Scaling API-first – The story of a global engineering organization
Ambient Air Radon Monitoring Report
1. Ambient Air Radon Monitoring Report on
Mosaic Riverview Phosphogypsum Stack
A joint effort of:
Florida Department of Health Environmental Protection Commission
Bureau of Radiation Control of Hillsborough County
Air Management Division
2. 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 ..........................................................................................11
Results .............................................................................................................................12
INTERPRETATION OF THE RESULTS .................................................................................12
CONCLUSIONS AND RECOMMENDATIONS .....................................................................13
APPENDICES ............................................................................................................................14
Appendix 1 – Winter Concentration Map .......................................................................14
Appendix 2 – Summer Concentration Map ....................................................................15
Appendix 3 – References and Suggested Resources ......................................................16
2|Page
3. EXECUTIVE SUMMARY
Ambient air radon measurements were taken to fulfill 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 measurements were taken over two separate
three month periods. One period was in the winter and one period was during the summer.
The measurements were taken from 16 locations around the active phosphogypsum stack, three
nearby schools (Gibsonton Elementary, Ippolito Elementary, and Progress 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 and thermoluminescent
dosimeters (TLD) mounted on six-foot high poles. Each of the 20 monitoring locations 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. However, differences in the two deployments were noted. The
first deployment showed a wider range of concentrations than the second deployment. 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 phosphogypsum stack to surrounding ambient air. The variation present between
the two deployments suggest that further study may allow more clear characterizations of the
data, possibly reconciling with seasonal variations. An additional two-year study would cover
these points and would make the use of the control site more statistically viable.
3|Page
4. INTRODUCTION
The Pre and Post Monitoring program of the Florida Department of Health, Bureau of Radiation
Control Environmental Section, conducted an ambient air radon monitoring project for the Air
Management Division of the Environmental Protection Commission of Hillsborough County
(EPC) spanning two separate three month periods. One period was conducted during the winter
and one period in the summer.
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
5. 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
Figure 2 it into a slurry that can then be
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
6. 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.
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
Figure 4 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
7. rem per hour, or microrem per hour to denote a millionths of a rem per hour. Millirem per hour
is used to denote a thousandth of the 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 risk of this dose is primarily in the form 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 affect 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
8. 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
The primary
measurement for
this study is the
amount of radon
in ambient air in
proximity to the
phosphogypsum
stack. This was
accomplished
Figure 5 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) 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
Figure 7 (TLD) that measured gamma radiation (see Figure 7).
8|Page
9. These items were deployed for a minimum of 90 days. Before deployment, they were kept in
airtight packaging and 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).
Deployments were made for two periods during
2010 of approximately 90 days each. One from
February 3rd to May 13th and the other from June
29th to October 1st.
Figure 8
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
10. 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
AMBIENT AIR RADON MEASUREMENTS
Winter 2010 Deployment
Date Date Exposed Radon TLD GAMMA
Deployment Site Latitude Longitude
Deployed Retrieved (days) (pCi/L) (uR/hr) (uR/hr)
Gypstack-1 2753.561 8222.833 2/3/2010 5/13/2010 99 0.8 7 7
Gypstack-2 2753.561 8222.618 2/3/2010 5/13/2010 99 BDL 6.8 7
Gypstack-3 2753.182 8222.453 2/3/2010 5/13/2010 99 0.5 8.5 9
Gypstack-4 2753.036 8222.453 2/3/2010 5/13/2010 99 BDL 7.7 9
Gypstack-5 2752.91 8222.451 2/3/2010 5/13/2010 99 BDL N/A 8
Gypstack-6 2752.775 8222.444 2/3/2010 5/13/2010 99 BDL 9.8 10
Gypstack-7 2752.481 8222.472 2/3/2010 5/13/2010 99 0.4 26 25
Gypstack-8 2752.48 8222.576 2/3/2010 5/13/2010 99 0.5 28.3 24
Gypstack-9 2752.481 8222.791 2/3/2010 5/13/2010 99 0.7 26.2 23
Gypstack-10 2752.364 8223.02 2/3/2010 5/13/2010 99 0.3 27 22
Gypstack-11 2752.478 8223.167 2/3/2010 5/13/2010 99 BDL 31 31
Gypstack-12 2752.785 8223.166 2/3/2010 5/13/2010 99 0.5 9.6 11
Gypstack-13 2752.94 8223.168 2/3/2010 5/13/2010 99 1.6 10 9
Gypstack-14 2753.041 8223.166 2/3/2010 5/13/2010 99 1.7 11.8 12
Gypstack-15 2753.164 8223.166 2/3/2010 5/13/2010 99 1.6 14.2 13
Gypstack-16 2753.385 8223.026 2/3/2010 5/13/2010 99 1.6 6.9 7
Gibsonton
2750.898 8222.133 2/3/2010 5/13/2010 99 0.7 4.9 5
Elementary School
Ippolito
2752.601 8221.418 2/3/2010 5/13/2010 99 0.6 6.3 6
Elementary School
Progress Village
2753.485 8221.948 2/3/2010 5/13/2010 99 0.5 6.5 5
Middle School
Control Site 2753.188 8218.552 2/3/2010 5/13/2010 99 1.6 5.6 5
See Appendix 1 for GIS Map of results
10 | P a g e
11. AMBIENT AIR RADON MEASUREMENTS
Summer 2010 Deployment
Date Date Exposed Radon TLD GAMMA
Deployment Site Latitude Longitude
Deployed Retrieved (days) (pCi/L) (uR/hr) (uR/hr)
Gypstack-1 2753.561 8222.833 6/29/2010 10/1/2010 94 0.3 5.5 7
Gypstack-2 2753.561 8222.618 6/29/2010 10/1/2010 94 0.4 6.1 7
Gypstack-3 2753.182 8222.453 6/29/2010 10/1/2010 94 0.3 6.1 9
Gypstack-4 2753.036 8222.453 6/29/2010 10/1/2010 94 0.5 7.8 9
Gypstack-5 2752.91 8222.451 6/29/2010 10/1/2010 94 0.4 6.9 8
Gypstack-6 2752.775 8222.444 6/29/2010 10/1/2010 94 0.5 6 10
Gypstack-7 2752.481 8222.472 6/29/2010 10/1/2010 94 0.7 8.1 25
Gypstack-8 2752.48 8222.576 6/29/2010 10/1/2010 94 0.5 24.5 24
Gypstack-9 2752.481 8222.791 6/29/2010 10/1/2010 94 0.6 24 23
Gypstack-10 2752.364 8223.02 6/29/2010 10/1/2010 94 BDL 24.5 22
Gypstack-11 2752.478 8223.167 6/29/2010 10/1/2010 94 0.5 23.8 31
Gypstack-12 2752.785 8223.166 6/29/2010 10/1/2010 94 0.5 26.4 11
Gypstack-13 2752.94 8223.168 6/29/2010 10/1/2010 94 0.4 7.7 9
Gypstack-14 2753.041 8223.166 6/29/2010 10/1/2010 94 0.4 8.8 12
Gypstack-15 2753.164 8223.166 6/29/2010 10/1/2010 94 0.5 10.2 13
Gypstack-16 2753.385 8223.026 6/29/2010 10/1/2010 94 0.9 13 7
Gibsonton
2750.898 8222.133 6/29/2010 10/1/2010 94 BDL 4.6 5
Elementary School
Ippolito
2752.601 8221.418 6/29/2010 10/1/2010 94 BDL 5.8 6
Elementary School
Progress Village
2753.485 8221.948 6/29/2010 10/1/2010 94 BDL 5.8 5
Middle School
Control Site 2753.188 8218.552 6/29/2010 10/1/2010 94 BDL N/A 5
See Appendix 2 for GIS Map of results
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
resulting deviations from these numbers will only be a few feet. The date deployed and date
retrieved show when the two deployments took place. 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. The TLD column refers to the gamma results from the TLD’s that were deployed with
11 | P a g e
12. 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 deployment.
RESULTS
Only data from the 16 sites in proximity to the gypstack are characterized. Both the median and
the mean are used due to their distinct representations of results. The median as used here is a
more accurate measure of the central tendency of the data because the presence of the cases
where a lower limit of detection is involved has no affect. 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.
The median for both the first and second deployment is 0.5 picocuries per liter, (pCi/L). The
mean of the first deployment is 0.73 pCi/L and the mean of the second deployment is 0.48 pCi/L.
The equal median for both deployments show that both data sets have a similar central tendency,
while the higher mean for the first data set shows the presence of more extreme values in that set.
In particular, the first data set has a higher number of measurements that were below the lower
limit of detection, and has four measurements that were above 1.0 pCi/L. It should also be noted
that sites with higher readings from the first deployment do not correspond to the sites of higher
readings on the second deployment.
For the first deployment, the measurements at the schools were from 0.5 to 0.7 pCi/L, and for the
second deployment, they were all below the lower limit of detection of 0.3 pCi/L. The control
site measured 1.6 pCi/L for first deployment, and was below lower limit of detection for second
deployment.
The averages of the gamma data from both the TLD’s and the hand held measurements ran from
12 to 13 microrem. The similarities were expected due to the contribution being from the nearby
soil, which was undisturbed during these deployments.
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.
Little can be said to characterize a given site, due to only having two measurements per site. In
addition, 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. Pre-construction data is used to help measure the environmental
impact of nuclear power plants, for example.
12 | P a g e
13. The most identifiable feature of the ambient air radon data is the difference of the distributions
between the two deployments. The first deployment showed a wider range of air radon
concentrations than the second set. By its very nature, an ambient air radon measurement taken
6 ft off the ground is clearly the sum total of many environmental affects at that point. In
addition, the amount of radon released from the surface of the gypstack itself is dependent on
many different environmental conditions. 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. 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. Though they are close enough to the gypstack to have radon present from 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 stack is higher than the radon measured at all
schools and equal to the highest level observed at the stack locations, (sites 13-16).
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 south of the
cooling pond are atypically higher, indicating additional contributions to gamma radiation levels.
The gamma measurements at the schools and the control site 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. 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 stack to surrounding ambient air. Because the ambient air levels are quite low,
the contribution from the stack can be both low in magnitude yet sizable as a percentage. For
example, 2 cents is twice as much as 1 cent, double the amount, or 100% higher; but it is still a
trivial amount of money.
The variation present between the two deployments suggests that further deployments may allow
more clear characterizations of the data, possibly reconciling with seasonal variations, both by
deploying during different seasons, and by redeploying during the same time periods as the
current study. A two-year study would cover both of these points, also making the use of the
control more statistically viable.
13 | P a g e
