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Geophysical Methods Used To Guide Hydrogeological
Investigation At An UMTRA Site Near Grand Junction, Colorado
Reference: Wightman, W. E., Martinek,
B. C., and Hammermeister, D., "Geophysical Methods Used to Guide
Hydrogeological Investigations at an Umtra Site Near Grand Junction,
Colorado," Current Practices in Ground Water and Vadose Zone
Investigations, ASTM STP 1118, David M. Nielden and Marin N. Sara, Eds,.
American Society for testing and Materials, Philadelphia, 1992.
Abstract
Surface geophysical surveys played a key role in identifying a suitable
hydrogeologic location for a Department of Energy Uranium Mill Tailings
Remedial Action (UMTRA) disposal cell near Grand Junction, Colorado.
Drilling and excavation work conducted as part of a first phase of site
characterization activities had determined that a previously identified
35 hectare (90 acre) disposal cell location was underlain by shallow
ground water (6 to 12 meters deep) suitable for domestic use. This
shallow ground water was found to be confined to a complex system of
alluvial-colluvial filled paleochannels eroded into the upper surface of
the underlying Mancos shale bedrock. Geophysical methods were then used
to cost-effectively direct a second phase of drilling and excavation
activities which delineated an adjacent disposal cell location that was
free of shallow ground water. Since this new disposal cell location was
within the original designated site area of approximately 240 hectare
(600 acres), considerable cost and time savings were realized by this
surface geophysics directed field characterization program.
KEYWORDS:
surface geophysics, electromagnetic conductivity, uranium mill tailings
Introduction
The Uranium Mill Tailings Remedial Action (UMTRA) program is a federal
program administered by the Department of Energy that is designed to
relocate uranium mill tailings in several western states to engineered,
environmentally stable containment cells. These cells are to be
constructed at a number of carefully selected sites in these states. The
selected sites are required to meet specific geologic and hydrogeologic
criteria that, together with a property designed containment cell,
insure that the environmental impact of the tailings will be minimized.
One of these criteria is that hydrogeologic conditions at the site are
such that the containment cell can be constructed, and the tailings
placed, above the water table.
The subject of this study is the 240 hectare (600 acre) Cheney Reservoir
(Cheney) UMTRA site at which a containment cell is to be constructed for
uranium mill tailings from the Grand Junction,
Colorado area. The Cheney site is located approximately 24 kilometers
(km) south of Grand Junction (Figure 1). Design plans for this site
called for the construction of a 35 hectare (90 acre) containment cell
at the location shown on Figure 1 (original cell location) which would
extend to a depth of 5 to 10 meters. Once constructed, the cell would be
filled with uranium tailings and subsequently capped. Initial site
characterization, which consisted of drilling and trenching at this
original cell location, indicated that ground water was not present to
depths of 15 meters and hence engineering designs for the cell were
prepared. However, during the drilling conducted as part of engineering
design, shallow ground water was unexpectedly encountered in some parts
of this original cell location at depths of 5 to 10 meters.
The discovery of shallow ground water within the cell area could,
depending on its extent, preclude the cell, or the entire Cheney
site, from meeting Department of Energy siting or design
criteria. The cost-related implications of this discovery were
significant and ranged from the requirement for cell design
modifications to abandoning the Cheney site altogether. The
costs of these options ran from several hundred thousand to
several million dollars. In addition, the tight schedule for
building the cell, and existing commitments for contractors and
equipment, meant that delays would also be costly. It was
therefore imperative to quickly, yet confidently, identify the
extent of the occurrence of the shallow ground water both in the
original cell location, and over the remainder of the Cheney
site if the original cell location proved unsuitable. It was
also apparent that this could not be done over such a large area
quickly enough and in sufficient detail solely by drilling.
Based on these considerations and favorable site conditions,
geophysical methods were selected to evaluate the extent of the
shallow ground water in the original cell location. Later, the
survey was expanded over the remainder of the 240 hectare site
in an attempt to identify a more suitable location for the
construction of the cell. The geology of the Cheney site, the
geophysical method selection, and the results and interpretation
of these two surveys are discussed below. |
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Fig. 1. Site Location Map |
Physical
Setting and Geology
The Cheney site is located at an elevation of about 1,550 meters and is
about 10 km west of the Grand Mesa, which is at an elevation of about
3,400 meters. The site generally slopes gently to the west with a
uniform gradient. Its northern boundary is marked by a gentle valley and
the southern boundary by a sharp valley with 5 to 10 meters of relief.
Both of these valleys contain stream beds that are dry for much of the
year. Rainfall in the area is less than 25 centimeters (cm) per year and
vegetation comprises desert grasses and shrubs.
The geology of the site consists of up to about 15 meters of alluvium
and colluvium resting on the Mancos shale. The alluvium and colluvium
consists mostly of mixtures of silty gravel with cobbles and boulders
derived from the basalt rock that caps Grand Mesa several kilometers
cast of the site [1]. Silty surface soils, derived from the erosion of
the Mancos and Mesa Verde formations, have developed on the gently
sloping surface pediment. Underlying the surface soils are a chaotic
mixture of particle sizes ranging from graded gravel and sand to clay.
Basalt boulders as large as 1 meter in diameter were encountered in test
pits and trenches. Locally, there are cemented zones of caliche directly
overlying perched, or formerly perched, water-bearing zones. Thick
gypsiferous deposits also overlie some of these same shallow perched or
formerly perched zones in the middle and upper portions of the
overburden.
The Mancos shale bedrock consists of finely bedded shale to depths of
approximately 15 to 25 meters [1]. Below these depths, the shaley
bedding appears only intermittently, with most of the formation
appearing as massive claystone. The degree of weathering varies from
severely to slightly weathered in the upper 15 meters of bedrock. Stress
relief fracturing and surfical erosion are determining factors in the
depth of the weathered zone. In the weathered rock, paleochannels have
been incised from 1.5 to 7.5 meters below adjacent bedrock surfaces.
Debris flows have filled most of these paleochannels. Fractures in the
weathered zone are commonly filled with gypsum, and iron and manganese
stains are prominent on partings. In the underlying unweathered bedrock,
most fractures arc unfilled and unstained, although some joints may have
clay or calcite filling.
Ground water is typically encountered at a depth of about 215 meters in
the area, in the Dakota Sandstone Formation underlying the Mancos shale.
The shallow perched ground water encountered during the engineering
design phase drilling was observed at depths of as little as 5 meters.
Information on the occurrence of this ground water was, prior to the
commencement of geophysical surveys at the site, limited to that
obtained from about 35 drillholes and three trenches that had been
excavated to the top of the shale. The trenches were about 150 meters
long and 10 meters wide. The perched water was observed directly above
the Mancos shale in alluvial and colluvial overburden that filled
topographic lows on the surface of the shale. Thus, the occurrence of
the water appeared to be controlled by the surface configuration of the
Mancos shale, probably occurring in channels on top of the shale. Apart
from these localized occurrences of water, the overburden and shale were
visually dry.
Geophysical Method Selection
Preliminary tests at the site were conducted using seismic techniques
along with surface conductivity measurements and in situ measurements of
the resistivity of the alluvium and the unsaturated and saturated shale.
Based on these results, along with site conditions, electromagnetic
conductivity methods using a
EM34 instrument (EM34)
were tentatively selected to delineate areas of shallow ground water at
the site. Since the electrical conductivity of a material is strongly
influenced by the presence of water, the selection of this method was
based on the hypothesis that the conductivity of wet shale/alluvium
would be higher than dry shale/alluvium and, moreover, that there would
be sufficient conductivity contrast between the wet and dry areas to
discriminate between them in the conductivity data.
Electromagnetic conductivity instruments, such as the EM34, measure the
conductivity of a volume of the earth using electromagnetic waves
[31,[41. The conductivity measured is influenced by the composition and
porosity of the soil and the conductivity of the fluids within the soil
pores [2] as well as the degree of saturation. The EM34 consists of
separate hand-held transmitter and receiver coils and power source.
During the operation of the EM34, the transmitter coil is energized by a
low frequency alternating current that radiates an electromagnetic field
into the earth. This primary field induces eddy currents in the ground
below the instrument. The receiver coil on the EM34 detects both the
primary field and the secondary magnetic field resulting from the eddy
currents. The ratio between the primary field and the quadrature
(out-of-phase with the primary field) component of the secondary field
is converted to conductivity which is displayed by the EM34. This
reading is a bulk measurement of the conductivity beneath the instrument
and is made up of the cumulative response to subsurface conditions
extending from the surface to the effective depth of penetration of the
instrument. More specific details of the instrument response can be
found in Technical Note TN-6 [3].
To test the hypothesis that shallow ground water is a cause of an
increase in conductivity that can be detected from surface measurements,
conductivity readings were taken along a traverse beside one of the
trenches excavated during the engineering design phase work. Tests were
conducted using both the horizontal and vertical dipole modes and with
spacings between the transmitter and receiver coils of 10, 20, and 40
meters (see 'Data Collection'). Ground water occurred in this trench
within a topographic low on the surface of the shale. The depth to the
wet shale, which occurred near the center of the trench, was
approximately 12 meters. To either side of this, the shale became
shallower and dryer. The water in the trench was about 1 meter deep.
Contrary to what was expected, there was not an increase in conductivity
over the wet shale. Instead, there were low conductivity values where
the shale was relatively deep regardless of whether it was wet or dry,
and higher conductivity values where the shale was shallow. These
results indicated that the shale, even where visually dry, was
significantly more conductive than the overburden and that the
occurrence of the shallow ground water had an insignificant effect on
the bulk conductivity. This relationship is shown schematically on
Figure 2. The conductivity values shown in Figure 2 are representative
of those found at the site. The reason for the unexpected results is due
to the relatively high conductivity of the shale, presumed to result
from the significant amounts of gypsum which could be observed. Under
these conditions, only very small amounts of moisture in the rock pores
are needed to produce high conductivity. Very little change in
conductivity occurs as the rocks become completely saturated. With
additional surface geophysical measurements, it was determined that this
relationship was consistently observed across the site. Thus, while
conductivity readings could not be used to detect the areas of shallow
ground water directly, they could be used to map the topography of the
shale.
Data Collection
The primary target for the geophysical survey was the location of
channels on the surface of the shale in the region of the original cell
location. Once these had been defined, and proved by drilling, the
survey was extended over the remainder of the site to determine if a
more suitable cell location could be identified.
Fig. 2. Hypothetical relationship of conductivity to depth
to top of shale |
As part of the final design of the survey, the instrument configuration
and line and station spacings were selected. The instrument
configuration variables consist of coil orientation and spacing.
Conductivity readings are made with the EM34 coils coplanar and oriented
either with their planes parallel to the ground surface (vertical dipole
mode) or at right angles to it (horizontal dipole mode). In addition,
readings can be taken with three different spacings between the
transmitter and receiver coils. Available coil spacings are 10, 20, and
40 meters. The frequencies of the transmitted electromagnetic waves at
these coil spacings, which are automatically selected by the instrument,
are 6.4 kilohertz (Khz), 1.6 Khz, and 0.4 Khz, respectively. Together,
the variable orientation of the coils and the transmitter-receiver
spacings result in a total of six different reading configurations.
These coil configurations control both the depth, and volume, of the
earth investigated and the response of the EM34 to lateral variations in
conductivity [31, [4].
The optimum coil separation and operating mode for the field survey of
the Cheney site were selected based on the results of several test
lines. These test lines were run using the EM34 at both 10 and 20 meter
coil spacing using both the horizontal and vertical dipole modes in
areas with drillhole or trench control. The horizontal dipole mode
provided much smoother data, and gave a larger amplitude response over
the shale undulations, than the vertical dipole mode. Of the two coil
spacings, the 20 meter spacing was chosen because the maximum
exploration depth required over the site was uncertain and the greatest
depth penetration could be achieved using the 20 meter spacing.
Using the horizontal dipole mode and 20 meter coil spacing, conductivity
readings were taken at nearly 5,000 stations across the site along lines
spaced about 60 meters apart (Figure 3). In the eastern area of the
site, in the vicinity of the original cell location and where rapid
changes in the conductivity were observed, the station spacing was 6
meters, while in the western area a 12 meters station spacing was
sufficient.
Data Interpretation
General
A
contour map of the conductivity values obtained over the whole site
together with the original cell location is shown on Figure 4. A small
regional trend of increasing conductivity values to the east has been
removed from the contoured data using a first order trend surface. The
relationship between conductivity and the depth to the Mancos shale that
was observed during the preliminary testing was the basis for
interpreting the contoured conductivity data. Areas of high conductivity
occur where the shale is shallow and the overburden thin; areas of low
conductivity occur where the shale is deep and the overburden thick. The
arrows on the map show the major paths taken by surface water across the
site, which usually occurs during summer thunderstorms and during spring
snow melt. The higher conductivity in these channels results from
topographic effects caused by stream valleys cutting into the
overburden. Since the Mancos shale is nearer to the surface in these
valleys, the measured conductivity is higher. The valley in the south of
the area is the most deeply incised and hence has the highest
conductivity values.
Fig. 3. Geophysical Survey Lines and Stations At Cheney Site
Paleochannel in Cell Area
The geophysical results in the cell area (Figure 4) show a conductivity
high in the center of the cell and a southwesterly trending, linear, low
conductivity feature to the west of this high. This conductivity low
extends southwestward, beyond the western margin of the cell, until it
is eclipsed by the high conductivity values in the southern strcam
valley. The pattern of low conductivity values was interpreted to
indicate a relatively large depth to the top of the shale reflecting a
paleochannel in this area.
This interpretation is consistent with the results of the engineering
design phase drilling which indicated that Mancos shale was at depths as
great as 14 meters just south of the center of the western edge of the
original containment cell. Additional drilling undertaken based on this
geophysical interpretation confirmed that this is a topographic
depression on the Mancos shale, and that this depression contains ground
water to the east of the western boundary of the original cell. A
section through the feature is shown on Figure 5 (the location of the
section line, marked A A', is shown on Figure 4 and is approximately
topographically flat). The conductivity high in the center of the cell
results from the shale being at a depth of only about 3 meters, while in
the depression the depth to the shale is as great as 15 meters. It was
also confirmed with subsequent drilling and trenching over the entire
cell area that this was the main feature controlling the occurrence and
flow of ground water. This feature, along with the interpreted
ground-water flow, is shown on Figure 6.
Conditions at the Remainder of the Site
As the geophysical survey was expanded to the remainder of the site
following the confirmatory drilling described above, it was observed
that the variability of the conductivity values to the west of the
original cell location (Figure 4) were, on average, much lower than
those in the cell area. This is shown on Figure 7 in which histograms of
the conductivity values from the two regions called the West Area and
East Area arc compared. A wide range of conductivity values, with two
predominant peaks at about 25 millisiemens per meter (mS/m) and 55 mS/m
is shown in the histogram for the East Area, whereas, as can be seen in
the plot for the West Area, the conductivity values are predominantly
concentrated between 25 and 30 mS/m with a much smaller peak at between
35 and 40 mS/m.
These differences in the conductivity values between the two areas are
interpreted to result from two factors. These are the depth of the
shale, the variability of which controls the amplitude variations in the
EM34 conductivity measurements, and the shale conductivity, which is
primarily a function of moisture content. Both of these factors
influence the magnitude of the measured conductivity. Overburden
conductivity variations were not found to be important since its
conductivity is much lower than that of the shale (15 mS/m versus 150 mS/m).
The lower average conductivity values in the West Area, along with their
smaller amplitude variations, suggest that either the shale is deep or,
if shallow, it has a relatively low conductivity and is comparatively
dry. Both of these factors are favorable for the location of the
containment cell.
In order to test these interpretations, a number of preliminary test
pits and boreholes were excavated in the West Area to identify depth and
moisture conditions for the shale. These test pits and borcholes
indicated that the depth to the shale was relatively shallow and that
the shale was comparatively dry. Based on these observations and the
geophysical data, it was therefore concluded that the shale was
comparatively dry and shallow throughout most of the West Area. The
subsequent excavation of additional confirmatory test pits found no
shallow ground water or significant palcochannels in the West Area. As a
result of this, the location of the containment cell was changed to a
new location on the dry shale areas, shown on Figure 6.
Fig. 4 Contoured EM34 Conductivity Data
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Fig. 5 Measured EM34 Conductivity Data (a) and Depth To Top of Mancos
Shale (b) Along Profile A-A' (see fig. 4 for line of section).
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Fig.6 Contoured conductivity Data Showing Interpreted Ground-Water Flow
Path Through Original Cell, and Proposed New Cell Location.
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Fig. 7 Histograms of measured EM34 Conductivity Values in West and East
Areas of the Cheney Site.
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Conclusions
With the use of the geophysical survey, in conjunction with geologic
studies and confirmatory drilling, it was possible to cost effectively
define the occurrence of shallow ground water at the Cheney site and
identify the location of an acceptable area for a tailings containment
cell on-site. Hundreds of thousands of dollars to millions of dollars
were savcd that might have otherwise been spent selecting an entirely
new site or drastically modifying the design of the cell if constructed
at the original location. These savings far outweighed the cost of the
survey, which was approximately $50,000 USD.
Key components in the success of the project are both technical and
organizational. Careful presurvey testing was necessary to develop the
correct understanding of how the geophysical information could be used
to investigate shallow ground water conditions at the site. Through the
close interaction between hydrogeologists and geophysicists involved in
the project it was possible for a geological-geophysical hypothesis to
be established prior to conducting routine measurements. In addition,
project managers were convinced of the importance of allocating time and
money for the pre-survey testing. By integrating the geophysical survey
with confirmatory drilling work, the confidence and accuracy of the
geophysical interpretation was enhanced while unnecessary drilling was
minimized. The resulting site characterization achieved by the
conductivity survey was more comprehensive and detailed than could be
achieved by drilling alone, and more accurate than could be obtained
solely through the collection of geophysical data.
References
(1) United States Department of Energy, Remedial Action Plan and Site
Design for Stabiliazation of hte Inactive Uranium Mill Tailings Site at
Grand Junction, Colorado, Attachment 2: Geology Report, August 1990.
(2) Keller, G.V. and Frank C. Frischknecht, Electrical Methods in
Geophysical Prospocting, International Series of Monographs in
Electromagnetic Waves, Volume 10, Pergamon Press.
(3) McNeill, J.D., Electromagnetic Terrain Conductivity Measurements at
Low Induction Numbers, Geonics Ltd., Technical Note TN-6, 1980, 15 pp.
(4) McNeill, J.D., Electrical Conductivity of Soils and Rocks, Geonics
Ltd., Technical Note TN-5, 1980, 22 pp.
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