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.
Fig. 1. Site Location Map
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.
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.
Fig. 2. Hypothetical relationship of conductivity
to depth to top of 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.
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.
