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Aquifer Tests

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ER–EC–13 Main Upper Zone and Main Lower Zone

Primary Investigators: Keith J. Halford, USGS

Well Data

USGS Site ID
Local Name Altitude Uppermost
Opening
Lowermost
Opening
Primary Aquifer Transmissivity (ft2/d)

Aquifer Test

All Aquifer Test Files (zip)

 

ER–EC–13 Main Upper Zone and Main Lower Zone

 

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

 

A pair of aquifer tests were conducted by Navarro–Intera, LLC (N-I) in wells ER–EC–13 main upper zone and ER–EC–13 main lower zone at Pahute Mesa on the Nevada National Security Site (NNSS) in southern Nevada (Figure 1). Hydraulic conductivities of tuff confining units and lava–flow aquifers within the Fortymile Canyon composite unit and total transmissivity were estimated. Drawdowns in all three observation wells at the ER–EC–13 well cluster site (Table 1, deep, intermediate and shallow) during both aquifer tests were interpreted simultaneously so that estimated hydraulic properties were consistent. Well ER–EC–13 main upper zone was pumped intermittently between June 22, 2012 and August 2, 2012. Well ER–EC–13 main lower zone was pumped for development and a constant–rate test between March 7, 2013 and March 29, 2013. Hydraulic properties estimated from the aquifer tests in wells ER–EC–13 main upper zone and ER–EC–13 main lower zone can be used to constrain estimates of radionuclide transport through volcanic rocks beneath Pahute Mesa, NNSS.

 

Figure 1— Well construction, lithology, and location of ER–EC–13 well cluster, Pahute Mesa, Nevada National Security Site and vicinity. (Observation and background wells were monitored, but not used in this interpretation.).

 

Table 1— Well location and construction data for analyzed wells in ER–EC–13 cluster, Pahute Mesa, Nevada National Security Site.

 

SITE AND GEOLOGY

The aquifer tests occurred beneath Pahute Mesa in the northwest corner of NNSS where transport of radionuclides is a concern (Laczniak and others, 1996). The three wells that were monitored during 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. The ER–EC–13 well cluster is located south of the Bench area (Figure 1). Well ER–EC–13 main; penetrates about 1,000 ft of unsaturated rock, and 2,000 ft of saturated rock where it produces water from lava–flow aquifers within the Fortymile Canyon composite unit, FCCM (Figure 2).

 

Figure 2— Lithology, alteration, hydrogeology, and well completion at ER–EC–13 well cluster Pahute Mesa, Nevada National Security Site.

 

The lithologies of major water–producing hydrostratigraphic units in the aquifertest area are stoney, vitrophyric, and pumiceous lavas (U.S. Department of Energy, 2011). An undifferentiated, lava–flow aquifer was identified as the contributing unit for both screens of the well ER–EC–13 main (Figure 2). The Lava–flow aquifer was differentiated into three zones so that heterogeneous hydraulic conductivities could be estimated (Figure 2). Ash–flow tuffs adjacent to the lava–flow aquifers are nonwelded and zeolitized. Similar units at the NNSS typically are characterized as confining units (Laczniak and others, 1996, p.11; U.S. Department of Energy, 1997).

 

PUMPING AND WATER–LEVEL CHANGES

Well ER–EC–13 main has upper and lower screened intervals that were isolated with a packer (Figure 2). The upper screen of well ER–EC–13 main is coincident with the open interval of well ER–EC–13 intermediate, and the lower screen of well ER–EC–13 main is coincident with the open interval of well ER–EC–13 deep. Upper and lower screens of well ER–EC–13 main produce water from the FCCM.

 

Approximately 3 million gallons were withdrawn from ER–EC–13 main upper zone for well development prior to the constant–rate test. The constant–rate test lasted about 123 hours from 7/3/2012 to 7/8/2012. Discharge during the constant–rate test averaged 303 gal/min with a total groundwater withdrawal of about 2.2 million gallons. Total withdrawal during the period of well development and testing (through July 21) was about 5.2 million gallons (Figure 3).

An additional 2.8 million gallons were pumped from the upper and lower screened intervals of well ER–EC–13 main between 7/21/2012 and 8/2/2012 (Figure 3). The pump and packer were reset prior to 7/21/2012 so that ER–EC–13 main lower zone could be developed and tested. Pumping ceased after realizing that significant flow inadvertently came from the upper zone because the packer was leaking. The additional pumpage does not affect this analysis, but is significant to responses in distant observation wells.

Water-levels were measured in wells ER–EC–13 shallow, ER–EC–13 intermediate, and ER–EC–13 deep during development and testing of ER–EC–13 main upper zone. Water levels measured in wells ER–EC–13 intermediate and ER–EC–13 deep were affected minimally by thermal expansion because transducers were submerged 1,057 and 1,608 ft, respectively, prior to pumping. Heating affected water columns at depths of less than 1,900 ft below land surface which affected water levels measured in well ER–EC–13 shallow where the transducer was submerged less than 50 ft.

Drawdowns in wells ER–EC–13 intermediate and ER–EC–13 deep were estimated by subtracting water levels prior to pumping from measured water levels. Environmental fluctuations were ignored in these wells because barometric and tidal changes were less than the measurement resolution of the transducers (0.1 ft). Maximum drawdown in well ER–EC–13 intermediate, adjacent to the pumping well, during the constant-rate test was about 60 ft (Figure 3).

 

Figure 3— Pumping from ER–EC–13 main upper zone during well development and aquifer testing, June&ndashJuly, 2012 and pumping from ER–EC–13 main lower zone after July 21, 2012 where packer leaked.

 

Drawdowns in well ER–EC–13 shallow were indeterminate because of thermal expansion of about 900 ft of water column. Water–levels changed about 0.2 ft during development and testing of well ER–EC–13 main upper zone. These changes could be resolved to within 0.01 ft because the transducer had a smaller measurement range than the deep transducers in wells ER–EC–13 intermediate and ER–EC–13 deep. Transducer submergence was less than 50 ft in well ER-EC-13 shallow, which allowed thermal expansion to effect water-level measurements. Most all of the observed change could be explained by thermal expansion. Maximum drawdown in well ER–EC–13 shallow was assumed to be less than 0.1 ft.

 

Approximately 1.8 million gallons were withdrawn from ER–EC–13 main lower zone for well development prior to the constant–rate test. The constant–rate test lasted about 219 hours and was conducted from 3/20/2013 to 3/29/2013. The analyzable period ended on 3/25/2013 after 6 days of pumping because packer leakage increased and water levels rose (Figure 4). Discharge during the constant–rate test averaged 303 gal/min with a total groundwater withdrawal of about 4.0 million gallons. Total withdrawal during the period of well development and testing was about 5.8 million gallons (Figure 4).

 

Water–levels were measured in wells ER–EC–13 shallow, ER–EC–13 intermediate, and ER–EC–13 deep during development and testing of ER–EC–13 main lower zone. Water-level changes measured in wells ER–EC–13 intermediate and ER–EC–13 deep were affected minimally by thermal expansion because transducers were submerged 1,061 and 1,607 ft, respectively, prior to pumping. This was because most of the thermal expansion occurred above the transducers so measured pressures were affected minimally. Heating did affect water levels measured in well ER–EC–13 shallow where the transducer was submerged less than 50 ft.

 

Drawdowns in wells ER–EC–13 intermediate and ER–EC–13 deep were estimated by subtracting water levels prior to pumping from measured water levels. Environmental fluctuations were ignored in these wells because barometric and tidal changes were less than the measurement resolution of the transducers. Maximum drawdown in well ER–EC–13 intermediate, adjacent to the pumping well, during the constant–rate test was about 290 ft (Figure 4).

Drawdowns in well ER–EC–13 shallow were indeterminate because of thermal expansion. Maximum drawdown in well ER–EC–13 shallow was assumed to be less than 0.1 ft.

 

Figure 4— Pumping from ER–EC–13 main lower zone during well development and aquifer testing, March, 2013

 

ANALYSIS

Hydraulic conductivities of Upper, Middle, and Lower zones of the lava–flow aquifer and adjacent tuff confining units (Figure 2) were estimated at the ER–EC–13 well cluster by simultaneously minimizing differences between simulated and measured drawdowns during both aquifer tests. Drawdowns were simulated with two–dimensional, radial MODFLOW models (Harbaugh and others, 2000). The models were identical except that well construction differed between upper–zone and lower–zone aquifer tests. Parameter estimation was performed by minimizing a weighted sum-of-squares objective function with PEST (Doherty, 2008).

 

The production well and aquifer system were simulated with an axisymmetric, radial geometry in a single MODFLOW layer in each model (Langevin, 2008). Radial distance increased with increasing column indices and depth increased with increasing row indices. Hydraulic conductivities and storages of the ith column were multiplied by 2πri to simulate radial flow where ri was the distance from the outer edge of the first column to the center of the ith column.

 

Models extended from a production well to more than 200,000 ft away and from the water table to 3,000 ft below land surface. The model domain was discretized into a layer of 41 rows of 53 columns. Cell widths ranged from 0.05 ft adjacent to the production well to 40,000 ft in the farthest column. Vertical discretization (row height) was a uniform 50 ft, except for a 1–ft thick layer at the water table so specific yield values could be specified directly. All external boundaries were at sufficient radial distance to be specified as no-flow. Changes in the wetted thickness of the aquifer were not simulated because the maximum drawdown near the water table was small relative to the total thickness. Discharges during the constant-rate tests were simulated with a single stress period. Initial heads were specified as 0.

 

The production well was simulated as a high conductivity zone with vertical conductances multiplied by 109. Water was removed from the uppermost node in a well and MODFLOW was allowed to apportion inflow to the well. Wellbore storage associated with the production well was simulated, but early–time data (where wellbore storage is active) was not compared in the objective function.

 

Potential leakage around packers also was simulated with two cells in the well between the upper and lower zones that could be estimated independently during each aquifer test. Packer leakage seemed likely because most of the drawdown in the unpumped interval occurred during the first hour of each aquifer test followed by very little additional drawdown over the remaining test period. For example, a drawdown of 1.5 ft was observed in well ER–EC–13 deep an hour after pumping began from well ER–EC–13 main upper zone (Figure 5). Less than 0.2 ft of additional drawdown occurred during the next 5 days of pumping well ER–EC–13 main upper zone.

 

Figure 5— Pumping from ER–EC–13 main lower zone during well development and aquifer testing, March, 2013

 

HYDRAULIC PROPERTY ESTIMATES

Hydraulic properties of the geohydrologic column and wells were estimated at the ER–EC–13 well cluster. Horizontal hydraulic conductivities of Upper, Middle, and Lower zones of the lava–flow aquifer were estimated independently. A single hydraulic conductivity was assigned and estimated for the tuff confining units that overlie and underlie the lava–flow aquifer (Figure 2). Two uniform hydraulic conductivities were assigned to the well screens of the upper and lower zones and were estimated independently. Different hydraulic conductivities were assigned and estimated for the packer during each aquifer test. A uniform vertical–to–horizontal anisotropy of 1 was assigned from the water table to the base of the aquifer. Uniform specific storage and specific yield of 2.0E–6 1/ft and 0.02 dimensionless, respectively, were assigned throughout the geohydrologic column and were not estimated (Halford and others, 2010).

 

Simulated and measured drawdowns matched within the error of the measurements (Figures 5 & 6). RMS errors of 0.3, 0.1, and 0.1 ft in wells ER–EC–13 intermediate, ER–EC–13 deep, and ER–EC–13 shallow, respectively, during the constant–rate aquifer test of well ER–EC–13 main upper zone were similar to the noise in the respective measurements (Figure 5). RMS errors of 0.9, 0.06, and 0.03 ft in wells ER–EC–13 deep, ER–EC–13 intermediate, and ER–EC–13 shallow, respectively, during the constant–rate aquifer test of well ER–EC–13 main upper zone were similar to the noise in the respective measurements (Figure 6).

 

Transmissivity of the Fortymile Canyon composite unit totaled 5,000 ft2/d. Transmissivity estimates for the Upper and Lower zones were 3,600 and 1,400 ft2/d, respectively (Table 2). Vertical leakances of the tuff confining units were indeterminate because drawdowns were not detected in well ER–EC–13 shallow during either aquifer test. Drawdowns ceased to be simulated in well ER–EC–13 shallow where hydraulic conductivities of the tuff confining units were less than 0.0001 ft/d or vertical leakances were less than 1.0E–7 1/day (Table 2). The middle zone of the lava–flow aquifer is a low permeability interval with a vertical leakance of less than 3.0E–7 1/day. Transmissivities and vertical leakances are reported because these values will change little if the geohydrologic units are reinterpreted and thicknesses change.

 

Table 2— Hydraulic property estimates at ER–EC–13 well cluster.

 

Drawdowns in unpumped intervals of the lava–flow aquifer resulted from packer leakage rather than flow across the middle zone. Alternative models were created and calibrated where hydraulic conductivities of 0.0001, 0.001, 0.01, and 0.1 were assigned to the middle zone. Simulated drawdowns in well ER–EC–13 intermediate during the constant-rate aquifer test of well ER–EC–13 main lower zone ceased to match measured drawdowns where hydraulic conductivities of the middle zone exceeded 0.001 ft/d(Figure 7). The shape of the drawdowns also diverged as simulated drawdowns increased gradually during the 5–day test while measured drawdowns increased rapidly during the first hour and little during the remainder of the pumping period. Simulated drawdowns in well ER–EC–13 deep during the constant–rate aquifer test of ER–EC–13 main upper zone (Figure 8) were similar to simulated drawdowns in well ER–EC–13 intermediate from the ER–EC–13 main lower zone test (Figure 7) because packer leakage controlled drawdowns in the unpumped zone.

 

Figure 6— Measured and simulated drawdowns during the constant–rate aquifer test of well ER–EC–13 main lower zone in wells ER–EC–13 shallow, ER–EC–13 intermediate, and ER–EC–13 deep.

 

Figure 7— Measured and simulated drawdowns in well ER–EC–13 intermediate during the constant–rate aquifer test of well ER–EC–13 main lower zone. Drawdowns were simulated with alternative models where different hydraulic conductivities of the middle zone were specified.

 

Figure 8— Measured and simulated drawdowns in well ER–EC–13 deep during the constant–rate aquifer test of well ER–EC–13 main upper zone. Drawdowns were simulated with alternative models where different hydraulic conductivities of the middle zone were specified.

 

REFERENCES

Cooper, H.H., and Jacob, C.E.. 1946. A generalized graphical method for evaluating formation constants and summarizing well field history. American Geophysical Union Transactions v. 27, pp. 526–534.

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

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

Halford, K.J., Fenelon, J.M., Reiner, S.R., and Sweetkind, D.S., 2011, Estimates of drawdowns from ER–20–7 and ER–20–8 main upper zone multi–well aquifer tests and simultaneous numerical analysis of four recent aquifer tests to estimate hydraulic properties on Pahute Mesa, Nevada National Security Site: U.S. Geological Survey Aquifer-Test Package, available at ’ Nevada Water Science Center Aquifer Tests ‘ webpage, accessed February 9, 2012, 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. http://pubs.er.usgs.gov/publication/wri964109

Langevin, C. D., 2008, Modeling Axisymmetric Flow and Transport. Ground Water, 46: 579–590. doi:10.1111/j.1745-6584.2008.00445.x

Theis, C.V., 1935, The relation between the lowering of the piezometric surface and the rate and duration of discharge of a well using groundwater storage: American Geophysical Union Transactions, v. 16, pp. 519-524.

U.S. Department of Energy, 1997, Completion report for well cluster ER–20–5: U.S. Department of Energy Report DOE/NV-466/UC-700.

U.S. Department of Energy, 2009, Phase II corrective action investigation plan for Corrective Action Units 101 and 102——Central and western Pahute Mesa, Nevada Test Site, Nye County, Nevada: U.S. Department of Energy Report DOE/NV——1312, Rev. 2, 255 p.

U.S. Department of Energy, 2011, Completion Report for Well ER–EC–13 Corrective Action Units 101 and 102: Central and Western Pahute Mesa: U.S. Department of Energy Report DOE/NV——1448.

 

APPENDIX A. CONSTRUCTION DIAGRAM WELL CLUSTER ER–EC–13

As-built diagram of the well completion for well cluster ER–EC–13 which includes the wells ER–EC–13 main, ER–EC–13 deep, ER–EC–13 intermediate, and ER–EC–13 shallow (U.S. Department of Energy, 2011).

 

Figure 7–1— As–Built Completion Schematic for Well ER–EC–13

 


 

 

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