United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S7-83-005 Mar. 1983
v>EPA Project Summary
Trace and Potentially Toxic
Elements Associated with
Uranium Deposits in South Texas
Christopher D. Henry
The environmentally sensitive trace
elements molybdenum, arsenic, and
selenium are concentrated with
uranium in ore deposits in South Texas.
Cattle grazing in some pastures in
mining areas have contracted molybde-
nosis, a cattle disease resulting from an
imbalance of molybdenum and copper.
To determine natural concentrations of
the elements in soils in the South Texas
area and to evaluate possible effects of
mining on adjacent agricultural land,
two sets of soil samples were collected
and analyzed for molybdenum, arsenic,
selenium, and copper. Two hundred
and fifty-six samples were collected in a
statistically random design from soils
developed on the Whitsett Formation,
Catahoula Formation, or Oakville
Sandstone, the major uranium hosts of
the area, and 182 samples were
collected nonrandomly from areas of
mining or mineralization to test specific
hypotheses concerning the presence
and origin of anomalously high concen-
trations of the elements.
Results of the random sampling show
that the different geologic formations
have different characteristic trace
element concentrations. The Whitsett
Formation has higher molybdenum
(resulting from minor near-surface
mineralization) and lower copper
concentrations than the other two
formations. With the exception of
molybdenum in the Whitsett Formation
and copper in all three formations, the
trace element concentrations are
similar to published average
concentrations in soils worldwide.
Sampling in areas of mining and
mineralization indicates that high con-
centrations of molybdenum, arsenic,
or selenium occur dominantly in two
situations: (1) in areas of shallow
mineralization, resulting from natural
processes, and (2) in drainages adjacent
to older abandoned mines, resulting
from runoff from the mines. Moderately
high concentrations also occur in a few
reclaimed areas. Windblown dust from
mining areas has not measurably
affected trace element concentrations
in adjacent areas.
Comparison of molybdenum and
copper concentrations in soils and
grasses and theoretical considerations
of the availability to plants of
molybdenum and copper in soils
suggest that forage in much of the area
studied could have anomalously low
copper/molybdenum ratios — low
enough to induce molybdenosis in
cattle.
This Project Summary was developed
by EPA's Industrial Environmental Re-
search Laboratory, Cincinnati, OH, to
announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
Open-pit mining in South Texas began
in the late 1950s and, in 1954, uranium
deposits were discovered in the area.
Mining and uranium production have
increased steadily and Texas is currently
ranked the nation's third leading
producer of uranium (behind Wyoming
and New Mexico), solely on the basis of
mining in South Texas.
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Several environmentally sensitive
trace elements (molybdenum, arsenic,
and selenium) are known to be associated
with the South Texas uranium deposits.
Mining brings to the surface material
previously buried, thus introducing the
potential for contamination of neighbor-
ing agricultural areas and water supplies.
Since some uranium mineralization also
occurs near the surface, high concentra-
tions of the trace elements could occur
naturally in soils in mineralized areas.
The purpose of this study is (1) to
establish baseline concentrations of the
potentially toxic elements throughout
most of the uranium mining area, (2) to
determine whether significantly high
concentrations exist and, if so, in what
settings they exist, and (3) to evaluate
several potential mechanisms of
pollution; e.g., the transport of high con-
centrations of trace elements away from
a mining site and the resultant
contamination of adjacent areas.
Systematically collected samples of soil
and grass were analyzed for the elements
molybdenum, arsenic, selenium, and
copper.
Physical Setting
To interpret the origin, distribution, and
environmental significance of trace ele-
ments in soils, it is necessary to under-
stand (1) the geologic substrate—the
parent material of soils; (2) the soils
themselves — their nature, origin, and
derivation from geologic substrates in a
particular climatic setting; and (3) the
geochemistry of the elements—how and
why they occur in various concentrations
in substrates and soils.
Geology
Within the South Texas area, three
geologic formations (Fig. 1) are of
immediate interest: (1) the Whitsett
Formation of the Jackson Group, (2) the
Catahoula Formation, and(3)theOakville
Sandstone. Almost all of the present
uranium mining and identified uranium
mineralization occurs in these three
formations.
The Whitsett Formation of the Jackson
Group consists of interbedded sands and
muds deposited in a strandplain-barrier
bar system. The sands are dominantly
strike oriented, were deposited in a
strandplain environment, and are major
hosts of uranium ore deposits. Lagoonal
or shelf muds were deposited between
the sands. Major dip-oriented, channel
sands occur irregularly along the line of
outcrop and are also mineralized.
The Catahoula Formation in the study
area (Karnes and Live Oak Counties)
consists of interbedded sands and
tuffaceous muds deposited in a fluvial
environment. Throughout most of Live
Oak and Karnes Counties, the Catahoula
outcrop consists of interchannel muds
deposited between major channels to the
northeast and the southwest.
The Oakville Sandstone was also
deposited by a major fluvial system, but in
the Karnes-Live Oak area the Oakville
has a much higher sand content than the
Catahoula Formation. The Oakville Sand-
stone was fed by four major river systems,
two of which occur in the Karnes-Live
Oak area. Thus, outcrops of the Oakville
are mostly sand-rich; a few muddier areas
occur in interchannel deposits.
Soils
The compositions of soils in the study
area are strongly influenced by the
compositions of the substrates. However,
individual soils are not restricted to
individual formations because similar
substrates commonly occur in different
formations. In general, four broad,
distinct but overlapping groups of soils
correlate with the distinct substrate
types. The largest group consists of
alkaline, calcareous clay-rich soils
developed on muddy parts of the Whitsett
Formation, on the mud substrates of the
Catahoula Formation, and on the rare,
high-mud parts of the Oakville
Sandstone. The clay soils are relatively
fertile and commonly heavily farmed.
Another group consists of sandy to rocky
soils developed on or adjacent to sands
and indurated sands of the Whitsett
Formation. Soils developed on the
nonindurated sands of the Oakville
Sandstone are of intermediate to sandy
texture. Bottomland soils developed on
Quaternary alluvium constitute a fourth
group.
Geochemistry of Molybdenum,
Arsenic, Selenium, and Copper
Molybdenum, arsenic, and selenium
are geochemically associated with
uranium and are concentrated in
uranium ore deposits. The elements are
soluble in oxidizing, neutral to alkaline
water as anions (Mo04=, AsO«3~, SeOs*,
or SeO*"). With decreasing Eh, the trace
elements are precipitated in a lower
valence state commonly as sulfides, as
minor constituents of pyrite, or as the
native element.
Unlike molybdenum, arsenic, selenium,
or uranium, copper is soluble in acidic
water and insoluble in the moderate or
high pH waters in which the other
elements are soluble. Copper thus is not
concentrated with uranium or
molybdenum.
Trace elements exist in various forms
in soil, and not all are equally available to
plants. Major factors that govern
availability of molybdenum, arsenic, and
selenium are pH and drainage status,
although organic content, sulfate
content, and other factors may also be
significant. At high pH, a large proportion
of the elements are desorbed, in solution,
and available to plants. Copper availa-
bility is also affected by pH and drainage
status. However, unlike the other
elements, copper is more available at low
pH and uptake by plants is severely
limited at pH of 6 or above. In the
predominately alkaline soils of the South
Texas uranium mining areas, molybden-
um, arsenic, and selenium should be
relatively more available to plants than
copper.
Soil Sampling
The soil samples collected fall'into two
general categories: background samples,
designed to determine the natural range
and scale of variation of concentrations in
soils of the uranium mining region, and
samples from mining and mineralized
areas. Background sampling is random.
Mining and mineralization area sampling
is nonrandom and includes sampling
areas of shallow mineralization and of
mining where anomalous concentrations
could be encountered.
Random-sampling background
concentrations were obtained for two
purposes: to determine the natural
geochemical environment and to provide
a baseline with which to compare con-
centrations in soils in mining and miner-
alized areas. Both purposes require
random sampling designs so that the
samples collected are representative of
the sampled area and show the normal
range of concentrations of an element.
Also, the design should allow statistical
treatment of the data. The sampling
design used here is generally called
stratified random sampling with natural
strata.
Random Sampling
Sampling was divided into a number of
levels; the highest level is the South
Texas uranium mining region. Within this
region, The Whitsett Formation,
Catahoula Formation, and Oakville Sand-
stone form the first sublevels (Fig. 1). To
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. Vjockson Group
_j Lower Whitsem
Anomaly Sampling
St Stoeltje
N Nieschwitz
B Boso
c Culpepper
W Weddington
F Felder
T Tordillo
246
-J_
_L
I—^
8 10 km
lOmi
_J
Figure 1. Index map of a part of the South Texas uranium mining district, showing geology and location of background sampling and mining and
mineralized area sampling. Large outlined areas are quadrangles of background sampling. Heavy line within each quadrangle is
approximate location of sampling barbell.
test geographic variation within each
formation, the uranium mining region
was divided into three geographic units:
southwestern, central, and northeastern.
Within each unit a 7.5-minute quadrangle
(Fig. 1) was selected to be the second
sublevel of sampling. The quadrangles
were limited in that one of the formations
had to occur within the quadrangle.
Variations in trace element
concentrations at different scales were
determined within each quadrangle by a
barbell sampling design (Fig. 2). Each
barbell is constructed by randomly
selecting a point and a direction (derived
from random number tables). The point is
used as the midpoint of a line 4.096 km
long that trends in the selected direction.
The end points of this line define the
midpoints of two new lines with randomly
selected directions. This process is
repeated three times with lines 512 m,64
m, and 8 m long. The end points of the 8 m
lines are sampling locations.
At each location, samples were
collected from the A and B soil horizons.
Thus there are a total of 16 locations and
32 soil samples from each quadrangle.
Standard statistical analysis, including
analysis of variance, was used on all
results.
Sampling in Areas of Mining
and Mineralization
Samples in areas of mining or shallow
mineralization were collected non-
randomly from specific locations to test
hypothesized processes or concepts.
Thus samples were collected upwind
(southeast) and downwind (northwest) of
mining areas to determine if wind
transport of exposed ore or overburden
could affect molybdenum or other trace
element concentrations in soils adjacent
to the mining area. In areas of suspected
or identified shallow mineralization,
sample traverses were made across the
mineralized zone. Where erosion of spoil
piles had broken down former protective
berms, samples were taken of the eroded
material and in drainages downstream.
Results of Random Sampling
Table 1 summarizes basic statistics of
the analyses from the random sampling
program; the results are tabulated by
formation and quadrangle.
Statistical Interpretation
Analysis of variance shows that within
each quadrangle of all three formations a
major part of the variance is at the lowest
level, between the A and B horizon
samples. The percentage of variance in
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•64m
Figure 2. General configuration of the barbell sampling scheme.
molybdenum concentrations at the
lowest level ranges from 19 percent
(Catahoula-Ecleto) to 70 percent
(Catahoula-Comanche Hills). Very little of
the variance occurs at the highest level,
between sample groups 4.096 km apart.
Only the Oakville (Ray Point) samples
have very much of the variance, 38
percent, at the highest level. For all other
quadrangles, variance at this level
comprises no more than 3 percent of the
total.
Most variance that is not at the lowest
level is at intermediate distance levels,
and mostly at 512 m or 64 m. For
example, all the variance in the Whitsett
(Fashing) samples is at the lowest level
(44 percent) or the 512-m level (56
percent). Variance at the 512-m level is
significant at the 0.01 probability level.
Other quadrangles have roughly similar
patterns.
In general, similar patterns exist for
selenium, copper and arsenic results.
Most variance is at the lowest level, much
of the rest is at an intermediate distance
level, and variance at the highest level is
very low with only a few exceptions.
To establish a geochemical baseline,
an "expected range" was designated as
the central 95-percent range of concen-
trations assuming lognormal distribution.
Only one concentration in 20 should fall
outside the central 95-percent range and
only one in 40 should lie above the range.
Because this study is concerned primarily
with anomalously high concentrations,
we have provided only the upper limit of
the expected range (Table 1). Because
almost all variance occurs at scales less
than 4.096 km, the upper limit for a
quadrangle should be applicable
throughout the area of the quadrangle.
Correlation with Geology
Several conclusions result from
relating the analytical results to geology.
First, the different geologic formations
have different trace element
concentrations. This pattern is most
obvious for molybdenum. The Whitsett
(Fashing) samples contain the highest
molybdenum concentrations and have
the highest variance — they have the
greatest range of molybdenum concentra-
tions of the various formations. Because
the Whitsett Formation was sampled only
in the Fashing Quadrangle, which in-
cludes areas of shallow mineralization
and anomalous radioactivity, character-
ization beyond the boundaries of the
quadrangle is unwarranted. The high
concentrations probably result from the
derivation of the soils from mineralized
rock.
The Catahoula Formation and Oakville
Sandstone have lower concentrations
and are geographically uniform; there is
not a regional pattern to the concentra-
tions. The Oakville Sandstone is
particularly homogeneous with very
similar means (0.82-0.91 ppm) and
ranges from each quadrangle. The
Catahoula Formation is also relatively
homogeneous, but the Catahoula (Ecleto)
samples (the northeasternmost quad-
rangle sampled) have slightly higher
means and higher variance than other
Catahoula samples, reflecting a few
distinctly higher molybdenum concentra-
tions.
Selenium shows a similar, but less
pronounced, pattern to molybdenum.
Arsenic was analyzed for only two
quadrangles, and too little data exist.to
make any major conclusions. Neverthe-
less, Whitsett (Fashing) samples have
greater arsenic concentrations than
Catahoula (Coy City) samples. This
relationship generally agrees with the
molybdenum and selenium patterns.
Copper, analyzed for five quadrangles,
shows the reverse pattern. Whitsett
samples have the lowest copper concen-
trations.
Comparison with
Concentrations in Other Soils
From comparison with published
summaries of trace element concentra-
tions in soils, it can be concluded that
most soils in the uranium mining areas
have molybdenum, arsenic, and
selenium concentrations similar to other
natural soils around the United States,
and worldwide. However, some soils
developed on the Whitsett Formation
have measurably higher molybdenum
concentrations, and all soils have distinc-
tively lower copper concentrations,
compared to natural soils from a variety of
locations.
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Table 1. Summary of Basic Statistics (all concentrations in ppm)
Means
Formation Quadrangle Range Arithmetic Geometric
Whitsett
Catahoula
Oakville
Whitsett
Catahoula
Oakville
Whitsett
Catahoula
Oakville
Whitsett
Catahoula
Fashing
Ecleto
Falls City
Coy City
Comanche Hills
Garfield
Kenedy
Ray Point
Fashing
Ecleto
Falls City
Coy City
Comanche Hills
Garfield
Kenedy
Ray Point
Fashing
Ecleto
Coy City
Comanche Hills
Ray Point
Fashing
Coy City
0.2 - 4.6
0.3 - 4.0
0.2 - 1.0
0.5 - 1.6
0.4 - 1.4
0.4 - 1.3
0.5 - 1.2
0.3 - 2.0
0.0? - 0.50
0.03 - 0.60
0.07 - 0.30
0.01 - 0.31
0.02 - 0.26
0.10 - 0.30
0.01 - 0.38
0.09 - 0.37
3.4 - 8.3
2.9 - 20
4.2 - 12
6.1 - 13
7.9 - 13
0.6- 17
0.2 - 6.9
2.1
1.1
0.69
0.99
0.73
0.84
0.82
0.91
0.18
0.15
0.18
0.07
0.13
0.19
0.14
0.19
5.5
8.0
8.6
10.8
10.1
5.3
3.4
Molybdenum
1.8
0.95
0.66
0.97
0.71
0.80
0.80
0.84
Selenium
0.10
0.117
0.16
0.04
0.117
0.19
0.09
0.18
Copper
5.4
7.9
8.4
10.7
10.0
Arsenic
4.6
2.7
Variance
0.86
0.56
0.039
0.046
0.026
0.066
0.043
0.146
0.038
0.019
0.005
0.004
0.0026
0.003
0.010
0.003
1.6
9.2
4.0
2.1
2.0
8.2
3.2
Standard
Deviation
0.93
0.75
0.20
0.22
0.16
0.26
0.21
0.38
0.20
0.14
0.07
0.07
0.05
0.05
0.10
0.05
1.3
3.0
2.0
1.4
1.4
2.9
1.8
Upper
Limit for
Expected
Range
6.05
3.19
1.33
1.60
1.10
1.53
1.34
1.89
1.10
0.47
0.53
0.32
0.32
0.32
0.87
0.32
8.39
14.0
13.8
14.0
13.1
13.8
12.7
No. of
Outlier
0
1
0
1
1
0
0
1
0
2
0
0
0
0
0
1
0
1
0
0
0
1
0
Results of Mining and
Mineralized Area Studies
The locations of seven areas of mining
or mineralization sampled for this study
are shown in Figure 1. To identify
anomalously high concentrations, these
samples are compared to the expected
ranges determined from the random
sampling. The particular range used is
determined by the formation and
quadrangle in which the tested sample
lies.
Anomalously high concentrations of
molybdenum, arsenic, and selenium exist
in three settings: (A) in areas of shallow
mineralization, (B) in drainages adjacent
'to older, abandoned mines, and (C) in
some reclaimed areas. High concentra-
tions have not apparently resulted from
wind transport.
U. S. GOVERNMENT PRINTING OFFICE-. 1983/659-095/1906
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Christopher D. Henry is with the University of Texas, Austin, TX 78712.
S. Jackson Hubbard is the EPA Project Officer (see below).
The complete report, entitled "Trace and Potentially Toxic Elements Associated
with Uranium Deposits in South Texas," (Order No. PB 83-154 054; Cost:
$13.00, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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