E C

/RIncorporated Providing Environmental Technical Support Since 1989

501 Eastowne Drive, Suite 250  Chapel Hill, North Carolina 27514

Telephone: (919)484-0222  Fax: (919) 484-0122

MEMORANDUM

Date: July 30, 2012

To: Anita Lee, U.S. EPA/Region 9

From: Steve Fudge and Stephen Edgerton, EC/R Incorporated 

Subject: Mobile Source Risk Estimates for Navajo Generating Station due to 

Transportation of Anhydrous Ammonia or Urea

1. Introduction

This memorandum describes two air dispersion modeling analyses conducted in response 

to comments that the EPA received on the Advanced Notice of Proposed Rulemaking (ANPR) of 

the Best Available Retrofit Technology (BART) for the Navajo Generating Station (NGS) in 

Arizona. The commenters noted that the transportation of anhydrous ammonia to the facility for

a selective catalytic reduction (SCR) system to reduce NOX emissions would generate emissions 

of NOx, CO, VOC, PM10, and other pollutants. In addition to SCR, selective non-catalytic 

reduction (SNCR) is being considered for NOx control at the NGS facility. For this system, urea 

would be used in place of anhydrous ammonia as a reagent to reduce NOx. The air dispersion 

modeling analyses were performed to estimate the emissions and risk that would result from the 

transportation of anhydrous ammonia to NGS in support of SCR, and urea crystals in support of 

SNCR.

Both modeling studies were conducted using the Human Exposure Model Version 3 

(HEM3) that contains as its air dispersion model the American Meteorological Society (AMS) 

and EPA Regulatory Model (AERMOD). Potential risks from toxic air pollutants were estimated 

by comparing the modeled air concentrations to EPA’s Office of Air Quality Planning and 

Standards’ (OAQPS’) dose response values contained within HEM3. Criteria pollutant air 

concentration estimates were compared to their National Ambient Air Quality Standards 

(NAAQS) and Class I increments. 

The commenters also expressed concern about the risks involved in the transport and 

storage of anhydrous ammonia. The final section of this memorandum, Section 6, presents 

background on anhydrous ammonia, discusses the EPA’s various programs for hazardous 

materials and the quantities that meet the threshold for regulation, and presents a qualitative 

assessment of the risks of the use of anhydrous ammonia at NGS.

2. Transportation Modeling 

For both the SCR and SNCR modeling analyses, the roadway emissions were modeled as 

20 area sources approximately overlying the roads between Flagstaff, Arizona and NGS. These 

area sources are shown in Figure 1. 
2

Figure 1. Approximate Locations of Line Sources from Flagstaff, Arizona to the 

Navajo Generating Station

Air concentrations at census blocks near the roadway were estimated based on the 

pollutant emissions rates and the site-specific meteorology and terrain input into the model. As 

shown in Figure 2, which shows the modeling of a representative segment of road (State Road 98

which connects the facility access road with U.S. Highway 89), two types of receptors were used 

in the modeling. The first set of receptors, represented as squares in Figure 2, are the locations of 

the census block (year 2000) centroids within 25 kilometers of the center of the road segment. 

The second receptor set, represented as circles, are the polar grid receptors that are located along 
3

13 rings of radial receptors, every 30 degrees beginning at half of the line length from the center 

of the line and extending to 50 km of center. A summary of the modeling options is presented in 

Table 1. Detailed model inputs are presented in Attachment 2.

Figure 2. Example of Receptor Locations: Line Source from the Navajo Generating 

Station Access Road to U.S. Highway 89

Table 1. Options for the HEM3 Air Dispersion Modeling of Transporting Reagent to 

NGS for SCR and SNCR

Options Value

Terrain used, rural land use, no 

deposition or depletion

Site specific terrain values for locations and hill 

heights from HEM3. 

Receptors Census blocks (year 2000) within 25 km of center of 

each line.

13 rings of radial receptors, every 30 degrees out to 

50 km of center of each line.

Meteorological Data 1991 AERMET file contained within HEM3 –

Prescott, AZ surface file, Tucson, AZ upper air file.

Acute Emissions Multiplier Annual emissions multiplied by 10 to account for 

variations in short-term emissions.
4

For the SCR modeling analysis, emissions were estimated from the two additional trucks 

daily cited by the commenter as being needed to transport the anhydrous ammonia from the 

railway station in Flagstaff, AZ to NGS (approximately 135 miles each way) based on the size of 

the plant. This corresponds to 728 trucks per year to transport anhydrous ammonia for SCR. 

Similarly, for the SNCR modeling analysis, emissions were estimated from the approximately 

1,583 truck trips per year (about 4 trucks per day) needed to transport the urea crystals along the 

same route. The number of truck trips needed to transport urea to the NGS plant was calculated 

from estimates provided by the plant and from data presented in a technical and economic 

comparison of SCR ammonia options.1 The report provided data on the number of trucks or 

railcars that would be needed to transport various forms of ammonia and estimated it would take 

19 truck deliveries to deliver 370 tons of urea, or 19.5 tons per truckload. The NGS plant 

estimated that 30,873 tons/yr of urea would be needed to support SNCR.

2 Using the figure of 

19.5 tons of urea per truckload, 1,583 truckloads of urea per year was computed.

Criteria pollutant emissions released by the trucks were developed from South Coast Air 

Quality Management District’s (SCAQMD’s) “Highest (Most Conservative) EMFAC2007 

(version 2.3) Emission Factors for On-Road Heavy-Heavy-Duty Diesel Trucks for 2010.”3

Table 2 presents the SCAQMD emission factors and the criteria pollutant emissions estimates 

used in both analyses. Toxic air pollutant emission factors were developed from the reactive 

organic gas (ROG) and PM2.5 emission factors using OAQPS’ Speciate Database.

4 Table 3 

presents the toxic air pollutant emissions estimates developed using the Speciate factors.

Table 2. Criteria Pollutant Emission Rates Used to Estimate Risk from Transporting 

Reagent to NGS for SCR and SNCR

Pollutant

SCAQMD EF 

(lb/mi)

Emissions 728 

trucks/yr, 270 miles 

roundtrip (TPY) for 

SCR

Emissions 1,583 

trucks/yr, 270 miles 

roundtrip (TPY) for 

SNCR

Reactive Organic Gases 

(ROG) 3.04E-03 2.99E-01 6.50E-01

CO 1.20E-02 1.17E+00 2.56E+00

NOX 3.82E-02 3.76E+00 8.16E+00

SOX 4.13E-05 4.06E-03 8.83E-03

PM10 1.83E-03 1.80E-01 3.91E-01

PM2.5 including tires & brakes 1.60E-03 1.57E-01 3.42E-01

PM2.5 engine exhaust 1.55E-03 1.53E-01 3.31E-01

 1 http://www.fwc.com/publications/tech_papers/files/TP_SCR_03_03.pdf

2 Grant Smedley, <grant.smedley@srpnet.com>, “Urea Cost Estimate,” email message to Anita Lee, March 23, 

2012.

3 http://www.aqmd.gov/ceqa/handbook/onroad/onroad.html

4 http://cfpub.epa.gov/si/speciate/
5

Table 3. Toxic Air Pollutant Emission Rates Used to Estimate Risk from

Transporting Reagent to NGS for SCR and SNCR

Pollutant

SCAQMD 

EF

(lb/mi)

Weight 

Percent of 

VOC from 

Speciate 

Calculated 

HAP 

Emission 

Factor 

(lb/mile) 

Emissions 728 

trucks/yr, 270 

miles roundtrip 

(TPY) for SCR

Emissions 1,583 

trucks/yr, 270 

miles roundtrip 

(TPY) for 

SNCR

Gas Phase Speciate Database Record # 4674: Diesel Exhaust - Medium Duty Trucks

ROG* 3.04E-03

1,3-Butadiene 1.18E-01 3.60E-04 3.53E-02 7.69E-02

2,2,4-

Trimethylpentane 4.73E-01 1.44E-03 1.41E-01 3.08E-01

Acetaldehyde 1.59E+01 4.85E-02 4.77E+00 1.04E+01

Acrolein 1.30E+00 3.94E-03 3.88E-01 8.42E-01

Benzene 1.05E+00 3.18E-03 3.12E-01 6.80E-01

Ethyl benzene 1.79E-01 5.45E-04 5.36E-02 1.16E-01

Formaldehyde 8.51E+00 2.59E-02 2.54E+00 5.53E+00

M & p-xylene 8.89E-01 2.70E-03 2.66E-01 5.77E-01

Naphthalene 2.35E-01 7.16E-04 7.03E-02 1.53E-01

O-xylene 3.17E-01 9.63E-04 9.46E-02 2.06E-01

Propionaldehyde 5.34E+00 1.62E-02 1.60E+00 3.46E+00

Toluene 1.52E+00 4.62E-03 4.54E-01 9.87E-01

Dibenzofuran 1.09E-02 3.33E-05 3.27E-03 7.12E-03

Acetophenone 1.95E+00 5.92E-03 5.81E-01 1.27E+00

Particulate Phase Speciate Database Record # 322082.5: Heavy Duty Diesel Trucks

Diesel engine 

emissions 1.55E-03 1.53E-01 3.31E-01

Arsenic compounds 1.50E-02 2.33E-05 2.29E-03 4.98E-03

Manganese 

compounds 9.00E-03 1.40E-05 1.37E-03 2.99E-03

Nickel compounds 2.70E-02 4.20E-05 4.12E-03 8.98E-03

* Assumed that reactive organic gases (ROG) are equivalent to volatile organic compounds (VOC).

3. Impacts of Criteria Air Pollutant Emissions

In order to place into perspective peoples’ exposure to criteria air pollutants, the modeled 

air concentration was compared to the NAAQS for the pollutant. The NAAQS for the criteria 

pollutants include both short-term and long-term average air concentrations (see Attachment 1). 

Table 4 presents the NAAQS levels used in this analysis, with the concentrations converted as 

necessary from expression as parts per million or billion to milligrams per cubic meter (mg/m3

).

As shown in Tables 5 and 6, the SCR and SNCR annual average and short-term air 

concentrations modeled at the most impacted census blocks (the populated census blocks with 
6

the highest modeled air concentration of a pollutant) are well below the NAAQS and Class I 

increments.

Table 4. NAAQS and Class I Increments Used in Criteria Pollutant Comparison

Pollutant

NAAQS Level 

as Codified

NAAQS Level for 

Comparison (mg/m3)

Class I 

Increments 

(mg/m3)

Averaging 

time

CO 10 mg/m3 10 8-hour 

40 mg/m3 40 1-hour 

NO2 53 ppb 0.1 0.0025 Annual

100 ppb 0.19 1-hour 

PM10 0.004 Annual

0.15 mg/m3 0.15 0.008 24-hour 

PM2.5 0.015 mg/m3 0.015 0.001 Annual

0.035 mg/m3 0.035 0.002 24-hour

SO2 0.03 ppm 0.08 0.002 Annual

0.14 ppm 0.37 0.005 24-hour

75 ppb 0.2 1-hour

Table 5. Comparison of Modeled Annual Average Air Concentrations from Emissions 

from Transporting Reagent to NGS for SCR and SNCR to NAAQS and 

Class I Increments

Pollutant

Annual Average 

NAAQS Level

(mg/m3)

Class I Annual 

Average 

Increments

(mg/m3)

SCR Annual 

Average Modeled 

Air Concentration

(mg/m3)

SNCR Annual 

Average Modeled 

Air Concentration

(mg/m3)

CO NA NA 0.000013 0.000031

NO2 0.1 0.0025 0.00004 0.000098

PM10 NA 0.004 0.0000019 0.0000047

PM2.5 0.015 0.001 0.0000017 0.0000041

SO2 0.08 0.002 0.000000043 0.00000011
7

Table 6. Comparison of Modeled Short-Term Concentrations from Emissions from 

Transporting Reagent to NGS for SCR and SNCR to NAAQS and Class I 

Increments

Pollutant

Short-Term 

Average NAAQS 

Level

(mg/m3)

Class I Short-Term 

Average Increments

(mg/m3)

SCR Short-term 

Average Modeled 

Air Concentration

(mg/m3)

SNCR Short-term 

Average Modeled 

Air Concentration

(mg/m3)

1-hour Average Air Concentrations

CO 40. NA 0.0043 0.0094

NOx 0.19 NA 0.014 0.030

SOx 0.2 NA 0.000015 0.000033

8-hour Average Air Concentrations

CO 10. NA 0.00094 0.0020

24-hour Average Air Concentrations

PM10 0.15 0.008 0.000064 0.00015

PM2.5 0.035 0.002 0.000056 0.00013

SOx 0.37 0.005 0.0000014 0.0000034

4. Impacts of Toxic Air Pollutants

National Ambient Air Quality Standards have not been established for the hundreds of 

toxic air pollutants that have the potential to cause cancer and other non-cancer adverse health 

effects. For these pollutants, toxicological dose response values have been established for use in 

estimating many toxic air pollutants’ adverse health effects. The OAQPS has established a table 

of pollutant-specific values that are used with modeled or measured air concentrations in order to 

quantify the impact of exposure to these pollutants. The long-term or chronic dose response 

values used in the analysis can be found at http://www.epa.gov/ttn/atw/toxsource/summary.html

in Table 1.

Cancer risk for exposure to a pollutant at a particular concentration is expressed in terms 

of the number of cancers that would be expected to be caused in a population exposed for a 

lifetime, for example one cancer per 1 million persons exposed (abbreviated as 1 x 10-6

). For 

purposes of this measure, a lifetime of exposure is defined as 70 years. As a general rule in the 

EPA’s evaluation of risks, a risk of 1 x 10-6 or less would be considered acceptable.

Non-cancer health effects are quantified using hazard quotients and hazard indices for 

various target organs. The hazard quotient (HQ) for a given chemical and target organ is the ratio 

of the ambient concentration of the chemical to the “reference concentration” (RfC) at which no 

adverse effects are expected from continuous inhalation for a lifetime (this is defined as 

70 years). Therefore, no adverse effects are expected as a result of exposure if the HQ is less than 

1.0. The chronic hazard index (HI) for a given target organ is the sum of hazard quotients for 

substances that affect that organ (e.g., respiratory). As with the HQ, aggregate exposures equal to 

or below an HI of 1.0 likely will not result in adverse non-cancer health effects over a lifetime of 

exposure and would ordinarily be considered acceptable.
8

As shown in Table 7 below, the maximum estimated cancer risk summed over all 

modeled toxic air pollutants for the SCR modeling analyses is less than 1 x 10-6

, and it is 1 x 10-6

for the SNCR modeling analysis. For non-cancer health effects, the respiratory system is the 

target organ most affected by the modeled pollutants that would be emitted by trucks 

transporting reagent to NGS. As shown in Table 7, the respiratory HIs for both SCR and SNCR 

are well below 1.0. In both the SCR and SNCR modeling analyses, the maximum cancer risk 

occurs at a census block located immediately adjacent to the road in Coconino County, Arizona 

with a population of 45 individuals. For the other impacted census blocks, the risks in the SCR 

analysis are estimated to be as low as 5 x 10-12 and the HIs are estimated to be as low as 

0.000002; for the SNCR analysis, as low as 1 x 10-11 and 0.000003.

Table 7. Modeled Risk Estimates for Toxic Air Pollutants from Emissions from 

Transporting Reagent to NGS for SCR and SNCR

Risks Due to Transportation of Anhydrous Ammonia for SCR

Cancer Risk 7 x 10-7 Seven excess cancer cases over a lifetime 

exposure per 10 million people exposed.

Non-Cancer Respiratory 

Hazard Index 0.22

A value of the HI at or below 1.0 indicates that 

the exposure is not likely to result in adverse 

health effects.

Risks Due to Transportation of Urea for SNCR

Cancer Risk 1 x 10-6 One excess cancer case over a lifetime 

exposure per 1 million people exposed.

Non-Cancer Respiratory 

Hazard Index 0.47

A value of the HI at or below 1.0 indicates that 

the exposure is not likely to result in adverse 

health effects.

5. Comparison of Modeled Results to Existing Concentrations and the 2005 NATA

In addition to estimating impacts by comparing modeled air concentrations to levels such 

as the NAAQS or toxicological dose response values, another way to characterize risk is to 

compare estimates to other values such as monitored concentrations or other estimated values. 

For the criteria pollutants, a large monitoring network has been established for the purpose of 

determining whether an area has attained the NAAQS. While the number of monitors for air 

toxics has increased in recent years, few areas of the nation are covered, and no air toxics 

monitors are sited in the area of interest in Arizona. Thus, for this analysis, we used the most 

recent National Air Toxics Assessment (NATA) analysis for comparisons. The NATA analysis 

is a county-level and census tract-level air toxics study that provides modeled air concentration 

and risk estimates for the entire United States and is completed every 3 years. The results of the 

2005 tract-level NATA are presented for comparison here.
9

As shown in Table 8, the expected impact of the increased truck traffic to NGS necessary 

to deliver either anhydrous ammonia for SCR or urea for SNCR is very low when compared to 

nationally monitored concentrations (http://www.epa.gov/airtrends/). Tables 9 and 10 show that 

for air toxics, in most cases the transportation of either anhydrous ammonia or urea to NGS 

would have minimal impact on the existing levels of risk or existing ambient concentrations

estimated in NATA. The values in Tables 9 and 10 for NATA represent the census tract that 

contains the census block with the highest concentrations resulting from reagent transportation as 

reported in the previous tables. Although the results shown in Table 9 indicate that the existing 

2005 total modeled respiratory HI in the most impacted census tract in Coconino County is 

already close to 1.0, levels well in excess of 1.0 are not uncommon nationally. For comparison, 

the 2005 NATA reported an average total respiratory HI of 3.2 for Maricopa County, AZ (the 

county containing Phoenix), with a total respiratory HI of 9.5 in the highest census tract within 

that county. For Pima County, AZ (the county containing Tucson), the 2005 NATA reported an 

average total respiratory HI of 1.5, with a value of 3.3 reported for the highest census tract within 

the county.

Table 8. Comparison of Annual Average Modeled Criteria Pollutant Concentrations 

from Emissions from Transporting Reagent to NGS for SCR and SNCR to 

National Monitored Values

Poll.

Avg. 

Period

SCR at 

NGS

Modeled 

Air 

Conc. 

(μg/m3)

SNCR at 

NGS

Modeled 

Air 

Conc. 

(μg/m3)

2008 Range of Monitored Values (μg/m3)

Poll

Avg. 

Period

10th 

%ile

Arithmetic 

Avg.

90th 

%ile

# of 

sites

CO Highest 

8-hour 0.94 2.0 CO 2nd Highest 8-hour 1,200 2,100 3,100 206

NOx Annual 0.040 0.098 NO2 Annual 8.7 25. 40. 151

PM10 Annual 0.0019 0.0047 PM10

Seasonal 

wt. annual 33. 56. 88. 325

PM2.5

Highest 

24-hour 0.056 0.13 PM2.5

2nd Highest 

24-hour 7.4 11. 14. 728

SOx Annual 0.000043 0.00011 SO2 Annual 3.7 9.1 15.5 141

Table 9. Comparison of Modeled Total Air Toxic Risks to NATA Risk Estimates for

Census Tract 04005000500 in Coconino County, AZ

Toxic Air Pollutants

SCR at NGS SNCR at NGS 2005 NATA Analysis

Modeled Risk Modeled Risk Modeled Total Risk

Cancer Risk 7 x 10-7 1 x 10-6 2.6 x 10-5

Non-Cancer Respiratory 

Hazard Index 0.22 0.47 0.98
10

Table 10. Comparison of Modeled Air Concentrations of Risk Driver Air Toxics to 

NATA Estimates for These Pollutants in Census Tract 04005000500 in

Coconino County, AZ

Toxic Air 

Pollutant 

SCR at NGS SNCR at NGS 2005 NATA Analysis

Modeled Annual 

Average Air Conc. 

(μg/m3)

Modeled Annual 

Average Air Conc. 

(μg/m3)

Modeled Total 

Annual Average Air 

Conc. (μg/m3)

Acetaldehyde 0.051 0.12 1.18

Acrolein 0.0041 0.010 0.021

Arsenic 

Compounds 0.000024 0.000060 0.00015

Diesel Engine 

Emissions 0.0016 0.0040 0.26

Formaldehyde 0.027 0.067 1.1

6. Transport and Storage of Anhydrous Ammonia

Anhydrous ammonia is a colorless gas with a pungent odor. It is stored and transported as 

a liquid under pressure. Ammonia vapor is lighter than air and tends to rise when released to 

atmosphere. 

Ammonia is used widely and in large quantities for a variety of purposes. Estimated use 

of ammonia in the United States in 2010 totaled 14.7 million metric tons.5 Approximately 

87 percent of this total was for fertilizer use, including direct application to the soil from tanks 

containing the liquefied gas as well as in the production of urea, ammonium nitrates, ammonium 

phosphates, and other nitrogen compounds.6 Other agricultural uses of ammonia include use as a 

source of protein in livestock feeds, as a pre-harvest cotton defoliant, as an anti-fungal agent on 

certain fruits, and as a preservative for the storage of high-moisture corn. 

Non-agricultural uses of anhydrous ammonia include refrigeration systems, water 

treatment, and heat treatment. It is used to produce nitric acid and other chemicals, dyes, sulfa 

drugs, vitamins, and cosmetics; to neutralize acid constituents of crude oil and to protect 

equipment from corrosion; to extract certain metals from their ores; for stabilizing natural and 

synthetic latex during transportation; for pulping wood; in the manufacture of synthetic fibers 

such as nylon and rayon; in the manufacture of phenolics and polyurethanes; as a curing agent 

and a slime and mold preventative in tanning liquors and as a protective agent for leathers and 

furs in storage; and in weak ammonia solutions as commercial and household cleaners and 

detergents.7

 5 http://minerals.usgs.gov/minerals/pubs/commodity/nitrogen/mcs-2011-nitro.pdf 6 Ibid.

7 R.M. Technologies, Mount Laurel, NJ. http://www.rmtech.net/Chemical%20Plants.htm
11

Anhydrous ammonia is toxic and can present a health hazard unless proper handling, 

operation, and maintenance procedures are followed. Effects of inhalation of anhydrous 

ammonia range from lung irritation to severe respiratory injuries, with possible fatality at higher 

concentrations. Anhydrous ammonia also is corrosive and can burn the skin and eyes. Anhydrous 

ammonia is stored as a liquid and has a boiling point of minus 28 degrees Fahrenheit. At this 

temperature it can cause freezing burns.8

Federal Risk Management Programs

There are a number of federal regulations that require reporting of the use and release of 

hazardous materials. The EPA programs are: 

• Emergency Planning and Community Right-to-Know Act (EPCRA), also known as 

Title III of the Superfund Amendments and Reauthorization Act of 1986 (SARA),

• Comprehensive Environmental Response, Compensation and Liability Act (CERCLA),

• Section 112(r) of the Clean Air Act (CAA).9 

EPCRA section 302 covers the presence of extremely hazardous substances (EHSs) in 

quantities at or above the Threshold Planning Quantity (TPQ). The TPQ for anhydrous ammonia 

is 50 pounds. Section 302 requires Local Emergency Planning Committees (LEPCs) to develop 

emergency response plans and facility owners or operators to notify the State Emergency 

Response Commission and LEPC if a chemical is present at the facility in quantities at or above 

the EHS’s TPQ. Releases of reportable quantities (RQ) of EHSs are subject to state and local 

reporting under section 304 of EPCRA. The RQ for anhydrous ammonia is 100 pounds under 

this program. EPCRA section 313 also pertains to toxic chemicals. Emissions, transfers, and 

waste management data for chemicals listed under section 313 must be reported annually as part 

of the community right-to-know provisions. Releases of CERCLA hazardous substances, in 

quantities equal to or greater than their RQ, are subject to reporting to the National Response 

Center under CERCLA. The RQ for anhydrous ammonia under CERCLA is also 100 pounds.

Under the accident prevention provisions of section 112(r) of the CAA, EPA developed a 

list of 77 toxic substances and 63 flammable substances. Threshold quantities (TQs) were 

established for these substances. The list and TQs identify processes subject to accident 

prevention regulations known as Risk Management Programs (RMP). Anhydrous ammonia is 

listed as a toxic substance under section 112(r) with a TQ of 10,000 pounds. Additional 

information is available at http://www.epa.gov/oem/content/rmp/caa_faqs.htm.

Transportation activities are exempted from compliance with the RMP regulations 

because these regulations apply only to stationary sources. Transportation-related chemical 

safety is the responsibility of the U.S. Department of Transportation (DOT). Specifically, the 

chemicals in the tank car are exempt only if the tank car is in transportation. The EPA considers 

a container to be in transportation as long as it is attached to the motive power (e.g., truck or 

 8 Ibid.

9 LIST OF LISTS: Consolidated List of Chemicals Subject to the Emergency Planning and Community RightTo-Know Act (EPCRA), Comprehensive Environmental Response, Compensation and Liability Act (CERCLA) and

Section 112(r) of the Clean Air Act. May 2010. Available at: http://www.epa.gov/emergencies/tools.htm#lol
12

locomotive) that delivered it to the site. If the tank car is detached from the motive power, and 

therefore no longer in transportation, the contents of the tank car must be considered in the TQ 

determination under the risk management program rule.

Specific to transportation, anhydrous ammonia is regulated domestically by the DOT as a 

non-flammable gas, but meets the definition of a Federal Motor Carrier Safety Administration 

(FMCSA) hazardous material because it is toxic by inhalation. It is covered under the FMCSA 

regulations that require a Hazardous Material (HM) Safety Permit when transported in a 

packaging having a capacity greater than 13,248 L (3,500 gallons).

Qualitative Assessment of Risks from the Transport and Storage of Anhydrous Ammonia

A FMCSA-supported risk assessment estimated that approximately 15,000 HM accidents 

and incidents occur each year. (Accidents and incidents involving anhydrous ammonia represent 

an unknown fraction of this total.) Of the HM incidents and accidents, about 75 percent represent 

loading and unloading incidents. En route HM accidents total about 2,500 annually with about 

700 (28 percent) of these being spill accidents. The estimated number of annual incidents and 

accidents were converted into rates using annual vehicle miles of HM operation based on the 

1997 Commodity Flow Survey. The average HM accident rate is 0.32 accidents per million 

vehicle-miles and the average HM incident rate is 0.51 incidents per million vehicle-miles.10

With a round-trip distance of 270 miles per delivery of anhydrous ammonia to NGS and

two deliveries per day, the total miles traveled by delivery tankers would be 197,100 miles per 

year. Only half of this total, or 98,550 miles per year, would be traveled by tanker trucks loaded 

with anhydrous ammonia. At the overall HM accident rate from the FMCSA risk assessment, 

one would expect one accident involving an NGS tanker truck approximately every 15 years. For 

the laden tanker trucks, the estimated frequency would be one accident in approximately 

30 years. According to the figures from the FMCSA risk assessment, only 28 percent of such 

accidents would be expected to involve a spill. Given that the vast majority of the route passes 

through a very sparsely populated area, the likelihood of injury to the populace from an accident 

is extremely small.

The infrequency of releases from transport and storage of anhydrous ammonia is 

confirmed by reported information on such releases. The U.S. Agency for Toxic Substances and 

Disease Registry (ATSDR) operates the National Toxic Substance Incidents Program (NTSIP).

The program collects and combines information from many resources, including the EPA and 

DOT, to protect people from harm caused by spills and leaks of toxic substances. A download 

from the National Database11 on anhydrous ammonia incidents between 2005 and 2010 

contained 45 entries. All of the entries were associated with the agricultural use of anhydrous 

ammonia. Of these incidents, the vast majority were associated with loading operations or soil 

applications rather than transport on highways or other areas where the public may be exposed. 

There were no incidents involving anhydrous ammonia use at a power plant or its transport to a 

 10 Comparative Risks of Hazardous Materials and Non-Hazardous Materials Truck Shipment Accidents/Incidents. 

Prepared for Federal Motor Carrier Safety Administration by Battelle Institute. Columbus, OH. March 2001.

http://www.phmsa.dot.gov/staticfiles/PHMSA/DownloadableFiles/Files/hazmatriskfinalreport.pdf 11 http://www.atsdr.cdc.gov/ntsip/national_database.html
13

power plant during that 5-year period. Given the very widespread use of anhydrous ammonia, the 

small number of incidents between 2005 and 2010 and the types of settings where they occurred 

show that the likelihood of a release associated with the transport and storage of anhydrous 

ammonia for use in SCR at NGS would be very small.

While the Federal Government’s safety net of programs were put in place to keep the 

public and workers safe while dealing with flammable, hazardous, or toxic materials, they do add 

an expense to the owners and operators of facilities dealing with these materials. Based on its use 

of anhydrous ammonia, Navajo Generating Station would fall under the CAA section 112(r)’s 

RMP requirements as well as notification requirements under other programs. However, the 

costs of complying with these programs would be a very small fraction of the total costs related 

to the use of SCR at the facility.
14

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Attachment 1

National Ambient Air Quality Standards
(This page intentionally left blank.)
National Ambient Air Quality Standards (NAAQS) (Source: http://epa.gov/air/criteria.html)

Primary Standards Secondary Standards

Pollutant Level Averaging Time Level Averaging Time

Carbon 

Monoxide

9 ppm 

(10 mg/m3) 

8-hour (1) None 

35 ppm 

(40 mg/m3)

1-hour (1)

Lead 0.15 μg/m3 (2) Rolling 3-Month Average Same as Primary

1.5 μg/m3 Quarterly Average Same as Primary

Nitrogen 

Dioxide

53 ppb (3) Annual 

(Arithmetic Average)

Same as Primary

100 ppb 1-hour (4) None 

Particulate 

Matter (PM10)

150 μg/m3 24-hour (5) Same as Primary

Particulate 

Matter (PM2.5)

15.0 μg/m3 Annual (6)

(Arithmetic Average)

Same as Primary

35 μg/m3 24-hour (7) Same as Primary

Ozone 0.075 ppm 

(2008 std) 

8-hour (8) Same as Primary 

0.08 ppm 

(1997 std) 

8-hour (9) Same as Primary 

0.12 ppm 1-hour (10) Same as Primary

Sulfur 

Dioxide

0.03 ppm Annual 

(Arithmetic Average) 0.5 ppm 3-hour (1)

0.14 ppm 24-hour (1)

75 ppb (11) 1-hour None 

(1) Not to be exceeded more than once per year.

(2) Final rule signed October 15, 2008.

(3) The official level of the annual NO2 standard is 0.053 ppm, equal to 53 ppb, which is shown here for the purpose 

of clearer comparison to the 1-hour standard

(4) To attain this standard, the 3-year average of the 98th percentile of the daily maximum 1-hour average at each 

monitor within an area must not exceed 100 ppb (effective January 22, 2010).

(5) Not to be exceeded more than once per year on average over 3 years.

(6) To attain this standard, the 3-year average of the weighted annual mean PM2.5 concentrations from single or 

multiple community-oriented monitors must not exceed 15.0 μg/m3.

(7) To attain this standard, the 3-year average of the 98th percentile of 24-hour concentrations at each populationoriented monitor within an area must not exceed 35 μg/m3 (effective December 17, 2006).

(8) To attain this standard, the 3-year average of the fourth-highest daily maximum 8-hour average ozone 

concentrations measured at each monitor within an area over each year must not exceed 0.075 ppm. (effective 

May 27, 2008) 

(9) (a) To attain this standard, the 3-year average of the fourth-highest daily maximum 8-hour average ozone 

concentrations measured at each monitor within an area over each year must not exceed 0.08 ppm. 

 (b) The 1997 standard—and the implementation rules for that standard—will remain in place for implementation 

purposes as EPA undertakes rulemaking to address the transition from the 1997 ozone standard to the 2008 ozone 

standard.

 (c) EPA is in the process of reconsidering these standards (set in March 2008).

(10) (a) EPA revoked the 1-hour ozone standard in all areas, although some areas have continuing obligations under 

that standard ("anti-backsliding").

 (b) The standard is attained when the expected number of days per calendar year with maximum hourly average 

concentrations above 0.12 ppm is < 1.

(11) (a) Final rule signed June 2, 2010. To attain this standard, the 3-year average of the 99th percentile of the daily 

maximum 1-hour average at each monitor within an area must not exceed 75 ppb.
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Attachment 2

Detailed Air Modeling Inputs
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Detailed Air Modeling Inputs Used to Estimate Risk from Transporting Anhydrous Ammonia and Urea to Navajo Generating Station for 

SCR and SNCR

Original Road Segment Location Identification Road Segment Location for Use in HEM3 Modeling Emissions Distribution

ID

End 1 of Road 

Segment 

(m) - Zone 12

End 2 of Road 

Segment 

(m) - Zone 12

Length of 

Road 

Segment 1

Rotated SW Corner 

(m) - Zone 12

Length in x 

direction 

Length in y 

direction 

Rotation 

Angle 2 Area of Road Segment

UTM E UTM N UTM E UTM N (m) UTM E UTM N (m) (m) degrees (m2) % 

A 456,327 4,083,346 464,762 4,083,385 8,435 456,327 4,083,346 8,435 10 0 84,348 4.00%

B 452,785 4,071,198 456,327 4,083,346 12,654 452,785 4,071,198 10 12,654 15 126,536 6.00%

C 451,295 4,071,211 452,785 4,071,198 1,490 451,295 4,071,211 1,491 10 0 14,905 0.71%

D 444,449 4,058,241 451,295 4,071,211 14,666 444,449 4,058,241 10 14,666 30 146,658 6.95%

E 443,631 4,058,247 444,449 4,058,241 818 443,631 4,058,247 818 10 0 8,179 0.39%

F 443,502 4,054,555 443,631 4,058,247 3,694 443,502 4,054,555 10 3,694 0 36,935 1.75%

FG 441,746 4,054,457 443,502 4,054,555 1,758 441,746 4,054,457 1,758 10 0 17,585 0.83%

G 441,205 4,050,393 441,746 4,054,457 4,100 441,205 4,050,393 10 4,100 10 40,999 1.94%

H 441,205 4,050,393 447,498 4,040,662 11,588 441,205 4,050,393 11,588 10 60 115,878 5.49%

I 447,498 4,040,662 464,083 4,003,616 40,589 447,498 4,040,662 40,589 10 65 405,895 19.23%

K 463,983 3,987,320 464,083 4,003,616 16,296 463,983 3,987,320 10 16,296 0 162,958 7.72%

L 463,983 3,987,320 464,574 3,983,514 3,852 463,983 3,987,320 3,852 10 85 38,517 1.83%

M 462,822 3,970,288 464,601 3,983,406 13,238 462,822 3,970,288 10 13,238 8 132,384 6.27%

N 460,686 3,966,609 463,009 3,970,296 4,357 460,686 3,966,609 10 4,357 30 43,572 2.06%

O 454,515 3,950,178 460,686 3,966,609 17,552 454,515 3,950,178 10 17,552 20 175,522 8.32%

P 447,270 3,915,624 454,515 3,950,178 35,305 447,270 3,915,624 10 35,305 12 353,051 16.73%

Q 447,270 3,915,624 450,695 3,909,008 7,451 447,270 3,915,624 7,451 10 65 74,505 3.53%

R 450,497 3,902,939 450,695 3,909,008 6,071 450,497 3,902,939 10 6,071 0 60,715 2.88%

S 447,930 3,899,615 450,559 3,903,568 4,747 447,930 3,899,615 10 4,747 40 47,468 2.25%

T 446,510 3,897,703 447,930 3,899,615 2,381 446,510 3,897,703 10 2,381 35 23,812 1.13% 1

 Width = 10 m

 

2 HEM3 requires angle be between 0 and 90 degrees. 2,110,421 = total area