points existed in any modified HSU because pilot points were not defined in locations where an HSU was absent. For example, the TMCM only is present south of the Timber Mountain caldera complex structural margin (Figures 2 and 3); therefore, for the TMCM, pilot points were not defined north of the Timber Mountain caldera complex structural margin where the TMCM does not exist in the model domain. Hydraulic conductivity was distributed with a total of 996 pilot points across all HSUs. Local hydraulic conductivity extremes were minimized by assigning homogeneous hydraulic conductivities around pumping and observation wells (tied pilot points on Figures 3 and 4). Homogeneous conditions around a well were defined by a ring of pilot points that were assigned a single, estimable hydraulic conductivity (Figure 4). This reduced the number of estimable pilot points by 251.

Hydraulic conductivities at background wells were assigned from previous aquifer–test results (Table 4) to bound hydraulic property estimates outside the area of investigation because hydraulic property estimates are insensitive to measurement observations. The area of investigation is defined where maximum drawdown was greater than or equal to 0.05 ft during simulation of any aquifer test (see section “Area Investigated” for details), and occurs within an 80 mi2 area shown in Figure 4. The numerical model domain for each aquifer test is about 5,400 mi2, and includes the area shown in Figure 3; the west and east model domain boundaries extend beyond the edge of Figure 3 by about 4 mi.

Hydraulic conductivity estimates at background wells that were assigned to HSUs at background well locations were interpreted from transmissivity estimates and flow logs in a previous investigation (Garcia and others, 2010). Assigned transmissivities at background wells in NNSS areas 19 and 20 bound hydraulic property estimates in the northern and eastern extents of the model domain (Figure 3). Assigned transmissivities at background wells in NNSS area 18 and south of the Timber Mountain caldera complex structural margin bound hydraulic property estimates in the southern and western extents of the model domain. About 60 percent of the hydraulic conductivity values were estimated after assigning values at previous aquifer–test sites and defining rings of homogeneous hydraulic conductivities around pumping and observation wells (Figure 3).

Assigned pilot points also were used to distribute specific yield and specific storage (Figure 3). Specific yield was distributed with 126 adjustable pilot points at the water table. Specific storage was distributed with 646 adjustable pilot points. Specific yield of fractured rocks was expected to range between 0.001 and 0.05. Specific storage initially was assigned as 1.5 x 10–6 1/ft and was allowed to range between 1 x 10–8 and 4 x 10–5 1/ft. The range of estimated specific storages is greater than the expected range (U.S. Geological Survey, 2014). This large range was permitted to compensate for potential errors in the framework model. For example, a specific storage estimate of 1 x 10–7 1/ft can be reasonable if the simulated feature is 5,000 ft thick and the actual transmissive feature is 400 ft thick. Vertical-to-horizontal anisotropy was assumed equal to 1 and was not estimated.

Hydraulic properties were laterally interpolated between pilot points with kriging to node locations defined within 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 (a nugget was not specified). Hydraulic properties within an HSU were assumed vertically constant. Therefore, laterally interpolated hydraulic properties from each HSU were assigned to all layers within the HSU in the groundwater–flow model (Figure 5).

 

Locations of pilot points in numerical models.

Figure 3. Locations of pilot points simulated in numerical models for each of the 16 multiple-well aquifer tests.

 

Locations of pilot points in numerical models near pumped and monitored observation wells.

Figure 4. Locations of pilot points near pumped and monitored observation wells that were simulated in numerical models for each of the 16 multiple-well aquifer tests.

 

Table 4. Transmissivity estimates from previous aquifer tests.

Transmissivity estimates from previous aquifer tests

 

Cross-section near ER-EC-12 main (upper and lower zone)

Figure 5. Cross–section showing vertical discretization and modified hydrostratigraphic units near the ER–EC–12 main (upper and lower zone) numerical groundwater–flow model.

 

Groundwater–Flow Models

Drawdowns from each multiple–well aquifer test were interpreted with three–dimensional MODFLOW models (Harbaugh and others, 2000). Each model was centered on the pumping well and each model grid extended laterally about 200,000 ft (38 mi) from the pumping well. All models were about 5,900 ft thick and extended vertically from 1,700 ft below sea level to 4,200 ft above sea level, which is the approximate water table (Figure 5). Rows and columns in the grid were assigned widths of 100 ft at the pumped well. Row and column widths increased successively by a factor of 1.25 away from the pumped well until groundwater–flow model cell widths equaled hydrogeologic framework cell widths of 820 ft. Row and column widths were a constant 820 ft until the hydrogeologic framework model edge was reached (delineated by red box in Figure 6). Row and column widths increased successively by a factor of 1.25 away from the hydrogeologic framework edge (red box in Figure 6) to the groundwater–flow model edge at a lateral distance of about 160,000 ft (30 mi). The number of rows and columns in each model was relatively consistent, with 113 to 114 rows and 115 to 116 columns (Table 5). All external boundaries were specified no–flow boundaries. Changes in the saturated thickness of the aquifer system were not simulated because the maximum drawdown near the water table was small relative to the total thickness. Variable discharge rates during each aquifer test were simulated with multiple stress periods, which were determined from simplified pumping schedules for each pumping well (Table 5).

A common vertical discretization was used in all groundwater–flow models to avoid structural inconsistencies and their potential effects on hydraulic property estimates. Discretization of groundwater–flow model layers was finer between 2,300 and 3,700 ft above sea level (Figure 5) where most pumped intervals occur. Groundwater–flow model layers gradually thickened from 2,300 ft above sea level to the base of the models where vertical discretization was relatively coarse.

All groundwater–flow models were discretized vertically into 29 layers. The top elevation of layer 1 defined the water table and the bottom elevation of layer 29 was equivalent to the elevation at the base of the hydrogeologic framework model (Figure 5). Layer 1 was 1–foot thick to better approximate drainage from the water table. Groundwater–flow model layers 2 to 4 were each 164–ft thick. Groundwater–flow model layers 5 to 20 were each 82–ft thick to capture thin HSUs, such as the mUPCU and LPCU in the vicinity of borehole ER–EC–11. Some modified HSUs such as the TCA and mCHZCM occur in multiple groundwater–flow model layers (Figure 5). Other modified HSUs such as the BA/SPA are locally absent in different parts of the groundwater–flow and hydrogeologic framework models.

 

Groundwater-flow model grid for the ER-20-11 aquifer test

Figure 6. Groundwater–flow model grid for the ER–20–11 aquifer test.

 

Table 5. Pumping wells and number of layers, rows, columns and stress periods included in each groundwater–flow model.

Table of included wells

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 1,768 pilot points where 1,250 pilot points were adjusted using Parameter ESTimation routine (PEST) (Doherty, 2008a). About 60 percent of adjustable pilot points defined the specific storage and specific yield distributions. Differences between measured and simulated observations defined the goodness–of–fit or improvement of calibration. These differences, or residuals, were weighted and summed in the objective function,

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,

Root-mean-square error

 

Measurement and regularization observations controlled model calibration. The models used 68,834 drawdowns as measurement observations (Table 6). Regularization observations guided hydraulic conductivity and specific yield estimates toward preferred conditions within similar HSUs and in areas that were insensitive to measurement observations. This approach is Tikhonov regularization (Doherty, 2008a).

The number of drawdown measurement observations was reduced by averaging drawdowns from each well to 6–hour intervals. Averaging reduced the number of measurement observations from more than 870,000 to 68,834 (Table 6) and suppressed high–frequency noise. Reliable observations were assigned weights greater than or equal to 0.5 (Table 6). Reliable drawdowns were defined as drawdowns not affected by pumping head losses, heating effects, wellbore storage, abridged records, or leaking bridge plugs (e.g., leakage across a bridge plug used to isolate multiple completion zones, resulting in drawdown responses from pumping upper and lower zones in a pumping well). Reliable observations totaled about 48,000 and were observed in 145 of 204 pumping–observation well pairs. Reliable observation weights were reduced to values between 0.5 and 1 at distant well clusters where similar drawdown responses were observed in multiple wells so that model calibration would not be skewed toward these well clusters. For example, drawdown responses in wells ER–EC–6 shallow, ER–EC–6 intermediate, and ER–EC–6 deep to the ER–EC–11 main aquifer test were similar, and therefore were assigned weights of 0.5 to reduce sensitivity to these observations when calibrating the ER–EC–11m groundwater–flow model.

Drawdowns in a number of observation wells directly adjacent to and monitoring the pumped intervals were uncertain because the pumping well cluster was affected by head losses, heating effects, packer leakage, or wellbore storage. Measured drawdowns in well ER–20–8 #2 main during the ER–20–8 #2 main aquifer test were uncertain because of the strong correlation between pumping well head losses and aquifer response. Measured drawdowns in wells ER–EC–11 main and ER–EC–11 upper intermediate during the ER–EC–11 main aquifer test were very uncertain because of pumping head losses and heating effects. Measured drawdowns in observation wells ER–EC–13 shallow, ER–EC–13 intermediate, and ER–EC–13 deep during the ER–EC–13 main upper and lower zone aquifer tests were uncertain because of packer leakage. Measured drawdowns in observation wells ER–EC–15 shallow, ER–EC–15 intermediate, and ER–EC–15 deep during the ER–EC–15 main upper zone aquifer test were uncertain because of entry losses to the well and wellbore storage. In the above cases, measured drawdowns in observation wells in the annulus of the pumping well that were not adjacent to the pumped intervals were uncertain because of the effects of heating and leakage across bridge plugs.

Compromised observations in wells that were affected by head losses, heating effects, packer leakage, or wellbore storage in the pumping well cluster were assigned small weights (between 0.0001 and 0.1) so that hydraulic conductivity estimates were minimally affected. These effects were not simulated and can be significant where drawdowns exceed 100 ft and transmissivity of the pumped interval is less than 1,000 ft2/d. Parameter sensitivity also is proportional to the magnitude of simulated drawdowns, which can skew calibration toward fitting less certain measurements that are simulated poorly. Weights between 0.0001 and 0.01 were assigned to these observations because of measurement uncertainty, simulation inadequacy, and sensitivity adjustment. Measured drawdowns in observation wells in the annulus of the pumping well, which were not adjacent to the pumped intervals, but were affected by heating and leakage across bridge plugs, were assigned weights between 0.01 and 0.1 to reflect the uncertainty associated with these observations. Effects of weighting are reported with un–weighted and weighted sum–of–squares errors for each hydrograph (Appendix B).

 

Table 6. Pumping wells and number of observation wells and drawdown observations included in each groundwater-flow model.

Table of included wells

 

Tikhonov regularization limited hydraulic conductivity and specific storage estimates at pilot points to reasonable values (Doherty and Johnston, 2003) in the absence of observation data indicating otherwise. Sharp differences between nearby values in similar modified HSUs were penalized to ensure relatively continuous hydraulic conductivity, specific yield, and specific storage distributions.

Regularization observations were equations that defined preferred relations between hydraulic conductivity estimates, specific yield estimates, and specific storage estimates. Regularization observations affected calibration most where the models were insensitive to measurement observations. Using regularization observations to impose preferred states, such as homogeneity within HSUs, was preferable to assigning fixed hydraulic property values within HSUs. A homogeneity condition allows hydraulic conductivity, specific yield, and specific storage estimates to differ where dictated by measured drawdowns.

Homogeneity within modified HSUs was the primary preferred relation between pilot points that defined hydraulic conductivity and specific storage. All pilot points defined specific yield at the water table (layer 1) regardless of modified HSUs present, which minimized the variance between specific yield estimates. About 29,000 regularization observations constrained hydraulic conductivity, specific yield, and specific storage estimates with these preferred relations.

Unrealistic hydraulic conductivity, specific yield, and specific storage distributions were avoided by limiting the fit between measured and simulated observations (Fienen and others, 2009). Irreducible measurement and numerical model errors were specified with the expected measurement error, known as PHIMEAS in PEST (Doherty, 2008a), which is a weighted, sum–of–squares error. Water–level modeling results suggest that the expected measurement RMS error should be about 0.02 ft (Garcia and others, 2013), which equals a PHIMEAS of 21 ft2. A PHIMEAS of 19 ft2 was specified and measurement error was reduced to 20 ft2.

Drawdown Estimates

Drawdown estimates and their classification (i.e., detected, ambiguous, not detected) are documented previously in nine supporting memoranda (Table 1). Drawdown was classified as detected based on the signal–to–noise ratio and correlation between the pumping signal and environmental fluctuations. The signal–to–noise ratio is defined as the ratio of maximum drawdown in a well during an aquifer test to the RMS error. Correlation between the pumping signal and environmental fluctuations becomes apparent where observed drawdown can be approximated by a linear trend during all or part of the period of analysis. Correlation typically is possible as hydraulic diffusivity decreases, distance between observation and pumping well increases, or recovery is truncated. Correlation is unlikely where sharply defined pumping signals (saw–tooth) exist or significant recovery has been observed. Drawdown was classified as detected where the signal–to–noise ratio was greater than 10 and recovery was observed. Drawdown was classified as ambiguous when the signal–to–noise ratio ranged between 5 and 10, or greater than 10 if there was correlation or recovery was not observed. Drawdown was classified as not detected when the signal–to–noise ratio was less than 5.

Differences between measured and simulated drawdowns in observation well ER–EC–6 shallow, as determined from 9 groundwater–flow models and 14 of the 16 aquifer tests, respectively, are shown in Figure 7 to illustrate differences between detected, ambiguous, and not detected drawdowns. Drawdown estimates in well ER–EC–6 shallow were reported as 1) detected during 6 aquifer tests in ER–20–7, ER–20–8 main upper and lower zones, ER–20–8 #2 main, ER–20–11, and ER–EC–11 main; 2) not detected during 7 aquifer tests in ER–EC–12 main upper and lower zones, ER–EC–14 main upper and lower zones, and ER–EC–15 main upper, intermediate, and lower zones; and 3) ambiguous during the ER–EC–13 main lower zone aquifer test. Drawdown was not estimated in well ER–EC–6 shallow during the ER–20–4 main aquifer test because drawdown was not detected in ER–20–8 deep, which is 6,800 ft closer to ER–20–4 main than ER–EC–6 shallow. Drawdown also was not estimated in well ER–EC–6 shallow during the ER–EC–13 main upper zone aquifer test because water–level data were corrupted (Damar and others, 2013a).

Drawdowns were detected at distances of up to 5 mi from pumping wells. The maximum distance where drawdown was detected occurred between pumping and observation well pair ER–EC–14 main and ER–EC–1, respectively (distance of 4.8 mi). Detected drawdowns in observation wells not located in the annulus of the pumping well ranged from 0.05 to 0.95 ft (Appendix B).

 

Simulated and estimated drawdowns

Figure 7. Simulated (BLUE) and estimated (RED) drawdowns in well ER–EC–6 shallow as determined from 9 groundwater–flow models and 14 aquifer tests.

 

Area Investigated

Areal extent and volume of investigation were defined using maximum simulated drawdown, which is the maximum simulated drawdown at a given location within the model domain from any of the 16 multiple–well aquifer tests. For example, the hydraulic head lowered a maximum of 0.91 ft in well ER–EC–6 shallow (Figure 7) during the 20th day of the ER–20–11 main aquifer test, and corresponds to the maximum simulated drawdown at this well site. A maximum simulated drawdown threshold of 0.05 ft was the criterion used for detecting simulated drawdown, which was supported by residual errors from water–level modeling to estimate drawdowns (Garcia et al., 2013). The extent of investigation where the maximum simulated drawdown exceeds a 0.05 ft drawdown threshold is a two–dimensional area that was defined by the maximum simulated drawdown at any depth (Figure 8). A total area of 60 mi2 was investigated where maximum simulated drawdowns exceeded 0.05 ft (Figure 8). The widest areal extent of the 0.05 ft contour ranged from 9.5 to 10.3 mi.

Thirty–two of the thirty–four observation wells analyzed are located within the 0.05 ft drawdown area (Figure 8). These wells are predominantly located within the Bench area. Fewer observation wells (i.e., ER–EC–2A, ER–EC–13, and ER–EC–14) are located southwest of the Bench in the Timber Mountain moat structural domain and northeast of the NTMMSZ (i.e., ER–20–1, ER–20–4, ER–20–5, and ER–20–7) (Figure 8). Observation wells ER–20–2–1 and UE–18r are located outside the 0.05 ft drawdown area. Well ER–20–2–1 occurs within the mCHZCM north of the NTMMSZ and well UE–18r occurs within one of the Timber Mountain Composite Units south of the Bench.

 

Maximum simulated drawdown

Figure 8. Maximum simulated drawdown that occurred at any time during one of the 16 aquifer tests and hydraulic connections between pumping-observation well pairs.

 

Hydraulic-Property Estimates

Transmissivity within modified HSUs around well sites was estimated from simultaneous analysis of groundwater–flow models. The spatial distribution of transmissivity estimates within the investigated area (≥0.05 ft of drawdown) is shown in Figure 9. Total transmissivity ranged from 300 to 120,000 ft2/d in the investigated area.

Transmissivity estimates were highest near the northern and southern margins of the Bench (Figure 9). High transmissivity zones occur in the northeast part of the Bench near wells ER–20–8, ER–20–8#2, ER–20–11, and ER–EC–11, which are close to the NTMMSZ. High transmissivity zones also occur near the Timber Mountain caldera complex structural margin that borders the Bench to the south. At these margins, faulted and displaced rhyolitic lava flows, ash–flow tuffs, and welded–tuffs abut the margin of the Silent Canyon caldera complex to the north and the Timber Mountain caldera complex to the south. High transmissivity estimates could be attributed to enhanced permeability of disturbed zones near the contacts between two different structural features.

In general, estimated transmissivity for modified HSUs around well sites were greater within the Bench than within the caldera complexes to the north and south of the Bench (Table 7). Note that Figure 9 shows transmissivity estimates were greater within the Timber Mountain caldera complex than within the Bench, whereas Table 7 shows transmissivity estimates were greatest within the Bench. This anomaly occurs because transmissivity estimates in Table 7 are computed by averaging transmissivity within a 300–ft radius around the wells. The BA/SPA, which comprises lava flows, is the most transmissive modified HSU, with a transmissivity estimate of 86,000 ft2/d around the ER–20–8 well cluster. The mFCCU is the second–most transmissive modified HSU modeled around well sites, where the highest transmissivity estimate is 24,000 ft2/d around well ER–20–11. Transmissivity estimates for the mFCCU are uncertain because no pumping or observations wells are open to the unit. The Paintbrush Group is the most transmissive unit north of the Bench with a transmissivity estimate of 17,000 ft2/d around well ER–20–7. South of the Bench, transmissivity estimates are uncertain for the TMCM, which is a composite of welded–tuff aquifers and non–welded tuff confining units, largely because few wells penetrate the TMCM, and the area of investigation does not extend far into the Timber Mountain caldera complex. Transmissivity estimates within the TMCM and mCHZCM, located north of the Bench, were similar and ranged between 380 and 10,000 ft2/d.

 

Transmissivity distribution >= .05 ft during simulation

Figure 9. Transmissivity distribution within the investigated area where the maximum drawdown was equal to or exceeded 0.05 ft during simulation periods.

 

Table 7. Transmissivity estimates for modified hydrostratigraphic units surrounding observation and pumping well sites.

Transmissivity estimates

 

Transmissivity estimates around well sites (Figure 9) that are hydraulically connected to multiple pumping wells (Figure 8) are the most reliable. Transmissivity around well site ER–EC–11 main totaled 17,000 ft2/d (Table 7) and distributions among the HSUs likely are correct as drawdown at this well site was detected during 12 of the 16 aquifer tests (Table 7). More than 75 percent of the transmissivity at this well site is attributed to the BA/SPA. The dominance of transmissivity in the BA/SPA is consistent with higher concentrations of tritium that were measured in this HSU while drilling ER–EC–11 main (U.S. Department of Energy, 2012). Results shown here and from flow logs (Garcia and others, 2010) also indicate that most of the transmissivity at well sites ER–EC–1, ER–EC–6, and ER–EC–12 main occurs in the BA/SPA.

Specific yield and specific storage estimates averaged 0.02 and 3 x 10–6 1/ft, respectively, for TMCM, mFCCU, BA/SPA, Paintbrush group, and mCHZCM units. About 50 percent of the specific yield estimates ranged between 0.01 and 0.05 and about 20 percent of the estimates exceeded the expected range between 0.001 and 0.05.

 

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