Ambient Air Radon Monitoring Report on Mosaic Riverview Phosphogypsum Stack
- 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
- 14. Appendix 1 14 | P a g e
- 15. Appendix 2 15 | P a g e
- 16. Appendix 3 References and Suggested Resources: www.fipr.poly.usf.edu www.Myfloridaeh.com/radiation www.epa.gov/radon www.radon.com 16 | P a g e