AAmmbbiieenntt AAiirr RRaaddoonn MMoonniittoorriinngg RReeppoorrtt oonn

MMoossaaiicc RRiivveerrvviieeww PPhhoosspphhooggyyppssuumm SSttaacckk

A joint effort of:

Florida Department of Health Environmental Protection Commission

Bureau of Radiation Control of Hillsborough County

Air Management Division

2 | P a g e

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

3 | P a g e

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.

4 | P a g e

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.

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.

Figure 1

5 | P a g e

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

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

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.

Figure 2

Figure 3

6 | P a g e

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

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

Figure 4

7 | P a g e

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

8 | P a g e

Figure 5

Figure 7

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

using Landauer

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

(TLD) that measured gamma radiation (see Figure 7).

Figure 6

9 | P a g e

Figure 10

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.

SITE LOCATIONS

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

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.

Figure 9

Figure 8

10 | P a g e

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

See Appendix 1 for GIS Map of results

AMBIENT AIR RADON MEASUREMENTS

WWiinntteerr 22001100 DDeeppllooyymmeenntt

Deployment Site Latitude Longitude Date

Deployed Date

Retrieved Exposed

(days) Radon

(pCi/L) TLD

(uR/hr) GAMMA (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 Elementary School

2750.898 8222.133 2/3/2010 5/13/2010 99 0.7 4.9 5

Ippolito Elementary School

2752.601 8221.418 2/3/2010 5/13/2010 99 0.6 6.3 6

Progress Village Middle School

2753.485 8221.948 2/3/2010 5/13/2010 99 0.5 6.5 5

Control Site 2753.188 8218.552 2/3/2010 5/13/2010 99 1.6 5.6 5

11 | P a g e

AMBIENT AIR RADON MEASUREMENTS

SSuummmmeerr 22001100 DDeeppllooyymmeenntt

Deployment Site Latitude Longitude Date

Deployed Date

Retrieved Exposed

(days) Radon

(pCi/L) TLD

(uR/hr) GAMMA (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 Elementary School

2750.898 8222.133 6/29/2010 10/1/2010 94 BDL 4.6 5

Ippolito Elementary School

2752.601 8221.418 6/29/2010 10/1/2010 94 BDL 5.8 6

Progress Village Middle School

2753.485 8221.948 6/29/2010 10/1/2010 94 BDL 5.8 5

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

12 | P a g e

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.

13 | P a g e

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.

14 | P a g e

Appendix 1

15 | P a g e

Appendix 2

16 | P a g e

Appendix 3

References and Suggested Resources:

www.fipr.poly.usf.edu

www.Myfloridaeh.com/radiation

www.epa.gov/radon

www.radon.com