United States
                 Environmental Protection
                 Agency
                 Office of Radiation Programs
                 Eastern Environmental
                 Radiation Facility
                 P.O. Box 3009
                 Montgomery, AL 36109
EPA-520/5-79-004
September 1979
                 Radiation
vyEPA
A Preliminary Radiological
Assessment of Radon
Exhalation From Phosphate
Gypsum  Piles and Inactive
Uranium Mill Tailings Piles

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                                                EPA-520/5-79-004
     A PRELIMINARY RADIOLOGICAL ASSESSMENT OF RADON
EXHALATION  FROM PHOSPHATE GYPSUM PILES  AND  INACTIVE
                URANIUM MILL TAILINGS PILES
                       Thomas R. Horton
              Eastern Environmental Radiation Facility
                        P. O. Box 3009
                  Montgomery, Alabama   36109
                        September 1979
           U.S. ENVIRONMENTAL PROTECTION AGENCY
                  Office of Radiation Programs
                      Waterside Mall East
                       401 M Street, S.W.
                    Washington, DC   20460

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                                EPA Review Notice

This report has been reviewed by the Environmental Protection Agency (EPA) and approved
tor publication. Approval does not signity that the contents necessarily reflect the views and
policies of the EPA, nor does mention of trade names or commercial products constitute
endorsement or recommendation for use.
                                    PREFACE

    The Eastern Environmental Radiation Facility (EERF) participates in the identification of
solutions to problem areas as defined by the Office of Radiation Programs (ORP). The Facility
provides  analytical capability for evaluation and assessment of radiation sources through
environmental studies and surveillance and analysis. The EERF provides technical assistance
to the State and local health departments in their radiological health programs and provides
special analytical support for Environmental Protection Agency Regional Offices and other
federal government agencies as requested.

    This  study is one of several current  projects which the EERF is conducting  to assess
environmental radiation contributions from naturally occurring radioactivity.
                                                     Charles R. Porter
                                                         Director
                                           Eastern Environmental Radiation Facility

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                              ACKNOWLEDGEMENT

    The Eastern Environmental Radiation Facility greatly appreciates the help of Mr. Harlan
Keaton and the staff of the Polk County Health Department, Winter Haven, Florida, in
obtaining the exhalation rate data for the two phosphate gypsum piles. Without their support
and cooperation, this technical report could not have been undertaken.

    The author would also like to thank the staff of the Eastern Environmental Radiation
Facility (Montgomery, Alabama), the Office of Radiation Programs - Las Vegas Facility, and
the Environmental Analysis Division (EPA headquarters)  for their  helpful comments and
suggestions during the preparation of this report.
                                     Abstract

    The EPA Office of Radiation Programs has conducted a series of studies to determine the
radiological impact of the phosphate mining and milling industry. This report describes the
efforts to estimate cumulative working  level months (CWLM) from radon-222 daughters
produced from radium-226 in phosphate gypsum piles and how these estimates compare with
CWLM from inactive uranium mill tailings piles.

    Two Florida  phosphate gypsum  piles were  selected for radon  exhalation  rate
measurements. Indoor radon concentration,  indoor working  level, and  individual  and
population CWLM were computed from the exhalation rate and source size for each source
category. The calculated results for each source category are tabulated and compared.

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                                CONTENTS

                                                                        Page

PREFACE	ii

ACKNOWLEDGEMENT	Mi

ABSTRACT	Mi

List of Tables  	v

I.   Introduction	1

II.   General Description of a Florida Phosphate Gypsum Pile 	2

III.  Estimation of Source Terms	2

IV.  Radiological Impact  Assessment	12

V.   Summary and Conclusions	13

REFERENCES	14
                                  TABLES

                                                                        Page

1.  Source Terms 	3

2.  Exhalation Rates	4

3.  Intercomparison of Exhalation Rate Measurements	8

4.  Air Concentration, Working Level and Annual Cumulative Working Level Months
   (CWLM)	11

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I.   Introduction

        Previous reports (1,2,3,4) have described  radiological conditions in and around
    phosphate mining and milling operations located in Florida. Information on radioactivity
    distributions in phosphate industry products, byproducts, and waste; dose assessment of
    phosphate industry personnel; dose estimates to the general population from particulate
    emissions;  and radium-226  concentrations  in  ground water are included in  these
    publications (1,2,3,4). In Polk County, Florida, and several neighboring counties large
    deposits  of phosphate ore are found. Associated with this phosphate  material are
    significant  quantities of natural radioactivity including economically  recoverable
    amounts  of uranium contained in the phosphoric acid which is  produced from the
    phosphate ore. The radium-226 activity tends to be concentrated in the waste gypsum (1).
    The waste gypsum pile then  becomes a potential source of radon.*

        For many years inactive uranium mill tailings piles have been recognized as a source
    of relatively large quantities of radon. To test the hypothesis that phosphate gypsum piles
    may also be a source of relatively large amounts of radon, radon exhalation rate studier
    were  undertaken and  recently  completed  at  two phosphate  gypsum piles.These
    exhalation rate data have been converted to radon source terms so that  for a nearby
    residence indoor  radon concentration,  indoor working level, and individual and
    population  cumulative working level months (CWLM) estimates can be determined by
    utilizing atmospheric dispersion  modeling. Similar calculations were undertaken for
    inactive uranium mill tailings piles. The uranium  mill tailings data used in the calculations
    were previously published in several EPA reports (5,6,7). The results for each source
    category are compared.

        Some initial work on this subject, which included other large area sources of radon,
    can be found in a previous EPA publication (5).  A theoretical relationship (8) which was
    developed for inactive uranium mill tailings piles was applied to all source categories
    because measured radon exhalation rate data were not available at that time for every
    source category of interest. Based on actual measurements of exhalation rate, the source
    terms for source categories other than inactive  uranium  mill tailings piles can be much
    lower than predicted by this theoretical relationship (8). While the use of this theoretical
    relationship gives a conservative estimate, there  is a danger of grossly overestimating the
    radon exhalation rates (e.g.,  the radon exhalation rate fora slag pile resulting from the
    combustion of  coal  in producing electrical power) for sources other than inactive
    uranium mill tailings piles. This theoretical relationship (8) can also give overestimated
    predictions for inactive uranium mill tailings piles.

        As implied  in the title of this publication, only radon from the waste  piles are
    considered in this report. Other sources of radon exist at both phosphate and uranium
    milling operations, but are not discussed in this report. These other sources of radon are
    thought to be at least an order of magnitude less  importantthan the sources presented. A
    discussion  of airborne particulates,  while very important, is not addressed in this
    publication.

    *    In this report radon means radon-222.

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II.  General Description of a Florida Phosphate Gypsum Pile

        Phosphate gypsum piles in central Florida range in size from 20 to 130 hectares with a
    height of 20 to 30 m. The piles range in age from new to several decades old. A gypsum
    pile is created by slurrying the waste gypsum from the phosphoric acid production
    building. The slurry is diverted to different portions of the active pile by creating dikes that
    channel the slurry. The gypsum settles out and part of the water is recycled back to the
    phosphoric acid production area. When the pile dries out, a crusty surface forms which
    minimizes wind erosion. If this crusty surface is disturbed, (e.g., by a bulldozer which is
    used  to construct the diking), the wind is  free to erode the powdery composition
    underneath the crusty surface. Modeling of this dusting or resuspension of particulate
    material is not addressed in this report.

        The previous description does not apply to all gypsum piles in all parts of the country.
    For example, wind erosion and resuspension of gypsum occur with gypsum piles in Idaho
    (9) because the pile has a sandy consistency which  is a function of the type of phosphate
    ore from which the gypsum is derived and ore benefication  prior to phosphoric  acid
    production (1,2). The difference in rainfall rates for Florida and Idaho also influences the
    amount of dust suppression.

III.  Estimation of Source Terms

        Table 1 summarizes the information that is required to estimate the radon-222 source
    term for each source category. The mean exhalation rates are presented for each type of
    pile. The exhalation  rate is expressed in terms of radon activity released per unit area per
    unit time (e.g., pCi/m2-min). The exhalation rate depends on a number of variable factors,
    including the radium-226 specific activity in the pile material, the emanating power (i.e.,
    the amount of  radon released per unit generated) of  the pile material, atmospheric
    pressure, and the diffusion coefficient (which includes the moisture content of the pile)
    for the radon in  the pile material.'Also, ice and water cover over the pile can drastically
    reduce the exhalation rate (10). This rate may be determined  experimentally by several
    methods, including  the two most common ones utilizing charcoal canisters (10) and  55
    gallon drums (accumulator technique) (11). Details of  these methods are adequately
    described in past reports (10,11).

     Phosphate Gypsum Pile Exhalation Rates

        Table 2 contains a series of exhalation rate results obtained by the charcoal canister
     method (10) at phosphate gypsum piles. Data are presented for two phosphate gypsum
     piles  sampled at various times and locations (i.e., old and new sections of each pile). The
     old (inactive) section of each pile constitutes a portion of the overall pile that is not
     presently being worked (i.e., new gypsum is not being added to this section of the pile). It
     may  include gypsum that has been present on the pile for a number of years. The new
     (active) section  is an area of the overall pile where new material is being slurried to the
     pile. For the most part, canisters were placed on relatively dry areas of each pile, primarily
     on the outer edges of the pile. Ideally, the canisters should have been distributed over the
     entire pile  to account for the spatial distribution of  radon flux. Since these  were
     operational piles, practical limitations precluded ideal sampling.

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Table 1
Source Terms
Inactive Uranium
Mill Tailings Piles
Maximum (U  )
Minimum (U ,, )
Shiprock (Usrt)
Average (Uav)
Range
Phosphate Gypsum Piles
PileA(PA) 
PileB(PB) 
Background
Worldwide bkg(Bw)
Polk County FL bkg (B )
Active Uranium
Mill Tailings Pile
Size (hectare)
43.3 (370)*[6J
0.8 ( 50)'|6]
35.3 (335)-(7]
15.1 <200)'[6,7]
0.8 - 43.3[6]
75.4 (490)'
81.7 (51 OK

NA
NA

!!6Ra Specific
Activity (pCi/g)
896(6]
356[6j
700[61
620[6J
50-980(63
Range
25(19.2-32,2)
27(12.8-42.8)

1
0.5

Exhalation Rate
pCi/m!-min
66,000
28,000
5,600
39,000
3,800-72,000
1,600
1,600
25[14]
18[15]

Source Term
Ci/yr
15,000[5]
120(5]
1 ,04017]
3,200
T20-15,000[5]
620
680
NA
NA

Background
"Ci/yr
 - *
5.7
0.1
4.6
25
NO
9.7
106
NA
NA

  Typical (U,
60.0(437)*[17]
                                                                 560[17]
                                                                                                            9,500
                                                                                                                                                 3,000[17|
                                                                                                                                                                               7.9
 E 1  Reference number inside brackets.
  "  Equivalent radius in meters within parentheses.
 "  Background soil source term if pile were not present (B w is used).
***  Piles are associated with same plants as given in Partridge et al3.
NA  Not applicable.
ND  No data.

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                                    Table 2

                               Exhalation Rates
       Phosphate Gypsum Pile

               Pile A

             Old section
Exhalation rate (pCi/m2 -min)
            New section
1.06 x 103
7.22 x 103
1.61 x 103
1.24x 103
5.07 x 102

1.17 x 103
1.00x 103
1.51 x 103
8.29 x 102
1.43x 102

6.30 x 103
1.26x 103
1.97x 103
1.71 x 102
1.39x 103

2.24 x 102
2.85 x 102
1.70x 103
1.24x 103
1.23x 103
                                                        8/28/78-9/01/78*
                                                        9/05/78-9/08/78*
                                                        8/28/78-9/01/78*
                                                        9/05/78-9/08/78*
Charcoal canister placement dates (e.g., the first group of canisters were placed on the old
section 8/28/78 and removed 9/1/78).

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                             Table 2 (Continued)

                               Exhalation Rates
      Phosphate Gypsum Pile

               Pile 8

            Old Section
Exhalation rate (pGi/m2-min)
            New Section
1.55 x 102
7,72 x 102
8.87 x 102
2.50 x 102
8.37 x 102

2.22 x 103
1.67 x 103
4.24 x 102
3.71 x 102
1.58x10*

8.00 x 102
2.36 x 103
1.14X102
2.46 x 103
5.39 x 102

3.79 x 102
2.40 x 103
8.11 x 102
1.09X 103
1.13X 102**

1.94x 103
8.07 x 103
1.96 x 103
1.38x 103

7.74 x 102
5.01 x 10*
3.00 x 103
1.12x 103
3.92 x 103
                                                       7/17/78-7/21/78*
                                                       7/24/78-7/28/78*
                                                       8/07/78-8/11/78*
                                                       7/17/78-7/21/78*
                                                       7/24/78-7/28/78*
                                                       8/07/78-8/11/78*
Charcoal canister placement dates.
Wet charcoal canister measurement.

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    An attempt was made to fit both sets of pile data (table 2) to both a normal distribution
and a  log normal distribution (12,13). Gypsum pile A results did not follow either
distribution while pile B results did approximate a log normal distribution. The arithmetic
mean exhalation  rate for pile A was calculated by assuming a normal distribution,
(because pile A results more closely approximate  a  normal  distribution), while pile B
exhalation rate is based on  an arithmetic mean estimated by assuming a log normal
distribution and calculating a geometric mean and a geometrfc standard deviation. The
arithmetic  mean  exhalation rate   for  each  pile  is 1600  pCi/m2-min, calculated
independently of each other  as described above.

    The exhalation rate measurements for each pile (table 2) vary over almost 2 orders of
magnitude. This variation can be explained in part by the nonuntform distribution of
radium-226 in the pile material and moisture content throughout the pile. One example of
the effect of moisture on exhalation rate can be seen in the pile B result (table 2) described
as a "wet charcoal canister measurement." This particular result is the lowest reported
individual exhalation rate for the two piles. The time of sampling is an important aspect of
attempting to explain these variations. For example, the arithmetic mean for both the old
and new sections of pile A for the sampling period, 8/28/78 - 9/1/78, is approximately the
same. This is also the case for the sampling period, 9/5/78 - 9/8/78, but the resulting
arithmetic means  for the two sampling  periods differ by more than a factor of 2. This
difference possibly can be accounted for by changes in average barometric pressure,
total rainfall (if any) during the sampling period, and charcoal canister placement (i.e.,
location on the pile). The ability of the charcoal canister technique to reproduce the same
result under identical conditions should also be mentioned as a plausible explanation for
variation. The ability of the counting system to analyze the charcoal canister with
precision and accuracy should also be considered.  Method reliability and sensitivity are
discussed by Countess (10). The reproducibility of this method is expressed in terms of
coefficients of variation, corrected for counting errors, ranging from 0.06 to 0.15 (10). A
typical lower limit of detection is 2 pCi/m2-min (10).

    Based  on an intercomparison study with the  Polk  County Health Department
(PCHD), Winter Haven, Florida,  it is felt the actual  analysis of the charcoal canister to
determine the exhalation rate is not a major contributor to the wide variation in exhalation
rates. Table 3 summarizes the results from this intercomparison. The charcoal canisters
used in the  intercomparison are  the same ones placed on the gypsum piles. After the
canisters were removed from the gypsum pile, they were first counted by the PCHD and
then by the  EERF. To determine whether there was any significant difference between
results, a simple statistical test was applied  to the data. The simplifying assumption was
made that if the mean of the differences between corresponding values of each data set
was zero or near zero, then there was no significant difference. The mean of the
differences  plus or minus one standard deviation  of the mean encompasses zero;
therefore, it is assumed that either set of measurements is equally valid. By choice EERF
values were used  in tables 1  and 2.

    Meteorological  parameters  such as barometric pressure,  wind speed, ambient
temperature, humidity or any other factors that might  potentially influence radon
exhalation rate are not available for this report. One of the purposes of this report is to
present phosphate gypsum pile radon exhalation rate data. To utilize this data, at least
one simplifying assumption must be made,  i.e., the arithmetic mean of the data for each
pile represents an annual average condition. A more representative value would result

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from an entire year's collection of data. A year's worth of data negates the need to know
the meteorological parameters identified previously. Unfortunately, collecting data for a
year is unfeasible at this time.One other option  is to apply the previously discussed
meteorological parameters to the radon  exhalation rate data and calculate a modified
arithmetic mean exhalation rate. The resulting exhalation rate would at best be highly
speculative since the relative effect of each meteorological parameter on exhalation rate
is not well understood. Therefore, it is felt that the previously indicated values of 1600
pCi/m2-min  for each phosphate gypsum  pile are  reasonable values to assume for the
purposes of this publication.

Inactive Uranium Mill Tailings Pile Exhalation Rates

    The  exhalation  rates for the uranium mill tailings piles  (except Shiprock)  are
estimated by using a theoretical relationship involving the radium-226 specific activity of
the tailings material (8). A correction for pile wetness (15% decrease), pile  thickness (5%
decrease), and stabilization (25% decrease) was also made (6). The 25% decrease for
stabilization was only applied to stabilized piles  (6). Since uranium mill tailings piles are
highly  variable in size,  radium-226 specific activity, and exhalation rate  (6),  several
situations are  included for comparison purposes (i.e., maximum and minimum source
terms,  average source  term,  range of source term dependent parameters, and the
Shiprock pile). The Shiprock pile exhalation rate was determined by averaging the results
from several different measurement techniques (7). The estimated exhalation rate of the
Shiprock pile is probably the most reliable value for the inactive uranium mill tailings pile
source category. The theoretical equation (8) used to generate the other exhalation rates
tends to be conservative, i.e.,  it overestimates the actual exhalation rate especially when
the radium-226 specific activity in the pile material is overestimated. For example, if the
theoretical relationship (8) had been used to compute the Shiprock pile exhalation rate.a
value of 41,000 pCi/m2-min (compared to 5,600 pCi/mz-min : table 1) would have been
reported after  accounting for pile wetness, pile thickness, and stabilization. The large
difference in values is due mostly  to the relative importance of stabilization on reducing
the exhalation rate. Measurements made before  and  after stabilization  showed  a
reduction in exhalation rate of slightly greater than 8 for several feet of earth cover over
the pile (7). In this report, stabilization was only given a 25% reduction credit for two feet
of earth cover (6,8). Before stabilization, the measured exhalation rate was approximately
46,000 pCi/m2-min  (7). A value of 54,000 pCi/m2-min is calculated  by the theoretical
relationship  (8) after accounting for pile wetness and pile thickness. Looking at a second
pile (U max) which is the same as the Salt Lake  City, UT, pile, the measured exhalation
rate has been  reported  to be about  19,000 pCi/m2-min for a series of short term
measurements (11), while the  value in table 1 (66,000 pCi/m2-min) was obtained from the
theoretical relationship (8) modified for pile wetness and thickness. Again, the predicted
value is greater than the measured one. Therefore, in general,  any comparisons with
phosphate gypsum piles tend to bias the inactive uranium mill tailings piles on the high
side since the source terms are thought to be overestimated for the inactive uranium mill
tailings pile  source category.

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                                    Table 3



                 Intercomparison of Exhalation Rate Measurements



                                         Exhalation Rate (aCi .  cm"2. sec
Charcoal Canister #
71
58
84
91
44
43
79
61
85
46
4
20
36
42
55
69
77
148
181
11
15
27
34.
47
52
71
74
154
121
82
9
126
75
48
68
vt
151
37
1
13
163
PCHD*
1709.79
357.17
1150.94
1572.69
558.33
4132.28
1489.86
3017.48
393.55
880.84
2996.49
2367.40
599.84
538.37
2770.23
2796.42
11968.23
2758.84
1830.78
1426.57
3750.83
178.99
3764.96
878.56
1322.76
961.40
5402.35
1888.22
6815.69
1480.04
1096.43
2315.52
2055.20
752.89
9139.21
1774.77
2617.57
205.84
1977.97
1698.39
1424.81
EERF**
1479.0
257.8
1287.0
1395.0
416.5
3993.0
1351.0
1813.0
187.7
630.4
3707.0
2791 .0
706.8
618.5
2639.0
3238.0
13450.0
3259.0
2304.0
1333.0
3936.0
190.4
4102.0
898.4
1290.0
835.6
4958.0
1869.0
6534.0
1767.0
1204.0
2677.0
2072.0
845.5
10500.0
2105.0
3277.0
284.4
2319.0
1957.0
1670.0
t   PCHD designator.

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                               Table 3 (Continued)

                              	    Exhalation Rate (aCi . cm "2. sec "1)***
Charcoal Canister #	PCHD*	EERF"

           54                           2385.03                            2513.0
           50                           1201.70                            1382.0
           33                            192.84                             238.5
          175                            396.82                             372.6
           42                            440.48                             475.3
          156                           2464.38                            2840.0
          125                           1984.42                            2059.0
           22                           1869.96                            2053.0
    Polk County Health Department, Winter Haven, FL; results that are given are as reported
    to EERF; no significance should be attached to the number of digits reported.

    Eastern Environmental Radiation Facility (USEPA), Montgomery, AL; computer gen-
    erated  results; no significance should be attached to the number of digits reported.

    Units used by PCHD; aCi = 10 ~18 Ci; 1 aCi/cm2-sec = 0.6 pCi/mz-min or 1 pCi/m* -min =
    1.67 aCi/cm2 -sec.
 f  PCHD designator.

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Background Exhalation Rates

    Referring to table 1, the worldwide background exhalation rate (25 pCi/m2-min) is an
average value for the entire world for background soil (14). The average U.S. value is
approximately the same as the worldwide rate (14). The exhalation rate for Polk County,
Florida, background soil was calculated to be 18 pCi/m2-min, the computed arithmetic
mean from accumulator method results (15).

Radon Source Terms

    Again referring to table 1, the radon source terms in units of Ci/yr are based on the
mean exhalation rate for each source and a representative source area expressed in terms
of hectares (1 hectare = 10,000 m2) and an equivalent radius. These source terms are used
as input into a computer code which calculates individual and population doses (16). The
computer generated doses are converted to radon concentration, working level, and
CWLM. The computer code (16) waswrittentooutputdosesdirectly without giving radon
concentration and working level; hence, the doses are transformed by hand calculations
to concentration and working level.

Discussion of Table 1 Results

    By addressing the source size and radium-226 specific activity information in table 1,
an almost reciprocal relationship exists between radium-226 specific activity and source
size which  accounts for the relatively large source terms  for phosphate gypsum piles.
Even though radium-226 specific activity for uranium mill tailings piles is large in most
cases, the pile areas are much smaller, in almost every instance, than the phosphate
gypsum piles. A pattern occurs when comparing exhalation rates and source terms. The
average uranium mill tailings pile (Uav) exhalation rate is much greater than either of the
two phosphate gypsum pile exhalation rates, which  reflects the greater radium-226
specific activity of the Uavpile. Looking at the source terms  for each  category,  the
difference is much smaller than was seen previously with the exhalation rate comparison.
The relatively large phosphate gypsum piles nullify a large portion of the difference.

    Of interest is the radon contribution background soil would make if each pile were
not present. The source terms for background soil are presented in table 1 in the last
column. In  all cases the pile radon source term is much greater than its corresponding
background soil component.

Active Uranium Mill Tailings Pile

    A typical active uranium mill tailings pile was included  for  academic interest. The
source size for the U act pile is approximately four times larger than that of the U av pile
(table 1) but only 25% of the pile is contributing to the radon source term (17). The other
75% of the pile includes the tailings  pond and wet beach areas which are assumed to
contribute negligibly to  the radon source term (17). The exhalation rate for U act in table 1
is averaged over the entire pile. This rate computed for the radon contributing portion of
the pile yields an exhalation rate of 38,000 pCi/m2-min or approximately the same as for
  av (
                                    10

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                                               Table 4

                                   Air Concentration, Working Level,

                                                and

                           Annual Cumulative Working Level Months (CWLM)
          Indoor Radon
          Concentration
Source      (pCi/l)(a)
     Indoor
Working Level (a)
 Population         Individual
Within 80 km    CWLM/Year(a)
   Population CWLM
(Person-CWLWyear) (b)
umax
min
Ush
Uav
PA
PB
"act
(C)
(d)
(e)
(d)
(f)
(0
(d)
3.2
0.07
0.45
1.8
0.19
0.21
1.4
.02
.0005
.003
.01
.001
.001
.01
850,000
36,000
43,000
36,000
1,300,000
1,200,000
36,000
0.45 (4.0E-3)*(g)
0.01 (1.0E-5)(g)
0.06 (2.0E-4)(g)
0.25 (3.0E-4)(g)
0.03 (2.0E-5)
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IV.  Radiological Impact Assessment

        The computer code AREAC (16) was used to assess the radiological impact of each
    radon source. A ground level release is assumed in every instance. The source term
    (Ci/yr) for each source is combined with representative meteorological data to calculate
    outdoor radon concentrations. The simplifying assumption is made that over a year's
    period the indoor radon concentration attributable to each pile will approach the annual
    average outdoor radon concentration resulting from atmospheric dispersion of the pile
    radon. Working level exposures associated with indoor radon are calculated assuming an
    indoor exposure at 70 percent equilibrium (18) (i.e., 100 pCi/l radon = 0.7 working level).
    All reported values (table 4) of radon concentration and working level are for a structure
    located 800 m from the center of the pile in the  maximum wind direction.

        To obtain individual CWLM estimates, the indoor radon concentration is multiplied
    by a CWLM conversion factor (1 pCi/l of radon-222 is equivalent to 0.14 CWLM/year at
    75% occupancy) (19). As would be expected, the individual  CWLM estimates for the
    uranium mill tailings piles are typically greater than for phosphate gypsum piles (table 4).
    The same simplifying assumptions made in calculating indoor radon concentration apply
    to CWLM predictions.

        The population CWLM predictions (person-CWLM/year : table 1) are noteworthy.
    Due to the relatively large population centers near the Polk County phosphate gypsum
    piles, the population CWLM for phosphate gypsum piles are significantly greaterthan for
    the average inactive uranium mill tailings pile (U av). The U av pile which is thought to be
    fairly typical in its population distribution for that source category (i.e., a low population
    density within 80 km of the pile). The preceding comparison also applies to a typical
    active uranium  mill tailings pile (Uact ) At least one exception to the aforementioned
    remarks  is the  uranium  mill tailings pile located close to Salt Lake City. With a
    combination of a large source term (same as U max )and proximity to Salt Lake City, the
    resulting population CWLM greatly exceeds those for P A and PB piles.
                                        12

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V.  Summary and Conclusions

        Florida phosphate  gypsum pile exhalation rate results are  provided. These are
    compared with those calculated for inactive uranium mill tailings. Indoor radon air
    concentration, indoor working level, and individual and population CWLM were derived
    from the exhalation rate and source size using the computer code AREACforeach source
    category. The computed results for each source category are tabulated and compared.
    The following conclusions are drawn.

         (A)  The maximum individual CWLM/year exposure due to radon emanation from a
         typical inactive uranium mill tailings pile is significantly greater than from a typical
         Florida phosphate gypsum pile which is attributable to the greater radon source
         term associated with a typical inactive uranium mill tailings pile.

         (B)  The population  CWLM/year  exposure within 80 km  of a typical Florida
         phosphate gypsum pile is as great or greater than from a typical inactive uranium
         mill tailings pile which is a reflection of a greater average population density within
         80 km of a typical  Florida phosphate  gypsum pile.

         (C)   In order to better define the total risk associated with each source category, a
         comprehensive radiological analysis is needed.  This involves the combining  of
         individual pile results from the entire United States. The best available estimate of
         the local meteorology, population distribution and source term ideally should be
         incorporated into this effort. Total health effects for each source category can then
         be determined.
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                                 REFERENCES

 1.  GUIMOND, R.J. and S. T. WINDHAM. Radioactivity Distribution in Phosphate Products,
    Byproducts, Effluents, and Wastes. ORP/CSD 75-3 (1975).

 2.  WINDHAM, S. T., J. E. PARTRIDGE and T. R. MORTON. Radiation Dose Estimates to
    Phosphate Industry Personnel. EPA-520/5-76-014 (1976).

 3.  PARTRIDGE, J.E., T. R. MORTON, E. L. SENSINTAFFAR and G. A. BOYSEN. Radiation
    Dose Estimates Due to Air Particulate Emissions from Selected Phosphate Industry
    Operations. Technical Note ORP/EERF-78-1 (1978).

 4.  KAUFMANN,  R.F. and J. D. BLISS.  Effects of Phosphate Mineralization and the
    Phosphate  Industry on  Radium-226  in Ground  Water of Central  Florida.  EPA/
    520-6-77-010 (1977).

 5.  MORTON, T. R. Estimates of Radon-222 Daughter Doses from Large-Area Sources. ANS
    Transactions, Vol. 27, San Francisco.CA (1977).

 6.  SWIFT, J. J., J. M. HARDIN and H. W. GALLEY. Potential Radiological Impact of Airborne
    Releases and Direct Gamma Radiation to Individuals Living Near Inactive Uranium Mill
    Tailings Piles. U.S. Environmental Protection Agency. EPA-520/1-76-001 (1976).

 7.  HANS, J.  M.,  T.  R. MORTON  and D.  PROCHASKA. Estimated  Average  Annual
    Radon-222  Concentrations Around the  Former  Uranium Mill Site in Shiprock, New
    Mexico. U.S. Environmental  Protection Agency, Office of Radiation Programs  - Las
    Vegas Facility. Technical  Note ORP/LV-78-7 (1978).

 8.  SCHIAGER, K. J. Analysis of Radiation Exposures on or Near  Uranium Mill Tailings
    Piles. Radiation Data and Reports, Vol. 15(7) 411-425 (1974).

 9.  U.S.  ENVIRONMENTAL  PROTECTION  AGENCY.  Radiological Surveys of Idaho
    Phosphate  Ore Processing - The Wet Process  Plant. Technical  Note ORP/LV-78-1
    (1978).

10.  COUNTESS, R. J. Measurement of Radon-222 Flux with Charcoal Canisters. Workshop
    on Methods for Measuring Radiation in and Around Uranium Mills. Atomic Industrial
    Form, Inc. (I977).

11.  BERNHARDT,  D. E., F. B. JOHNS and R. F. KAUFMANN.  Radon Exhalation from
    Uranium Mill Tailings Piles. Description and Verification of the Measurement Method.
    Technical Note ORP/LV-75-7(A)  (1975).

12.  TOY, A. J.  and C. L. LINDEKEN. The Implications of Sampling from a Log-Normal
    Population. Lawrence Livermore Laboratory. Preprint UCRL-76936 (1975).

13.  DENHAM, D.  H. and D. A. WAITE. Some Practical Applications  of the Log-Normal
    Distribution for Interpreting  Environmental Data. 20th Annual Meeting of the Health
    Physics Society, Buffalo, NY (1975).
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14.  WILKENING, M. H., W. E. CLEMENTS and D.STANLEY. Radon-222 Flux Measurements
    in Widely  Separated  Regions.  Natural Radiation  Environment  II,  Houston,  TX,
    CONF-720805 (1972).

15.  UNIVERSITY OF FLORIDA. Natural Radiation Exposure Assessment. Radioactivity of
    Lands and  Associated  Structures. Third Semiannual Technical Report submitted to
    Florida Phosphate Council. Lakeland, FL. Covering the period March 1,1977-August 31,
    1977.

16.  MICHLEWICZ, D. Area Source Radiological Emission Analysis Code  (AREAC). U.S.
    Environmental Protection Agency. Technical Note ORP-EAD-76-6 (1976).

17.  MAGNO. P. Radon-222 Releases from Milling Operations, Testimony before the Atomic
    Safety and Licensing Board in the Matter of Perkins Nuclear Station (1978).

18.  GEORGE, A. C. and A. J. BRESLIN. The Distribution of Ambient Radon and Radon
    Daughters in Residential Buildings in the New Jersey-New York Area. Presented at
    Symposium on the National Radiation Environment III, Houston, Texas (1978).

19.  GUIMOND, R. J., W. H. ELLETT, J. E. FITZGERALD. S. T. WINDHAM and P. A. CUNY.
    Indoor Radiation Exposure Due to Radium-226 in Florida Phosphate Lands. EPA 5207
    4-78-013 (1979).
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