WM’00 Conference, February 27 – March 2, 2000, Tucson, AZ
ALTERNATIVE FLOW MODELS AT YUCCA MOUNTAIN,
NEVADA;
STATE OF NEVADA-FUNDED RESEARCH
Linda Lehman and Tim P. Brown
Technical & Regulatory Evaluations Group, (T-Reg), Inc.
13231 Henning Circle NE * Prior Lake, MN 55372
Phone (612) 496-0594 Fax (612) 495-2097
ABSTRACT
In the hydrogeologic assessments of ground water pathways in complex systems such as
exist at Yucca Mountain, Nevada, heat may be used as a flow path tracer. Using heat
along with hydraulic head and chemistry measurements serves to constrain the results of
flow path analyses that have non-unique solutions. Heat and chemical tracers can give a
much more reliable answer to ground water flow directions in a complex system than the
use of hydraulic head alone. It has been known for some time that the Yucca Mountain
ground water system has a range of spatially distributed temperatures associated with it.
It also has been hypothesized that certain features in the water table surface
(embayments) are coincident with major faults. Unfortunately, the latest Total System
Performance Assessment (TSPA) of Yucca Mountain (November 1998) did not include
geologic structure, temperature, or chemistry data in their determination of saturated zone
ground water flow paths. It is our contention that DOE must utilize all relevant data
available to them in determining ground water flow paths and subsequent dose to
potential receptors, not just selective data sets.
A numerical model was constructed to evaluate flow paths to the accessible environment
at Yucca Mountain, Nevada. The model is fully three-dimensional, evaluates thermal
transport, and explicitly considers geologic structures as controls on the flow field. The
latest model results indicate major differences exist in flow path direction and velocity
when compared to the latest TSPA, which characterized the performance behavior of
Yucca Mountain in the DOE Viability Assessment document.
These conceptual differences are presented, along with supporting evidence from field
data generated by the Yucca Mountain Project and Nye County studies. Results of
calibrations against existing temperature and hydraulic head data will be shown. Recent
chemistry analyses performed by the USGS also support a different flow path than
analyzed in the TSPA or the DEIS.
The conclusions of the DOE Viability Assessment and the Draft Environmental Impact
Statement are questionable, since they have failed to analyze viable alternative models of
the groundwater flow field; alternatives which may have adverse impacts to local
populations.
WM’00 Conference, February 27 – March 2, 2000, Tucson, AZ
INTRODUCTION
In order to evaluate the conclusions of the Viability Assessment or the Draft
Environmental Impact Statement (DEIS) for Yucca Mountain, the basic underlying
assumptions in the latest Performance Assessment (PA) must be examined. The relative
importance of these assumptions to the PA also needs to be evaluated.
In conducting a performance assessment for Yucca Mountain, an accurate view of the
groundwater flow field is essential. The velocity of the groundwater is one of the most
important parameters in the transport equation. The direction of the groundwater
pathway is important as it dictates the hydrologic and geochemical character of the
pathway that influence sorption and other variables such as dilution in the saturated zone.
It has been known for some time that temperatures in the Yucca Mountain ground water
system show considerable spatial variability. It also has been observed that certain
features in the water table surface (embayments) are coincident with major faults. In the
hydrogeologic assessments of ground water pathways in complex systems such as exist at
Yucca Mountain, heat may be used as a flow path tracer. Using heat along with
hydraulic head and chemistry measurements serves to constrain the results of analyses of
flow paths, which have non-unique solutions. Heat and chemical tracers can give a much
more reliable answer to ground water flow directions in a complex system than the use of
hydraulic head alone. Unfortunately, the latest Total System Performance Assessment
(TSPA) of Yucca Mountain (1) did not include geologic structure, temperature, or
chemistry data in their determination of saturated zone ground water flow paths. It is our
contention that DOE must utilize all relevant data available to determine ground water
flow paths to potential receptors.
CONCEPTUAL MODEL
A numerical model was constructed to evaluate flow paths to the accessible environment
at Yucca Mountain, Nevada. The model is fully three-dimensional, accounts for thermal
transport, and explicitly considers geologic structures as controls on the flow field. The
latest model results indicate that major differences exist in groundwater flow vectors
(velocity and direction) when compared to the latest TSPA. The latest TSPA
characterized and calculated the performance behavior of Yucca Mountain in the DOE
Viability Assessment, in terms of dose to the Critical Group.
In order to evaluate an alternative conceptual model of saturated zone flow for the area
around and including Yucca Mountain, a numerical model, based on previous work done
by ourselves and other State of Nevada contractors, was assembled and tested. This
conceptual model postulates that faults and fractures dominate the flow of groundwater
through the volcanic tuffs underlying Yucca Mountain. The initial basis for this
conceptual model lies in observations of the potentiometric surface as interpreted by the
USGS and our own analysis of water table fluctuations and groundwater temperature
measurements.
WM’00 Conference, February 27 – March 2, 2000, Tucson, AZ
The implications of this conceptual model on performance may be significant, because
the potential for these fractured zones to transmit groundwater rapidly and transport
contaminants with minimal dispersion or adsorption, is high. In addition, seismic activity
in the region may result in unpredictable changes in the hydrogeologic system over time,
because earthquakes may adjust the flow properties of conduits or create new fracture
zones, which could cause major realignment of the system.
The above information resulted in a different conceptualization of the flow field than that
currently considered in the TSPA/VA or the DEIS. The proposed model is structurally
controlled by fault and fracture zones. Fracture zone intersections play a key roll in the
distribution of recharge, velocity fields and pathways. The proposed conceptual model is
also dynamic rather than static, and has the potential to change rapidly due to tectonic
movements.
The proposed conceptual model postulates that some water movement occurs across the
mountain block from east to west, primarily via discrete northwest trending fracture
zones. The Solitario Canyon Fault zone creates a resistance to eastern flow but does not
totally prevent it. A steep hydraulic gradient exists at the location of the Solitario Canyon
Fault and is equal to about 35 meters of head difference over a lateral distance of about
1000 meters. Water movement across this fault probably occurs as a result of
intersections with northwest trending shear zones and creates cascading flow to the next
lower level of the water table.
The proposed model of flow is shown in Figure 1. This figure shows the potentiometric
surface and the proposed flow paths. Some colder flow also enters the Yucca Mountain
block from the northwest across a very steep hydraulic gradient. This gradient is equal to
over 300 meters of head change across 2500 meters distance. The faults in the Drill Hole
Wash region no doubt play a role in the transport of water across this hydraulic barrier.
Where the Drill Hole Wash Fault or those near it intersect the northern extension of the
Solitario Canyon Fault, a potential breach may occur and allow the colder water north of
the steep gradient to move down this fault zone and subsequently into the Ghost Dance
Fault or Midway Valley Fault. The proposed model and the potentiometric surface
suggested that another fault zone exists just to the south of the repository footprint. This
fault zone was later identified when the “C- Well” tests were performed. This zone may
also be transporting water from the Solitario Canyon side of the block toward the east.
WM’00 Conference, February 27 – March 2, 2000, Tucson, AZ
Our conceptual model emphasizes fracture flow paths in the saturated and unsaturated
zones. We hypothesize that fault and fracture zones are inter-connected and dominate
saturated zone flow. The fracture “conduits” dominate flow paths in the proposed
repository block, as well as important connections between this block and adjacent
WM’00 Conference, February 27 – March 2, 2000, Tucson, AZ
structurally separated blocks. Further, these fracture conduits have hydrologic properties
that are sensitive to seismic events, especially fracture apertures.
Two-dimensional non-isothermal modeling efforts conducted in 1994 (2), by the State of
Nevada, concluded that upward flow from the carbonates was important and must be
accounted for by Yucca Mountain flow models, to be credible. This idea was also
independently developed by John Bredehoeft (3) in his work for Inyo County, California.
At well number UE-25 p#1, Bredehoeft calculated upward flux rates and fault
permeabilities derived from earth tide calculations. At the UE-25p#1 location, head in
the carbonates was approximately 20 meters higher than in the volcanics. (This location
remains the only measurement of temperature and head in the Paleozoic carbonate
aquifer in the area of Yucca Mountain.) Bredehoeft concluded that the upward
movement of hot water was coincident with major extensional faults, such as the Bow
Ridge in Midway Valley, which breach a very tight confining unit. This work was
published in January of 1998, though performed much earlier. Farrell et al. (4) and
Painter and Armstrong (5) state that buoyancy resulting from thermal instability may also
contribute to the observed heat effects.
Large temperature and hydraulic gradients also exist across the mountain block. These
observations and those mentioned above led to the conclusion that three-dimensional
non-isothermal models would be required to accurately model the flow field at Yucca
Mountain.
Stratigraphy and Hydrostratigraphic Units
The stratigraphic section utilized by the 3D model is assumed to be as described by
Dudley (6). In this conceptualization, flow in the carbonates is upward and discharges to
the south of the Yucca Mountain block. Flow in the volcanics is downward in the
northern reaches of the block and also discharges south of Yucca Mountain where the
volcanics are pinched out by the carbonates.
This hydrostratigraphic section was modeled as three layers. The first layer represents
the aquifer series of upper Miocene tuffs. Layer 2 represents the lower Miocene volcanic
confining units. The Eleana Formation is not known to be present under Yucca
Mountain. Since its properties would also be that of a confining unit, it has not been
specifically singled out in these analyses. Layer 3 represents the Paleozoic carbonate
aquifer. In this modeling exercise Layer 3 is implicitly included through the use of
boundary conditions.
Structure
The conceptual model is that of a fracture flow system where flow paths and velocities
are controlled by the existing fracture networks. The two dimensional analyses done in
1994, (2) indicated that a significant upward component of flow would be possible on the
west side of Yucca Mountain due to the high temperatures noted at the water table at
WT-10 and WT-7 along the alignment of the Solitario Canyon Fault indicating potential
vertical permeability. The Solitario Canyon Fault also appears to be influencing the
WM’00 Conference, February 27 – March 2, 2000, Tucson, AZ
hydrology in a lateral sense by creating a “medium hydraulic gradient” from west to east
near the repository block and further south, possibly ponding water behind it. Measured
heads range from about 775 meters to the west of the fault and drop abruptly to about 730
meters on the eastern side. The Solitario Canyon Fault is a major north-striking scissors
fault, which to the south is down thrown on its western side and to the north is down
thrown on the eastern side (8) and may exhibit differing hydraulic properties along its
length.
Northwest trending strike-slip faults exhibiting right lateral movement also seem to play a
role in influencing the hydraulic potentials at the site. Strike-slip faulting may provide
vertical conduits to flow and in some cases barriers to horizontal flow and have been
linked to tectonic activity in the Walker Lane Belt, a large northwest-trending structural
zone that parallels most of the southwest border of Nevada (7). For example, note the
potentiometric surface contours at the Drill Hole Wash fault, Sun Dance fault and the
southerly fault location, as shown earlier on Figure 1. This figure is a potentiometric
surface map drawn by Lehman and Brown (2). Areas of fault intersections are important
and may act as drains in the northern regions and conduits for upwelling in the southern
regions of the mountain block.
These fault and structural related flow properties are different from those assumed by the
US DOE in their analyses of the performance of Yucca Mountain in both the DEIS and in
the Viability Assessment. The analyses performed by the US DOE do not utilize heat,
chemistry or structures such as faults and fractures. Rather, potentiometric surface maps
utilized by the US DOE show very smooth contour intervals, which fail to show
embayments and ignore their consequences. Failure to include these data may lead to
erroneously conservative estimates of repository performance and ultimately, dose
consequences to the citizens of Nevada.
Modeling Objectives
The primary objective was to calculate a steady state flow field in which the measured
head and temperature measurements will be matched as closely as possible. In addition
to calculating the steady state velocity field, one objective of this exercise was to
ascertain which faults exhibit control on the flow field and which do not.
Model Grid
There are three layers each possessing 396 cells in an 18 x 22 grid. In this layering, a few
of the faults are explicitly included and specific hydrologic properties have been
individually assigned to them. The key, which defines the hydrologic properties assigned
to each grid zonation, is given as Table I.
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Table I. Parameters used for the VTOUGH Saturated Zone Model
Material
Porosity
X Permea- Y Permea- Z Permeability (m2) bility (m2) bility (m2)
Wet Conductivity
(W/m-oC)
TUFF1
TRANS
TIGHT
TUFF2
FRAC1
FRAC2
FRAC3
FZON1
FZON2
FZON3
CONF1
CARB
0.3
0.2
0.2
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.2
0.2
9.000E-10
8.000E-10
1.000E-12
1.000E-09
9.000E-05
7.000E-08
1.000E-08
1.000E-04
1.500E-11
5.000E-07
1.000E-13
4.000E-12
9.000E-10
8.000E-10
1.000E-12
1.000E-09
9.000E-05
7.000E-08
1.000E-08
1.000E-04
1.500E-11
5.000E-07
1.000E-13
4.000E-12
9.000E-10
8.000E-11
1.000E-13
0.000E-09
9.000E-05
7.000E-08
1.000E-08
1.000E-08
1.500E-11
5.000E-07
1.000E-13
4.000E-12
1.35
1.35
1.35
1.35
1.35
1.35
1.35
1.35
1.35
1.35
1.35
1.35
Specific
Hear
Capacity
(J/kg-oC)
50
50
50
50
50
50
50
50
50
50
50
50
Initially, all permeabilities, vertical and horizontal were set equal, as there is no
information available to indicate differences. The initial permeabilities of the tuff units
were assigned the average value as measured by the USGS and the DOE. The
permeabilities in the fault zones were then adjusted arbitrarily until the potentiometric
surface, including the embayments were matched.
Layer 1 represents the volcanic aquifer. Thickness of this unit generally decreases from
north to south. In this modeling exercise, the layer is set uniformly at 500 meters in
thickness. At UE-25-p#1 the thickness is approximately 800 meters. While this
difference in thickness may influence the outcomes somewhat, we felt that for this very
simple model, it was more important to assess the effects of structure. Permeability
increases generally from north to south. The Tight Unit comprises the northern boundary
and creates the large hydraulic gradient conditions. The Transitional Unit (TRANS)
represents a permeability transition to the generally more transmissive tuff properties
(TUFF1 & 2). This unit was necessary to maintain numerical stability and to simulate the
observed potentiometric surface, although in terms of numerical stability, smaller grid
cell size may have accomplished the same result. Fault hydraulic properties range from
less permeable than or more permeable than the tuffs, in some cases causing barriers and
in others conduits. Explicitly included in Layer 1 are the Solitario Canyon, the Ghost
Dance and the Bow Ridge faults. Three northwest trending strike-slip faults are included
as Drill Hole Wash, Sun Dance and a third unknown fault zone (name unknown to us at
the time of this document) just south of the repository horizon.
Boundary conditions are head and temperature on the northern boundary and head and
temperature at two positions along the southern boundary at WT 11 and WT-12. In a few
simulations, points for head and temperature were also placed along Solitario Canyon to
the west of the mountain, near WT-7 and WT-10. The eastern boundary represents the
WM’00 Conference, February 27 – March 2, 2000, Tucson, AZ
Forty Mile Wash. It is represented as a no flow boundary condition, but no pressures or
temperatures were assigned along this boundary.
Layer 2 has the same number of elements (396) as does Layer 1. This layer represents
the lower volcanic confining unit. This unit is about 400 meters in thickness at UE-25p#1. In these simulations it is held constant at 500 meters in thickness. It is represented
by the Lithic Ridge and older tuffs down to the Tertiary/Paleozoic contact. The volcanic
system appears to be acting as an aquitard at its lower boundary with the carbonates, as
interpreted from UE - 25p#1 data by Bredehoeft, (3) 1998. The head increases slightly in
the Lithic Ridge Tuff and significantly increases in the Older Tuffs beneath the Lithic
Ridge, to a potential of 752.2 meters ASL. At UE - 25p#1 this aquitard lies at a depth of
873 meters. The Tertiary/Paleozoic contact is at a depth of approximately 1,200 meters.
Rock properties assigned to this layer are the same for the fault zones in Layer 1, but the
rest of the grid is assigned the lowest permeability of 10-12 m2 (TIGHT). Bredehoeft, (3)
indicates that this confining unit must be quite tight in order to sustain earth tides. This
permeability was adjusted to best match measured head and temperature values in the
lower volcanic confining units. Upward vertical gradients are apparent in a number of
the 10 wells that have measured data with depth. No boundary conditions are assigned to
this unit and it is allowed to float, or equilibrate naturally.
Layer 3 is the carbonate aquifer, which apparently discharges in Ash Meadows, and
generally to the south, southeast and possibly southwest of the Yucca Mountain block.
As mentioned earlier, it is simulated via constant temperature and head boundary nodes
of 752 meters and 57 degrees C, and does not actually exist as a separate layer. Flow will
therefore be upward from the carbonates into the tuffs. In the southeastern section of the
grid heads are 20 meters above those in the volcanics.
Data Sets
In 1994, the USGS published a report entitled Revised Potentiometric Surface Map,
Yucca Mountain and Vicinity, Nevada (9). In this report, the USGS has undertaken to
correct the earlier water level measurements by re-surveying the elevations of the wellheads and by correcting for temperature and density. The earlier potentiometric surface
map showed an embayment in the vicinity of Drill Hole Wash. The USGS explains that
they had removed the embayments by selectively discarding the revised water level data,
because they saw no physical reason for the hydraulic lows. Fortunately, this report
contained the actual revised data, so we were able to re-plot the potentiometric surface
utilizing all the new data and the embayments are clearly visible again. There are three
major embayments, which appear to be coincident with the NW-SE trending strike-slip
Drill Hole Wash Fault, Sundance Fault and an unnamed fault just south of the repository
footprint. We believe that this is the correct potentiometric surface and we have
attempted to reproduce this surface in our numerical model.
Other data are available in terms of hydraulic head on which to calibrate model outputs.
There are 10 sites that have been measured at multiple depth intervals. These sites and
WM’00 Conference, February 27 – March 2, 2000, Tucson, AZ
values are given in Luckey et al (7). Many of these wells exhibit increasing head with
depth, especially when the lower volcanic confining unit is reached.
Additional information on the pathways in which groundwater may be moving is
obtained from the temperature distribution. The data set utilized for this work is that of
Sass, et al. (10). The temperature distribution is shown as Figure 2. This temperature
distribution was utilized to calibrate the flow model.
Zell Peterman of the USGS has developed plots of major ion and isotope chemistry.
These plots were distributed at the NWTRB January 1998 meeting in Amargosa. These
plots should also be helpful in delineating flow paths. Models should be consistent with
these data, to the extent possible. More recently, the USGS has analyzed split samples
obtained by Nye County Early Warnng Drilling Program (11), which also supports a
southerly flow path.
Model Results
Numerous simulations were carried out as part of this modeling exercise in the
calibration and sensitivity analyses phases. We have selected two simulation outputs as
our final product and have examined the resultant velocity fields. These model results
are believed to be representative of the flow system at Yucca Mountain. The model,
while simple, allowed us to examine the relationship of the head distribution to the
position of major faults. It is our conclusion that the major faults included in this model
significantly affect the observed head distributions. The head and temperature
distributions for the immediate vicinity of the repository on the Yucca Mountain block
are believed to be correct.
Figure 3 represents the Layer 1 potentiometric surface. We believe that the general shape
of the surface matches quite well with Figure 1. This plot has reproduced the observed
embayments by adjusting the permeability of the northwest trending shear zones. One
feature that is significant is the flattening of the 729.6 meter contour line. As can be seen,
this flattened area coincides with the Ghost Dance Fault nodes and allows for the
southerly movement of water along the fault.
Figure 4 is the temperature distribution in Layer 1. We believe that we have matched to
shape of the Sass distribution. One could argue that the 30-degree contour is a little far to
the south, and that this position should represent the 32-degree contour line. However, we
felt that given the limited data for this site, coming within a degree or two of the actual
distribution was an acceptable error.
Where we have had difficulty with these simulations is in the higher than observed
temperatures in the southeastern quadrant, just west of the Forty Mile Wash region. The
authors believe that this condition could be corrected by adding additional cold recharge
to the area. For our final runs we added 10 cm/yr recharge along the top of the mountain
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block and along the Drill Hole Wash area eastward to Midway Valley. The additional
recharge in this area is justified as potential recharge could be coming from higher
elevations at Yucca Mountain, or along Forty Mile Wash. The amount of recharge is
consistent with average estimated recharge at Yucca Mountain.
We believe that our latest 3D runs have matched the Yucca Mountain and Forty Mile
Wash temperatures fairly accurately. However, the very much higher heads and
temperatures utilized as boundary conditions in the carbonates dominate all attempts to
lower the temperature through varying the vertical permeability tensor. The only way to
cool the upper layers was to add more advective flux or recharge.
In a comparison of measured temperatures and heads with modeled temperatures and
heads, the heads at WT 7 and WT 10 are much to low, 740 meters ASL as opposed to the
actual of 775 meters. The general trend of the modeled heads, i.e., upward or downward
appears to be accurate over the mountain block with a few exceptions. As mentioned
previously, the western side of the modeled region has several instances of heads being
too low. Another instance is with H-5. This could indicate that there are areas of
ponding, such as west of Solitario Canyon in lower Crater flat, or in the case of H-5,
upward head has been underestimated for the head boundary condition in the carbonates
(750 meters as opposed to the measured 775 meters)
The temperature distribution in the lower Crater Flat area is also too low, running near 30
degrees rather than the observed 38 degrees. One reason for this is that the actual well
placement is right in the Solitario Canyon Fault, while our simulations place the Solitario
Canyon Fault three grid cells to the east. Also the Solitario Canyon Fault dips to the
west, where in our simulations the fault is vertical. The higher temperatures arise in this
simulation coincident with the simulated Solitario Canyon Fault and come within two
degrees of those measured at WT-7 and WT-10.
CONCLUSIONS
In summary, we believe that our simple 3-D non-isothermal model of Yucca Mountain
has been instructive in ascertaining areas where more data are needed to sort out
alternative flow models and alternative flow paths.
The conclusion of this research and reviews conducted for the State of Nevada indicate
that major deficiencies exist in the current TSPA and those utilized in the DEIS and the
Viability Assessment. Therefore, no confidence can be placed in conclusions of these
documents unless all data are analyzed in the determination of potential groundwater
flow pathways and radionuclide transport to potential receptors. Reasonable alternative
flow paths must be analyzed to ascertain significance to outcomes. While the US DOE
acknowledges that reasonable alternatives exist, they have not analyzed their significance
in the latest performance assessments.
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REFERENCES
1.
USDOE, “Viability Assessment for Yucca Mountain and Total Systems
Performance Assessment”, Washington, D.C. November (1998).
2.
Lehman L.L. and T.P. Brown, “Alternate Conceptual Models in the Saturated
Zone at Yucca Mountain”, Presentation to the Nuclear Waste Technical Review
Board, Reno, NV, April (1994).
3.
Bredehoeft, J.B, Water Resources Research, January (1998).
4.
Farrell et al., “Structural Controls on Groundwater Flow in the Yucca Mountain
Region”, Center for Nuclear Waste Regulatory Analyses, SRI, San Antonio, TX
(1999).
5.
Painter, S. and Armstrong, A., “On the Origin of the Groundwater Temperature
Variations Near Yucca Mountain, Nevada”, Proc. AGU Fall Meeting, Vol. 80,
No.46, (1999).
6.
Dudley, A.L., R.R.Peters, J.H. Gauthier, M.L. Wilson, M.S. Tierney and E.A.
Klavetter, Total System Performance Assessment Code (TOSPAC) Volume 1:
“Physical and Mathematical Bases”. Sandia National Laboratories, SAND850002, Albuquerque, New Mexico. (1988).
7.
Luckey, R, P. Tucci, C. Faunt, E. Ervin, W. Steinkampt, F. D’Agnese and G.
Patterson. “status of Understanding of the Saturated-Zone Ground-Water Flow
System at Yucca Mountain, Nevada, as of 1995”, USGS WRI Report 96-4077,
(1996)
8.
Scott, R.B. and J. Bonk, Preliminary Geologic Map of Yucca Mountain, Nye
County, Nevada, with Geologic Sections, USGS Open File Report 84-494.
(1984).
9.
Ervin, E.M., R.R. Luckey and D.J. Burkhardt, Revised Potentiometric Surface
Map, Yucca Mountain and Vicinity, Nevada, USGS, Water Resources
Investigations Report 93-4000, (1994).
10.
Sass, J.H., A.H. Lachenbruch, W.W. Dudley Jr., S.S. Priest, and R.J. Munroe,
Temperature, Thermal Conductivity and Heat Flow Near Yucca Mountain,
Nevada: Some Tectonic and Hydrologic Implications, USGS Open File Report
87-649, (1988).
11.
Stellavato, Nick, “Transmittal letter for the Nye County Early Warning Drilling
Program (EWDP) Phase I Data Report”, Pahrump, NV, April 20, (1999).