xvEPA
United States
Environmental Protection
Agency
Office of Radiation Programs
Las Vegas Facility
P.O. Box 15027
Las Vegas NV 89114
Technical Note
ORP/LV-78-7
August 1978
Radiation
Estimated Average Annual
Radon-222 Concentrations
Around the Former
Uranium Mill Site
in Shiprock, New Mexico
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Technical Note
ORP/LV-78-7
ESTIMATED AVERAGE ANNUAL RADON-222 CONCENTRATIONS
AROUND THE FORMER URANIUM MILL SITE
IN SHIPROCK, NEW MEXICO
Joseph M. Hans, Jr.
*Thomas R. Horton
Daphne Prochaska
V.J
AUGUST 1978
OFFICE OF RADIATION PROGRAMS - LAS VEGAS FACILITY
U.S. ENVIRONMENTAL PROTECTION AGENCY
LAS VEGAS, NEVADA 89114
^EASTERN ENVIRONMENTAL RADIATION FACILITY
U.S. ENVIRONMENTAL PROTECTION AGENCY
MONTGOMERY, ALABAMA 36109
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DISCLAIMER
This report has been reviewed by the Office of Radistton Programs -
Las Vegas Facility, Environmental Protection Agency, and approved for publi-
cation. Mention of trade names or commercial products does not constitute
endorsement or recommendation for use.
tt
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PREFACE
The Office of Radiation Programs of the U.S. Environmental Protection
Agency carries out a national program designed to evaluate population exposure
to ionizing and non-ionizing radiation, and to promote development and controls
necessary to protect the public health and safety. This report describes
methods used to evaluate the average annual radon-222 concentrations around a
former uranium mill site. It also addresses the apparent effect of mill site
decontamination on the tailings piles radon-222 source term. Readers of this
report are encouraged to inform the Office of Radiation Programs of any
omissions or errors. Comments or requests for further information are also
invited.
Donald W. Hendricks
Director, Office of
Radiation Programs, LVF
m
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ACKNOWLEDGMENT
The authors gratefully acknowledge the assistance provided by Mr. Benny
Benally of the Navajo Environmental Protection Conmission and Mr. Edward
Burris of the Navajo Engineering and Construction Authority for collecting
meteorological data and assisting with the sample collection to establish the
radon-222 source terms on which this study is based.
IV
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ABSTRACT
Uranium mills, as a part of the nuclear fuel cycle, produce large volumes
of wastes which contain both the long and short-lived radionuclides from the
naturally-occurring uranium decay chain. A relatively short-lived member of
the chain, radon-222, is a noble gas and can diffuse from the wastes and be
transported from its point of origin by prevailing winds. The airborne
radon-222 decays into other short-lived radioactive progeny which may result
in human inhalation exposures at considerable distances from the point of
origin. Since the parent of radon-222 has a half-life of 1620 years, exposure
to radon and its progeny will persist for thousands of years.
This study was made to estimate the annual average radon-222 concentra-
tions around the former Shiprock, New Mexico uranium mill site in order to
identify areas which may require limited or restricted occupancy and to
evaluate the effectiveness of the site decontamination activities in reducing
the ambient airborne radon-222 concentrations. Using on- and off-site meteoro-
logical data and radon source term estimations, average annual radon-222 iso-
concentration lines were determined by computer modeling for the mill site
area. The lines were determined for both pre- and post-decontamination
periods.
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CONTENTS
Page
PREFACE Hi
ACKNOWLEDGMENT iv
ABSTRACT v
INTRODUCTION 1
RADIOLOGICAL ASPECTS 5
PURPOSE OF REPORT 7
STUDY DESCRIPTION 8
ESTIMATES OF THE RADON-222 SOURCE TERM 9
Wind Speed and Direction Frequencies 10
Stability Frequencies 10
Dispersion Model 11
STUDY RESULTS 14
DISCUSSION AND CONCLUSIONS 22
Contemporary Radon-222 Concentrations and Health Effects Risks 22
Pre-Decontamination Radon-222 Concentrations 27
Establishment of Exclusion or Restricted Areas 29
REFERENCES 31
APPENDIX 33
vn
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LIST OF FIGURES
Number Page
1 Shiprock, New Mexico and Vicinity 2
2 Major Element of the Workplan for the Decontamination of the
Shiprock Uranium Mill Site 4
3 Equivalent Area Circle 12
4 Geometry of AREAC - Plan View 13
5 Radon-222 Iso-Concentration Lines in the Vicinity of the Shiprock,
New Mexico Uranium Mill Tailings Piles (Pre-Decontamination) 20
6 Radon-222 Iso-Concentration Lines in the Vicinity of the Shiprock,
New Mexico Uranium Mill Tailings Piles (Post-Decontamination) 21
7 Contemporary Health Effects Risks Around the Former Uranium Mill
Site at Shiprock, New Mexico 25
8 Radon-222 Iso-Concentration Line Location (1 pCi/1) 30
LIST OF TABLES
Number Page
1 Radon-222 Source Term Estimates for the Shiprock, New Mexico
Uranium Mill Tailings Piles 9
2 Shiprock Joint Frequency Distribution (10 m Level) 15
3 Wind Frequency Distribution (Percent) 16
4 Atmospheric Stability Distribution for the Farmington, New Mexico
Airport 15
5 Radon-222 Iso-Concentration Distance (m) for Each Sector
(Pre-Decontamination) 18
6 Radon-222 Iso-Concentration Distance (m) for Each Sector
(Post-Decontamination) 19
7 Conversion Factors and Equivalents for Radon-222 and
Progeny 23
8 Iso-Risk Rate Calculations Based on Iso-Concentrations Using
Conversion Factors and Equivalents 26
VI1 1
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INTRODUCTION
The Shiprock uranium mill site is located on a bench overlooking the
south side of the San Juan River in the community of Shiprock, New Mexico
(Figure 1). Both Shiprock and the mill site are located on the Navajo Indian
Reservation.
The mill was constructed and operated from 1954 to 1963 on a leased 230-
acre tract by Kerr-McGee Oil Industries, Inc. The Vanadium Corporation of
America and its successor, the Foote Mineral Company operated the mill from
1963 to 1968. During its operating life, 1.5 million short tons of uranium
ore were processed and approximately 3711 tons of ILOo concentrate were
produced and subsequently purchased by the U. S. Atomic Energy Commission.
During the milling process, the uranium ore was crushed to less than 35
mesh and acid leached to remove the uranium. Uranium was removed from the
leach solution by solvent extraction and the uranium-barren solution was
discharged into raffinate ponds for disposal by evaporation and seepage. The
spent solids were pulped with waste process fluids and water and discharged to
form tailings piles. The piles (ponds, dams) were formed with earth starter
dikes and the dikes were raised using the sand fraction of the tailings. The
slime fraction of the tailings was discharged into the diked impoundment.
Since only a small percentage (0.25 percent) of the ore was U.,0R by
O O
weight, essentially all of the ore bulk ended up as wastes. Approximately 1.5
million tons of ore were processed, resulting in approximately 1.5 million
tons of wastes. The wastes contain nearly all of the radionuclides of the
uranium decay chain except for most of the uranium which was removed by
milling. Some carry-over of the uranium decay chain radionuclides also
occurred in the barren solution.
The mill was shut down and partially dismantled before the lease expired
in 1973 and the remainder of the mill and wastes (tailings) reverted back to
1
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Figure 1. Shiprock, New Mexico and Vicinity
2
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the Navajo Tribe which was the lessor. After expiration of the lease, por-
tions of the mill site were occupied by the Navajo Engineering and Construction
Authority (NECA). The NECA is a tribal enterprise and does construction and
excavation work on the Reservation. In order to provide skilled personnel to
operate earth moving equipment for both NECA and local construction projects,
a heavy equipment operators training school was established on the mill site.
In November 1973, the Tribal Chairman requested assistance from the
Environmental Protection Agency (EPA), Atomic Energy Commission (AEC) and the
Public Health Service - Indian Health Service (IMS) concerning the stabiliza-
tion of four uranium mill tailings piles on the reservation. Both the IHS and
EPA responded by conducting radiation surveys to delineate the spread of wind
and water eroded tailings in order that cost estimates could be made for their
removal. In addition, costs were to be determined for "interim stabilization"
of the tailings piles. The Shiprock site was the last of the four reservation
sites surveyed (Douglas and Hans, 1975). During the survey, it was noted that
the earth-moving training activities were aggravating the radiological condition
of the mill site. It was recommended at that time, in April 1974, that the
training activities be directed toward the decontamination of the mill site
and its environs and toward the interim stabilization of the tailings piles.
This would, in effect, reduce potential internal and external exposures to the
Shiprock and mill site inhabitants by controlling erosion of the tailings,
returning eroded tailings back to the piles, and cleaning up areas contaminated
by mill processes. The decontamination work was officially started by the
Tribal Vice-Chairman at ground breaking ceremonies on January 8, 1975 and is
near completion.
The decontamination work is proceeding along the general outline provided
in a 10-task work plan (Figure 2). The plan is being administered and executed
by staff from EPA, IHS, Navajo Environmental Protection Commission (NEPC),
NECA, and the New Mexico Environmental Improvement Agency (NMEIA). Environ-
mental sampling and measuring networks have been established to document the
on- and off-site radiological improvement as a result of the decontamination
activities.
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Major Elements of the Work Plan for Decontamination of the Shiprock
Uranium Mil! Site
(1)
ADJUST UNSTABILSZED
PILE BOUNDARY
DECONTAMINATE
ORE STORAGE
AREA YARD AREA F
(5)
DECONTAMINATE
MILL SITE AREA D
(7)
DECONTAM8NATE
AREA B
(9)
COVER
UNSTABILIZED PILE
(2)
DECONTAMINATE
TAILINGS POND AREA E
(4)
COVER STABILIZED
PILE AND BANK
AREA
(6)
DECONTAMINATE
AREA C
DECONTAMINATE
AREA A
(10)
INSTALL
FENCE
Figure 2. Major Elements of the Work Plan for the Decontamination
of the Shiprock Uranium Mill Site
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RADIOLOGICAL ASPECTS
A uranium mill may be considered as having four major operations. These
are 1) the stockpiling and preparation of the ore for leaching, 2) the leach-
ing of the uranium from the ore, 3) the removal and processing of the uranium
from the leachate, and 4) the disposal of the liquid and solid wastes. Ore
effluents and wastes from all of these operations may contribute to human
exposure while the mill is operating. After the mill has ceased operating,
some of the residual effluents and ore may be potential sources of exposure.
Solid wastes, however, appear to constitute the principal source of exposure.
The solid wastes are disposed of by above-ground storage and the liquid
wastes are commonly disposed of by evaporation and seepage which is commonly
called the "crib" method. Human exposure, resulting from the waste disposal,
occurs from several major sources which are listed.
1. Direct gamma radiation exposure from the tailings piles, ponds, and
wind and water eroded tailings.
2. Leaching of radionuclides from the tailings piles and ponds into
ground and surface water.
3. Wind erosion and subsequent deposition of tailings.
4. Diffusion of radon-222 from the tailings piles and ponds through the
air - man pathway.
Environmental sampling and measurement data indicate that the decontami-
nation of the Shiprock mill site has resulted in a reduction of potential
exposures from all of the listed sources except leaching. The gamma radiation
source has been reduced substantially by returning eroded tailings and ore
remains back to the original tailings piles. Their placement on the unstabilized
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tailings pile surface has, in effect, substantially reduced the above-ground
exposure rates around and adjacent to the pile.
Substantial leaching has occurred while process fluids were being dis-
charged on the tailings piles, in the raffinate ponds and through the plant
drains (DHEW-PHS, 1960). Some small scale leaching, observed in exploratory
trenches, is presently occurring to the San Juan River. The source of the
water is not known and it is also not known if the interim stabilization will
have any effect in diminishing the seepage.
Wind erosion of the south pile has been substantially reduced also by the
placement on it of a lightly contaminated soil cover resulting from the decon-
tamination activities. The north tailings pile was stabilized with 0.5 to 2.0
feet of pit-run soil and gravel prior to the mill closing. In addition,
approximately three feet of sand and cobble was added to 40 percent of the
surface area of the north pile to provide a roadbed in order to keep wheeled
vehicles from sinking into the tailings while adding cover to bare areas of
the pile sides. Although both of the tailings piles are covered, some minor
wind erosion is probably occurring from the decontamination activities, the
off-pile areas that have not been decontaminated, and from the pile covers.
Radon-222 is capable of moving through relatively large thicknesses of
earth. Because of this, some fraction of the total radon produced within the
tailings pile can become airborne and be transported by prevailing winds,
resulting in human exposure at several miles from the mill site. Decontamina-
tion of the mill site and cover added to the tailings piles have reduced the
radon-222 source term.
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PURPOSE OF REPORT
The decontamination activities at the former Shiprock uranium mill site
have been following a master work plan. Step 10 of the work plan is directed
at designating exclusion and restricted use areas around the mill site after
the decontamination and interim stabilization have been completed. The exclu-
sion and restricted use area locations will be dependent on the residual gamma
exposure rate levels, airborne radioactive particulate and radon-222 concentra-
tions. It appears, for the most part, that the gamma exposure rates and the
airborne particulates will have little influence concerning the designation of
off-pile exclusion areas. Radon-222 exhalation, although reduced by the
decontamination activities, is now considered to be the dominant factor for
determining any exclusion areas. The primary purpose of this report is to
estimate the average annual radon-222 concentrations in the local environment
around the tailings piles in order that the IHS and the Navajo Tribe can
determine if any exclusion or restricted-use areas should be designated in
light of available radon-222 concentration guides or health risks posed by
radon-222 and its progeny.
In addition to satisfying the work plan task, techniques and procedures
for evaluating ambient levels of radon-222 have been developed which are
applicable to other active and inactive mill sites. Some estimates of the
tailings pile radon-222 source term, prior to the decontamination work, have
been made. Using these and contemporary estimates, the effects of the decon-
tamination work on the radon-222 source term can be appraised.
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STUDY DESCRIPTION
With reference to the purpose of the report, radon-222 concentrations
needed to be evaluated in areas around th.e mill site in order to determine if
any measures should be taken to minimize human exposure. This means, in
effect, that the average annual radon-222 concentrations in all areas
adjoining the mill site should be evaluated. Assuming a constant average
radon-222 source term for the tailings piles, its concentration in the adjoin-
ing areas is dependent upon local meteorological conditions. These conditions
are variable from hour to hour and season to season; however, average meteoro-
logical conditions are similar from year to year- With regard to the afore-
mentioned, the radon concentrations in adjacent areas should be determined for
at least one year and preferably longer.
Two approaches may be taken to estimate the average annual radon-222
concentrations in areas around the mill site. The first is to place samplers
or measuring devices in areas of interest and monitor the radon-222 levels
continuously for a year, or at specific intervals throughout the year. In
order to establish a distance versus concentration relationship, at least two
measurements should be made at different distances along each radial from the
tailings piles. The second approach is to use annual meteorological data with
the tailings pile radon-222 source term and compute the average concentrations
as a function of distance from the tailings piles.
The latter approach was used because of the logistics problems involved
with the former approach. The latter requires an estimate of the radon-222
source term for the tailings piles, local wind speed and direction frequencies,
and the frequency of the various atmospheric stability classes. In addition,
an atmospheric dispersion model is needed to calculate the average annual
radon-222 concentrations resulting from large area sources. Elements of the
latter approach are described in further detail.
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ESTIMATES OF THE RADON-222 SOURCE TERM
Decontamination of the Shiprock uranium mill site began in late 1974 and
has resulted in major changes of the mill site. The changes include a major
alteration in the tailings pile configuration, the removal of a former raf-
finate pond, and the addition of contaminated earth on the south tailings
pile. The contaminated earth now covers the south tailings pile to a depth of
approximately four feet. The north tailings pile cover averages one and one-
half feet in depth. These changes have reduced the ambient radioactive par-
ticulate concentrations and gamma ray exposure rate levels around the mill
site. Also they have apparently been effective in substantially reducing the
radon-222 source term for the tailings piles.
During the very early stages of the site decontamination, radon exhalation
measurements of the tailings piles were made with the barrel accumulation
technique (Bernhardt, et al, 1975) and calculations using borehole logging
data (Rodgers, 1977). The radon-222 source term was also estimated (Swift,
1976) using a calculation method described by Schaiger, 1974. Contemporary
estimates of the source term which reflect the effect of the major changes
resulting from the decontamination activities, have been made by Hans, 1977.
It is not expected that any significant changes in the contemporary source
term estimation will occur as a result of the remaining decontamination
activities. Early and contemporary radon-222 source term estimates are listed
in Table 1.
TABLE 1. RADON-222 SOURCE TERM ESTIMATES FOR THE SHIPROCK, NEW MEXICO
URANIUM MILL TAILINGS PILES
Early Source Contemporary Source
Term (pCi/sec) Method Term (pCi/sec) Method
O 7
3.6 x 10 From exhalation data 2.7 x 10 Pile edge flux measure-
(Bernhardt, et al 1975) ment (Hans, 1977)
O 7
2.4 x 10 From bore hole logging 2.0 x 10 Downwind sampling
(Rodgers, 1977) (Hans, 1977)
O 7
2.2 x 10 Calculations 5.8 x 10 Downwind sampling
(Swift, 1976) (Hans, 1977)
2.8 x 10 Charcoal canister
(FBDU, 1977)
2.7 x 108 (Average) 3.3 x 107 (Average)
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The average difference between the early and contemporary source term
estimates, confirmed with on-site working level measurements, is a measure of
the effect of the decontamination activities.
WIND SPEED AND DIRECTION FREQUENCIES
A meteorological station was established on the mill site in 1974 in
order to collect local weather information in support of the decontamination
activities. The station consists of a small travel trailer and two meteoro-
logical units, each containing wind speed, wind direction, and temperature
sensors. One meteorological unit is mounted on a telescoping mast at an
elevation of 10 meters above the ground and the other unit is mounted at one
meter above the ground. The mast of the elevated unit is attached to the
trailer and the stand for the other has sufficient cabling to permit it to be
located away from air disturbances that may be caused by the trailer wake.
Electrical power and signal processing for the units are provided from the
trailer.
Wind speed is measured with a cup anemometer and wind direction is
determined with a vane. Interrogation of the wind speed and direction occurs
every 15 minutes and the data are recorded on paper punch tape for computer
processing.
STABILITY FREQUENCIES
Although temperature measurements are recorded for two different heights,
they are not considered suitable for estimating atmospheric stability frequen-
cies on site. Farmington, New Mexico, which lies approximately 40 km east of
Shiprock, is the closest community at which long-term stability observations
have been made. Farmington, like Shiprock, lies in the San Juan River Valley
and is subject to down-valley winds during stable atmospheric conditions. The
river valley at Farmington runs in an east-west direction, while the valley in
Shiprock runs southeast to northwest. Stability frequencies for Farmington
were acquired from the National Climatic Center, Asheville, North Carolina.
10
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DISPERSION MODEL
In order to estimate annual average radon-222 concentrations around the
Shiprock tailings piles, the source term and meteorological parameters must be
incorporated into an atmospheric dispersion model. Models exist for dispersion
calculations, however, most of them are derived for point-source releases.
Unfortunately, tailings piles constitute large area source releases and are
not readily amenable for point-source calculations except at large source to
receptor distances. A computer code, "Area Source Radiological Emission
Analysis Code (AREAC)" was developed by EPA for such uses (Michlewicz, 1976).
The code was specifically written to accurately assess close-in air concentra-
tions resulting from a large area source such as a uranium mill tailings pile.
The EPA area source computer model (AREAC) takes into account parameters
such as an area source shape, cloud dispersion, ground and inversion-lid
reflections and radionuclide decay by time of flight. Several simplifying
assumptions were made concerning the source and are listed:
1. A circular tailings pile configuration was assumed with an equiva-
lent area of the tailings piles (Figure 3)
2. The radon-222 source is uniformly distributed within the tailings
pile
3. The radon-222 release rate is constant
4. The radon-222 concentrations are average over each sector for a
given distance downwind
5. A ground level release of radon-222 is assumed.
In this application, the circular equivalent area was divided into 100
point sources and the radon-222 concentration due to each source was summed at
the desired downwind distances. A standard continuous point source sector
averaged Gaussian diffusion equation (Appendix) was used for the summing.
Radon-222 concentrations were computed for distances along each of 16 sectors.
The sectors are depicted in Figure 4.
11
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D
LEGEND
TAILINGS PILE
BLUFF RIDGE
335 M EQUIVALENT
RADIUS
Figure 3. Equivalent Area Circle
12
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UP TO 12
ANNULAR RINGS
16 SECTORS
STABILITY
CLASSES
ER SECTOR
EGMEN
Figure 4. Geometry of AREAC - Plan View
13
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STUDY RESULTS
A summation of the local meteorology is given in Table 2, i.e., wind
speed classes versus wind direction or the joint frequency distribution
summed over all stability classes for the Shiprock on-site data. Calms (0 to
1 miles per hour) were distributed proportionally in the lowest wind speed
class (1-3 miles per hour), resulting in a 0-3 miles per hour wind speed
class. The resulting wind frequency distributions, using on-site data for
Shiprock and Farmington, New Mexico is presented in Table 3. The Farmington
airport, which is about 40 km (25 mi) east of Shiprock was the nearest station
collecting meteorological data available for comparison with the Shiprock
data. Both Farmington and Shiprock lie in the San Juan River Valley, however,
the valley runs east to west in the Farmington area and southeast to northwest
in the Shiprock area. In addition, the valley is generally steeper in the
Farmington area. Because of these topographical differences, the wind frequency
distributions for both locations are somewhat different. The down-valley
winds, which are generally associated with stable conditions, are spread
through the west to north-northwest sectors in Shiprock while they are spread
through the southwest to west-northwest sectors in Farmington. This reflects
the differences in the San Juan River Valley directions at these locations.
The same directional frequencies are more pronounced for the Farmington area
than the Shiprock area. This is probably due to the differences in the valley
slopes at the two locations.
Unfortunately, the on-site meteorological data does not include stability
information. Consequently, the overall stability distribution for the Farm-
ington airport (Table 4) has been applied to the joint frequency distribution
(Table 2) to simulate a joint frequency distribution versus stability type for
Shiprock. The overall stability distribution for Shiprock probably does not
vary much from that of the Farmington airport.
Radon-222 concentrations were calculated for distances of 435 m to 72 km
from the center of the tailings piles for two different radon-222 source
terms. This was done to illustrate the effects of decontamination of the mill
14
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TABLE 2. S
W
0 - 3
HIPROCK JO
IND SPEED
4 - 7
INT FREQUENCY DISTRIBUTION (10 m Level)
CLASS (MILES PER HOUR)
8 -12
13 - 18
19 - 24
Over 24
1/7
T3
t-
(O
S
O
1
"ZL
O
s= c
t '
O
LU
an
H- <
* *
o
ZL
3
S
ssw
sw
wsw
W
WNW
NW
NNW
N
NNE
NE
ENE
E
ESE
SE
SSE
.00824
.00620
.00864
.01630
.02581
.03251
.02945
.01735
.01371
.01173
.01469
.01710
.01945
.01735
.01454
.00957
.00540
.00336
.00627
.01753
.03788
.06019
.06538
.02581
.01043
.00818
.01272
.01719
. 02772
.02729
.01935
.00858
.00244
.00102
.00232
.00963
.03016
.02828
.02806
.01213
.00617
.00800
.01278
.00929
.01809
,02738
.01954
.00633
.00049
.00049
.00034
.00262
.00710
.00624
.00432
.00355
.00204
.00383
.00883
.00577
.00997
.02081
,01398
.00225
.00034
.00022
.00015
.00015
.00105
.00213
.00179
.00170
.00185
.00228
.00596
.00478
.00562
.01482
.00917
.00086
.00003
.00003
.00000
.00006
.00006
.00025
.00040
.00065
.00034
.00068
.00225
.00139
.00120
.00491
.00445
.00025
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TABLE 3. WIND FREQUENCY DISTRIBUTION (PERCENT)
Wind Direction
(Towards )
N
NNE
NE
ENE
E
ESE
SE
SSE
S
ssw
sw
wsw
w
WNW
NW
NNW
TABLE 4.
Stability Class
A
B
C
D
E
F
Shiprock (On-site Data
3.45
3.47
5.72
5.55
8.21
11 .26
8.10
2.78
1.69
1 .13
1 .77
4.63
10.21
12.96
12.94
6.12
ATMOSPHERIC STABILITY
FOR THE FARMINGTON, N
Stability Description
Extremely unstable
Moderately unstable
SI ightly unstabl e
Neutral
Slightly stable
Moderately stable
) Farmington Airport
3.79
2.82
5.71
7.02
1 2.87
6.07
2.80
1 .88
4.38
4.82
9.18
10.02
16.35
6.64
3.63
1 .96
DISTRIBUTION
.M. AIRPORT
Frequency (Percent)
4.68
8.74
12.96
15.81
12.73
45.08
16
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site on the source term as well as depict the prevailing radon-222 concentra-
tions that will occur after the site decontamination has been completed. The
estimated radon-222 source term before decontamination began was 8500 Ci/yr
and 1040 Ci/yr after decontamination has been completed. Iso-concentrations
of radon-222 versus distance in each sector for pre- and post-decontamination
conditions are tabulated in Tables 5 and 6.
Figure 5 locates radon iso-concentration lines around the mill site on a
local map for the pre-decontamination period. The 1 pCi/1 iso-concentration
line (Figure 5) extends for a distance of 1500 m (M mile) to the northwest of
the mill site and reaches into the more heavily inhabited areas. The 3 pCi/1
iso-concentration line extends off-site from the south-southwest to the
north-northwest sectors and encompasses only very lightly inhabited areas.
Only a small projection of the 10 pCi/1 iso-concentration line could be made
to the northwest area. This line is on-site and is located close to the
tailings piles.
Figure 6 locates iso-concentration lines on a local map for the post-
decontamination period. In Figure 6, the 0.1 pCi/1 iso-concentration line is
well off-site, extending into the more heavily inhabited areas from the
southwest to the north-northwest sectors. The 1 pCi/1 iso-concentration line
is located on-site except for a small area on the southwest corner. A 3 pCi/1
iso-concentration line could not be plotted.
17
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TABLE 5. RADQN-222 ISO-CONCENTRATION DISTANCE (m] for
EACH SECTOR CPRE-DECONTAMINATION)
Sector
N
NNE
NE
ENE
E
ESE
SE
SSE
S
ssw
sw
NSW
W
WNW
NW
NNW
C 0 N C E I
10 1
800
800
920
1 ,000
1 ,100
1 ,125
980
760
620
600
700
1 ,000
410 1S325
420 1,575
400 1,500
380 15025
^ T R A T
0.1
3,300
3,000
3,700
3,900
4,700
4,900
4,000
2,700
2,300
1 ,900
2,350
3,800
5,600
6,700
6,600
4,200
I 0 N (pCt/
0.01
15,000
14,000
17,500
19,000
22,500
23,000
19,000
12,000
9,800
8,200
10,700
18,000
28,000
34,000
32,000
20,000
1)
0.001
66,000
54,000
42,000
56,000
96,000
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TABLE 6. RADON-222 ISO-CONCENTRATION DISTANCE Cm) for
EACH SECTOR [POST-DECONTAMINATION)
Sector
N
NNE
NE
ENE
E
ESE
SE
SSE
S
SSW
SW
WSW
W
WNW
NW
NNW
1.0
400
375
370
460
470
460
400
C
1
1
1
1
1
1
1
1
1
0 N C E F
0.1
900
900
970
,125
,280
,280
,100
830
670
640
780
,100
,500
,750
,700
,200
1 T
3
3
4
4
5
5
4
3
2
2
2
4
6
7
7
4
RAT
0.01
,550
,500
,200
,500
,300
,400
,500
,100
,550
,100
,700
,300
,500
,600
,600
,800
I 0 N
0
17
16
20
22
26
27
22
14
12
9
12
20
32
40
38
23
CpCt/1)
.001
,500
,000
,000
,000
,500
,500
,000
,000
,000
,100
,000
,500
,500
,000
,000
,000
0.0001
76,000
62,000
48,000
63,000
19
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Figure 5. Radon-222 Iso-Concentration Lines in the Vicinity of the
Shiprock, New Mexico Uranium Mill Tailings Pile
(Pre-Decontamination)
20
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Figure 6. Radon-222 Iso-Concentration Lines in the Vicinity of the
Shiprock, New Mexico Uranium Mill Tailings Pile
(Post-Decontamination)
21
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DISCUSSIONS AND CONCLUSIONS
CONTEMPORARY RADON-222 CONCENTRATIONS AND HEALTH EFFECTS RISKS
The decontamination activities at the former uranium mill site fn Ship-
rock, New Mexico are proceeding into the final stages of the overall work
plan. One final task in the work plan is to determine what areas, if any,
should be restricted around the mill site because of radon-222 exhalation from
the tailings piles.
Current regulations and guides concerning radon are excerpted (Swift
et al., 1976):
"Federal regulations 10 CFR, Part 20 presently permit
AEC-licensed operations to release average concentrations
of radon-222 to the air in unrestricted areas of 3 x 10
uCi/ml, (3 pCi/1) above natural background. (Such releases
may be subject to reduction if it appears that a suitable
sample of an exposed population may have an intake of the
decay product radionuclides greater than they would have if
exposed to average concentrations of one-third of the
3 x 10"9 uCi/ml (1 pCi/1).)"
"Recommendations of the International Commission on
Radiological Protection (ICRP) state that the annual radiation
dose limits for individual members of the general public
should be one-tenth of the corresponding annual occupational
dose for continuous exposure as listed in the Committee II
report. Application of these recommendations to the 168 hour
concentration (10 pCi/1] for occupational exposure to radon-222
(with daughter products present at equilibrium concentrations)
leads to a value of 1 pCi/1 for continuous exposure to individuals
in the general population."
22
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Since the Shiprock mill site's Atomic Energy Commission license has been
terminated, the radon-222 concentrations stated and discussed in the excerpt
do not have a legal status and can only be used for planning and guidance.
Although radon-222 is an alpha emitter and is respirable, its contribution
to a respiratory tract dose is relatively negligible when compared to its
short-lived progeny. A majority of the epidemiological evidence relating lung
cancer with exposures to radon-222 and its progeny has been made with the unit
of working level (WL) instead of the usual radiological units of rems or rads
and present regulations and guides use this unit. Table 7 lists conversion
factors and equivalents for radon-222 and progeny.
TABLE 7. CONVERSION FACTORS AND EQUIVALENTS FOR RADON-222 AND PROGENY
1. One working level is equal to a combination of short-lived radon
daughters in one liter of air that will result in an ultimate
emission of 1.3 x 105 MeV of alpha energy.
2. A working level month (WLM) is the exposure to one WL concentration
of radon daughters for 173 hours or an equivalent product of radon
daughter concentrations and time.
3. Assumed radon-222 and daughter ratios for a living accommodation
with normal ventilation (1.0/0.9/0.5/0.35).
4. Continuous exposure to Continued exposure
1 pCi/m3 of radon-222 of 5 x 10"6 WL
WL
Year
= 25 MLM for the general public (USEPA, 1978)
6. A continuous exposure of one WLM is assumed to increase an
individual's bronchial cancer risk by three percent.
23
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Occupied structures on the mill site lie close to the 1 pCi/1 iso-
concentration line. Occupancy of the permanent structures is generally
limited to normal working hours (40 hrs/week) or about 23 percent of full
occupancy. This would amount to about an average effective exposure to 0.23
pCi/1 of radon-222. Since the occupancy is also limited to daylight hours,
the effective radon exposure will be less because stable atmospheric conditions
occur less frequently during the daylight hours.
Figure 7 depicts contemporary iso-risk rate locations based on radon iso-
concentrations around the mill site. The risks were calculated using conver-
sion factors and equivalents for radon-222 and daughters and are listed in
Table 7. The results of the risk calculations are contained in Table 8.
The iso-risk line values indicated in Figure 7 are contemporary annual
risks and must be multiplied by the number of years that an individual will be
exposed to the risk. For example, if an individual lives on the 0.39 percent
risk line for 10 years, his relative bronchial cancer risk will be increased
by (0.39%/yr x 10 years) or 3.9 percent.
Presently the 0.39 percent risk per annum iso-risk line (1 pCi/1) extends,
at its most distant point, about 180 m (600 ft) in the northwest direction
from the tailings piles and it is well within the mill site boundaries in the
northwest quadrant. It does, however, extend off-site for a distance of
approximately 60 m (200 ft) in the southwest quadrant Figure 8. An area
approximately 30 m (MOO ft wide) adjacent to the southwest edge of the pile
is taken up by a haul road.
Other sampling, conducted on the mill site, tends to support the annual
average radon-222 concentrations estimated with AREAC. This sampling was made
to determine working level values in the NECA office and the new maintenance
shop building. Twenty-six working level samples were taken in the office
building during the period of January through October, 1977. The average net
working level was 0.007. Using the previously described conversion factors,
this would correspond to an outdoor average radon concentration of approxi-
mately 1.4 pCi/1. The average net working level for the shop was 0.0035 for
the same time period the office building was sampled. Using the conversion
24
-------
p
-
I*
Figure 7. Contemporary Health Effects Risks Around the Former
Uranium Mill Site at Shiprock, New Mexico
25
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TABLE 8. ISO-RISK RATE CALCULATIONS BASED ON ISO-CONCENTRATIONS USING
CONVERSION FACTORS AND EQUIVALENTS (TABLE 7)
Percent Relative
iOWOUMVrfCIIUI UWIV.
(PCI/I)
i
0.7
0.3
0.1
-HI .,.
WL
3
5 x 10"
3
3.5 x 10
3
1.5 x 10
5 x 10"4
WLM/YR
0.13
0.088
0.038
0.013
i\ i j> r\ i 1 1 v_- 1 ^uo^_
Per Year
0.39
0.26
0.11
0.039
Assuming 50 percent equilibrium.
26
-------
factor as before, this would correspond to an outdoor average radon concentra-
tion of 0.7 pCi/1. Both buildings are close to each other and they are about
the same distance from the tailings piles. The probable reasons that the
office building has approximately twice the WL of the shop are that the office
was built at the beginning of the milling activities and tailings have been
tracked into it over a longer period of time by the former operators as well
as NECA staff. Some radium-226 is known to be entrained in the office carpet-
ing and a planter, containing approximately two cubic yards of tailings, was
located against the office outer wall.
The outdoor radon concentrations for the two buildings were calculated
*
based on the conversion factor which assumes 50 percent equilibrium ratio.
Outdoor radon-222 measurements were made concurrently with indoor WL measure-
ments for the office building during a 24-hour period on February 28, 1976
(FBDU-1977). The results of these measurements indicates that the relation-
ship between outdoor radon and indoor WL is that 1 pCi/m3 corresponds to
_6
approximately 1.8 x 10 WL (18 percent equilibrium ratio). Using this
relationship with the net long term WL measurements, the average outdoor
radon-222 concentrations would be approximately 4 pCi/1. The outdoor radon
concentration estimated from WL measurements in the shop would be 2 pCi/1
using the FBDU data. It is believed that the shop is more representative of
actual WL than the office for reasons previously cited. It is also assumed
that the lower equilibrium ratio derived from the FBDU-1977 report data is due
to less radon daughter ingrowth because of the short distance between the
office building and the tailings piles. Further discussion of the two equi-
librium ratios is contained in the next section concerning pre-decontamination
radon-222 concentrations.
PRE-DECONTAMINATION RADON-222 CONCENTRATIONS
With reference to Figure 5, it would appear that during the latter part
of the milling activities the annual average radon-222 concentrations beyond
the boundry of the mill site may have exceeded the Atomic Energy Commission's
Regulations (USAEC, 1965).
* Percent equilibrium equals the working level divided by the ambient radon
concentration divided by 100 pCi/1.
27
-------
The average net working level in the NECA office from February 1974 to
March 1975 was 0.025 NL. Again, using the stated conversion factorCSO percent
equilibrium ratio), this would correspond to an average outdoor radon-222
concentration of 5.0 pCi/1. Using a conversion factor of 1 pCi/m3 =
1.6 x 10"6 WL (FBDU, 1977), the average outdoor radon-222 concentrations would
be
-------
ESTABLISHMENT OF EXCLUSION OR RESTRICTED AREAS
Since the site is largely decontaminated and if a guide of 1 pCi/1
average annual concentration of radon-222 is applied to the area around the
tailings piles, all areas with radon-222 concentrations equal to or greater
than 1 pCi/1 will lie within the confines of the mill site boundary (Figure 8)
except for a small area to the southwest of the tailings piles. The occupied
buildings on site probably experience an average annual concentration of
radon-222 of approximately 1 to 2 pCi/1. It is likely that, if the site
occupancy of the general area around and including the office, workshop and
classroom is limited to normal working hours, the average exposure to individ-
uals will be less than 1 pCi/1 radon-222. Occupancy of any areas near the
southwest corner of the tailings piles would be unlikely because of the heavy
equipment traffic on the haul road.
With regard to working level guidance, the Public Health Service Surgeon
General's Guidelines for Grand Junction, Colorado (U.S. DHEW, 1960) stipulate
that remedial action should be considered for structures that have mill
tailings use when their average WL ranges from 0.01 to 0.05. Remedial action
is necessary when WL are >0.05.
The EPA Office of Radiation Programs is preparing WL guidance concerning
the Florida phosphate industry (USEPA, 1978). These guides take into account
more recent data and information and they may be used for guidance concerning
non-occupational exposures around uranium mills as well as other sources of
radon progeny exposures. The guides stipulate that the WL in frequently
occupied buildings is unacceptable if the gross WL is >0.03. Working levels
ranging from 0.005 WL above ambient background to a gross WL of 0.03 should be
reduced to levels as low as reasonably achievable.
With regard to the EPA guidance, the new shop and office building net
working levels are 0.0035 and 0.0070 respectively. Although the office build-
ing exceeds the lower level of the guides, its occupancy factor is 23 percent
as compared to an occupancy factor of 80 percent on which the guides are
based.
29
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CO
o
NORTH TAILINGS PILE
SOUTH TAILINGS PILE
DWASHROOM
r-j-NECA OFFICE
Hi
OLD SHOP
TRAINING
CLASSROOM
HAUL ROAD
Figure 8. Radon-222 Iso-Concentration Line Location (1 pCi/1)
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REFERENCES
Bernhardt, D. et al (1975), "Radon Exhalation from Uranium Mill Tailings
Piles", ORP/LV-75-7(A), November 1975, U.S. Environmental Protection Agency,
Office of Radiation Programs, Las Vegas Facility, Las Vegas, NV 89114.
Douglas, R. and J. Hans (1975), Gamma Radiation Surveys at Inactive Uranium
Mill Sites, Technical Note ORP/LV-75-5. U.S. Environmental Protection Agency,
Office of Radiation Programs, Las Vegas Facility, Las Vegas, NV 89114.
Ford, Bacon & Davis Utah Inc. (1977). Phase II - Title I, Engineering
Assessment of Inactive Uranium Mill Tailings, Shiprock Site, Shiprock, New
Mexico, March 31, 1977. ERDA Contract No. E(05-l) - 1658.
Hans, J. M., Jr., (1977). Techniques for the Estimation of Radon-222 Source
Terms for Uranium Mill Tailings Piles. Proceedings of the Workshop on Methods
for Measuring Radiation in and around Uranium Mills. May 23-26, 1977. Atomic
Industrial Forum,Inc.
Michlewiez, D. (1977). Area Source Radiological Emission Analysis Code.
U.S. Environmental Protection Agency, Office of Radiation Programs, Washington,
D.C. 20460. October 1976.
Rodgers, V. (1977), Ford Bacon & Davis Utah Inc., Private communication with
J. Hans.
Schaiger, K. J, (1974), "Analysis of Radiation Exposures on or Near Uranium
Mill Tailings Piles, Radiation Data and Reports", Vol. 15, No. 7, U.S. Environ-
mental Protection Agency, July 1974.
Swift, J. et al (1976), Potential Radiological Impact of Airborne Releases and
Direct Gamma Radiation to Individuals Living near Inactive Uranium Mill
Tailings Piles, U.S. Environmental Protection Agency, Office of Radiation
Programs, Washington, D.C. 20460.
U.S. Atomic Energy Commission (1965), Standards for Protection Against
Radiation, Title 10, Code of Federal Regulations, Part 203 U.S. Atomic Energy
Commission, Washington, D.C.
U.S. Department of Health, Education and Welfare-Public Health Service (1960).
Stream Surveys in Vicinity of Uranium Mills IV. Area of Shiprock, New Mexico.
November 1960.
U.S. Department of Health, Education and Welfare-Public Health Service (1962).
Region VIII, Denver, Colorado. Colorado River Basin Water Quality Control
Project. December 1962.
31
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U.S. Environmental Protection Agency (1978), Indoor Radiation Exposure of
Persons Due to Radium-226 Associated with Florida Phosphate-Related Land -
Technical Support Document for Radiation Protection Recommendations, Office of
Radiation Programs, Criteria and Standards Division, Washington, D. C. 20460
(in preparation).
U.S. Surgeon General (1970), Recommendations of Action for Radiation Exposure
Levels in Dwellings Constructed on or with Uranium Mill Tailings, IN: Hearings
on the Use of Uranium Mill Tailings for Construction Purposes, Joint Committee
on Atomic Energy (1971), p. 51-54.
32
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APPENDIX
Continuous Point Source Sector Averaged Gaussian Diffusion Equation
J * , ___ ___ ^
_ q
Where: x-j = ground level concentration for the i th sector (Ci/m )
Q = source release rate (Ci/sec)
.. = fractional wind frequency in the i th sector for the
' v
j th stability class
x = distance downwind (m)
h = effective release height (m)
o . = standard deviation of the vertical distribution of an
^ J
assumed Gaussian plume for the j th stability class (m)
u- = average wind speed in the i th sector for the j th
' J
stability class (m/sec)
33
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
-7
2.
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
Estimated Average Annual Radon-222 Concentrations
Around the Former Uranium Mill Site in
Shiprock, New Mexico
5. REPORT DATE
August 1978
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Joseph M. Hans, Jr., Thomas R.
Daphne Prochaska
Morton,
8. PERFORMING ORGANIZATION REPORT NO
). PERFORMING ORGANIZATIONNAME AND ADDRESS
Office of Radiation Programs - Las Vegas Facility
U.S. Environmental Protection Agency
P.O. Box 15027
Las Vegas, NV 89114
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
12. SPONSORING AGENCY NAME AND ADDRESS
Same as above
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
16. ABSTRACT
Uranium mills, as a part of the nuclear fuel cycle, produce large volumes of
wastes which contain both the long and short-lived radionuclides from the naturally-
occurring uranium decay chain. A relatively short-lived member of the chain, radon-
222, is a noble gas and can diffuse from the wastes and be transported from its point
of origin by prevailing winds. The airborne radon-222 decays into other short-lived
radioactive progeny which may result in human inhalation exposures at considerable
distances from the point of origin. Since the parent of radon-222 has a half-life of
1620 years, exposure to radon and its progeny will persist for thousands of years.
This study was made to estimate the annual average radon-222 concentrations
around the former Shiprock, New Mexico uranium mill site in order to identify areas
which may require limited or restricted occupancy and to evaluate the effectiveness
of the site decontamination activities in reducing the ambient airborne radon-222
concentrations. Using on- and off-site meteorological data and radon source term
estimations, average annual radon-222 iso-concentration lines were determined by
computer modeling for the mill site area. The lines were determined for both pre-
and post-decontamination periods.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Radon-222
Uranium series
Uranium mill tailings piles
Environmental surveys
Radiation surveys
Uranium mill tail ings
1806
1807
1808
3. DISTRIBUTION STATEMENT
Release to Public
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
42
20. SECURITY CLASS (This page)
Unclassified
22. PRICE
EPA Form 2220-1 (Rev. 4-77) PREVIOUS EDITION is OBSOLETE
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