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APPENDIX D: REVIEW OF AIR QUALITY MODELING STUDIES
ATSDR views environmental sampling data as critical inputs to the public health assessment process. As evidence of this, ATSDR strongly recommends the use of validated sampling data as the basis for public health decisions. In some circumstances, however, sampling data are not sufficient to characterize all site-specific exposures. For instance, few air samples were collected on Vieques between the early 1970s and 1999–the years when the Navy's military training exercises using live bombs were most extensive–and the few samples that were collected are of questionable quality. In such cases, models are arguably the best tools available to evaluate the nature and extent of contamination. ATSDR emphasizes that models are only capable of estimating exposure concentrations, based on a scientific understanding of how chemicals move in the environment. All models, however, have assumptions and uncertainties and may not accurately represent actual environmental conditions. Therefore, ATSDR carefully reviews all modeling applications to determine whether they provide meaningful estimates of environmental contamination and whether they can be used in the public health assessment process.
When evaluating the four key questions in this PHA (see Section V), ATSDR determined that the available sampling data were sufficient to address two of the key questions, without the need for modeling. On the other hand, insufficient sampling data were available to characterize air quality during live bombing exercises and to evaluate releases from the Navy's periodic use of certain materials (e.g., depleted uranium, chaff). ATSDR decided to use modeling analyses to put these two exposure scenarios into perspective.
The remainder of this appendix presents ATSDR's review of the modeling studies available for the island of Vieques. This includes modeling studies conducted by contractors to the Navy (Appendix D.1), by an engineer from Vieques (Appendix D.2), and by contractors to ATSDR (Appendix D.3). Sections V.C and V.D describe how ATSDR used these modeling analyses to reach public health conclusions.
D.1 Review of the Navy's Modeling Study of Live Bombing Activities (IT 2000, 2001)
In February 2000, contractors to the Navy completed an air dispersion modeling study of selected air emissions sources on the island of Vieques (IT 2000). The modeling study had two objectives: to determine whether certain environmental regulations apply to the Navy's operations on Vieques and to estimate ambient air concentrations of contaminants released to the air during military training exercises and open detonation of unexploded ordnance. In May 2001, the Navy released a revision to this air dispersion modeling study to correct a computational error (IT 2001). Once corrected, the estimated emission rates (and likewise the estimated ambient air concentrations) increased slightly, by less than 5% for most contaminants. Copies of both versions of this dispersion modeling report are in the Vieques site's records repositories, which are located at Biblioteca Publica on Vieques, the Vieques Conservation and Historical Trust, and the University of Puerto Rico School of Public Health.
D.1.A Overview of the Navy Contractor's Modeling Approach
The following paragraphs review three key features of the Navy contractor's modeling analysis: how emission rates were estimated, how atmospheric fate and transport was simulated, and how results were presented and interpreted. Refer to Section D.1.B for ATSDR's evaluation of the scientific rigor of the Navy contractor's modeling analysis.
The Navy contractor estimated emissions for a single calendar year, 1998–a year the authors asserted was representative of prior years' activities at Vieques. Further, emissions were estimated only on an annual basis. Estimating emissions over shorter time frames, such as highest daily emissions, was not conducted. The following paragraphs describe how emissions were estimated for different categories of contaminants. ATSDR's comments on these approaches are presented in Section D.1.B.
Emissions of particulate matter. Particulate matter emissions were estimated using a model that the Army Research Laboratory developed to predict how much dust, smoke, and debris is released to the air during realistic battlefield situations (Army Research Laboratory 2000). In general, this model characterizes the size of craters formed by explosions and then quantifies the amount of particulate matter that may be released to the air as a result. According to this model, the emission rate of particulate matter following an explosion is a function of several parameters, including the net explosive weight (NEW) of the ordnance fired and soil properties.
When calculating emission rates using this model, the Navy contractor assumed that only those bombs that contain at least 10 pounds of high explosives will generate craters that release particulate matter to the air upon detonation. During 1998, the base year for the modeling analysis, only four types of ordnance–all air-to-ground bombs–met this 10-pound criterion. Because all land-based and ship-to-shore ordnance used in 1998 contained less than 10 pounds of NEW, this approach effectively asserted that none of the ship-to-shore or land-based activities caused emissions of particulate matter.
To estimate particulate matter emissions, the Navy contractor first estimated the volume of craters generated by bombs. The mass of soil apparently ejected from craters was estimated by multiplying the crater volumes listed above by an assumed soil density (1.5 g/cm3). To estimate how much of the soil released is emitted as PM10, the Navy contractor multiplied the mass of soil ejected by a scaling factor documented by the Army Research Laboratory as the proportion of crater ejecta that is believed to be released as "small particles," or particles with radii less than 10 microns (Army Research Laboratory 2000). This scaling factor was 0.01007, meaning that roughly 1% of the crater ejecta is assumed to be released as PM10 emissions, and the remaining crater ejecta will be larger particles that settle to the ground in the vicinity of the impact location.
Following the aforementioned approach, the Navy contractor estimated that total PM10 emissions from the air-to-ground bombing exercises and open detonation activities were 76 tons per year. The combined PM10 emissions from all other sources evaluated (e.g., wind-blown dust, dust from driving on dirt roads, generator exhaust) was less than 5 tons per year. Thus, by the Navy contractor's approach, emissions from the bombing exercises account for an overwhelming majority of the estimated PM10 emissions. ATSDR has many comments on the approach described in the previous paragraphs, as Appendix D.1.B describes further.
Emissions of explosion by-products. The Navy contractor used emission factors to estimate releases of inorganic and organic explosion by-products. The emission factors were derived from a series of source tests, known as "Bangbox" studies, that measured the amounts of selected inorganic and organic chemicals released during the open detonation of various types of ordnance. The Bangbox is a flexible structure in which ordnance is detonated. Because the Bangbox is completely enclosed, pollutants released during the detonation do not escape the structure and can be measured by air sampling equipment. The Bangbox has thus allowed scientists to estimate emission factors for various types of ordnance, many of which are similar to those the Navy uses at Vieques. The emission factors used estimate the amounts of chemicals released to the air per weight of NEW detonated.
The Navy contractor reviewed many Bangbox emission factors to select appropriate factors to apply to the military training activities at Vieques. Emission factors were identified for several types of contaminants, including criteria pollutants, metals, volatile organic compounds, and semi-volatile organic compounds. The Navy contractor selected two different sets of Bangbox emission factors to estimate air releases from the following sources:
Once the Navy contractor selected emission factors for the two types of activities, air emissions were estimated by multiplying the chemical-specific emission factors by the corresponding weight of explosives (expressed as NEW) used.
Emissions of metals. To estimate the amounts of metals released during military training exercises, the Navy contractor used emission factors published in an open detonation burn plan conducted for another Navy installation (Radian 1996). ATSDR notes that these emission factors combine together the amounts of metals detected during Bangbox experiments and the entire mass of four metals–aluminum, copper, manganese, and zinc–commonly found in bomb casings. Therefore, the emission factors assume that the entire metallic content of the casings is vaporized during an explosion.
As with the emission factors for the by-products of explosions, two different sets of emission factors for metals were selected. First, emission factors for air-to-ground exercises were selected from the Bangbox studies involving TNT, RDX, and aluminum powder explosives. Second, emission factors for all other exercises were determined from the highest factors reported for all Bangbox studies identified.
The INPUFF simulations calculated ambient air concentrations of contaminants at downwind locations from a limited set of input parameters. To run INPUFF, the Navy contractor used meteorological conditions observed at the US Naval Air Station Roosevelt Roads, located on the eastern shore of the main island of Puerto Rico, less than 10 miles from the western shore of Vieques. The Navy contractor also selected values for the locations and size of dust clouds generated by explosions during the military training exercises. Section D.1.B presents ATSDR's comments on the selected input values.
D.1.B ATSDR's Review of the Navy Contractor's Modeling Analysis
Because limited environmental sampling data are available to characterize how the Navy's live bombing activities affected air quality at Vieques, ATSDR thoroughly evaluated all modeling studies of these activities to determine if the modeling results can be used to reach scientifically defensible public health conclusions. ATSDR's specific comments on the Navy contractors' dispersion modeling analysis follow, organized by topic.
By design, the Navy contractor's modeling study was based entirely on range utilization statistics for 1998, and the amount of ordnance used in 1998 was assumed to be representative of amounts used in previous years (IT 2000). This is a critical assumption, because the model predictions for 1998 will not be representative of other years if the range utilization statistics for 1998 were unusually high or low. To evaluate whether 1998 is an adequate base year for the modeling application, ATSDR thoroughly evaluated range utilization statistics available for the years 1983 to 1998–the time frame for which the most complete statistics are available. An overview of ATSDR's review of those statistics follows:
Based on these observations and on the fact that emissions from the bombing exercises depend directly on the number of exercises and amounts of high explosives used, ATSDR finds that 1998 indeed appears to be an adequate base year for the modeling analysis. Because range usage in 1998 was greater than the long-term average, using 1998 as a base years might lead to a slight overestimate of emission rates.
For several reasons, ATSDR believes COMBIC is an adequate basis for estimating emissions of particulate matter from bombing exercises. First, ATSDR doubts that COMBIC grossly underestimates emissions of dusts and particles, because significant underestimates may cause modelers to reach incorrect conclusions that have potentially serious consequences to military personnel in battlefield environments (e.g., predicting that critical electro-optical equipment will function, when they might not). Further, according to the model documentation, several key input parameters have been established empirically from field studies involving high explosive ordnance, much like the ordnance the Navy uses at Vieques. Moreover, the model predictions have proven consistent with observations documented in other publications.
Though ATSDR believes COMBIC is a reliable model for estimating particulate emissions from bombing exercises, assumptions made when applying the model may lead to biases in emissions estimates. ATSDR identified the following potential shortcomings in the Navy contractor's application of COMBIC at Vieques:
ATSDR further believes the Navy contractor's approach used to select emission factors from the available Bangbox studies was appropriate. For instance, to characterize emissions from air-to-ground exercises, the Navy contractor first identified the subset of Bangbox studies that tested explosives with similar compositions to those used at Vieques, and then selected the highest emission factor for every chemical from the various tests. As a result, the emission factors used are the highest measured releases of chemical by-products from the available Bangbox studies. Moreover, when applying the emission factors to the net explosive weight of explosives in ordnance, the Navy contractor included quantities of aluminum dust in the explosive charge toward the net explosive weight. This approach likely leads to overestimates of organic by-products of explosions, because the aluminum dust is not an explosive chemical that releases energy (and forms organic by-products) during explosions.
Though the Navy contractor's approach includes assumptions that appear to overstate emissions of explosion by-products, it remains unclear exactly how representative the Bangbox studies are to live bombing exercises. Appendix D.3 discusses this issue further. Ultimately, ATSDR used the same set of emission factors, with one exception, to estimate releases of chemical by-products of explosions. As the exception, the emission factor for 2-nitrophenylamine was apparently transcribed incorrectly in the Navy contractor's modeling analyses. This error caused the Navy contractor to underestimate this chemical's emissions by a small margin (7%).
D.2 Review of Rafael Cruz Pérez's Modeling Study of Live Bombing Activities (Cruz Pérez 2000)
In 2000, Dimension Magazine, a publication of the College of Engineers and Surveyors of Puerto Rico, released an article written by Rafael Cruz Pérez, PE, about environmental contamination at Vieques (Cruz Pérez 2000). ATSDR has identified additional releases of this article from earlier years, but bases its review of the article on the most recent version. The article summarizes levels of environmental contamination, both measured and modeled, in multiple media, including soil, surface water, groundwater, and air. This review focuses specifically on an air modeling analysis documented in the article of high explosives used at Vieques. Refer to Appendix C.4 and C.5 for ATSDR's review of this article's summary of ambient air sampling on Vieques.
D.2.A Overview of Rafael Cruz Pérez's Modeling Approach
The following paragraphs review three key features of Rafael Cruz Pérez's modeling analysis: how emission rates were estimated, how atmospheric fate and transport was simulated, and how results were presented and interpreted. Refer to Section D.2.B for ATSDR's evaluation of the scientific rigor of this modeling analysis.
To estimate air emissions, Rafael Cruz Pérez reported that firing a single 105 mm high explosive mortar will displace 400 kg of soil. Of this amount, 80% (or 320 kg) was assumed to fall to the ground immediately in the vicinity of the impact location. The remaining 80 kg of particles that remain airborne were assumed to be available for downwind transport. Rafael Cruz Pérez further estimated that 94% of these remaining airborne particles will fall to the ground within several hundred feet of the impact location. With this assumption, 4.8 kg of the soil particles released are considered available for longer range transport. Information on the assumed particle sizes is not provided. The publication by Rafael Cruz Pérez cites no references for any of the aforementioned assumptions and emissions estimates.
To predict actual exposure point concentrations, Rafael Cruz Pérez presented several additional data points. First, the article indicates that an exercise involving the use of several 105 mm high explosive mortars can increase ambient air concentrations of particulate matter in the residential areas of Vieques by 98.38 µg/m3, but the article does not present the equations used to estimate this concentration nor does it indicate the averaging time for this reported increase. Next, the article indicates that actual exposure point concentrations would be higher than 197 µg/m3–a level apparently calculated by adding the 98.38 µg/m3 increase in concentration to an assumed background concentration of 99 µg/m3. No averaging period is given for the estimated concentration of 197 µg/m3 or the assumed background concentration. The article concludes by asserting that the estimated concentrations are higher than EPA's primary and secondary air quality standards, which are cited as 75 µg/m3 (annual average) and 60 µg/m3 (highest 24-hour average), respectively. Section D.2.B, below, presents ATSDR's review of Rafael Cruz Pérez's modeling analysis.
D.2.B ATSDR's Review of Rafael Cruz Pérez's Modeling Analysis
As with the Navy contractor's modeling analysis, ATSDR thoroughly reviewed Rafael Cruz Pérez's publication on environmental contamination at Vieques. ATSDR's specific comments on this modeling analysis is presented below, organized by the same three topics presented in Section D.2.A:
Next, ATSDR notes that Rafael Cruz Pérez's study and the Navy contractor's study are quite similar in terms of estimating the proportion of displaced soil that is available for longer range downwind transport. Specifically, Rafael Cruz Pérez assumed that 1.2% of the soil displaced by an explosion will travel in the plume for long distances, though information on the particle sizes is not provided. The Navy contractor, on the other hand, assumed that 1.007% of the soil displaced will be emitted as PM10 and will remain airborne for long distances. Section D.3.B presents ATSDR's approach to estimating emissions of airborne particles, as well as more detailed information on the particle sizes.
Finally, to get a sense for how the two modeling studies compare, ATSDR used the Navy's total annual usage of high explosives to extrapolate Rafael Cruz Pérez's emissions estimates to an annual value. To do this calculation, ATSDR noted that Rafael Cruz Pérez's study predicts that 4.8 kg of particulate matter (available for long-range transport) are generated for every 5.1 pounds of high explosives used; further, the Navy's usage of high explosives in 1998 was 771,734 pounds (IT 2000). Assuming, to a first approximation, that particulate matter emissions vary linearly with the amount of high explosives in the ordnance, Rafael Cruz Pérez's emissions estimates imply that the annual releases of particulate matter may be as high as 800 tons per year. This emission rate is 10 times higher than the particulate matter emission rate used in the Navy contractor's modeling analysis. Therefore, the approaches used by the Navy contractor and Rafael Cruz Pérez lead to considerably different emissions estimates. Section D.3.B presents ATSDR's best estimate of particulate matter emissions from military training exercises at Vieques. ATSDR's estimate is higher than the Navy's, and lower than Rafael Cruz Pérez's.
The appropriate value of the concentration decay term notwithstanding, ATSDR emphasizes that Rafael Cruz Pérez's approach to estimating ambient air concentrations likely provides only a very rough approximation of actual air quality. Many years of research have established that atmospheric dispersion is not only a function of downwind distance, but is also a function of atmospheric stability. Further, the approach used by Rafael Cruz Pérez does not account for varying wind speeds, wind directions, mixing heights, and other meteorological phenomena that affect how contaminants move through the atmosphere. As Section D.3.C describes, ATSDR used a model that accounts for how site-specific meteorological conditions at Vieques affect atmospheric fate and transport.
ATSDR has several comments on the article's interpretation of the estimated ambient air concentrations. First, ATSDR notes that not enough information is provided to evaluate how Rafael Cruz Pérez estimated the increase in particulate matter concentrations resulting from the mortar firing (i.e., 98.38 µg/m3), which is apparently based on a 24-hour averaging period. Nonetheless, the interpretations of this estimated concentration appear to be flawed. Specifically, ATSDR notes that the assumed background concentration used in the article is the highest ambient air concentration of TSP (99 µg/m3) measured in two different studies (see Appendix C.4 and C.5). The background concentration selected is more than twice as high as the average TSP levels that PREQB recently measured at Vieques using rigorous sampling methods. Therefore, the estimated background concentration appears to be more representative of a maximum concentration than of an average concentration.
More importantly, ATSDR does not believe comparing an estimated 24-hour average concentration to an annual average health-based standard is appropriate. A more appropriate interpretation would compare the estimated 24-hour average concentration (197 µg/m3) to the former 24-hour average health-based standard for TSP (260 µg/m3), assuming the estimated concentrations are indeed total suspended particulates. ATSDR's modeling analysis, documented throughout Appendix D.3, estimates both annual average and maximum 24-hour average concentrations of particulate matter, and compares these estimates to the appropriate health-based standards.
Finally, to assess whether the predicted air quality impacts are reasonable, ATSDR extrapolated the predicted concentrations to a larger-scale bombing event: firing a single 2,000-pound bomb containing 1,000 pounds of high explosives. To extrapolate to this scenario, ATSDR notes that Rafael Cruz Pérez analysis predicts that firing a single round of ordnance containing 5.1 pounds of high explosives would cause short-term average air concentrations in the residential areas of Vieques to increase by at least 33 µg/m3. To a first approximation, firing a single bomb that contained 200 times as much high explosives would cause approximately a 200-fold higher air quality impact, or a short-term concentration of 6,600 µg/m3. Although extensive sampling data are not available to determine whether or not such predicted concentrations are reasonable, these increases in concentrations, if correct, would likely be associated with significantly impaired visibility throughout the residential areas of Vieques. ATSDR has heard no accounts of such air quality impacts and has not witnessed such effects on visibility during open detonation events involving much more ordnance than a single bomb. These observations, combined with the previous comments, suggest that Rafael Cruz Pérez's modeling analysis may overstate air quality impacts from military training exercises on Vieques.
D.3 ATSDR's Modeling Study of Navy Exercises Using Live Bombs (ERT 2001)
Much of ATSDR's efforts evaluating this site have focused on air quality between the 1970s and 1999–the years when the Navy conducted military training exercises on Vieques using live bombs. Though three parties conducted air sampling projects during this time frame, all of which did not find ambient air concentrations of pollutants at levels above EPA's air quality standards, the quality of the sampling data are not known because original documentation on the sampling projects is limited or not available. As a result, ATSDR used modeling studies to evaluate potential exposures to contaminants released from live bombing activities.
Before estimating emissions and modeling fate and transport, ATSDR first obtained and thoroughly reviewed the two air quality modeling studies that were readily available for Vieques. In so doing, ATSDR not only could build upon the strengths of the work already completed but also could identify and improve upon potential shortcomings noted in Appendix D.1 and D.2. Key features of ATSDR's dispersion modeling analysis are reviewed in the following sections.
D.3.A Goal of ATSDR's Modeling Study
ATSDR designed its modeling study to generate reasonable estimates of how air-to-ground, ship-to-shore, and land-based military activities at Vieques affect air quality in the residential areas of the island. Because this PHA is evaluating potential inhalation exposures, the emphasis in ATSDR's modeling was to make reasonable estimates of ambient air concentrations; characterizing deposition of air particles was not considered in this study, since ATSDR's other PHAs have already addressed (or will soon address) levels of contamination present in other environmental media, including drinking water supplies, soils, and biota. Recognizing that military training exercises at Vieques are not continuous and vary in intensity from one exercise to the next, ATSDR estimated both annual average and maximum 24-hour average exposure point concentrations. These concentrations were then used to evaluate chronic exposure scenarios and acute exposure scenarios, respectively.
The rest of this appendix describes the approaches ATSDR used to estimate emissions from the various military training exercises (Section D.3.B) and to model the atmospheric fate and transport of these emissions (Section D.3.C). Section D.3.D then presents key findings from the modeling analyses. ATSDR's public health interpretations of the modeling results are documented in Sections V.C and V.D.
This section describes how ATSDR estimated emission rates from the Navy's military training exercises, including both maximum 24-hour emissions and annual average emissions. Consistent with the goal of the modeling study, ATSDR estimated the combined emissions from the use of high explosives ordnance during air-to-ground, ship-to-shore, and land-based exercises. ATSDR notes that the Navy periodically collects unexploded ordnance from the LIA and destroys the explosive charges in open detonation events. ATSDR's approach to estimating emissions assumed that all ordnance fired on the LIA explodes upon impact. With this approach, performing separate calculations for open detonation events is unnecessary, because ATSDR has already accounted for the potential explosion by-products in its calculations for the bombing exercises.
ATSDR estimated emissions using the range utilization statistics for 1998–the same base year that the Navy contractor used in its modeling analysis (see Appendix D.1.A). ATSDR selected this base year for several reasons, but primarily because 1998 has the most detailed range utilization statistics of all years of data that ATSDR has reviewed. Further, the Navy's use of the range in 1998 is representative of that of previous years. More specifically, ATSDR found that the number of days the Navy used the range in 1998 and the amount of high explosives that were fired on the range in 1998 exceed the long-term average for these parameters over a 16-year period (see Appendix D.1.A). Finally, by using the same base year as the Navy contractor, ATSDR can compare emissions estimates between the studies on the same basis. The remainder of this section describes how ATSDR estimated emissions for different classes of pollutants released during military training exercises:
ATSDR's use of the COMBIC model differs from the evaluation performed by the Navy contractor (see Appendix D.1) in three important regards. First, ATSDR considered emissions to the "skirt" of the explosion cloud, which the Navy contractor did not–a factor that results in approximately a 2-fold difference in the emission rates, all other inputs considered equal. Second, ATSDR assumed that all ordnance used during military training exercises, and not just those with more than 10 pounds of high explosives, generate particulate emissions. Third, although bombs at Vieques are fuzed to detonate on impact, ATSDR assumed that the bombs penetrate the surface, which leads to higher emissions estimates than a surface detonation. Table D-1 lists several key inputs used to, and assumptions made when, estimating emissions of particulate matter. Based on these inputs, including a detailed distribution of high explosive ordnance types (as documented in IT 2000), ATSDR estimated that the military training exercises release 277 tons of particulate matter into the air per year. A much greater amount of soil is displaced during the explosions and falls back to the ground in the immediate vicinity of the craters. ATSDR assumed that the 277 tons of emissions are in the form of PM10, even though the COMBIC model documentation indicates that these particles range in sizes from 0-20 microns. More detailed information on particle size distributions is not available.
To estimate maximum daily particulate emissions, ATSDR reviewed nearly 6 years of daily range utilization statistics to characterize the most intense bombing activity over a 24-hour time frame. Only 6 years were considered because only annual range utilization statistics are available for other years. The daily bombing activity selected to calculate the highest 24-hour average emission rate occurred in October 1995, when 94.5 tons of ordnance containing 39 tons of high explosives were used on a single day. Based on this level of activity and the assumption that the distribution of ordnance types used was the same as the annual average, ATSDR estimated the daily worst-case emission rate to be 28 tons of PM10 released on this one day identified as being representative of the most intense military training exercises. The emission rate for this one day should not be viewed as being representative of typical conditions. In fact, the range utilization statistics indicate that less than 10 tons of ordnance were fired per day of military training exercises scheduled.
ATSDR acknowledges that these estimated emission rates have inherent uncertainties, and the actual emission rates may be higher or lower than the levels calculated. The estimated emission rates used in these analyses are believed to be based on the best information currently available. Though predicting the amount of emissions from a single explosion is extremely difficult, due to the variability in blast behavior and soil properties from one event to the next, the COMBIC model is designed to given reasonable predictions for a series of events, such as those that occur over a year or a day of intense activity.
When selecting emission factors, ATSDR first identified all of the BangBox studies that considered high explosives of similar composition to those the Navy used at Vieques, namely those that contain some combination of TNT, RDX, and aluminum powder. From this set of studies, the highest emission factor was selected for this modeling analysis for every chemical measured. Table D-2 lists the chemicals by-products of explosions that ATSDR considered, along with their emission factors, emission rates, and estimated annual average air concentrations. Section D.3.D comments on the uncertainties associated with the data presented in Table D-2.
Table D-3 lists the annual emission rates that ATSDR calculated for metals, organized by the four different factors that contribute to these emissions. The table also lists the estimated annual average air concentrations. Section D.3.D comments on the uncertainty associated with the metals emissions estimates.
Approximately 93% of the high explosive material used during the base year was from three different types of air-to-ground ordnance, which contain TNT and RDX at concentrations up to 80% and 45.1%, by weight. ATSDR used these maximum levels to estimate the total quantity of these chemicals in the charges, even though both chemicals clearly cannot be present at these concentrations in the same mixture. The remaining 7% of high explosive material has widely varying compositions. Rather than calculating the quantities of each component in the charges, ATSDR instead calculated a single emission rate for "all other" high explosive chemicals.
After calculating the amounts of chemicals present in the charges, ATSDR then estimated the proportion of the high explosives that are consumed during the detonation. Although destruction efficiencies for high explosives have not been measured for live bombing exercises, ATSDR notes that the BangBox emission factors suggest that open burning and open detonation activities are typically more than 99% efficient at destroying organic high explosive chemicals. High destruction efficiencies are assumed to apply to the military training exercises at Vieques, primarily because rapid destruction of the charge is needed for ordnance to be effective. The fact that only trace amounts of high explosive chemicals remain in the LIA soils (ATSDR 2001b) is consistent with the assumed high destruction efficiency. To calculate emissions for the dispersion modeling analysis, ATSDR assumed that 10% of the organic chemicals in high explosive charges are emitted. In other words, ATSDR assumed that the explosions have a 90% destruction efficiency for the organic chemicals in the charges.
ATSDR's emissions estimates for high explosives, along with the estimated ambient air concentrations that result from the modeling analysis, are summarized below:
| Chemical | Annual Amount Used | Estimated Air Concentration |
| TNT | 31 tons/year | 0.003 µg/m3 |
| RDX | 19 tons/year | 0.002 µg/m3 |
| All others | <2.8 tons/year | <0.0003 µg/m3 |
D.3.C Atmospheric Fate and Transport
ATSDR used the CALPUFF dispersion model to evaluate the atmospheric fate and transport of air emissions. This model was selected because it has been designed to assess many types of sources, including non-continuous (or "puff") sources, and can also assess deposition, which other "puff" models (like INPUFF) cannot do. The modeling was performed using CALPUFF Version 5.5, Level 010730_1. The following paragraphs describe key inputs selected for this application; a complete listing of these inputs is available in the final modeling report (Trinity Consultants 2002):
Modeling results were reported as normalized concentrations, based on unit emission rates (Trinity Consultants 2002). For all three initial cloud dimensions considered, the highest normalized concentrations occurred for receptors along the property line that separates the residential areas of the island from Navy property. These receptor locations are at least 1 mile upwind from the most heavily populated areas on Vieques.
At the location with highest predicted air quality impacts, the annual average normalized concentrations varied with initial cloud height and the year of meteorological data considered. Table D-4 summarizes the main model outputs for the various scenarios considered. The modeling results showed that concentrations did not change dramatically with initial cloud height, as annual average ambient air concentrations varied by less than a factor of two between the 500-pound and 2,000-pound bombing events, whose initial cloud heights differ by 160 meters.
The approach used to calculated air concentrations from the normalized concentrations depends on the averaging time and contaminant of concern. The normalized concentrations for particles (i.e., considering deposition) were used to estimate air concentrations for both metals and particulate matter, while those for vapors (i.e., not considering deposition) were used to estimate air concentrations for chemical by-products of explosions and high explosive chemicals. The highest daily emission rate was multiplied by the 24-hour maximum normalized concentrations when assessing worst case air quality impacts over the short term. This approach assumes that the most intense bombing activity occurred on the day that had the least favorable meteorological conditions–an unlikely scenario, but one that helps ensure that the modeling analysis does not underestimated 24-hour average concentrations. To calculate annual average air concentrations, the annual average emission rates were multiplied by the corresponding annual average normalized concentrations.
As acknowledged throughout this section, air dispersion modeling analyses have inherent uncertainties and limitations, and the concentrations predicted in this analysis may be higher or lower than the actual impacts that occurred on Vieques during military training exercises with live bombs. Specific comments on uncertainties associated with individual contaminants follow:
Table D-1. Review of Selected Inputs to COMBIC and CalPUFF Models
| Parameter | Input/Assumption Selected | Comments |
| Approach used to estimate particulate emissions | COMBIC model | This model was developed by the Army to estimate airborne dust levels during battlefield scenarios. Accurate prediction of emissions is necessary to ensure that critical equipment will operate during combat situations. |
| Annual amount of high explosives in the ordnance used | 386 tons, based on calendar year 1998 range utilization statistics | This annual usage rate of high explosive chemicals is higher than the average (353 tons) for 1983 to 1998, the longest period of record for which detailed utilization statistics are available. Selection of the 1998 base year will therefore not understate the annual air quality impacts, when averaged over the long term. Note that this amount of high explosives is based on firing 1,295 total tons of ordnance. The total tonnage is greater, because it includes contributions from casings, fuzes, and fillers. |
| Maximum daily amount of high explosives in the ordnance used | 39 tons, based on a review of daily range utilization statistics from 1993 to 1998) | This usage was determined from reviewing nearly 6 complete years of daily range utilization statistics. The amount of high explosives assumed to be fired on the day with most intense activities equals roughly 10% of the annual usage. This value appears to be reasonable, especially when noting that military training exercises occurred on approximately 200 days per year prior to 1999. |
| Percent of bombs that detonate upon impact | 100% | Not all bombs detonate upon impact. Site documents imply that over 90% of the bombs fired on the LIA do detonate. Assuming that all bombs detonate will lead to an overestimate of emissions. |
| Soil type | Dry cohesive soils | This soil type is most consistent with the soils on the LIA. Of the six soil types considered by COMBIC, "dry sandy soils" leads to the highest proportion of small particles in the emissions cloud. |
| Depth of burst | 1 foot | Bombs fired on the LIA are fuzed to detonate upon impact. To be conservative, ATSDR assumed that the center of a bomb penetrates up to 1 foot of soil before the bomb explodes. This assumption leads to predicted emission rates approximately 40% higher as compared to the emissions from surface detonations. |
| Particle size distribution in emissions | 100% PM10 | The COMBIC model reports that "small particle" emissions have diameters less than 20 microns. Therefore, the emission rates that ATSDR calculated include both PM10 and larger particles. For a conservative evaluation of air quality impacts, however, ATSDR assumed that all of the "small particle" emissions have diameters less than 10 microns. This assumption leads to lower deposition estimates, and therefore higher estimates of ambient air concentrations. Moreover, by assuming that all of the emissions are in particle size ranges that are more likely to be inhaled, this approach also overstates the toxicity of the particles. Thus, assuming the particles are all PM10 is a conservative approach to assessing the emissions. |
| Approach to estimate emissions for chemical by-products of explosions | BangBox emission factors | BangBox emission factors have been widely used in estimating air quality impacts resulting from the detonation of high explosives. The only air emissions and dispersion model available from EPA's Support Center for Regulatory Air Models that is specifically designed to evaluate these detonations estimates emissions using the BangBox emission factors. |
| Approach to estimate emissions of metals | Multiple considerations | Section D.3.B lists the different assumptions made when estimating air emissions of metals. Assuming that the bomb casings and aluminum powder completely vaporize likely leads to an overstated emission rate. Adding the BangBox emission factors to the estimated releases from casings may be "double-counting," and therefore overstating, emissions. |
| Approach to estimate emissions of high explosives | Assumed explosions are 90% efficient in consuming organic chemicals in the high explosive charge | The chemical bonds in the organic chemicals in an explosive charge (e.g., TNT and RDX) contain the energy released during a detonation. These chemicals react quickly during an explosion, releasing large amounts of energy as they break up into smaller molecules. A considerable fraction of these organic chemicals must react in order for a bomb to be effective. ATSDR assumed that the bombs at Vieques consume 90% of the organic chemicals in the high explosive charges. This percentage is relatively low (and therefore leads to overstated emission rates for these chemicals), when compared to the destruction efficiencies (>99%) typically reported for open detonation activities. |
| Modeling deposition of particulate matter | Used regulatory default procedures in modeling analysis | The COMBIC model predicts that the "small particle" emissions (i.e., those considered in this modeling analysis) have a settling velocity of 0.3 cm/s. Therefore, over the course of an hour, or the time it generally takes wind to blow from the LIA to the residential areas of Vieques, particles would be expected to settle approximately 10 meters, on average. This would result in essentially the entire "skirt" of the emissions cloud, or the near ground-level emissions, to settle to the surface well before plumes reach the residential areas of Vieques. To be conservative, ATSDR assumed that these emissions transport downwind in the "puff" generated during an explosion, which has the greater potential for long-range transport. |
Table D-2. Emission Factors, Emission Rates, and Estimated Annual Average Concentrations for Chemical By-Products of Explosions
| Chemical | Emission Factor (grams emitted per grams of NEW used) |
Emission Rate (pounds per year) |
Estimated Annual Average Air Concentration in Residential Areas (µg/m3) |
| Carbon dioxide | 1.33e+00 | 9.58e+05 | 9.54e-02 |
| Carbon monoxide | 7.17e-03 | 5.17e+03 | 5.14e-04 |
| Nitrogen dioxide | 2.60e-03 | 1.87e+03 | 1.87e-04 |
| Nitric oxide | 1.46e-02 | 1.05e+04 | 1.05e-03 |
| Sulfur dioxide | 2.23e-04 | 1.61e+02 | 1.60e-05 |
| 2,4-Dinitrotoluene | 3.51e-06 | 2.53e+00 | 2.52e-07 |
| 2,6-Dinitrotoluene | 4.39e-07 | 3.16e-01 | 3.15e-08 |
| N-2,4,6-Tetranitroaniline | 2.20e-08 | 1.59e-02 | 2.98e-09 |
| 1,2-Methylnaphthalene | 3.00e-05 | 2.16e+01 | 2.15e-06 |
| 1,1,3-Trimethyl-3-Phenylindane | 5.70e-07 | 4.11e-01 | 4.09e-08 |
| 1,3,5-Trinitrobenzene | 1.97e-06 | 1.42e+00 | 1.41e-07 |
| 1,3-Butadiene | 4.09e-06 | 2.95e+00 | 4.73e-07 |
| 1,4-Dichlorobenzene | 3.15e-07 | 2.27e-01 | 2.26e-08 |
| 1-Nitropyrene | 1.06e-06 | 7.64e-01 | 7.61e-08 |
| 2,5-Diphenyloxazole | 7.23e-05 | 5.21e+01 | 5.19e-06 |
| 2-Methylnaphthalene | 1.77e-06 | 1.28e+00 | 1.27e-07 |
| 2-Methylphenol (o-cresol) | 6.84e-07 | 4.93e-01 | 5.19e-08 |
| 2-Nitrodiphenylamine | 6.01e-07 | 4.33e-01 | 4.31e-08 |
| 2-Nitronaphthalene | 6.43e-07 | 4.63e-01 | 4.61e-08 |
| 4-Methylphenol (p-cresol) | 5.68e-07 | 4.09e-01 | 4.08e-08 |
| 4-Nitrophenol | 2.59e-06 | 1.87e+00 | 1.86e-07 |
| Acetophenone | 1.50e-05 | 1.08e+01 | 1.08e-06 |
| Dimethylphenethylamine | 0.00e+00 | 0.00e+00 | 5.20e-09 |
| Acetylene | 1.82e-05 | 1.31e+01 | 1.31e-06 |
| Ammonia | 2.92e-04 | 2.10e+02 | 2.10e-05 |
| Benzene | 9.62e-04 | 6.93e+02 | 6.90e-05 |
| Benzo(a)pyrene | 4.77e-06 | 3.44e+00 | 3.42e-07 |
| Benzyl alcohol | 1.41e-07 | 1.02e-01 | 1.01e-08 |
| Biphenyl | 5.20e-08 | 3.75e-02 | 3.73e-09 |
| Bis(2ethylhexyl)phthalate | 2.93e-06 | 2.11e+00 | 2.10e-07 |
| Butylbenzylphthalate | 1.03e-06 | 7.42e-01 | 7.49e-08 |
| Carbon tetrachloride | 6.30e-06 | 4.54e+00 | 4.52e-07 |
| Dibenz(a,h)anthracene | 1.73e-06 | 1.25e+00 | 1.24e-07 |
| Dibenzofurans | 1.32e-06 | 9.51e-01 | 9.47e-08 |
| Diethyl phthalate | 3.04e-07 | 2.19e-01 | 2.70e-08 |
| Dimethyl phthalate | 8.64e-07 | 6.22e-01 | 6.20e-08 |
| Di-n-butyl phthalate | 8.32e-05 | 5.99e+01 | 5.97e-06 |
| Di-n-octyl phthalate | 1.87e-06 | 1.35e+00 | 1.34e-07 |
| Diphenylamine | 7.73e-08 | 5.57e-02 | 5.55e-09 |
| Methane | 5.88e-03 | 4.24e+03 | 4.27e-04 |
| Naphthalene | 1.50e-04 | 1.08e+02 | 1.08e-05 |
| Nnitrosodiethylamine | 1.18e-07 | 8.50e-02 | 8.47e-09 |
| Nnitrosodiphenylamine | 5.86e-06 | 4.22e+00 | 4.20e-07 |
| Non-benzene aromatics | 3.16e-03 | 2.28e+03 | 2.27e-04 |
| Olefin (VOCs) | 1.35e-03 | 9.73e+02 | 9.89e-05 |
| Paraffins (VOCs) | 1.81e-04 | 1.30e+02 | 1.30e-05 |
| Phenol | 2.52e-05 | 1.82e+01 | 1.81e-06 |
| Total PAHs | 1.74e-05 | 1.25e+01 | 1.25e-06 |
| Vinyl chloride | 1.23e-06 | 8.86e-01 | 8.83e-08 |
Notes:
- Emission factors and emission rates listed are for air-to-ground activities only. ATSDR used different sets of emission factors for ship-to-shore and land-based activities, but these activities consistently accounted for approximately 5% of the total concentrations and are not summarized in this table.
- The ambient air concentration listed is for the location in the residential area of Vieques found to have the highest air quality impacts from the military training exercises. The concentrations reflect contributions from all three types of military training exercises.
Table D-3. Estimated Emission Rates and Annual Average Concentrations for Metals
| Metal (or Element) |
Estimated Contribution (kg/year) to Emissions from Different Factors | Estimated Annual Average Ambient Air Concentration in Residential Areas (µg/m3) | |||
| Casings | BangBox Data | Aluminum Powder | Crater Ejecta (Soil) | ||
| Aluminum | 1.14e+04 | 7.54e+03 | 9.39e+04 | 4.08e+03 | 2.04e-02 |
| Antimony | 0.00e+00 | 1.84e+01 | 0.00e+00 | 2.87e-01 | 3.27e-06 |
| Arsenic | 0.00e+00 | 5.20e-01 | 0.00e+00 | 1.98e+00 | 4.37e-07 |
| Barium | 0.00e+00 | 3.28e+02 | 0.00e+00 | 2.65e+01 | 6.19e-05 |
| Beryllium | 0.00e+00 | 0.00e+00 | 0.00e+00 | 6.08e-02 | 1.06e-08 |
| Boron | 4.07e-01 | 0.00e+00 | 0.00e+00 | 3.96e+00 | 7.62e-07 |
| Cadmium | 0.00e+00 | 5.27e+02 | 0.00e+00 | 4.31e-01 | 9.22e-05 |
| Calcium | 0.00e+00 | 1.66e+03 | 0.00e+00 | 2.87e+04 | 5.31e-03 |
| Chromium | 4.07e+01 | 4.14e+01 | 0.00e+00 | 9.53e+00 | 1.60e-05 |
| Chromium VI | 0.00e+00 | 2.07e+00 | 0.00e+00 | 0.00e+00 | 3.62e-07 |
| Cobalt | 0.00e+00 | 0.00e+00 | 0.00e+00 | 3.68e+00 | 6.43e-07 |
| Copper | 4.79e+03 | 1.29e+04 | 0.00e+00 | 9.86e+00 | 3.10e-03 |
| Iron | 1.90e+05 | 0.00e+00 | 0.00e+00 | 8.44e+03 | 3.46e-02 |
| Lead | 0.00e+00 | 6.28e+02 | 0.00e+00 | 2.14e+00 | 1.10e-04 |
| Manganese | 3.71e+03 | 0.00e+00 | 0.00e+00 | 1.82e+02 | 6.79e-04 |
| Mercury | 0.00e+00 | 5.75e-02 | 0.00e+00 | 5.44e-03 | 1.10e-08 |
| Molybdenum | 2.04e+00 | 0.00e+00 | 0.00e+00 | 0.00e+00 | 3.56e-07 |
| Nickel | 2.04e+01 | 1.14e+01 | 0.00e+00 | 4.01e+00 | 6.25e-06 |
| Potassium | 0.00e+00 | 5.86e+02 | 0.00e+00 | 0.00e+00 | 1.02e-04 |
| Scandium | 0.00e+00 | 0.00e+00 | 0.00e+00 | 3.15e+00 | 5.50e-07 |
| Selenium | 0.00e+00 | 0.00e+00 | 0.00e+00 | 3.10e-01 | 5.42e-08 |
| Sodium | 0.00e+00 | 1.50e+02 | 0.00e+00 | 0.00e+00 | 2.63e-05 |
| Strontium | 0.00e+00 | 0.00e+00 | 0.00e+00 | 3.93e+01 | 6.87e-06 |
| Tin | 0.00e+00 | 0.00e+00 | 0.00e+00 | 1.23e+00 | 2.14e-07 |
| Titanium | 2.04e+01 | 1.07e+02 | 0.00e+00 | 4.16e+02 | 9.49e-05 |
| Vanadium | 0.00e+00 | 0.00e+00 | 0.00e+00 | 2.67e+01 | 4.67e-06 |
| Yttrium | 0.00e+00 | 0.00e+00 | 0.00e+00 | 5.24e+00 | 9.16e-07 |
| Zinc | 9.16e+02 | 8.21e+03 | 0.00e+00 | 1.20e+01 | 1.60e-03 |
| Zirconium | 0.00e+00 | 0.00e+00 | 0.00e+00 | 1.49e+01 | 2.60e-06 |
Note:
- Section D.3.B discusses the assumptions made to estimate the emission rates for metals. Several assumptions are highly conservative (e.g., the casings from all high explosives completely vaporize upon impact) and most likely cause these emissions estimates to overstate actual emissions levels.
Table D-4. Normalized Concentrations Predicted by CALPUFF
| Emissions Scenario | Particle or Vapor | Averaging Period | Normalized Concentration (µg/m3)/(lb/hr) |
| 500-lb air-to-ground bombing event | Particle | Annual average | 0.000464 |
| 24-hour maximum | 0.00317 | ||
| Vapor | Annual average | 0.000543 | |
| 24-hour maximum | 0.00366 | ||
| 1,000-lb air-to-ground bombing event | Particle | Annual average | 0.000338 |
| 24-hour maximum | 0.00273 | ||
| Vapor | Annual average | 0.000393 | |
| 24-hour maximum | 0.00319 | ||
| 2,000-lb air-to-ground bombing event | Particle | Annual average | 0.000258 |
| 24-hour maximum | 0.00230 | ||
| Vapor | Annual average | 0.000299 | |
| 24-hour maximum | 0.00269 |
Note:
- The annual average normalized concentrations are averages of the annual average concentrations output for the five different years of meteorological data; the 24-hour average normalized concentrations are the highest daily-average level predicted for the five years of meteorological data.
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