Ambient Air Radon Monitoring Report on
Mosaic Riverview Phosphogypsum Stack
Part II
A joint effort of:
Florida Department of Health
Bureau of Radiation Control
Environmental Protection Commission
of Hillsborough County
Air Management Division
Table of Contents
EXECUTIVE SUMMARY ...................................................................................................... 3
INTRODUCTION .................................................................................................................. 4
BACKGROUND ON PHOSPHOGYPSUM AND RADIATION ............................................ 5
Phosphogypsum ........................................................................................................... 5
Radiation ...................................................................................................................... 6
Radon ........................................................................................................................... 7
Gamma Radiation ......................................................................................................... 8
MEASUREMENTS TAKEN IN THIS PROJECT ................................................................... 8
SITE LOCATIONS .................................................................................................................. 9
DATA .................................................................................................................................... 10
Data Tables ................................................................................................................ 10
Explanation of Table Entries ....................................................................................... 13
Results ........................................................................................................................ 14
INTERPRETATION OF THE RESULTS .............................................................................. 15
CONCLUSIONS AND RECOMMENDATIONS .................................................................. 16
APPENDIX – REFERENCES AND SUGGESTED RESOURCES........................................ 17
2|Page
EXECUTIVE SUMMARY
Ambient air radon measurements were taken as a follow up to the outstanding radon monitoring
requirement of the Development of Regional Impact (DRI) #242 for the construction and
operation of the expansion of the phosphogypsum stack near The Mosaic Company’s fertilizer
manufacturing plant in Riverview, Florida. The original study (Part I) was of two separate three
month periods in 2010. The measurements for Part II were taken over four consecutive three
month periods, primarily during 2013. As much as possible, the locations for this follow up
were kept the same as the original study for comparative purposes. Among the minor monitor
site movements, seven of the monitoring locations were relocated slightly (approx. 10-15 feet)
due to construction of a fence since the original study that limited access to the former locations.
The minor location changes were not considered significant enough to impact comparisons to the
original study.
The measurements were taken from 16 locations around the active phosphogypsum stack, three
nearby schools (Gibsonton Elementary, Ippolito Elementary, and Progress Village Middle) and
one control site to determine if the phosphogypsum stack was contributing elevated levels of
radiation to nearby residents.
Ambient air radon was measured by using Landauer Alpha Tracks mounted on six-foot high
poles or on fence lines. Each of the 20 monitors were left in place for approximately 90 days
before being retrieved for laboratory analysis. During the deployment of the ambient air radon
monitors, gamma measurements were taken utilizing a handheld device giving real-time data.
None of the measurements taken in this project constituted remedial actions as recommended by
various public health agencies. Differences in the four deployments were noted. The first two
deployments showed higher radon concentration than the second two.
Given the amount of data collected, no exact estimates can be made about the contribution of
radon from the phosphogypsum stack to surrounding ambient air. The variation present between
the deployments suggest that the variation in the original study was not an isolated case. Further
study may allow more clear characterization of the data variability, but may still not clarify the
contribution of the phosphogypsum stack.
3|Page
INTRODUCTION
The Pre and Post Monitoring section of the Florida Department of Health, Bureau of Radiation
Control Environmental Radiation Program, conducted a follow up ambient air radon monitoring
project for the Air Management Division of the Environmental Protection Commission of
Hillsborough County (EPC). The monitoring spanned four consecutive three month periods
beginning in November 2012.
Figure 1
The monitoring site is the phosphogypsum stack east of U.S. Highway 41, near the Mosaic
company’s fertilizer manufacturing plant at 8813 U.S. Highway 41 South, in Riverview, Florida
(see Figure 1). Three nearby elementary schools and a control site were included in the study.
The schools included were Gibsonton Elementary, Ippolito Elementary, and Progress Village
Middle School. The control site was located about 4.3 miles east of the study area at Mosaic’s
groundwater pumping site.
4|Page
Measurements were taken to fulfill the outstanding radon monitoring requirement of the
Development of Regional Impact (DRI) #242 Specific Condition A.8.a. of the Development
Order for the construction and operation of the expansion taking place at this phosphogypsum
stack. It will encompass about 376 acres and will be 260 feet high when completed. The stack
also serves as a reservoir for storing and supplying process water for the industrial processes at
the fertilizer production facility.
BACKGROUND ON PHOSPHOGYPSUM AND RADIATION
Phosphogypsum
Phosphate mining is accomplished in surface mines (see Figure 3) with a dragline (see Figure 2),
which uses a large bucket to remove a mixture known as matrix, which consists of phosphate
rock, clay and sand. The phosphate ore is found 15-50 feet below the earth’s surface and is
Figure 3
about 10-20 feet thick. The rock is
dumped in a pit at the mining site
and high-pressure water guns turn
it into a slurry that can then be
Figure 2
pumped to a beneficiation plant
where the phosphate will be separated from the sand and clay. After going through
beneficiation, the clay slurry is pumped to a settling pond. The sand is sent back to the mine site
to be used in reclamation and the phosphate is sent to the chemical processing plant where it is
processed for use in fertilizer and other products.
Phosphogypsum is a by-product of the chemical processing plant’s chemical reaction called the
“wet process”, whereby sulfuric acid is reacted with phosphate rock to produce the phosphoric
acid needed for fertilizer production. There are approximately five tons of phosphogypsum
produced for every ton of phosphoric acid produced.
5|Page
Phosphogypsum, like natural gypsum, is calcium sulfate, a relatively innocuous material that is
used in materials such as wallboard. Phosphogypsum, however, is slightly more radioactive than
natural gypsum. The radium that is found naturally associated with phosphate rock becomes
associated
with
the
phosphogypsum after the rock is
reacted with sulfuric acid.
Figure 4
In
1989,
stacking
of
phosphogypsum (see Figure 4) in
the U.S. became a legal necessity
when the U.S. Environmental
Protection Agency (EPA) banned
its use due to radioactivity. There
are currently about one billion tons
of phosphogypsum stacked in 25
stacks in Florida and about 30
million new tons are generated
each year and added to those
stacks.
Radiation
When an atom transforms, it gets rid of excess energy in the form of particles or electromagnetic
waves (like gamma rays). Sometimes the atom transforms into another unstable atom and will
continue transforming and releasing energy until it is completely stable. The uranium found with
the phosphate in Florida’s earth, for instance, transforms through four intermediate elements to
radium and then to radon, a gaseous element. It goes through a chain of seven more
transformations into unstable elements before reaching a point of stability. Because radon is a
gaseous element the impacts of its radioactive effects may be present at distances far from a
phosphogypsum stack.
It is customary to refer to the quantity of radioactive material in terms of activity, which is the
number of atoms that undergo transformation in the material over a given period of time. A
common unit of activity is the curie, named after Marie Curie, who discovered radium. One
curie is equal to 37 billion transformations per second. A curie is considered a large amount of
activity. To conveniently discuss common amounts of radioactivity, the term picocurie (pCi) is
used. A picocurie is one trillionth of a curie.
Activity is related to a given mass or volume of material, like a liter or a gram, depending on
whether the material is liquid, solid or gas. The derived unit pCi/gram denotes the number of
transformations occurring per unit time for a given quantity of material. The derived unit pCi/L
denotes the activity present in a given liter of gas, usually the atmosphere.
Radiation is also quantified in terms of the exposure received by a human body. The amount of
the exposure is often generalized as the dose. The amount of gamma radiation present at a given
site is usually given as the dose that a person would receive from standing there, in the units of
6|Page
rem per hour (R/hr) or microrem per hour (uR/hr) to denote a millionths of a rem per hour.
When rem measurements are expressed without time, it refers to a total dose received,
irrespective of exposure time.
We are exposed to many common sources of radiation during our everyday lives. Some
common sources of radiation include medical imaging, smoking, air travel, building materials,
and consumer electronics. Even the food we eat, the water we drink, and the air we breathe
exposes us to radiation in very small doses.
Radon
Radon is a colorless, odorless gas. You can’t see it, taste it, or smell it, but it is always present in
the atmosphere. Its radiation travels a very short distance, not even able to penetrate an outer
layer of a person’s skin cells. Its primary mode of effect on human health is from inhalation,
where it can deposit a dose to lung tissue. The effect of this dose is primarily in the risk of lung
cancer.
Radon has a half-life of 3.8 days, meaning that on average one half of a given collection of
Radon atoms will decay in this time period. This gives it enough time after formation to diffuse
from the ground, and travel considerable distance through the atmosphere. That is why it can
contribute to the radon present in the environment far from the gypstack (phosphogypsum stack).
There is no build up factor for radon around a gypstack. Diffusion and convection (wind)
removes any buildup of radon close to the stack.
Because it has to diffuse through the ground before it can escape, its rate of emanation from the
soil is related to the diffusion properties of the soil. Equilibrium is not reached from diffusion
because if the radon takes too long to reach the surface it will decay before it can become
airborne. Anything that affects the rate of diffusion through soil will affect the rate of emanation
of radon. Ground properties that affect diffusion rates include soil composition, compaction,
moisture, vegetation, and various other factors. All these variables make emanation rates
impractical to estimate, and variable over time due to any changes to the ground, for example
from seasonal changes, such as rainfall.
The difference should be distinguished here between outdoor and indoor radon. They are the
same gas of course, but the environmental factors that affect its concentration are very different.
Outdoor radon concentrations consist of the amount of radon present in the atmosphere. It
includes the radon from all sources including the gypstack. The contributions from different
sources can be taken to be variable, as in the case of the gypstack possibly contributing a higher
percentage at sites in closer proximity to the stack. Other sources, such as ground water, soil and
construction materials all contribute to outdoor radon.
Indoor radon can have a build up factor not present in outside air, due to possible lack of mixing
with outside air. When this is the case, even small sources can have a magnified effect on indoor
radon concentrations. Such sources include cracks in solid floors, construction joints, cracks in
walls, gaps in suspended floors, gaps around service pipes, space inside walls and from the water
supply. Most all of these sources are due to radon emanation from soil underneath the house,
and most all of them can be reduced with appropriate construction methods. The EPA
7|Page
recommends testing a house water supply and other mediations if the indoor radon concentration
is over 4 pCi/L.
Ventilation has the opposite effect on the buildup factor of indoor radon. Just a small amount of
ventilation can mitigate build up from most all of the sources mentioned above. Just opening
windows in a dwelling can readily reduce indoor levels and bring them in line with outdoor
levels. As such, the contribution of outdoor ambient air radon levels on indoor levels is broadly
varied depending on a dwellings degree of ventilation.
Gamma Radiation
Gamma radiation is in the form of electromagnetic waves. Most nuclear transformations involve
the release of gamma waves, resulting in a broad spectrum of energies over most Florida soil.
They do not travel a long distance in the atmosphere, so the exposure level at a particular
location is due to radioactive sources in the ground directly below and nearby. Because of its
limited range, it is not considered as much a public health risk to surrounding properties near the
gypstack.
The most common unit of measurement for environmental levels is the
microroentgen, due to average background levels being just a few millionths of a rem. Much of
the source of gamma radiation in Florida soils is uranium and radium. When reduction is
warranted a few feet of fill dirt can reduce exposure.
MEASUREMENTS TAKEN IN THIS PROJECT
Figure 5
Figure 7
The
primary
measurement for
this study is the
amount of radon
in ambient air in
proximity to the
phosphogypsum
stack. This was
accomplished
using Landauer
Figure 6
Alpha Tracks.
These consist of
a small tape protected in a casing that allows
ventilation (see Figure 5). The alpha particles from
radon decay leave visible tracks in the tape, which can
be counted. These devices were deployed at the top
of PVC poles (see Figure 6) or along fence lines to
keep them at about 6 feet off the ground (see Figure
10). The height is chosen to represent breathing
height. Mounted with each Alpha Track was a
thermoluminescent dosimeter (TLD) that measured
8|Page
gamma radiation (see Figure 7). These items were deployed for a minimum of 90 days. Before
deployment, they were kept in airtight packaging. After retrieval, they were again sealed from
outside air so that they only accumulated radon exposure during the deployment period. The
Alpha Tracks were returned to the Landauer
Company, which read their exposure, and the
TLD’s were sent to the Florida Bureau of
Radiation Controls’ environmental laboratory for
their exposure reading. At each site another
gamma reading was also taken with a hand held
meter (see Figure 8).
Figure 8
Deployments were made for four periods of
approximately 90 days each. The total deployment
time spanned 405 days from November 28th 2012
to January 7th 2014.
SITE LOCATIONS
Figure 10
To get a representative measure of air
around the phosphogypsum stack, 16 sites
around the outside of the perimeter were
chosen (see Figure 9). These locations
Figure 9
were in most cases on the outside edge of
a perimeter road. In this way they could
stand undisturbed and be close enough to the stack to be representative. These sites were also
close enough to show increased gamma readings, due to the proximity of the stack and the
perimeter roadbed.
9|Page
Additional sites were chosen at three local schools, and one site as a control site. The three
schools were Progress Village Middle School, Gibsonton Elementary School, and Ippolito
Elementary School. The control site was at a Mosaic owned groundwater pumping site several
miles east of the phosphogypsum stack. The school sites were located apart from the building
structures to remove any influence from these structures in the form of interior radon or gamma
rays from building materials.
DATA
Data Tables
AMBIENT AIR RADON MEASUREMENTS
First Quarter Deployment
Deployment Site
Latitude
Longitude
Date
Deployed
Date
Retrieved
Exposed
(days)
Radon
(pCi/L)
TLD
(uR/hr)
GAMMA
(uR/hr)
Gypstack-1
2753.563
8222.835
11/28/12
3/5/13
97
1.8
1.25
8
Gypstack-2
2753.562
8222.625
11/28/12
3/5/13
97
1.7
1.13
7
Gypstack-3
2753.185
8222.452
11/28/12
3/5/13
97
1.6
3.63
9
Gypstack-4
2753.040
8222.451
11/28/12
3/5/13
97
1.7
0.87
8
Gypstack-5
2752.907
8222.452
11/28/12
3/5/13
97
2.1
2.32
7
Gypstack-6
2752.775
8222.441
11/28/12
3/5/13
97
2
2.2
10
Gypstack-7
2752.479
8222.478
11/28/12
3/5/13
97
0.9
2.78
26
Gypstack-8
2752.48
8222.577
11/28/12
3/5/13
97
1.5
2.41
26
Gypstack-9
2752.479
8222.790
11/28/12
3/5/13
97
1.5
1.75
24
Gypstack-10
2752.363
8223.024
11/28/12
3/5/13
97
1.1
3.13
25
Gypstack-11
2752.479
8223.168
11/28/12
3/5/13
97
1.9
16.44
29
Gypstack-12
2752.785
8223.167
11/28/12
3/5/13
97
2.4
16.4
12
Gypstack-13
2752.941
8223.167
11/28/12
3/5/13
97
1.9
16.31
9
Gypstack-14
2753.041
8223.170
11/28/12
3/5/13
97
2
16.11
14
Gypstack-15
2753.163
8223.169
11/28/12
3/5/13
97
2.5
17.26
15
Gypstack-16
2753.385
8223.026
11/28/12
3/5/13
97
1.9
4.08
8
Control Site
Progress Village
Middle School
Ippolito
Elementary School
Gibsonton
Elementary School
2753.191
8218.552
11/28/12
3/5/13
97
2
3.44
6
2753.488
8221.952
11/28/12
3/5/13
97
1.3
4.36
6
2752.603
8221.417
11/28/12
3/5/13
97
1.7
6.34
6
2750.897
8222.137
11/28/12
3/5/13
97
1.2
2.16
5
10 | P a g e
AMBIENT AIR RADON MEASUREMENTS
Second Quarter Deployment
Deployment Site
Latitude
Longitude
Date
Deployed
Date
Retrieved
Exposed
(days)
Gypstack-1
2753.563
8222.835
3/5/13
6/18/13
105
Gypstack-2
2753.562
8222.625
3/5/13
6/18/13
105
Gypstack-3
2753.185
8222.452
3/5/13
6/18/13
Gypstack-4
2753.040
8222.451
3/5/13
Gypstack-5
2752.907
8222.452
Gypstack-6
2752.775
Gypstack-7
Radon
(pCi/L)
TLD
(uR/hr)
GAMMA
(uR/hr)
3.1
2.31
8
3.6
2.95
7
105
3.2
3.5
9
6/18/13
105
3
2.85
8
3/5/13
6/18/13
105
3.5
2.28
7
8222.441
3/5/13
6/18/13
105
2752.479
8222.478
3/5/13
6/18/13
105
2.1
2.2
3.88
18.67
10
26
Gypstack-8
2752.48
8222.577
3/5/13
6/18/13
105
2
19.17
26
Gypstack-9
2752.479
8222.790
3/5/13
6/18/13
105
2.3
18.87
24
Gypstack-10
2752.363
8223.024
3/5/13
6/18/13
105
2
18.28
25
Gypstack-11
2752.479
8223.168
3/5/13
6/18/13
105
0.5
20.71
29
Gypstack-12
2752.785
8223.167
3/5/13
6/18/13
105
0.8
5
12
Gypstack-13
2752.941
8223.167
3/5/13
6/18/13
105
0.6
4.26
9
Gypstack-14
2753.041
8223.170
3/5/13
6/18/13
105
0.6
5.51
14
Gypstack-15
2753.163
8223.169
3/5/13
6/18/13
105
0.7
7.48
15
Gypstack-16
2753.385
8223.026
3/5/13
6/18/13
105
1.1
2.65
8
Control Site
Progress Village
Middle School
Ippolito
Elementary School
Gibsonton
Elementary School
2753.191
8218.552
3/5/13
6/18/13
105
0.8
1.32
6
2753.488
8221.952
3/5/13
6/18/13
105
0.6
1.71
6
2752.603
8221.417
3/5/13
6/18/13
105
0.7
1.67
6
2750.897
8222.137
3/5/13
6/18/13
105
0.7
0.78
5
11 | P a g e
AMBIENT AIR RADON MEASUREMENTS
Third Quarter Deployment
Deployment Site
Latitude
Longitude
Date
Deployed
Date
Retrieved
Exposed
(days)
Radon
(pCi/L)
TLD
(uR/hr)
GAMMA
(uR/hr)
Gypstack-1
2753.563
8222.835
6/18/13
10/2/13
106
3.7
1.76
N\A
Gypstack-2
2753.562
8222.625
6/18/13
10/2/13
106
3.9
2.2
N\A
Gypstack-3
2753.185
8222.452
6/18/13
10/2/13
106
1.1
2.43
N\A
Gypstack-4
2753.040
8222.451
6/18/13
10/2/13
106
N\A
1.73
N\A
Gypstack-5
2752.907
8222.452
6/18/13
10/2/13
106
1.3
1.4
N\A
Gypstack-6
2752.775
8222.441
6/18/13
10/2/13
106
Gypstack-7
2752.479
8222.478
6/18/13
10/2/13
106
1.2
1.4
2.57
14.76
N\A
N\A
Gypstack-8
2752.48
8222.577
6/18/13
10/2/13
106
1.1
15.13
N\A
Gypstack-9
2752.479
8222.790
6/18/13
10/2/13
106
BDL
15
N\A
Gypstack-10
2752.363
8223.024
6/18/13
10/2/13
106
BDL
17.18
N\A
Gypstack-11
2752.479
8223.168
6/18/13
10/2/13
106
BDL
15.95
N\A
Gypstack-12
2752.785
8223.167
6/18/13
10/2/13
106
BDL
2.81
N\A
Gypstack-13
2752.941
8223.167
6/18/13
10/2/13
106
0.4
2.7
N\A
Gypstack-14
2753.041
8223.170
6/18/13
10/2/13
106
BDL
3.09
N\A
Gypstack-15
2753.163
8223.169
6/18/13
10/2/13
106
BDL
5.3
N\A
Gypstack-16
2753.385
8223.026
6/18/13
10/2/13
106
0.4
1.15
N\A
Control Site
Progress Village
Middle School
Ippolito
Elementary School
Gibsonton
Elementary School
2753.191
8218.552
6/18/13
10/2/13
106
0.4
0.51
N\A
2753.488
8221.952
6/18/13
10/2/13
106
BDL
0.64
N\A
2752.603
8221.417
6/18/13
10/2/13
106
BDL
0.64
N\A
2750.897
8222.137
6/18/13
10/2/13
106
N\A
0.08
N\A
NOTE: The presence of “N\A” in the data tables refers to data being “not available”. In the case of gamma measurements it was
due to redundancy (hand held meter not on-site), and in the case of some radon measurements it was due to possible damage to
the devices or unreliable readings.
12 | P a g e
AMBIENT AIR RADON MEASUREMENTS
Fourth Quarter Deployment
8222.835
Date
Deployed
10/2/13
Date
Retrieved
1/7/14
Exposed
(days)
97
TLD
(uR/hr)
GAMMA
(uR/hr)
2753.562
8222.625
10/2/13
1/7/14
97
BDL
2.2
8
Gypstack-3
2753.185
8222.452
10/2/13
1/7/14
97
BDL
2.98
7
0.4
3.62
8
Gypstack-4
2753.040
8222.451
10/2/13
1/7/14
97
N/A
2.7
8
Gypstack-5
2752.907
8222.452
10/2/13
1/7/14
97
0.4
2.43
8
Gypstack-6
2752.775
8222.441
10/2/13
1/7/14
97
0.5
3.8
9
Gypstack-7
2752.479
Gypstack-8
2752.48
8222.478
10/2/13
1/7/14
97
BDL
17.6
25
8222.577
10/2/13
1/7/14
97
Gypstack-9
BDL
17.61
26
2752.479
8222.790
10/2/13
1/7/14
97
BDL
18.09
26
Gypstack-10
2752.363
8223.024
10/2/13
1/7/14
97
BDL
19.09
29
Gypstack-11
2752.479
8223.168
10/2/13
1/7/14
97
0.4
26.6
29
Gypstack-12
2752.785
8223.167
10/2/13
1/7/14
97
BDL
5.27
12
Gypstack-13
2752.941
8223.167
10/2/13
1/7/14
97
BDL
4.11
10
Gypstack-14
2753.041
8223.170
10/2/13
1/7/14
97
BDL
5.28
13
Gypstack-15
2753.163
8223.169
10/2/13
1/7/14
97
BDL
7.26
14
Gypstack-16
2753.385
8223.026
10/2/13
1/7/14
97
0.5
2.55
9
Control Site
2753.191
8218.552
10/2/13
1/7/14
97
BDL
1.33
6
2753.488
8221.952
10/2/13
1/7/14
97
BDL
1.79
6
2752.603
8221.417
10/2/13
1/7/14
97
BDL
2.06
6
2750.897
8222.137
10/2/13
1/7/14
97
BDL
1.05
3
Deployment Site
Latitude
Longitude
Gypstack-1
2753.563
Gypstack-2
Progress Village
Middle School
Ippolito
Elementary School
Gibsonton
Elementary School
Radon
(pCi/L)
NOTE: The presence of “N\A” in the data tables refers to data being “not available”. In the case of radon measurement it was
due to possible damage to the device or unreliable reading.
Explanation of Table Entries
The deployment sites label the schools by name, and the stack site locations by number one
through 16. The site labeled control is always at the same site explained above. The Latitude
and Longitude numbers are GPS coordinates for accurate mapping and location of the sites. The
date deployed and date retrieved show the timespan of that deployment. Exposed (days) column
is the number of days the monitors were in place collecting cumulative exposure.
The data in the Radon column is the primary concern for this project. Radon is measured in
picocuries per liter of air (pCi/L). It is calculated from the total accumulated measurement of the
Alpha Tracks and the total time they were deployed. The presence of BDL in this column
indicates a measurement that is below 0.3 pCi/L, which is the lower limit of detection for these
devices. N\A refers to not available. The TLD column refers to the gamma results from the
TLD’s that were deployed with the Alpha Tracks for the full time period. The GAMMA column
refers to the gamma measurements taken from a hand held instrument at the time of retrieval.
13 | P a g e
Results
This data will be primarily characterized using the median rather than the mean. The median as
used here is a more accurate measure of the central tendency of the data because the presence of
the cases where an unknown value below the lower limit of detection is involved has been
minimized. The median result represents a value in which half the data results are below and
half are above it. The mean is the average, and is a measure of central tendency, being also more
susceptible to extreme values.
For the radon measurements, the median for the first deployment for all 20 monitoring points
combined is 1.75 picocuries per liter (pCi/L), the second is 1.55 pCi/L, the third is 0.4 pCi/L and
the fourth is 0.3 pCi/L. In the latter case there were 14 measurements that were BDL (below the
detection limit of 0.3 pCi/L). Therefore, even using the most conservative assumption of 0.3 for
each BDL reading, the median in that case is actually either at or below 0.3 pCi/L.
As expected, when summarizing the data from only the 16 monitors immediately surrounding the
gypstack, the mean and median values were equal to or higher than the values representing all 20
sites combined.
The medians for all 3 schools combined for Quarters 1-4 were 1.3, 0.7, 0.3 and 0.3 pCi/L,
respectively. The resulting data taken from the schools alone show that the medians (and means)
were primarily lower when compared to the gypstack monitors, most significantly during
Quarter 2. The combined median values for the schools had a similar trend to the other monitor
groupings throughout the deployment in that they typically decreased from Quarter 1 to Quarter
4, although the gypstack monitors increased slightly from Quarter 1 to Quarter 2.
The control site is located several miles from the gypstack and therefore not considered to be
influenced by Mosaic’s operation at the gypstack. Interestingly, the radon readings at the control
site were equal to or higher than the average of the readings from the 3 schools combined
throughout the monitoring period.
For the gamma measurements, the averages of the gamma data from hand held instruments for
all 20 monitoring points combined were 13 microrem per hour (uR/hr). The readings from the
TLD’s ran about half that, largely due to systematic losses from storage of control devices. Both
were consistent from period to period, as expected due to the contribution being from the nearby
soil, which was undisturbed during these deployments.
The average gamma readings for the 16 gypstack monitors were consistently higher than the
combined 20 monitors, averaging 15 uR/hr on the hand held instrument. The average of the
readings with the hand held monitors at the 3 schools combined were typically about 1/3 of the
values recorded at the gypstack monitors. The control site’s readings were consistent with the
readings at the schools.
14 | P a g e
INTERPRETATION OF RESULTS
For comparison, a couple of statistics should be mentioned at this point. The average national
outdoor air concentration of radon is 0.4 pCi/L. The national average for indoor radon is 1.3
pCi/L, and the U.S. EPA’s recommended action level for indoor radon concentration is 4.0
pCi/L. The U.S. EPA has also estimated that a phosphogypsum stack can contribute 0.2 pCi/L to
surrounding ambient air, while other estimates run lower.
Some difficulties arise from drawing conclusions with the measurements taken for this project.
Low-end results such as is common with environmental data are hard to characterize with
statistics when number sets are small. It should also be pointed out that data were not taken prior
to gypstack formation. Preconstruction data is used to help measure the environmental impact of
nuclear power plants for example.
The most identifiable feature of the ambient air radon data is the trend in regard to the difference
of the medians of the four deployments. Consistently, the first deployment showed the highest
radon readings with the following deployments each being lower, with the exception of the
combined 16 gypstack monitors which increased slightly from the first to second quarters. The
readings at the control site followed this same pattern. The control site is too far from the
gypstack to be influenced by it. This suggests that the cause of the differences in the medians is
due to outside influences not related to the gypstack.
Although variations in weather strongly effect both radon emanation from the ground and
ambient levels, the effects are so numerous and varied that it is impossible to reference without
being speculative. Some generalizations can be mentioned as having possible relevance. Wind
speed and vertical mixing can reduce ambient are concentrations. Continental air masses have
significantly higher levels of radon than marine air masses. Given Florida’s location it could
certainly have varied exposure due to these effects. Since radon is almost completely distributed
by air movement, something as simple as higher average wind speed could make the data for a
given deployment less varied, and lower than average wind speed could make the data more
varied. Other differences such as rainfall can also affect results, which may help explain the
lower readings in summer and fall months for both phases of the study. A precise juxtaposition
of weather during deployment periods to the measured results is beyond the scope of this report.
The variation in the levels measured at the schools can also be attributed to environmental
effects. Although the schools are close enough to the gypstack to potentially be impacted and
have an increased radon level related to it, the presence of other possible environmental effects
precludes attributing the variance to just the gypstack. Also, given the distance of the schools
from the gypstack, small changes in wind direction would greatly affect any possible
contribution of its radon. Note that the radon level at the control site, which is not influenced by
the gypstack, is higher than the radon measured at all schools.
The variation in gamma measurements at different locations around the stack reflects the amount
of phosphogypsum in soil near the deployment sites. Since these sites are along a road around
the site, different locations will have different compositions as a result of soil relocation in
making the road. The higher gamma measurements found in this study are typical of Florida
phosphogypsum stacks. Deployments 1-6 and 12-16 around the stack are consistent and
representative of low-level exposure rates from phosphogypsum. Deployments 7-11 are located
15 | P a g e
closest to the active stack and on the edge of the cooling pond. The readings at these points are
atypically higher, indicating additional contributions to gamma radiation levels from the
gypstack. The gamma measurements at the schools and the control site were comparable to one
another and represent only local conditions, and were typical for background levels in Florida.
CONCLUSIONS AND RECOMMENDATIONS
None of the measurements taken in this project constituted remedial actions as recommended by
various public health agencies. They are generally low compared to background levels in most of
the country. As such, no remedial actions are recommended on the phosphogypsum stack itself,
in terms of radon mitigation.
Given the amount of data collected, no exact estimates can be made about the contribution of
radon from the gypstack to surrounding ambient air. The variation in the medians is not likely to
be caused by the gypstack, and the variation between the individual measurements is not
dissimilar to other outdoor measurements in central Florida. Although further studies may
characterize the range of ambient values of the offsite locations more completely, they may not
more clearly determine the contribution of the gypstack.
16 | P a g e
Appendix
References and Suggested Resources:
www.fipr.poly.usf.edu
www.Myfloridaeh.com/radiation
www.epa.gov/radon
www.radon.com
17 | P a g e