Prepared for: Prepared by:

Arizona Public Service AECOM

Phoenix, AZ Chelmsford, MA

April 5, 2013

Environment

Four Corners Power Plant and Navajo 

Mine Environmental Impact Statement 

NAAQS Modeling Study

Draft 
Prepared for: Prepared by:

Arizona Public Service AECOM

Phoenix, AZ Chelmsford, MA

April 5, 2013

Environment

Four Corners Power Plant and Navajo 

Mine Environmental Impact Statement 

NAAQS Modeling Study

Draft 

_________________________________

Prepared By Mary Kaplan

_________________________________

Reviewed By Robert J. Paine
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Contents

1.0 Introduction...................................................................................................................... 1-1

1.1 Project Overview.................................................................................................................. 1-1

1.2 Purpose of Modeling Study ................................................................................................. 1-2

1.3 Contents of the Modeling Study Report.............................................................................. 1-2

2.0 Description of Power Plant and Mine Sources............................................................ 2-1

2.1 Source Location................................................................................................................... 2-1

2.2 Emission Sources ................................................................................................................ 2-1

2.2.1 Point Sources at FCPP......................................................................................... 2-1

2.2.2 Coal, Ash, and Lime Handling Volume Sources at FCPP .................................. 2-4

2.2.3 Vehicular Traffic at FCPP..................................................................................... 2-4

2.2.4 Lime and Ash Piles at FCPP................................................................................ 2-4

2.2.5 Navajo Mine North Sources ................................................................................. 2-5

2.2.6 Navajo Mine South Sources................................................................................. 2-6

3.0 Modeling Procedures...................................................................................................... 3-1

3.1 Model Selection ................................................................................................................... 3-1

3.1.1 Low Wind Speed Improvements for AERMOD ................................................... 3-1

3.1.2 Plume Depletion and Secondary Particulate Modeling....................................... 3-2

3.2 Meteorological Data for AERMOD ...................................................................................... 3-3

3.2.1 Available Meteorological Data.............................................................................. 3-3

3.2.2 AERMET Processing............................................................................................ 3-4

3.3 Good Engineering Practice Stack Height Analysis........................................................... 3-10

3.4 Receptor Grid and AERMAP Processing ......................................................................... 3-13

3.5 Class II Area Modeling Analyses ...................................................................................... 3-14

4.0 Background Concentrations ......................................................................................... 4-1

4.1 Regional Ambient Background............................................................................................ 4-1

4.2 Nearby Sources ................................................................................................................... 4-2

5.0 Modeling Results............................................................................................................. 5-1

6.0 References ....................................................................................................................... 6-1
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List of Appendices

Appendix A Emissions Calculations

Appendix B 2011 EPA Clarification Memo

Appendix C Alternative Model Justification for Low Wind Speed Beta Options: AERMET and AERMOD

Appendix D AERSURFACE Comparison

Appendix E 30-Year Precipitation Data

Appendix F Significant Concentration Gradient Analysis

Appendix G Modeling Archive
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List of Tables

Table 2-1 Unit 4 and 5 Stack Parameters.......................................................................................... 2-1

Table 2-2 Emission Rates for Units 4 and 5 ...................................................................................... 2-9

Table 2-3 Emission Rates and Source Parameters for Lime, Coal, and Ash Handling Point Sources2-10

Table 2-4 Emission Rates and Source Parameters for Lime, Coal, and Ash Handling Volume Sources

.......................................................................................................................................... 2-11

Table 2-5 Emission Rates and Source Parameters for Vehicular Traffic ....................................... 2-12

Table 2-6 Emission Rates and Source Parameters for Lime and Ash Pile Area Sources............. 2-13

Table 2-7 Emission Rates and Area Source Parameters for Navajo Mine North .......................... 2-14

Table 2-8 Emission Rates and Volume Source Parameters for Navajo Mine North...................... 2-15

Table 2-9 Emission Rates and Source Parameters for Navajo Mine South................................... 2-16

Table 3-1 Material Handling Depletion Parameters........................................................................... 3-3

Table 3-2 Vehicle Fugitive Dust Depletion Parameters..................................................................... 3-3

Table 3-3 Data Capture for Navajo Met 1 .......................................................................................... 3-5

Table 3-4 Data Capture for Navajo Met III ......................................................................................... 3-6

Table 3-5 Meteorological Data Used in Running AERMET ............................................................ 3-10

Table 3-6 AERSURFACE Bowen Ratio Condition Designations.................................................... 3-14

Table 4-1 1-hour SO2 Season and Hour of Day Background Concentrations (ppb)........................ 4-1

Table 4-2 1-hour NO2 Season and Hour of Day Background Concentrations (ppb) ....................... 4-2

Table 4-3 Monitored Background Concentrations for CO, PM10 and PM2.5, Lead, 3-hour SO2 and Annual 

NO2 ..................................................................................................................................... 4-5

Table 5-1 NAAQS Modeling Results.................................................................................................. 5-1
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List of Figures

Figure 2-1 FCPP and Navajo Mine Location ...................................................................................... 2-2

Figure 2-2 FCPP/Navajo Mine North Source Locations................................................................... 2-17

Figure 2-3 Navajo Mine South Source Locations ............................................................................. 2-18

Figure 3-1 Land Use Sectors for Navajo Met 1................................................................................... 3-8

Figure 3-2 Land Use Sectors for Navajo Met 3................................................................................... 3-9

Figure 3-3 Wind Rose for Navajo Met 1 Station ............................................................................... 3-11

Figure 3-4 Wind Rose for Navajo Met 3 Station ............................................................................... 3-12

Figure 3-5 Buildings and Stacks Included in GEP Analysis ............................................................. 3-15

Figure 3-6 Near-Field Receptor Locations ........................................................................................ 3-16

Figure 3-7 Far-Field Receptor Locations........................................................................................... 3-17

Figure 4-1 Ambient Background Monitor Locations............................................................................ 4-4

Figure 5-1 Maximum Modeled Impact Locations (North) ................................................................... 5-2

Figure 5-2 Maximum Modeled Impact Locations (South)................................................................... 5-3
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List of Acronyms

AERMAP AERMOD Terrain Preprocessor

AERMET AERMOD Meteorological Preprocessor

AERMOD AMS/EPS Regulatory Model

AERSURFACE AERMET Surface Characteristics Preprocessor

AIG AERMOD Implementation Guide

APS Arizona Public Service

ASOS Automated System Observing Station

BART Best Available Retrofit Technology

BIA Bureau of Indian Affairs

BNCC BHP Navajo Coal Company

BPIP Building Profile Input Program

BPIPPRM Building Profile Input Program with PRIME algorithm

CASAC Clean Air Scientific Advisory Committee

CO Carbon Monoxide

EIS Environmental Impact Statement

EPA Environmental Protection Agency (also US EPA)

FCPP Four Corners Power Plant

GEP Good Engineering Practice

GAQM Guideline on Air Quality Models

H2SO4 Sulfuric Acid Mist

NAD North American Datum

NAAQS National Ambient Air Quality Standards

NCDC National Climatic Data Center

NED National Elevation Dataset
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NEPA National Environmental Policy Act

NMED New Mexico Environment Department

NO2 Nitrogen Dioxide

NWS National Weather Service

OAQPS Office of Air Quality Planning and Standards

PM2.5 Particulate Matter less than 2.5 microns

PM10 Particulate Matter less than 10 microns

SJGS San Juan Generating Station

SRMG Salt River Materials Group

SCR Selective Catalytic Reduction

SO2 Sulfur Dioxide

USGS United States Geological Survey

UTM Universal Transverse Mercator
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1.0 Introduction

1.1 Project Overview

The Arizona Public Service Company (APS) operates the Four Corners Power Plant (FCPP), a 

privately owned and operated coal-fired power plant located on the Navajo Nation, about 25 miles 

west of Farmington, New Mexico. The facility currently consists of five boiler electric generating units 

(Units 1-5), with a total capacity of approximately 2,060 megawatts. This modeling demonstration 

does not include Units 1, 2, and 3 which will be permanently retired prior to the FCPP lease expiration. 

Units 4 and 5 with a total capacity of 1,500 MW will continue to operate after 2016.

APS contracted AECOM to provide support in preparation for the environmental impact statement 

(EIS) to address the renewal of the lease and rights-of-way for the FCPP and associated transmission 

lines for the 25-year period of 2016 through 2041. The current FCPP lease expires in 2016. During 

the National Environmental Policy Act (NEPA) process, the following APS actions will be evaluated in 

the EIS: 

• Lease Approval: The Bureau of Indian Affairs (BIA) would evaluate whether to 

approve a lease extension to 2041 allowing for continued operation of the FCPP 

and the associated transmission lines across the Navajo Nation.

• Grant Issuance: The BIA would evaluate whether to approve the issuance of 

federal grants of rights-of-way, pursuant to 25 U.S.C. § 323, for the plant, 

associated transmission lines, and ancillary facilities. 

Concurrent with the proposed FCPP lease amendment approval and renewed rights-of-way grant 

actions, BHP Navajo Coal Company (BNCC) proposes to develop a new permit area, called the 

Pinabete Permit, which is located adjacent to the FCPP on tribal trust lands on the Navajo 

Reservation. The new Pinabete Permit area would, in conjunction with the mining of any reserves 

remaining within the existing Navajo Mine Permit area (Federal SMCRA Permit NM0003F), supply 

low-sulfur coal to the FCPP at a rate of approximately 5.8 million tons per year. Development of the 

Pinabete Permit area and associated coal reserves would use surface mining methods and, based on 

current projected customer needs, would supply coal to FCPP for up to 25 years beginning in 2016. 

The proposed Pinabete Permit area would include previously permitted but undeveloped coal 

reserves within Area IV North of the Navajo Mine Lease, and unpermitted and undeveloped coal 

reserves in a portion of Area IV South of the existing Navajo Mine Lease. Full implementation of the 

proposed Pinabete Permit is expected to require specific federal actions that will be addressed in the 

EIS, including: 

• Approval from OSM of the new SMCRA permit. 

• Approval from the BLM of a revised Mine Plan developed for the proposed 

maximum economic recovery of coal reserves. 

• Approval of a Section 404 Individual Permit from the USACE for the impacts to 

waters of the United States from proposed mining activities. 
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• Approval of a Section 402 National Pollutant Discharge Elimination System 

(NPDES) permit or permit revision from the EPA. 

• Approval by the BIA of a proposed realignment for approximately 2.8 miles of BIA 

3005/Navajo Road N-5082 (Burnham Road) in Area IV South to avoid proposed 

mining areas. 

• Approval or grant of other associated facilities, permits, or rights-of-way for access 

and haul roads, power supply for operations, and related facilities. 

One technical study required as part of the NEPA process involves determining whether the criteria 

pollutant emissions from the Four Corners Power Plant (FCPP) and Navajo Mine meet the National 

Ambient Air Quality Standards (NAAQS). This analysis is particularly important because the NAAQS 

for SO2 and NO2 have recently been strengthened. In addition to SO2 and NO2, whether projected 

emissions meet the NAAQS has been assessed for PM2.5, PM10, CO, and lead. Determination of 

ozone NAAQS will be addressed separately using photochemical modeling and submitted in a future 

report. 

1.2 Purpose of Modeling Study

The purpose of this document is to present the modeling procedures and results for the NAAQS 

modeling that will support the NEPA process and EIS being conducted for the FCPP lease approval. 

Emissions calculations, modeling methods and assumptions, including model selection and options, 

meteorological data and source parameters used in the modeling analysis are presented in this 

document.

1.3 Contents of the Modeling Study Report

This report consists of six sections and five appendices. Section 1 provides an introductory 

presentation. Section 2 contains a project description, including information regarding the FCPP and 

Navajo Mine equipment, locations and modeled air pollutant emissions. Sections 3 and 4 present a 

detailed description of the modeling procedures used in evaluating NAAQS for the FCPP including 

model selection criteria, good engineering practice stack height determination, refined modeling 

analyses, and ambient air quality. Section 5 presents the modeling results submitted as part of the 

EIS. References are provided in Section 6. 

Appendix A contains emissions calculations for the equipment at FCPP and associated mining 

activities. Appendix B cites text from the March 1, 2011 Clarification Memo, Appendix C contains 

the Alternative Model Justification for Low Wind Speed Beta Options: AERMET and AERMOD. 

Appendix D shows the comparison between the output of the older version of AERSURFACE to the 

output of the recently released version of AERSURFACE. Appendix E contains the 30-years of 

monthly precipitation used to determine if each month is wet, dry, or average for Bowen ratio 

calculations. Appendix F describes a significant concentration gradient for a background source 

considered for modeling. Appendix G contains the modeling files and archive for the NAAQS 

analysis. 
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2.0 Description of Power Plant and Mine Sources

2.1 Source Location

The FCPP and Navajo Mine are located 5 miles south of State Route 64, approximately 13 miles 

southeast of Shiprock, New Mexico and 15 miles west-southwest of Farmington, New Mexico. A 

topographical map of the site region is shown in Figure 2-1. 

2.2 Emission Sources

The criteria pollutant air emissions associated with the FCPP and Navajo Mine primarily occur due to 

the combustion of coal in the main coal-fired boilers, Units 4 and 5, and the material handling 

operations of coal, lime, and ash. Smaller amounts of NAAQS pollutant air emissions are emitted 

from the ancillary equipment such as the recently permitted gas-fired auxiliary boiler and vehicle 

exhaust. As the auxiliary boiler will primarily run when Units 4 and 5 are off-line, it is not included in 

the analysis as the emissions from the auxiliary boiler are significantly less than Units 4 and/or 5.

2.2.1 Point Sources at FCPP

2.2.1.1 Units 4 and 5

Units 4 and 5 are cell burner, pulverized coal-fired boilers equipped with lime spray towers and 

baghouses for SO2 and PM control. Selective Catalytic Reduction (SCR) will be installed as part of 

the Best Available Retrofit Technology (BART) agreement. The exhaust gases from Units 4 and 5 are 

ducted into one stack containing two flues. Table 2-1 summarizes the current exhaust stack 

parameters for Units 4 and 5 that will be used for the criteria pollutant NAAQS assessment. The 

temperature and exhaust velocity are based on actual hourly data from 2009 through 2011.

Table 2-1 Unit 4 and 5 Stack Parameters1

Units Units 4-5 

Merged Stack

UTM-X, Zone 12, NAD83 Meters 725349.5

UTM-Y, Zone 12, NAD83 Meters 4063098.5

Stack Height Meters 115.824

Base Elevation Meters 1631.29

Effective Diameter Meters 12.28

Gas Exit Velocity m/s 20.60

Stack Gas Exit Temperature deg K 327.00

 

1 The exhaust parameters coincide with a 99th percentile heat input rate of 7,411 MMBtu/hr for each of Units 4 and 5.
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Figure 2-1 FCPP and Navajo Mine Location
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Emission rates from the Units 4 and 5 stack are derived from consideration of actual hourly data, the 

current Title V operating permit2 for FCPP, as well as from the final BART rule3 for NOx and particulate 

matter. Table 2-2 provides a list of the emission rates. Per EPA guidance4 issued on February 7, 

2013, AECOM modeled three years of actual 1-hour SO2 emissions from Units 4 and 5 with 

concurrent exhaust temperature and velocity. Since the BART emission rate for NOX is based upon a 

30-day rolling average (which is appropriate for visibility impacts), we provide both a 30-day and 1-

hour average emission rate for NOX. In addition, the PM10 and PM2.5 emission rates used in the 

modeling include condensable emissions consistent with increased sulfuric acid mist (H2SO4) 

emissions from future planned SCR operation.

2.2.1.2 Coal, Lime, and Ash Handling Point Sources at FCPP

FCPP maintains two lime storage silos which process dry lime into a slurry used in the scrubbers for 

Units 4 and 5. Each lime silo is equipped with a baghouse for particulate control, and two lime dust 

scrubbers for particulate control on each silo. Currently, the lime dust scrubbers exhaust horizontally 

but will be modified to discharge vertically. These scrubbers operate twelve hours per day, from 7am 

to 7pm. Emissions are estimated using the outlet grain loading (grains/acf) of the baghouses and the 

volumetric displacement of the air from the lime loaded into the silo. 

The coal processed and stored by the Navajo Mine is transported on a series of conveyors to FCPP. 

The North and South Surge Bins hold coal coming from the processing area. The South Surge Bin 

currently services Units 1, 2, and 3, but will be repurposed to feed Units 4 and 5 with the North Surge 

Bin. The North Surge Bin baghouse and South Surge Bin rotoclone control particulate from these 

sources. From these bins, the coal is transported via open conveyor belts to a tripper deck, where it is 

separated and transported via open conveyor to, one of nine coal silos. Transfer of coal from the coal 

silos to the pulverizers is in closed piping. A surfactant is applied at the bottom of the surge bins to 

mitigate fugitive dust on the open conveyor leading to the tripper deck. Additionally, a project is 

currently underway to install an additional surfactant dust suppression system at the tripper deck to 

further reduce fugitive dust. The coal is conveyed from the tripper deck to nine storage bunkers with 

two rotoclones installed for particulate control. Emissions from the coal handling sources are 

estimated using the AP-425 “drop equation” from Section 13.2.4 and applying appropriate control 

factors as mitigation.

The fly ash waste disposal area has five silos, all equipped with baghouses. Two silos are for ash 

storage, one for fly ash sales/export, and two for on-site fly ash disposal. In addition to the silos, there 

are three pug mills with scrubbers for processing fly ash. These are not currently operated but are 

including in the dispersion modeling should they be used in the future. These sources would operate 

twelve hours per day, 7AM to 7PM. The locations and release parameters are listed in Table 2-3. 

The emissions calculations for all sources are presented in Appendix A. Baghouse and scrubber 

 

2 NNEPA 2008. Arizona Public Service Four Corners Power Plant Title V Operating Permit (August 1, 2008). 

http://www.navajonationepa.org/airqty/Pdf_files/FCPPpermit8-1-2008.pdf. 

3 US EPA 2012a. Source Specific Federal Implementation Plan for Implementing Best Available Retrofit Technology for Four 

Corners Power Plant. 77FR 51620, August 24, 2012.

4 US EPA 2013a. http://www.epa.gov/airquality/sulfurdioxide/pdfs/20130207SO2StrategyPaper.pdf, (pages 2-3).

5 US EPA 1995. AP-42, Compilation of Air Pollutant Emission Factors, Fifth Edition. (January 1995).
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emissions are estimating using the outlet grain loading for each control device and the airflow through 

each piece of equipment.

2.2.2 Coal, Ash, and Lime Handling Volume Sources at FCPP

As part of the lime slurry mixing process, lime grit that settles out is dropped into one of two bins and 

then dropped onto one of two piles. These drops are modeled as volume sources. Open transfer 

points as part of the coal transport from the surge bins to the nine silos are also modeled as volume 

sources. Saleable fly ash truck loading and on-site disposal fly ash truck loading are drops into trucks 

for transport at the bottom of each of these silos. Fly ash truck loading occurs from 7 AM to 7 PM. 

The locations and source parameters are summarized in Table 2-4. The emissions calculations for all 

sources are presented in Appendix A. Lime pile drops, coal transfer point, and ash load out

emissions are estimated using the AP-42 “drop equation” from Section 13.2.4.

2.2.3 Vehicular Traffic at FCPP

Paved and unpaved road sources were developed to represent vehicular traffic at FCPP. The 

roads are represented by lines of volume sources, each having the following characteristics, 

calculated using the haul road configuration tool based on the Haul Road Workgroup Final Report 

Submission to EPA-OAQPS (March 2012)6 in the Lakes Environmental AERMOD View software:

• The sources are assumed to be adjacent, elevated sources.

• Vehicle height is a weighted average based on vehicles traveling on each road segment. 

• Plume height, equal to vehicle height x 1.7 (top of plume height per haul road report).

• Release height = Plume height / 2.

• Initial Sigma-z = Plume height / 2.15.

• Lane type = Most roads are characterized as two lanes. 

• Plume width = Road width + 6m.

• Initial Sigma-Y = Plume width / 2.15. 

AECOM accounted for fly ash haul trucks, saleable fly ash haul trucks, lime delivery trucks, SCR 

catalyst deliver trucks, watering trucks, and company and employee vehicles. The road segments are 

listed in Table 2-5 and the calculations are provided in Appendix A. Vehicular traffic during overnight 

hours is minimal and this was accounted for in the modeling as fly ash disposal and deliveries occur 

during daylight hours. Roads are watered between 4am and 8pm, daily, and during high wind periods 

are watered during overnight hours as well. Emissions are calculated using AP-42 Sections 13.2.1 for 

paved roads and 13.2.2 for unpaved roads.

2.2.4 Lime and Ash Piles at FCPP

Area sources at FCPP consist of stockpiles located at the lime processing area and the ash disposal 

area. The stockpile base areas were derived from the reported annual average surface area of each 

 

6 US EPA 2012b. Haul Road Workgroup Final Report. (March 2, 2012) 

http://www.epa.gov/ttn/scram/reports/Haul_Road_Workgroup-Final_Report_Package-20120302.pdf. 
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pile, and the actual area modeled was a best-fit polygon or circle over the plot plan stockpile 

“footprint.” The average heights of the stockpiles were from estimates provided by plant personnel.

The fly ash has a high moisture content when transported and unloaded by the haul trucks. Over 

time, it dries into a cement-like solid. Surfactant is applied regularly to reduce the amount of fugitive 

dust that can become airborne during high-wind events. In general, wind erosion from stock piles is 

highly intermittent due to the high threshold wind speeds involved. Consideration of triggering wind 

events can be accomplished simply by inspection of the available meteorological data, since the onset 

of windblown dust is based upon threshold friction velocity guidance as listed in AP-42 Section 13.2.5. 

For the potential surface pile emission sources, the AP-42 threshold friction velocity values will be 

used to determine appropriate fastest mile velocities from Farmington Airport for an anemometer7

height of 10 meters. The threshold velocity for the ash piles is 19.25 m/s. To be conservative, 

AECOM modeled this source such that the piles generated emissions when the wind speed is greater 

than 12.49 m/s. The lime grit piles are much smaller in scale and are subject to wind erosion at all 

wind speeds. Emissions from the lime grit piles are estimated using the Mojave Desert Air Quality 

District (MDAQMD) equation for wind erosion from stockpiles from their emissions inventory guidance 

for mineral handling and processing industries.

Fly ash from Units 4 and 5 is transported and unloaded in the Dry Fly Ash Disposal Area (DFADA) 

and compacted using bulldozers. Unloading and bulldozing activities occur 8 hours per day. Another 

byproduct of combustion, cenospheres, are transported and unloaded in a different area. This

material accumulates until it is sold and transported off-site. Unloading and bulldozing activities occur 

less frequently at the cenosphere pile.

The older ash disposal ponds are inspected and maintained on a regular basis to determine if 

additional surfactant needs to be applied to reduce fugitive dust emissions. Source parameters and 

emission rates for each pile are listed in Table 2-6. The fastest mile emission calculations are in 

Appendix A.

2.2.5 Navajo Mine North Sources

The Navajo Mine coal preparation plant in the northern portion of the mine lease, is adjacent to FCPP 

on the east side. A train transports coal from the mine to the processing area where the train cars are 

unloaded into one of two hoppers. Water sprays located inside each receiving hopper are activated 

when coal from a railcar is being unloaded into the hopper. This activity, however, is not 

representative of a traditional "transfer point" because air inside the hopper is displaced by the 

incoming coal, resulting in some coal particles being entrained in that air as it exits the top of the 

hopper. Due to this, particulate emissions from the hoppers have been estimated by first applying the 

standard drop equation from AP-42, § 13.2.4.3 and then multiplying the resultant emission rate by an 

“activity factor” of 10, i.e., a subjective, but conservative, approximation of the amount of hopper 

emissions compared to the amount from a “standard drop.” In addition, it is conservatively assumed 

that the water sprays inside the hoppers provide no more than 50% particulate removal. 

There is a substantial drop with the transfer of coal from the hopper/feeder to the inclined reclaim 

conveyor (transfer points TP1 and TP7). This height is greater than the typical “drop” height for 

conveyor-to-conveyor transfers. Consequently, uncontrolled particulate emissions from this atypical 

transfer point were again first estimated by applying the standard drop equation from AP-42, § 

 

7 Since fastest mile information is not available from the on-site meteorological stations, it is obtained from Farmington airport for 

the purpose of determining the frequency of windblown dust events.
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13.2.4.3 and then multiplying the resultant emission rate by an “activity factor” of 10, a subjective, but 

conservative, approximation of the amount of emissions from this atypical drop compared to the 

amount from a “standard drop.” 

There are also two particulate controls (enclosure and water spray) at the transfer from the 

hopper/feeder to the inclined reclaim conveyor. The Texas Natural Resource Conservation 

Commission’s CHEER Manual estimates that control efficiencies for water sprays on transfer points 

range from 30% to 70%. Because this drop height is atypical, Navajo Mine conservatively used 30% 

as the control efficiency. This atypical drop is fully enclosed in an underground vault. The CHEER 

Manual (page 34) identifies 90% as the estimated control efficiency for fugitive particulate matter 

emissions from enclosed transfer points. Combining the efficiencies of the two controls gives a factor 

of 10 * (1-0.3) * (1-0.9) = 0.7. This factor has been applied as the overall control factor for TP1 and 

TP7.

The two inclined reclaim conveyors transfer raw coal to the crusher towers where two fully enclosed 

secondary crushers reduce the size of the coal to meet product specifications. The CHEER Manual 

identifies 90% as the estimated control efficiency for fugitive particulate matter emissions from fully 

enclosed crushing equipment. In addition to the enclosure, a foam-binder mixture is applied to the 

coal at the crusher. The CHEER Manual identifies 98% as the estimated control for an enclosed 

crusher with foam addition. Therefore, for crusher emission estimates a control value of 98% has 

been used for emission points TP2/CR1 (CRUSHER1) and TP8/CR2 (CRUSHER2).

Through a system of transfer towers, conveyors and flop gates, crushed coal is stacked onto one of 

ten open storage piles. The binder which had been added at the crusher provides residual dust 

suppression in the stockpiles. The CHEER Manual suggests an estimated control efficiency of 30% to 

50% for stacking operations with “wet” material. A value in the middle of this range, 40%, is selected. 

When the coal is ready to be conveyed to FCPP, it is reclaimed from the appropriate storage piles by 

two bucketwheel reclaimers which drop the product coal onto collection conveyors that in turn feed 

two parallel conveyor susyems that transport the coal to the north and south surge bins described in 

Section 2.2.1.2. With the planned shutdown of Units 1, 2, and 3, the future throughput for the Navajo 

Mine will be 6 million tons per year of coal, down from the current 8.2 million tons per year of coal.

As described in Section 2.2.4, particulate emissions due to wind erosion of open stockpiles is highly 

intermittent due to the high threshold wind speeds involved. Consideration of triggering wind events 

can be accomplished simply by inspection of the available meteorological data, since the onset of 

windblown dust is based upon threshold friction velocity guidance as listed in AP-42 Section 13.2.5. 

The threshold velocity for the coal piles has been calculated to be 21.15 m/s. To be conservative, 

AECOM modeled this source such that the piles generated emissions when the wind speed is greater 

than 12.49 m/s. Emissions and source parameters are listed in Table 2-7.

2.2.6 Navajo Mine South Sources

Sources in the southern portion of the Navajo Mine will include the mining in the Pinabete permit

area, transport of coal from the Pinabete permit area to the coal stockpile, unloading at the pile and 

loading onto the train, blasting, and overburden replacement. It is assumed that active mining will 

occur in two locations within the permit area at any time, labeled as Pinabete East and Pinabete 

North. Emission factors, control efficiencies, and equipment were supplied by the Navajo Mine.
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2.2.6.1 Proposed Active Mining Sources

Proposed mining in the Pinabete East and Pinabete North areas are characterized as open pit 

sources consistent with previous modeling for the Navajo Mine performed by Class One Technical 

Services in July 20118

. These sources include mining operations such as: drilling, dragline activity, 

bulldozing, loading coal into trucks with front end loaders, scraping, road grading and blasting for PM10

and PM2.5. Coal and overburden blasting also emits CO, NOx, and SO2. Emission factors from AP-42 

Section 13.3 (1995) were used to calculate emissions, with the exception of SO2. The Navajo Mine 

uses ultra-low sulfur diesel fuel in its ammonium nitrate/fuel oil (ANFO); therefore, the SO2 emissions 

are significantly less than that cited in the 1995 emission factors. Coal blasting can occur between 11 

AM and 3 PM, Monday through Friday, and typically occurs three times per week. Although all 

blasting emissions would occur within one hour each day and three times per week, the full blast 

emission rate was conservatively modeled in each hour for this four hour period each weekday (20 

hours per week) to simplify the modeling. Each blast is in a different location in the mine and 

meteorological conditions will vary, therefore the modeled concentrations are conservative. Overburden blasting occurs once per month over a larger area and does not occur in the same location 

twice. Due to the infrequent and unscheduled nature of overburden blasting, it was not included in the 

modeling based upon guidance for intermittent sources as noted in EPA’s March 1, 2011 memo5 cited 

above. The applicable text in the memo is provided in Appendix B. 

In the case of overburden blasting at the mine, this event occurs only 1 hour per month, and not in the 

same location ever. Given that there is only one emission event per year from any given location, this 

intermittent event cannot contribute significantly to affect whether the NAAQS is met for any standard, 

even the annual distribution of daily maximum 1-hour concentrations for either the NO2 or SO2

NAAQS. This is true not only due to the non-stationary aspect of the source, but also because the 

1-hour NAAQS is a statistical standard that would involve a number of different days of emissions as 

well as the fact that the wind would not be expected blow in the same direction each emission event.

2.2.6.2 Vehicular Traffic

Traffic from coal haul trucks, watering trucks, scrapers, end dump trucks, and pick-up trucks on the 

roads between the active mining areas and the Lowe coal stock pile are characterized in the 

modeling. The roads are represented in the model as discussed in Section 2.2.3. Traffic between the 

mining areas and the stockpile can occur 24 hours per day. Watering trucks apply water to the 

unpaved roads to control fugitive dust. A Chemical suppressant is also applied to major unpaved 

roads annually. 

2.2.6.3 Stockpiles and Wind Erosion 

Coal from the active mining areas is transported and unloaded onto the Lowe stockpile and 

compacted using bulldozers. Unloading and bulldozing activities occur 24 hours per day. The coal is 

loaded from the pile onto a train for transport to the Navajo Mine Coal Preparation Plant. As 

described in Section 2.2.4, wind erosion from stock piles is highly intermittent due to the high 

threshold wind speeds involved. Consideration of triggering wind events can be accomplished simply 

by inspection of the available meteorological data, since the onset of windblown dust is based upon 

threshold friction velocity guidance as listed in AP-42 Section 13.2.5. Using the AP-42 approach, the 

threshold velocity for wind erosion of the field stockpiles is estimated to be 21.15 m/s. To be 

 

8 Class One Technical Services 2011. BNCC Navajo Mine Area 3 and 4 Modeling, Four Corners Area, NM. Environmental 

Assessment Dispersion Modeling. (July 2011).
AECOM Environment

Four Corners Power Plant and Navajo Mine EIS / NAAQS Modeling Study April 2013

2-8

conservative, AECOM modeled this source such that the piles generated emissions when the wind 

speed is greater than 12.49 m/s.

Wind erosion from over-burden reclaim piles is also calculated using AP-42 Section 13.2.5. The 

threshold velocity for the over-burden and reclaim piles is 19.25 m/s. To be conservative, AECOM 

modeled this source such that the piles generated emissions when the wind speed is greater than 

12.49 m/s.

Figures 2-2 and 2-3 show the configurations of the FCPP/Navajo Mine North and Navajo Mine South 

sources, respectively.
AECOM Environment

Four Corners Power Plant and Navajo Mine EIS / NAAQS Modeling Study April 2013

2-9

Table 2-2 Emission Rates for Units 4 and 5

Criteria Pollutant

Factor

Factor Reference Notes

Unit 4 Unit 5 Combined

lb/

mmBTU

mmBTU/

hr lbs/hr

mmBTU/

hr lbs/hr lbs/hr g/sec

Sulfur Dioxide (SO2)

1 0.190 1-hour Average Emission Rate; actual 

hourly emissions used in the modeling 7,411 1,408 7,411 1,408 2,816 354.83

Nitrogen Oxides (NOX) 0.098 40 CFR 49 BART Rule, 30-day Rolling 

Average 7,411 726 7,411 726 1,453 183.02

Nitrogen Oxides (NOX) 0.190 1-hour average 7,411 1,408 7,411 1,408 2,816 354.83

Carbon Monoxide (CO) 0.028 AP-42 Table 1.1-3; 17.632 mmBTU/ton 7,411 208 7,411 208 415 52.29

Lead (Pb) 1.2E-06 40 CFR 63 Subpart UUUUU Table 2 7,411 0.01 7,411 0.01 0.02 0.0022

Filterable Particulate (PM)2 0.01500 40 CFR 49 BART Rule 7,411 111.17 7,411 111.17 222 28.01

Total Filterable PM10 0.01380 AP-42 Table 1.1-6; 92% of filterable PM 7,411 102.27 7,411 102.27 205 25.77

"Coarse" Filterable PM10 0.00585 difference (total filterable - fine filterable) 7,411 43.35 7,411 43.35 87 10.93

Fine Filterable PM2.5 0.00795 AP-42 Table 1.1-6; 53% of filterable PM 7,411 58.92 7,411 58.92 118 14.85

Fine "Soil" PM2.5 0.00766 difference (fine filterable - fine elemental 

carbon) 7,411 56.74 7,411 56.74 113 14.30

Fine Elemental Carbon PM2.5 0.00029 EPA 68-D-98-046 Table 6; 3.7% of PM2.5 7,411 2.18 7,411 2.18 4 0.55

Total Condensable PM10 / PM2.5 0.00835 sum (sulfuric acid + organics) 7,411 61.91 7,411 61.91 124 15.60

Condensable Sulfuric Acid 

(H2SO4) 0.00435 Stack Test and EPRI Removal (%) 7,411 32.26 7,411 32.26 65 8.13

Condensable Organics 0.00400 AP-42 Table 1.1-5; 20% of 0.02 lb/mmBTU 7,411 29.64 7,411 29.64 59 7.47

Grand Total PM10 0.02215 Total Filterable + Total Condensable 7,411 164.18 7,411 164.18 328 41.373

Grand Total PM2.5 0.01630 Fine Filterable + Total Condensable 7,411 120.83 7,411 120.83 242 30.448 1

 1-hour SO2 NAAQS determined by modeling actual hourly emission rate and stack parameters over 3 years. 2

 PM modeling results are dominated by near-surface releases, which are composed primarily of direct PM emissions and have minimal precursor PM emissions. 

Secondary PM emissions from the FCPP stack are already accounted for in the conservative background monitoring concentration. Note that with future emission 

reductions at FCPP and area-wide sources, the secondary PM emission component that is part of the background concentration will be reduced, so the current 

background overestimates this future component of PM concentrations.
AECOM Environment

Four Corners Power Plant and Navajo Mine EIS / NAAQS Modeling Study April 2013

2-10

Table 2-3 Emission Rates and Source Parameters for Lime, Coal, and Ash Handling Point Sources

ID Orientation

UTM E 

(NAD83)

UTM N 

(NAD83)

Stack 

Height 

(m)

Stack 

Diameter 

(m)

Velocity 

(m/s)

Temperature 

(K)

PM10 ST 

(g/s)

PM2.5 ST 

(g/s)

PM2.5 Ann 

(g/s) Comments

EU20_8L Down 

Facing 725274.3 4062992.5 21.641 0.628 0.001 Amb. 0.0222 0.0222 0.0203

EU21_11L Down 

Facing 725246.3 4062968.4 21.641 0.628 0.001 Amb. 0.0222 0.0222 0.0203

EU22_6L Will be 

vertical 725268.4 4063001.3 12.192 0.330 18.11 366.483 0.2371 0.2371 0.2371 12 hours/day

EU22_7L Will be 

vertical 725280.5 4062983.9 12.192 0.330 18.11 366.483 0.2371 0.2371 0.2371 12 hours/day

EU24_9L Will be 

vertical 725240.6 4062977.4 12.192 0.330 18.11 366.483 0.2371 0.2371 0.2371 12 hours/day

EU25_10L Will be 

vertical 725251.9 4062959.2 12.192 0.330 18.11 366.483 0.2371 0.2371 0.2371 12 hours/day

SRMGBH Horizontal 725177.3 4063271.3 22.860 0.330 0.001 16.7 + 

Ambient 0.1893 0.1893 0.1893

EU29_15A Horizontal 725159.9 4063263.6 32.918 0.330 0.001 16.7 + 

Ambient 0.1893 0.1893 0.1798

EU28_14A Horizontal 725155.2 4063253.8 32.918 0.330 0.001 16.7 + 

Ambient 0.1893 0.1893 0.1798

EU27_13A Horizontal 725152.5 4063232.0 25.298 0.330 0.001 16.7 + 

Ambient 0.1893 0.1893 0.1798

EU26_12A Horizontal 725153.0 4063233.1 25.298 0.330 0.001 16.7 + 

Ambient 0.1893 0.1893 0.1798

EU30_16A Horizontal 725165.2 4063229.2 11.278 1.219 0.001 Amb. 0.1058 0.1058 0.1058 12 

hours/day

EU31_17A Horizontal 725168.1 4063221.5 11.278 1.219 0.001 Amb. 0.1058 0.1058 0.1058 12 

hours/day

EU32_18A Horizontal 725172.9 4063231.4 11.278 1.219 0.001 Amb. 0.1058 0.1058 0.1058 12 

hours/day

NSURGEBH Horizontal 725021.6 4063493.4 5.000 0.300 0.001 Amb. 0.0081 0.0081 0.0081

SSRGERTC Horizontal 725038.4 4063473.4 19.668 0.200 0.001 Amb. 8.629E-05 1.307E-05 1.307E-05

CROTO1 Horizontal 725005.0 4063308.6 19.290 0.200 0.001 Amb. 5.752E-05 8.711E-06 8.711E-06

CROTO2 Horizontal 725034.0 4063275.0 19.290 0.200 0.001 Amb. 5.752E-05 8.711E-06 8.711E-06
AECOM Environment

Four Corners Power Plant and Navajo Mine EIS / NAAQS Modeling Study April 2013

2-11

Table 2-4 Emission Rates and Source Parameters for Lime, Coal, and Ash Handling Volume Sources

Volume Source Name Source ID

UTM E 

(NAD83)

UTM N 

(NAD83)

Release 

Height 

(m)

Sigma-y 

(m)

Sigma-z 

(m)

PM10 ST 

(g/s)

PM2.5 ST 

(g/s)

PM2.5 Ann 

(g/s) Controls

Drop into Lime Grit Bin GRTDRP1A 725276.24 4062930.5 2.438 0.425 0.709 1.04E-03 1.57E-04 1.57E-04

Drop into Lime Grit Bin GRTDRP2A 725276.24 4062930.5 1.219 0.425 0.425 1.04E-03 1.57E-04 1.57E-04

Drop onto Lime Grit Piles GRTDRP1B 725311.10 4062960.1 2.438 0.425 0.709 1.04E-03 1.57E-04 1.57E-04

Drop onto Lime Grit Piles GRTDRP2B 725311.10 4062960.1 1.219 0.425 0.425 1.04E-03 1.57E-04 1.57E-04

North Surge Bin onto 

Conveyor 1 TP1A 725032.76 4063489.3 2.000 0.465 5.671 5.75E-04 8.71E-05 8.71E-05 Surfactant

South Surge Bin onto 

Conveyor 1 TP1B 725042.52 4063476.7 2.000 0.465 5.671 5.75E-04 8.71E-05 8.71E-05

Transfer from Conveyor 

1 to 2 TP2 725081.64 4063426.8 2.000 1.151 0.930 5.75E-04 8.71E-05 8.71E-05

Conveyor 2 to Tripper 

Deck TP3 724973.63 4063342.6 17.000 1.509 0.930 5.75E-04 8.71E-05 8.71E-05 Surfactant

Tripper Deck to Enclosed 

System (9 Silos) TP4 724979.50 4063336.4 18.000 1.221 0.930 5.75E-04 8.71E-05 8.71E-05

Fly Ash Truck Loading 1 FAT_1 725150.56 4063227.9 3.000 0.472 1.395 5.55E-03 8.40E-04 8.40E-04

Fly Ash Truck Loading 2 FAT_2 725154.75 4063237.2 3.000 0.472 1.395 5.55E-03 8.40E-04 8.40E-04

Saleable Fly Ash Truck 

Loading SFAT 725180.77 4063270.3 3.000 0.472 1.395 3.23E-03 4.89E-04 4.89E-04
AECOM Environment

Four Corners Power Plant and Navajo Mine EIS / NAAQS Modeling Study April 2013

2-12

Table 2-5 Emission Rates and Source Parameters for Vehicular Traffic

Road Location Road ID

# 

Volumes

24-hour PM10 g/s / 

volume

24-hour PM2.5 g/s / 

volume

Annual PM2.5 g/s / 

volume

Release Height 

(m)

Sigma-y 

(m)

Sigma-z 

(m)

Plant Area 1 to 2 3 3.73E-03 9.16E-04 7.29E-04 4.25 7.91 1.98

Plant Area 2 to 3 24 4.60E-03 1.13E-03 8.82E-04 4.25 7.91 1.98

Plant Area 3 Loop 26 2.64E-03 6.47E-04 5.15E-04 4.25 5.58 1.98

Plant Area 3 to 4 5 8.05E-04 1.98E-04 1.67E-04 4.25 5.58 1.98

Plant Area 1 to 5 7 9.81E-04 2.41E-04 2.06E-04 4.25 6.98 1.98

Plant Area 5 to 2 8 1.07E-03 2.64E-04 2.13E-04 4.25 6.98 1.98

Plant Area 5 to 6 15 3.98E-04 9.77E-05 9.04E-05 4.25 6.98 1.98

Plant Area 1 to 7 41 3.68E-03 9.02E-04 7.43E-04 4.25 7.91 1.98

Fly Ash Area 7 to 8 75 3.65E-03 8.95E-04 7.37E-04 4.25 7.91 1.98

Fly Ash Area 8 to 9 43 3.69E-03 9.05E-04 7.45E-04 4.25 7.91 1.98

Fly Ash Area 9 Loop 111 7.13E-03 7.13E-04 5.87E-04 4.25 5.58 1.98

Fly Ash Area 8 to 10 17 1.38E-03 1.38E-04 1.38E-04 2.55 7.91 1.19

Fly Ash Area 10 to 11 46 1.40E-03 1.40E-04 1.40E-04 3.40 7.91 1.58

Fly Ash Area 11 to 12 69 1.39E-03 1.39E-04 1.39E-04 3.40 7.91 1.58

Fly Ash Area 11 to 13 44 1.40E-03 1.40E-04 1.40E-04 3.40 7.91 1.58

Fly Ash Area 12 to 13 29 1.38E-03 1.38E-04 1.38E-04 3.40 7.91 1.58

Fly Ash Area 12 to 14 123 1.39E-03 1.39E-04 1.39E-04 3.40 7.91 1.58

Fly Ash Area 13 to 14 39 9.68E-04 9.68E-05 9.68E-05 3.40 7.91 1.58

Fly Ash Area 14 to 15 54 9.76E-04 9.76E-05 9.76E-05 3.40 7.91 1.58

Fly Ash Area 10 to 16 77 9.64E-04 9.64E-05 9.64E-05 3.40 7.91 1.58

Fly Ash Area 15 to 7 56 5.63E-03 5.63E-04 4.63E-04 3.40 7.91 1.58

Fly Ash Area 15 Loop 36 3.90E-03 3.90E-04 3.21E-04 4.25 5.58 1.98
AECOM Environment

Four Corners Power Plant and Navajo Mine EIS / NAAQS Modeling Study April 2013

2-13

Table 2-6 Emission Rates and Source Parameters for Lime and Ash Pile Area Sources

Area Source Name Source ID

UTM E 

(NAD83)

UTM N 

(NAD83)

Area 

(m2)

Release 

Height 

(m)

PM2.5

(ton/yr)

PM2.5

(lb/time)

PM10

(lb/hr)

PM10

(lb/time)

Annual 

PM2.5

(g/m2/s)

24-hour 

PM2.5

(g/m2/s)

24-hour 

PM10

(g/m2/s)

Lime Grit Pile 1 GRITPIL1 725276.1 4062929 4.0 1.0 2.65E-05 N/A 0.000176 N/A 1.27E-06 1.9E-07 1.9E-07

Lime Grit Pile 2 GRITPIL2 725310.8 4062959 4.0 1.0 2.65E-05 N/A 0.000176 N/A 1.27E-06 1.9E-07 1.9E-07

Ash Pond 6 ASHPOND6 722426.4 4063915 527205.4 1.0 0.126943 244.2714 N/A 1074.794 3.46E-10 1.22E-07 5.35E-07

Ash Pond 5 ASHPOND5 722932.4 4063102 90268.07 1.0 0.021735 41.82413 N/A 184.0262 3.46E-10 1.22E-07 5.35E-07

Lined Decant Water Pond LDWPNRTH 722302.8 4062912 61088.24 1.0 0.014709 28.30416 N/A 124.5383 3.46E-10 1.22E-07 5.35E-07

Lined Decant Water Pond LDWPSTH 722282.2 4062406 194640.8 1.0 0.046867 90.18338 N/A 396.8069 3.46E-10 1.22E-07 5.35E-07

Lined Ash Impoundment LAIPOND 722734.1 4062402 347518.8 1.0 0.083678 161.0168 N/A 708.4737 3.46E-10 1.22E-07 5.35E-07

Dry Fly Ash Disposal Area DFADA 722689.0 4062225 162430.6 3.0 3.376536 483.6055 N/A 2127.864 4.19E-07 1.09E-05 4.81E-05

Cenospheres CENOSPHR 723471.5 4062645 2922.47 5.0 0.017717 3.2605 N/A 4.347333 4.36E-08 1.46E-06 1.95E-06

Area Source Name Source ID

UTM E 

(NAD83)

UTM N 

(NAD83)

Area 

(m2)

Release 

Height 

(m)

PM2.5

(ton/yr)

PM2.5

(lb/day)

PM10

(lb/day)

Annual 

PM2.5

(g/m2/s)

24-hour 

PM2.5

(g/m2/s)

24-hour 

PM10

(g/m2/s)

DFADA Unloading DFADA_MH 722689 4062225 162430.6 3.0 0.036868 0.202018 13.34084 1.31E-08 1.31E-08 8.62E-07

Cenosphere Unloading CENOS_MH 723471.5 4062645 2922.47 5.0 0.000552 0.003023 0.199632 1.09E-08 1.09E-08 7.17E-07

Area Source Name Source ID

UTM E 

(NAD83)

UTM N 

(NAD83)

Area 

(m2)

Release 

Height 

(m)

PM2.5

(ton/yr)

PM2.5

(lb/hr)

PM10

(lb/hr)

Annual 

PM2.5

(g/m2/s)

24-hour 

PM2.5

(g/m2/s)

24-hour 

PM10

(g/m2/s)

DFADA Bulldozing DFADA_B 722689 4062225 162430.6 3.0 2.401746 1.649551 5.398529 4.25E-07 1.28E-06 4.19E-06

Cenosphere Bulldozing CENOSP_B 723471.5 4062645 2922.47 5.0 0.031906 0.613582 2.008087 3.14E-07 2.65E-05 8.66E-05
AECOM Environment

Four Corners Power Plant and Navajo Mine EIS / NAAQS Modeling Study April 2013

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Table 2-7 Emission Rates and Area Source Parameters for Navajo Mine North

Name Source ID

UTM E 

(NAD83)

UTM N 

(NAD83)

24-hour 

PM10

(g/s/m2)

24-hour 

PM2.5

(g/s/m2)

Annual 

PM2.5

(g/s/m2)

Release 

Height (m) X (m) Y (m)

Angle from 

North 

(degrees)

System 2 Hopper TL2TP7 725565.74 4063328.12 8.65E-05 1.31E-05 1.31E-05 0.305 3.048 29.108 50.35

System 1 Hopper TL1TP1 725502.93 4063308.44 2.27E-04 3.43E-05 3.43E-05 0.305 3.048 25.908 48.32

Stacker, Hopper, 

Reclaim PILES12 725293.99 4063302.39 5.03E-07 7.62E-08 7.62E-08 5.258 59.74 255.118 -41.135

Stacker, Hopper, 

Reclaim PILES34 725339.21 4063342.18 5.05E-07 7.65E-08 7.65E-08 5.258 59.741 255.118 -41.135

Stacker, Hopper, 

Reclaim PILES56 725384.44 4063381.72 5.06E-07 7.66E-08 7.66E-08 5.258 59.741 255.118 -41.135

Stacker, Hopper, 

Reclaim PILES78 725429.36 4063421.55 3.96E-07 6.00E-08 6.00E-08 5.258 59.741 255.118 -41.135

Stacker, Hopper, 

Reclaim PILES910 725474.89 4063461.08 5.03E-07 7.61E-08 7.61E-08 5.258 59.741 255.118 -41.135

Piles 1 and 2 Wind 

Erosion PILES12W 725293.99 4063302.39 6.58E-05 9.87E-06 2.04E-07 5.258 59.74 255.118 -41.135

Piles 3 and 4 Wind 

Erosion PILES34W 725339.21 4063342.18 6.58E-05 9.87E-06 2.04E-07 5.258 59.741 255.118 -41.135

Piles 5 and 6 Wind 

Erosion PILES56W 725384.44 4063381.72 6.58E-05 9.87E-06 2.04E-07 5.258 59.741 255.118 -41.135

Piles 7 and 8 Wind 

Erosion PILES78W 725429.36 4063421.55 6.58E-05 9.87E-06 2.04E-07 5.258 59.741 255.118 -41.135

Piles 9 and 10 Wind 

Erosion PILE910W 725474.89 4063461.08 6.58E-05 9.87E-06 2.04E-07 5.258 59.741 255.118 -41.135
AECOM Environment

Four Corners Power Plant and Navajo Mine EIS / NAAQS Modeling Study April 2013

2-15

Table 2-8 Emission Rates and Volume Source Parameters for Navajo Mine North

Name Source ID

UTM E 

(NAD83)

UTM N 

(NAD83)

24-hour 

PM10 (g/s)

24-hour 

PM2.5 (g/s)

Annual 

PM2.5 (g/s)

Release 

Height (m)

Sigma-y 

(m)

Sigma-z 

(m)

System 1 Crusher CRUSHER1 725465.23 4063369.75 3.35E-03 2.97E-03 2.97E-03 5.334 2.147 2.481

System 2 Crusher CRUSHER2 725514.31 4063405.82 1.44E-03 1.27E-03 1.27E-03 4.724 2.147 2.197

Sample Tower SAMPTWR 725371.61 4063286.30 3.20E-03 4.84E-04 4.84E-04 4.877 1.665 2.268

Tower 1 TOWER1 725358.33 4063274.71 2.56E-03 3.87E-04 3.87E-04 3.658 1.665 1.701

Tower 1A TOWER1A 725351.83 4063284.89 3.20E-03 4.84E-04 4.84E-04 1.829 1.665 0.851

Tower 2 TOWER2 725394.69 4063320.58 2.56E-03 3.87E-04 3.87E-04 2.896 1.389 1.347

Tower 3 TOWER3 725440.27 4063360.33 1.92E-03 2.90E-04 2.90E-04 2.134 1.205 0.992

Tower 4 TOWER4 725485.86 4063399.14 1.27E-03 1.93E-04 1.93E-04 2.896 1.205 1.347

Tower 5 TOWER5 725531.73 4063438.13 6.39E-04 9.70E-05 9.70E-05 2.134 1.134 0.992

Transfer Point TP57 725142.02 4063521.53 6.39E-04 9.70E-05 9.70E-05 0.305 0.567 0.284

Transfer Point TP58 725187.09 4063561.42 6.42E-04 9.70E-05 9.70E-05 0.305 0.567 0.284

Transfer Point TP59 725232.40 4063600.51 6.43E-04 9.70E-05 9.70E-05 0.305 0.567 0.284

Transfer Point TP60 725277.33 4063639.60 6.34E-04 9.60E-05 9.60E-05 0.305 0.567 0.284

Transfer Point TP61 725322.63 4063678.88 6.39E-04 9.70E-05 9.70E-05 0.305 0.567 0.284

Transfer Point TP62TP70 725246.72 4063613.79 2.56E-03 3.87E-04 3.87E-04 2.134 1.163 1.985

Tower 6 TOWER6 725219.48 4063643.93 2.56E-03 3.87E-04 3.87E-04 2.743 1.205 1.276

Tower 7 TOWER7 725145.94 4063580.78 2.56E-03 3.87E-04 3.87E-04 2.896 1.205 1.347

Flop Gate A/B FLPGTEAB 725037.56 4063482.96 2.56E-03 3.87E-04 3.87E-04 12.192 1.205 5.671

Drop into N. Surge NSURGE 725032.78 4063489.34 1.29E-03 1.95E-04 1.95E-04 18.668 1.205 5.671

Drop into S. Surge SSURGE 725042.53 4063476.62 1.28E-03 1.94E-04 1.94E-04 18.668 1.205 5.671
AECOM Environment

Four Corners Power Plant and Navajo Mine EIS / NAAQS Modeling Study April 2013

2-16

Table 2-9 Emission Rates and Source Parameters for Navajo Mine South

Name Source ID Type

UTM E 

(NAD83)

UTM N 

(NAD83)

24-hour 

PM10

(g/s/m2)

24-hour 

PM2.5

(g/s/m2)

Annual 

PM2.5

(g/s/m2)

Release 

Height 

(m) X (m) Y (m)

Pit 

Volume 

(m3)

Angle 

from 

North 

(degrees)

Pinabete North Pit Strip STRIP1 OPENPIT 724454 4039298 2.17E-05 2.65E-06 9.63E-07 4.0 106.4 2490 7935003.8 -1.45

Pinabete East Pit Strip STRIP2 OPENPIT 720558.9 4042970 2.17E-05 2.65E-06 9.63E-07 4.0 2943.6 90 7934632.8 -2.01

Name Source ID Type

UTM E 

(NAD83)

UTM N 

(NAD83)

24-hour 

PM10

(g/s/m2)

24-hour 

PM2.5

(g/s/m2)

Annual 

PM2.5

(g/s/m2)

Release 

Height 

(m) X (m) Y (m)

Angle 

from 

North 

(degrees)

Lowe Coal Pile Wind 

Erosion COALPILE AREA 721204.7 4046534 7.07E-05 1.06E-05 2.19E-07 2.0 190 1160 0

Lowe Coal Pile 

Loading/Unloading COALPLLU AREA 721204.7 4046534 1.63E-06 5.11E-07 9.03E-08 2.0 190 1160 0

Pinabete East Overburden PEOVRBDN AREA 724557.7 4039303 1.13E-04 1.69E-05 6.79E-07 2.0 106.4 2490 -1.45

Pinabete North Overburden PNOVRBDN AREA 720557.4 4043065 1.12E-04 1.68E-05 6.75E-07 2.0 2943.6 90 -2.01

Pinabete North Reclamation PNRECLAM AREA 723210.3 4043269 2.21E-06 3.32E-07 1.33E-08 2.0 274.932 2208 -91.85

Pinabete East Reclamation PERECLAM AREA 724670.8 4039508 2.21E-06 3.32E-07 1.33E-08 2.0 274.932 2208 -1.28

Name Road ID

# 

Volumes

24-hour 

PM10 g/s 

/ volume

24-hour 

PM2.5

g/s / 

volume

Annual 

PM2.5

g/s / 

volume

Release 

Height 

(m)

Sigma-Y 

(m)

Sigma-Z 

(m)

Lowe Coal Pile to Split LOWESPLT 93 4.69E-02 9.93E-03 4.96E-03 5.64 13.95 2.23

Split to Pinabete North SPLT2PN 70 1.58E-02 3.36E-03 2.71E-03 5.64 13.95 2.23

Split to Pinabete East SPLT2PE 148 3.07E-02 6.51E-03 2.21E-03 5.64 13.95 2.23
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Figure 2-2 FCPP/Navajo Mine North Source Locations
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Figure 2-3 Navajo Mine South Source Locations
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3.0 Modeling Procedures

The dispersion modeling analyses conducted for the FCPP and Navajo Mine are based on certain 

procedures referenced in the EPA “Guideline on Air Quality Models” (GAQM, which is contained in 40 

CFR Part 51, Appendix W) (US EPA 2005)9

. EPA is currently considering procedures for determining 

compliance with the 1-hour SO2 NAAQS, and suggestions provided in the previously-mentioned 

February 7, 2013 draft guidance4 have also been considered for this analysis. The following sections 

present the procedure for assessing NAAQS for the FCPP and the standards to which the predicted 

impacts are compared.

3.1 Model Selection

The suitability of an air quality dispersion model for a particular application is dependent upon several 

factors. The following selection criteria were evaluated:

• stack height relative to nearby structures,

• dispersion environment,

• local terrain, and

• representative meteorological data.

The EPA GAQM prescribes a set of approved models for regulatory applications for a wide range of 

source types and dispersion environments. Based on a review of the factors discussed below, the 

latest version of AERMOD (12345)

10 is used to assess NAAQS for the FCPP and Navajo Mine. 

3.1.1 Low Wind Speed Improvements for AERMOD

In 2010, the results of an evaluation11 of low wind speed databases for short-range modeling 

applications were provided to EPA. The reason for the study was that some of the most restrictive 

dispersion conditions and the highest model predictions occur under low wind speed conditions, but 

there had been very little model evaluation for these conditions. The results of the evaluation 

indicated that in low wind conditions, the friction velocity formulation in AERMOD results in underpredictions of this important planetary boundary layer parameter. There were multiple outcomes of 

this under-prediction: mechanical mixing heights that were very low (less than 10 meters), very low 

effective dilution wind speeds, and very low turbulence in stable conditions. In addition, the evaluation 

study concluded that the minimum lateral turbulence (as parameterized using sigma-v) was too low by 

at least a factor of 2.

 

9 US EPA 2005. Guideline on Air Quality Models (Revised). Codified in the Appendix W to 40 CFR Part 51. (November 2005). 

10 US EPA 2004a. User’s Guide for the AMS/EPA Regulatory Model (AERMOD). EPA-454/B-03-001 (September 2004 – 

Addendum March 2011).

11 Paine, R.J., J.A. Connors, and C.D. Szembek. AERMOD Low Wind Speed Evaluation Study: Results and Implementation. 

Paper 2010-A-631-AWMA, presented at the 103rd Annual Conference, Air & Waste Management Association, Calgary, 

Alberta, Canada. 2010.
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After these issues were once again stated at the 10th EPA Modeling Conference in March 2012, EPA 

made some revisions in late 2012 to the AERMOD12 modeling system to correct the model 

deficiencies in this area. This culminated in EPA releasing AERMET and AERMOD Versions 12345, 

which include “beta” options in AERMET for a revised u* formulation under stable conditions and two 

different low wind speed options in AERMOD. 

Because the beta options constitute a non-guideline application of AERMOD, an alternative model 

demonstration has been provided for justification of the use of the beta option. This discussion is 

provided in Appendix C. 

3.1.2 Plume Depletion and Secondary Particulate Modeling

To be consistent with previous modeling performed by Class One Technical Services for the Navajo 

Mine13, AECOM modeled PM10 and PM2.5 emission sources using the plume depletion option. Plume 

depletion simulates the deposition of particles form the plume to the ground as the plume travels. 

Therefore, the farther the plume travels from the emission point to the receptor, the greater the effect 

of plume depletion. Particle size distribution, particle mass fraction and particle density are required 

inputs to AERMOD for this option.

The particle size distribution data used in AERMOD for the material handling sources are based on

data from the City of Albuquerque Air Quality Bureau (CAQB) “Air Dispersion Modeling Guidelines for 

Air Quality Permitting” (01/21/10, Table 1)14. Particle size distribution for paved and unpaved roads 

were obtained from the particle size k factors found in the AP-42 13.2.1 and 13.2.2 emission 

equations for paved and unpaved roads, respectively. Particle size distribution for combustion 

emissions assumes all particles are less than 10 μm.

The mass-mean particle diameter was calculated using the formula below:

 d = ((d3

1 + d21d2 + d1d22 + d32) / 4)1/3

Where: d = mass-mean particle diameter

 d1 = low end of particle size category range

 d2 = high end of particle size category range

An average particle density for soil is 2.5 g/cm3 as obtained from the “Handbook of Chemistry and

Physics”, published by the CRC. The size distribution and densities are summarized in Tables 3-1

and 3-2. 

PM modeling results are dominated by near-surface releases, which are composed primarily of direct 

PM emissions and have minimal precursor PM emissions. Secondary PM emissions from the FCPP 

 

12 US EPA 2004b. User’s Guide for the AERMOD Meteorological Preprocessor (AERMET). (EPA-454/B-03-002, November 

204 – Addendum December 2012)

13 Class One Technical Services 2011. BNCC Navajo Mine Area 3 and 4 Modeling, Four Corners Area, New Mexico. 

Environmental Assessment Dispersion Modeling (July 2011).

14 CAQB 2010. Air Dispersion Modeling Guidelines for Air Quality Permitting. (January 21, 2010). 

http://www.cabq.gov/airquality/documents/pdf/agd_model_guidelines.pdf. [Accessed 11/15/2012].
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stack are already accounted for in the conservative background monitoring concentration. Note that 

with future emission reductions at FCPP and area-wide sources, the secondary PM emission 

component that is part of the background concentration will be reduced, so the current background 

overestimates this future component of PM concentrations.

Table 3-1 Material Handling Depletion Parameters

Particle Size 

Category (μm)

Mass Mean

Particle Diameter(μm)

Mass Weighted

Size Distribution (%) Density (g/cm3

)

2.5 – 5 3.88 22.6 2.5

5 – 10 7.77 77.4 2.5

Table 3-2 Vehicle Fugitive Dust Depletion Parameters

Particle Size 

Category (μm)

Mass Mean

Particle Diameter(μm)

Mass Weighted

Size Distribution (%) Density (g/cm3

)

0 – 2.5 1.57 10.0 2.5

2.5 – 10 6.92 90.0 2.5

3.2 Meteorological Data for AERMOD

There are several factors that need to be weighed when selecting the source of meteorological data to 

be used in a dispersion modeling study. For AERMOD, the most important factor to consider is site 

representativeness with respect to terrain considerations and land use between the AERMOD 

application site and the meteorological measurement site, as these parameters potentially have a 

large effect on the predominant wind direction and boundary layer parameterization of the 

meteorological inputs into AERMOD. Other factors such as data capture, quality of data, elevation 

differences, and distance from the measurement site and the application site need also to be 

considered. For this application, since the meteorological measurements come from a meteorological 

tower less than 4 kilometers from the FCPP, site representativeness is not in question. Therefore, the 

meteorological data processing discussion is focused on data capture and quality of data.

Meteorological data required for AERMOD include hourly values of wind speed, wind direction, and 

ambient temperature. Since the AERMOD dispersion algorithms are based on atmospheric boundary 

layer dispersion theory, additional boundary layer variables are derived by parameterization formulas, 

which are computed by the AERMOD meteorological preprocessor, AERMET. These parameters 

include sensible heat flux, surface friction velocity, convective velocity scale, vertical potential 

temperature gradient, convective and mechanical mixing heights, Monin-Obukhov length, surface 

roughness length, Bowen ratio, and albedo.

3.2.1 Available Meteorological Data

The AERMOD modeling for FCPP and Navajo Mine was conducted with data available from nearby 

10-meter meteorological towers, referred to as Navajo Met Tower 1, 2 and 3. The database was 

provided by APS and Navajo Mine and is of sufficient quality for air dispersion modeling applications. 
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The data and instrumentation are routinely maintained to the quality that would be required for 

modeling in support of a New Source Review permit according to EPA’s ambient monitoring guidance.

The location of Navajo Met Towers 1, 2 and 3 are shown in Figure 2-1. The meteorological towers 

include an instrumented 10-meter meteorological tower. Specifically, the tower records the following 

measurements (all at 10 meters except for precipitation): 

1. Wind speed 

2. Wind direction 

3. Sigma Theta (standard deviation of the wind direction)

4. Temperature 

5. Relative Humidity 

6. Precipitation.

The five years of meteorological data chosen for this analysis were April 2006 through March 2011. 

Temperature data for Navajo Met 1 in 2007 did not meet USEPA’s ambient monitoring guidance

relative to quarterly data capture (as shown in Table 3-1) therefore temperature data from Navajo Met 

2 was substituted for this period. Navajo Met 3 data meets the guidance relative to quarterly data 

capture for all parameters in all quarters as shown in Table 3-2. 

Upper air data for the concurrent period was obtained from Albuquerque International Airport, NM. 

The Albuquerque upper air data was provided by Eric Peters of New Mexico Environment Department 

(NMED). The missing wind speed and direction data were not filled given the low number of missing 

hours. Variables not available from the Navajo Met Tower 1 (such as cloud cover) were taken from 

the Farmington Regional Airport, NM. 

3.2.2 AERMET Processing 

The five years of hourly surface meteorological data from Navajo Met Tower 1 and 3 along with 

concurrent upper air data from Albuquerque International Airport, NM were processed with AERMET 

(US EPA 2012) with the beta option discussed above. Table 3-3 gives site locations and information 

on these data sets. 

AECOM utilized the latest version of AERMET (12345), the meteorological preprocessor, to create 

AERMOD-ready meteorological inputs. Since wind data from Farmington Regional Airport, NM is not 

being substituted during Stage 3 of the AERMET processing, the incorporation of 1-minute ASOS 

data from Farmington and AERMINUTE is not necessary for this application.
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Table 3-3 Data Capture for Navajo Met 1

Quarter

10-Meter 

Wind 

Speed

10-Meter 

Wind 

Direction

10-Meter 

SigmaTheta

10-Meter 

Temperature

EPA 

Threshold

2006

1st Qtr 100% 100% 100% 100% 90%

2nd Qtr 100% 100% 100% 100% 90%

3rd Qtr 100% 100% 100% 100% 90%

4th Qtr 96% 96% 96% 93% 90%

2007

1st Qtr 95% 95% 95% 95% 90%

2nd Qtr 100% 100% 100% 44% 90%

3rd Qtr 100% 100% 100% 64% 90%

4th Qtr 93% 100% 100% 100% 90%

2008

1st Qtr 100% 100% 100% 100% 90%

2nd Qtr 100% 100% 100% 100% 90%

3rd Qtr 100% 100% 100% 100% 90%

4th Qtr 100% 100% 100% 100% 90%

2009

1st Qtr 100% 100% 100% 100% 90%

2nd Qtr 100% 100% 100% 100% 90%

3rd Qtr 100% 100% 100% 100% 90%

4th Qtr 100% 100% 100% 100% 90%

2010

1st Qtr 100% 100% 100% 100% 90%

2nd Qtr 100% 100% 100% 100% 90%

3rd Qtr 100% 100% 100% 100% 90%

4th Qtr 100% 100% 100% 100% 90%

2011

1st Qtr 100% 100% 100% 100% 90%

2nd Qtr 100% 100% 100% 100% 90%

3rd Qtr 100% 100% 100% 100% 90%

4th Qtr 100% 100% 100% 100% 90%

Note: Red numbers indicate that temperature data for Navajo Met 2 were substituted for the missing 

Navajo Met 1 temperature data in Quarters 2 and 3 of 2007 (instrument malfunction/replacement).
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Table 3-4 Data Capture for Navajo Met 3 

Quarter

10-Meter 

Wind 

Speed

10-Meter 

Wind 

Direction

10-Meter 

SigmaTheta

10-Meter 

Temperature

EPA 

Threshold

2006

1st Qtr N/A N/A N/A N/A 90%

2nd Qtr 100% 100% 100% 100% 90%

3rd Qtr 100% 100% 100% 100% 90%

4th Qtr 100% 100% 100% 100% 90%

2007

1st Qtr 99% 99% 99% 99% 90%

2nd Qtr 100% 100% 100% 100% 90%

3rd Qtr 100% 100% 100% 100% 90%

4th Qtr 100% 100% 100% 100% 90%

2008

1st Qtr 98% 100% 100% 100% 90%

2nd Qtr 100% 100% 100% 100% 90%

3rd Qtr 100% 100% 100% 100% 90%

4th Qtr 100% 100% 100% 100% 90%

2009

1st Qtr 96% 96% 96% 96% 90%

2nd Qtr 100% 100% 100% 100% 90%

3rd Qtr 100% 100% 100% 100% 90%

4th Qtr 100% 100% 100% 100% 90%

2010

1st Qtr 100% 100% 100% 100% 90%

2nd Qtr 100% 100% 100% 100% 90%

3rd Qtr 100% 100% 100% 100% 90%

4th Qtr 100% 100% 100% 100% 90%

2011

1st Qtr 100% 100% 100% 100% 90%

2nd Qtr 100% 100% 100% 100% 90%

3rd Qtr 100% 100% 100% 100% 90%

4th Qtr 77% 77% 77% 77% 90%

Note: Red text indicates that these data were not used due to missing information for all parameters. 
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AERMET creates two output files for input to AERMOD:

• SURFACE: a file with boundary layer parameters such as sensible heat flux, surface 

friction velocity, convective velocity scale, vertical potential temperature gradient in the 

500-meter layer above the planetary boundary layer, and convective and mechanical 

mixing heights. Also provided are values of Monin-Obukhov length, surface 

roughness, albedo, Bowen ratio, wind speed, wind direction, temperature, and heights 

at which measurements were taken.

• PROFILE: a file containing multi-level meteorological data with wind speed, wind 

direction, temperature, sigma-theta (σθ) and sigma-w (σw) when such data are 

available. For this application involving representative data from the nearest NWS 

station, the PROFILE file will contain a single level of wind data (10.0 meters) and the 

temperature data (2 meters).

The AERMOD Implementation Guide (AIG) recommends that the surface characteristics be 

determined based on digitized land cover data. EPA has developed a tool called AERSURFACE 

(US EPA 2008a)15 that can be used to determine the site characteristics based on digitized land 

cover data in accordance with the recommendations from the AIG discussed above. 

AERSURFACE incorporates look-up tables of representative surface characteristic values by land 

cover category and seasonal category. AERSURFACE will be applied with the instructions 

provided in the AERSURFACE User’s Guide.

AECOM utilized AERSURFACE (Version 08009) which supports the use of land cover data from the 

USGS National Land Cover Data 1992 archives16 (NLCD92). The NLCD92 archive provides data at a 

spatial resolution of 30 meters based upon a 21-category classification scheme applied over the 

continental U.S. The AIG recommends that the surface characteristics be determined based on the 

land use surrounding the site where the surface meteorological data were collected, in this case 

around Navajo Met Tower 1 and 3. EPA released a new version of AERSURFACE on January 18, 

2013, after much of the modeling for this report was completed. AECOM compared the output from 

the two versions of AERSURFACE and noted minimal change. The comparison is presented in 

Appendix D. 

As recommended in the AIG for surface roughness, the 1-km radius circular area centered at the 

meteorological station site can be divided into sectors for the analysis; each chosen sector has a mix 

of land uses that is different from that of other selected sectors. Sectors used to define the 

meteorological surface characteristics for Navajo Met 1 and 3 are shown in Figures 3-1 and 3-2. 

In AERSURFACE, the various land cover categories are linked to a set of seasonal surface 

characteristics. As such, AERSURFACE requires specification of the seasonal category for each 

month of the year. The following five seasonal categories are supported by AERSURFACE, with the 

applicable months of the year specified for this site.

• Midsummer with lush vegetation (June-September); 

• Autumn with un-harvested cropland (October-November);

 

15 US EPA 2008a. AERSURFACE User’s Guide. (EPA-454/B-08-001, January 2008). Office of Air Quality Planning and 

Standards, Research Triangle Park, NC.

16 USGS 1992. USGS Landcover Institute. http://edcftp.cr.usgs.gov/pub/data/landcover/states/ [Accessed 07/15/2012] 
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Figure 3-1 Land Use Sectors for Navajo Met 1
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Figure 3-2 Land Use Sectors for Navajo Met 3
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• Late autumn after frost and harvest, or winter with no snow (December-March); 

• Winter with continuous snow on ground (none); and

• Transitional spring with partial green coverage or short annuals (April-May).

For Bowen ratio, the land use values are linked to three categories of surface moisture corresponding 

to average, wet and dry conditions. The surface moisture condition for the site may vary depending 

on the meteorological data period for which the surface characteristics will be applied. 

AERSURFACE applies the surface moisture condition for the entire data period. Therefore, if the 

surface moisture condition varies significantly across the data period, then AERSURFACE can be 

applied multiple times to account for those variations. As recommended in AERSURFACE User’s 

Guide, the surface moisture condition for each month is determined by comparing precipitation for the 

period of data to be processed to the 30-year climatological record, selecting “wet” conditions if 

precipitation is in the upper 30th-percentile, “dry” conditions if precipitation is in the lower 30thpercentile, and “average” conditions if precipitation is in the middle 40th-percentile. The 30-year 

precipitation data set used in this modeling is taken from Farmington Regional Airport, New Mexico. 

Appendix E contains the 30 years of monthly precipitation data obtained from NCDC. The 30-year 

period of record used to establish the 30-year average monthly precipitation totals include 1981 

through 2010.

The monthly designations of surface moisture input to AERSURFACE are summarized in Table 3-4. 

Wind roses for Navajo Met 1 and Navajo Met 3 are shown in Figures 3-3 and 3-4. 

Table 3-5 Meteorological Data Used in Running AERMET

Met Site Latitude Longitude

Base 

Elevation 

(m)

Data 

Source Data Format

Albuquerque 

International Airport, NM 35.05N 106.62W 1619.0 NMENV FSL

Farmington Regional 

Airport, NM 36.744N 108.229W 1678.0 NMENV ISHD

Navajo Met 1 Monitoring 

Station, NM 36.680N 108.450W 1646.0 APS/Navajo Mine free format

Navajo Met 3 Monitoring 

Station, NM 36.499N 108.512W 1656.6 APS/Navajo Mine Free format

3.3 Good Engineering Practice Stack Height Analysis

A Good Engineering Practice (GEP) stack height analysis17 was performed based on the current 

geometry of stacks and buildings at the FCPP to determine the potential for building-induced 

aerodynamic downwash for all modeled stacks. The analysis procedures described in US EPA’s 

Guidelines for Determination of Good Engineering Practice Stack Height (US EPA 1985), Stack 

Height Regulations (40 CRF 51), and current Model Clearing house guidance was used.

 

17 US EPA 1985. Guideline for the Determination of Good Engineering Practice Stack Height (Technical Support Document for 

the Stack Height Regulations) – Revised. EPA-450/4-80-023R, US EPA, Research Triangle Park, NC 27711.
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Figure 3-3 Wind Rose for Navajo Met 1 Station
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Figure 3-4 Wind Rose for Navajo Met 3 Station
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The GEP formula height is based on the observed phenomena of disturbed atmospheric flow in the 

immediate vicinity of a structure resulting in higher ground level concentrations at a closer proximity to 

the building than would otherwise occur. It identifies the minimum stack height at which significant 

aerodynamic downwash is avoided. The GEP formula stack height, as defined in the 1985 final 

regulations, is calculated from:

HGEP = HBLDG + 1.5L

Where:

• HGEP is the maximum GEP stack height

• HBLDG is the height of the nearby structure, and

• L is the lesser dimension (height or projected width) of the nearby structure

For a squat structure, i.e., height less than projected width, the formula reduces to:

HGEP = 2.5HB

Both the height and width of the structure are determined from the frontal area of the structure 

projected onto a plane perpendicular to the direction of the wind. In all instances, the GEP stack 

height is based on the plane projections of any nearby building which result in the greatest justifiable 

height. For purposes of the GEP analysis, nearby refers to the “sphere of influence”, defined as five 

times the height or width of the building, whichever is less, downwind from the trailing edge of the 

structure. In the case where a stack is not influenced by nearby structures, the maximum GEP stack 

height is defined as 65 meters.

The Units 4 and 5 dual-flue stack and other point sources at the FCPP are all greater than 65 meters, 

but are all below the GEP formula stack height based on the formula shown above. As such, all 

stacks are modeled with their actual stack height. In addition, the USEPA’s Building Profile Input 

Program (BPIP-Version 04274) version that is appropriate for use with PRIME algorithms in AERMOD 

is used to incorporate downwash effects in the model for all modeled stacks. The building dimensions 

of each structure are input in BPIPPRM program to determine direction specific building data. PRIME 

addresses the entire structure of the wake, from the cavity immediately downwind of the building, to 

the far wake. Figure 3-5 shows the buildings and stacks input to BPIPPRM and included in 

AERMOD.

3.4 Receptor Grid and AERMAP Processing

The receptor grid developed for the FCPP and Navajo Mine NAAQS modeling consists of the 

following levels in a Cartesian grid:

• 100-meter spacing out to 2 kilometers;

• 250-meter spacing out to 3 kilometers;

• 500-meter spacing out to 5 kilometers;

• 1000-meter spacing out to 10 kilometers; and

• 3000-meter spacing out to 15 kilometers for PM2.5 and PM10, 30 kilometers for all other 

pollutants. 
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Table 3-6 AERSURFACE Bowen Ratio Condition Designations

Month 2005 2006 2008 2009 2010

Jan Wet Average Wet Average Wet

Feb Wet Dry Average Average Wet

Mar Average Wet Dry Average Average

Apr Wet Average Dry Average Average

May Average Dry Average Wet Dry

Jun Average Average Average Wet Average

Jul Average Wet Dry Dry Average

Aug Wet Dry Wet Dry Wet

Sep Dry Wet Dry Dry Average

Oct Average Wet Average Average Average

Nov Dry Dry Average Average Dry

Dec Dry Wet Wet Average Wet

In addition, receptors spaced at 50 meters were placed along the FCPP and Navajo Mine boundary 

and were also placed at 100-meter spacing on the high-terrain Hogback Ridge in areas where the 

terrain elevations were equal to or greater than stack top (5660 ft/1724 m). The receptor grid used in 

the modeling analysis is based on NAD 83 datum and in UTM Zone 12.

The extent of this grid was sufficient to capture maximum impacts near the FCPP. To refine the levels 

of the peak predictions, 100-m spaced receptors were used to resolve the highest concentrations if 

the receptor spacing was more than 100-m at the maximum impact locations. Figures 3-6 and 3-7

show the locations of the near- and far-field receptors for the FCPP and Navajo Mine NAAQS 

modeling.

AERMAP (version 11103) (US EPA 2004c)18, AERMOD terrain preprocessor program, was used to 

calculate terrain elevations and critical hill heights for the modeled receptors (NAD83 datum and 

Zone 12) using National Elevation Data (NED)19. The dataset downloaded from the USGS website 

consists of 1 arc second (~30 m resolution) NED. As per the AERMAP User’s Guide (US EPA

2004a), the domain is sufficient to ensure that all significant nodes are included such that all terrain 

features that exceed a 10% elevation slope from any given receptor are considered.

3.5 Class II Area Modeling Analyses

A refined modeling analysis was conducted using AERMOD (version 12345) in default mode with beta 

AERMET option to determine NAAQS for the FCPP and Navajo Mine. The analysis was conducted to 

demonstrate whether both state and federal applicable ambient air quality standards would be met. 

Impacts from sources other than FCPP and the Navajo Mine and the use of monitoring data are 

discussed in Section 4. The modeled concentrations for the NAAQS modeling (that were added to 

regional background values for the total concentration) were calculated as follows: 

 

18 US EPA 2004c. User’s Guide for the AERMOD Terrain Preprocessor (AERMAP). (EPA-454/B-03-003, 

October 2004 – Addendum March 2011) Office of Air Quality Planning and Standards, Research Triangle Park, 

NC.

19 USGS. National Elevation Dataset. (http://seamless.usgs.gov/) [Accessed 07/15/2012] 
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Figure 3-5 Buildings and Stacks Included in GEP Analysis
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Figure 3-6 Near-Field Receptor Locations
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Figure 3-7 Far-Field Receptor Locations
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• For lead, a rolling 3-month average was compared to the NAAQS20. 

• The short-term modeled concentrations of PM10 (24-hour) was based on the highest sixth

highest (H6H) concentrations over all the modeled receptors and 5 years of meteorological 

data. The FCPP and Navajo North sources were modeled with Navajo Met I and the 

Navajo South sources were modeled with Navajo Met III at all receptors. The H6H at each 

receptor from each model run were summed for the total modeled impact. 

• The 24-hour PM2.5 modeled concentration was based on highest ranked modeled 

concentration at each receptor per year averaged over 5 years. The FCPP and Navajo 

North sources were modeled with Navajo Met I and the Navajo South sources were 

modeled with Navajo Met III at all receptors. The highest 5-year average at each receptor 

from the two model runs were summed for the total modeled impact. 

• The annual PM2.5 evaluation was based on the highest 5-year average concentration. The 

FCPP and Navajo North sources were modeled with Navajo Met I and the Navajo South 

sources were modeled with Navajo Met III at all receptors. The highest 5-year average at 

each receptor from the two model runs were summed for the total modeled impact.

• The 1-hour SO2 modeled concentration was based on the 99th percentile peak daily 1-hour 

concentration averaged over 5 years including hourly seasonal background, as shown in 

Table 4-1 from the Substation monitor. Use of this monitor is conservative as it may 

double-count emissions from the FCPP. The FCPP and Navajo North sources were 

modeled with Navajo Met I and the Navajo South sources were modeled with Navajo Met III 

at all receptors. The 99th percentile peak daily 1-hour concentration at each receptor from 

the two model runs were summed for the total modeled impact.

• The 1-hour NO2 modeled concentration was based on the 98th percentile peak daily 1-hour 

concentration averaged over 5 years including hourly seasonal background, as shown in 

Table 4-2 from the Substation monitor. Use of this monitor is conservative as it may 

double-count emissions from the FCPP. The modeling of NO2 impacts is subject to 

application of a tiered approach, as mentioned in EPA’s March 1, 2011 guidance on NO2

modeling. A Tier 2 approach (ambient ratio method) multiplies the model impacts by a 

conversion factor of NO to NO2 by 80%. Since hourly seasonal background is being added 

concurrently in AERMOD, AECOM multiplied the emission rate by 80% such that the hourly 

background would not have to be subtracted from the design concentration, convert the 

modeled impact, then add background again. The FCPP and Navajo North sources were 

modeled with Navajo Met I and the Navajo South sources were modeled with Navajo Met III 

at all receptors. The 98th percentile peak daily 1-hour concentration at each receptor from 

the two model runs were summed for the total modeled impact.

• The annual modeled concentrations of NO2 were based on the highest concentration over 

all modeled receptors and 5 years of meteorological data. The FCPP and Navajo North 

sources were modeled with Navajo Met I and the Navajo South sources were modeled with 

Navajo Met III at all receptors. The annual average at each receptor from the two model 

runs were summed for the total modeled impact.

• The short-term modeled concentrations of CO (1- and 8-hour) were based on the highest 

second highest (H2H) concentrations over all modeled receptors and 5 years of 

 

20 US EPA 2012. National Ambient Air Quality Standards (40CFR50). http://www.epa.gov/air/criteria.html
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meteorological data. The FCPP and Navajo North sources were modeled with Navajo Met I 

and the Navajo South sources were modeled with Navajo Met III at all receptors. The H2H 

at each receptor from each model run were summed for the total modeled impact.
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4.0 Background Concentrations

4.1 Regional Ambient Background 

AECOM located ambient background monitors for SO2, NO2, PM2.5, PM10, CO and lead in the Farmington 

vicinity as shown in Figure 4-1. For 1-hour SO2 and NO2, AECOM developed hourly seasonal background 

values, as shown in Table 4-1 and Table 4-2 from the Substation monitor. Use of this monitor is 

conservative as it may double-count emissions from the FCPP. For PM10, PM2.5, CO, annual NO2 and 3-

hour SO2, AECOM reviewed the available data for the nearest monitors as summarized in Table 4-3. 

NMED does not maintain any lead monitors in the state of New Mexico. The closest monitor is located in 

the Denver, Colorado area. The EPA Air Data website21 (US EPA 2013b) does not currently report 

quarterly lead concentrations for comparison with the NAAQS. The highest 24-hour value at a monitor in 

Denver in 2011 is 0.02 μg/m3. The most recent Colorado Monitoring Report from 200922 lists the highest 

quarterly value as 0.006 μg/m3.

Table 4-1 1-hour SO2 Season and Hour of Day Background Concentrations (ppb)

3-yr Average 0:00 1:00 2:00 3:00 4:00 5:00 6:00 7:00

Winter 1.7 1.3 1.3 1.0 3.0 1.7 1.0 1.3

Spring 1.0 1.3 1.0 1.7 2.0 1.3 2.7 4.0

Summer 3.3 3.7 4.3 3.3 3.0 3.0 3.7 13.0

Fall 2.7 2.3 2.0 3.7 5.0 6.0 3.7 5.7

3-yr Average 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00

Winter 3.7 3.7 6.3 8.3 8.7 8.7 8.3 5.0

Spring 6.0 5.3 4.0 1.7 1.3 3.0 1.3 1.3

Summer 10.0 11.0 9.0 5.0 3.3 2.3 2.0 2.0

Fall 7.3 10.7 16.7 13.0 9.3 8.0 7.7 7.0

3-yr Average 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00

Winter 5.0 2.7 3.0 3.0 2.0 1.7 1.3 1.7

Spring 1.7 1.7 2.0 1.0 0.7 2.0 1.0 1.0

Summer 2.3 2.3 3.3 2.3 4.0 5.7 4.3 3.7

Fall 5.3 5.0 3.7 3.3 2.7 2.0 3.0 1.7

 

21 EPA 2013b. EPA Air Data website: (http://www.epa.gov/airdata/ad_rep_mon.html) 

22 CDPHE 2010. 2009 Air Quality Data Report. 

http://colorado.gov/airquality/tech_doc_repository.aspx?action=open&file=2009AnnualDataReport.pdf
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Table 4-2 1-hour NO2 Season and Hour of Day Background Concentrations (ppb)

3-yr Average 0:00 1:00 2:00 3:00 4:00 5:00 6:00 7:00

Winter 28.0 28.7 31.3 32.0 31.7 32.7 33.0 31.3

Spring 16.7 20.3 25.7 24.7 22.3 22.3 24.0 26.7

Summer 18.0 17.0 18.3 17.0 19.3 18.7 20.3 26.7

Fall 23.7 22.0 23.0 23.3 25.0 24.7 25.0 22.7

3-yr Average 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00

Winter 27.7 26.0 26.0 28.7 29.0 29.3 27.3 25.7

Spring 34.0 28.3 23.0 21.0 19.3 17.7 15.0 15.3

Summer 30.7 31.0 30.0 25.3 19.0 15.3 13.0 13.7

Fall 21.0 22.3 27.7 27.3 26.3 21.7 23.3 20.3

3-yr Average 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00

Winter 28.3 30.3 31.7 31.3 29.3 31.7 30.0 29.0

Spring 17.3 19.0 22.0 16.3 16.3 19.3 18.3 19.7

Summer 14.0 15.7 16.7 14.0 18.0 21.7 22.3 16.7

Fall 23.0 21.0 19.7 20.7 19.0 21.0 20.7 23.3

4.2 Nearby Sources

AECOM reviewed the locations of large emission sources (e.g., with emissions exceeding 250 tons per 

year) out to distances as large as 40 km from Four Corners Power Plant. Of the two sources located, San 

Juan Generating Station (SJGS) and the San Juan River Gas Plant, the latter has retired their major 

emissions source, a Sulfur Recovery Unit23. Therefore, we concluded that the only background source to 

be evaluated (determining whether it would have a significant concentration gradient) for modeling is the 

SJGS, located about 13 km from FCPP. It is noteworthy that our monitored background data uses a 

monitor close to that facility, so the impacts from SJGS are already included in the monitored background. 

SJGS will be reducing emissions within the next five years; therefore, the use of this monitored background 

is a conservative estimate of future ambient background. 

To determine if there is a significant concentration gradient beyond the FCPP location from SJGS 

emissions, AECOM modeled impacts from that facility along a line of receptors passing through and beyond 

the Four Corners Power Plant. The analysis, described in Appendix F, indicates that the relative change in 

modeled concentrations between the location of FCPP and the peak impact location of FCPP along the 

same direction would be too small to be detected by an array of SO2 monitors along that same line. 

 

23 See http://www.docstoc.com/docs/106180605/Public-Notice-Western-Gas-R-San-Juan-River-Gas-Plant-2013-M3-

28July10; the resulting SO2 annual emissions would be less than 150 tons per year, and the facility would be screened 

out with a “20D” rule due to its distance of over 12 km from FCPP.

 
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Therefore, the concentration gradient is insignificant, and impacts from that the source impact from that 

source are accommodated within the monitored background. 
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Figure 4-1 Ambient Background Monitor Locations
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Table 4-3 Monitored Background Concentrations for CO, PM10 and PM2.5, Lead, 3-hour SO2 and Annual NO2

Pollutant

Averaging 

Period Ranking Year

Concentration(1)

(μg/m3) Units Monitor ID

Monitor 

Address County/City State

PM10 24-hour

H2H 2009 44.0 μg/m3

35|45|0019

3400 

Messina 

Drive

H2H 2010 19.0 μg/m Farmington NM 3

H2H 2011 29.0 μg/m3

PM2.5

24-hour

98th p’tile 2009 10.4 μg/m3

35|45|0019

3400 

Messina 

Drive

Farmington NM

98th p’tile 2010 18.0 μg/m3

98th p’tile 2011 12.3 μg/m3

3-year AVG 2009-2011 13.6 μg/m3

Annual

H 2009 4.4 μg/m3

H 2010 4.8 μg/m3

H 2011 4.1 μg/m3

3-year AVG 2009-2011 4.4 μg/m3

Lead Quarterly H 2209 0.006 μg/m3 Denver CO

SO2 3-hour

H2H 2009 15 ppb

35|45|01005

USBR 

Shiprock 

Substation 

(Farmington)

San Juan 

County NM H2H 2010 9.6 ppb

H2H 2011 17

(44.5)

ppb 

(μg/m3)

NO2 Annual

H 2009 18.2 ppb

35|45|01005

USBR 

Shiprock 

Substation 

(Farmington)

San Juan 

County H 2010 NM 19.9

(37.4)

ppb 

(μg/m3)

H 2011 19.3 ppb

CO

1-hour

H2H 2009 1.4 (1610) ppm 

(μg/m3)

08|67|7001

1 MI. NE OF 

IGNACIO ON 

COUNTY 

RD. 517

La Plata 

County CO

H2H 2010 1.2 ppm

H2H 2011 1.3 ppm

8-hour

H2H 2009 0.9 (1035) ppm 

(μg/m3)

H2H 2010 0.7 ppm

H2H 2011 0.7 ppm

(1) Bold shaded values indicate for each pollutant and averaging period the background concentrations that are used in the NAAQS modeling analysis. 

• PM10 24-hr used the highest 2nd highest over three years.

• PM2.5 24-hr used the 3-year average 98th percentile.

• PM2.5 Annual used the 3-year annual average.

• SO2 3-hr used the highest 2nd highest over three years.

• NO2 Annual used the highest annual average over the last three years.
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5.0 Modeling Results

AECOM conducted dispersion modeling with AERMOD for 1-hour and 3-hour SO2, 1-hour and annual 

NO2, 24-hour and annual PM2.5, 24-hour PM10, 1-hour and 8-hour CO and quarterly rolling average for 

Lead. Table 5-1 summarizes the modeling impacts for all pollutants with the additional of ambient 

background concentrations. Total concentrations for all pollutants and averaging periods are below 

the NAAQS. The maximum impact locations are shown in Figure 5-1.

Twenty-four hour and annual PM2.5 impacts are dominated by low level sources such as coal 

preparation, lime and ash handling and roadway emissions. Quarterly lead, 1-hour and 3-hour SO2, 

1-hour and annual NO2 impacts are from the Units 4 and 5 stack. Active mining and mine vehicle 

traffic emissions dominate the impact for 24-hour PM10. 1-hour and 8-hour CO impacts are controlled 

by the coal blasting emissions, which was modeled for four hours per day at the full blasting rate 

although these emissions will occur in only one hour per day. The modeling archive is presented in 

Appendix G. 

Table 5-1 NAAQS Modeling Results

Pollutant

Avg 

Period Standard

Modeled 

Impact 

(μg/m3)

Background 

(μg/m3)

Total 

Impact 

(μg/m3)

NAAQS 

(μg/m3)

CO

1-hour High-2nd High 954.65 1610.00 2564.65 40,000.00

8-hour High-2nd High 181.04 1035.00 1216.04 10,000.00

NO2

1-hour

98th-percentile of 

Daily Max Over 5 

Years

— — 168.96 188.00

Annual High-1st-High 3.43 37.40 40.83 100.00

PM2.5

 1 

24-hour Highest of 5-yr 

Average 19.56 13.60 33.16 35.00

Annual Highest of 5-yr 

Average 3.83 4.40 8.23 12.00

PM10

(1) 24-hour High-6th-High Over 5 

Years 43.42 44.00 87.42 150.00

SO2

1-hour

99th-percentile of 

Daily Max Over 5 

Years

194.50 196.50 

3-hour High-2nd High 186.06 44.50 230.56 1,300.00

Lead

Rolling 

3-month 

Average

High-1st-High 0.001 0.006 0.007 0.15

Due to a recent change in stack parameters and condensable PM emission rate, the PM2.5 impacts 

have been scaled to approximate the increased emission rate. The modeling will be updated 

during the document review process.
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Figure 5-1 Maximum Modeled Impact Locations (North)
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Four Corners Power Plant and Navajo Mine EIS / NAAQS Modeling Study April 2013

5-3

Figure 5-2 Maximum Modeled Impact Locations (South)
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6-1

6.0 References

City of Albuquerque (CABQ) 2010. Air Dispersion Modeling Guidelines for Air Quality Permitting. 

(January 21, 2010). http://www.cabq.gov/airquality/documents/pdf/aqd_model_guidelines.pdf. 

[Accessed 11/15/2012].

Colorado Department of Public Health and Environment (CDPHE) 2010. 2009 Air Quality Data 

Report. 

http://www.colorado.gov/airquality/tech_doc_repository.aspx?action=open&file=2009AnnualDataRepo

rt.pdf. [Accessed 11/15/2012].

Class One Technical Services 2011. BNCC Navajo Mine Area 3 and 4 Modeling, Four Corners Area, 

New Mexico, Environmental Assessment Dispersion Modeling. (July 2011).

Navajo Nation Environmental Protection Agency (NNEPA) 2008. Arizona Public Service Four 

Corners Power Plant Title V Operating Permit. (August 1, 2008). 

http://www.navajonationepa.org/airqty/Pdf_files/FCPPpermit8-1-2008.pdf. [Accessed 05/01/2012].

Texas Natural Resource Conservation Commission (TNRCC) 1996. Coal Handling Emissions 

Evaluation Roundtable (CHEER) Workshop, May 23. 

US EPA 1985. Guideline for the Determination of Good Engineering Practice Stack Height (Technical 

Support Document for the Stack Height Regulations), Revised. EPA-450/4-80-023R, US EPA, 

Research Triangle Park, NC 27711.

US EPA 1995. AP-42, Compilation of Air Pollutant Emission Factors, Fifth Edition (January 1995). 

http://www.epa.gov/ttnchie1/ap42/. Office of Air Quality Planning and Standards, Research Triangle 

Park, North Carolina. [Accessed 01/04/2013].

US EPA 2004a. User’s Guide for the AMS/EPA Regulatory Model (AERMOD). EPA-454/B-03-001 

(September 2004 – Addendum December 2012). Office of Air Quality Planning and Standards, 

Research Triangle Park, North Carolina.

US EPA 2004b. User’s Guide for the AERMOD Meteorological Preprocessor (AERMET). (EPA454/B-03-002, November 2004 – Addendum December 2012). Office of Air Quality Planning and 

Standards, Research Triangle Park, North Carolina.

US EPA 2004c. User’s Guide for the AERMOD Terrain Preprocessor (AERMAP). (EPA-454/B-03-

003, October 2004 – Addendum March 2011). Office of Air Quality Planning and Standards, 

Research Triangle Park, North Carolina.

US EPA 2005. Guideline on Air Quality Models (Revised). Codified in the Appendix W to 40 CFR 

Part 51. Office of Air Quality Planning and Standards, Research Triangle Park, NC. November 2005.

US EPA 2008a. AERSURFACE User’s Guide. (EPA-454/B-08-001, January 2008). Office of Air 

Quality Planning and Standards, Research Triangle Park, North Carolina.
AECOM Environment

Four Corners Power Plant and Navajo Mine EIS / NAAQS Modeling Study April 2013

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US EPA 2008b. QA Handbook for Air Pollution Measurement System, Volume II. (EPA-454/B-08-

003, December 2008).

US EPA 2009. AERMOD Implementation Guide (Revised) (March 19, 2009). Office of Air Quality 

Planning and Standards, Research Triangle Park, North Carolina. 

US EPA 2011. Additional Clarification Regarding Application of Appendix W Modeling Guidance for 

the 1-hour NO2 NAAQS. (March 1, 2011). Office of Air Quality Planning and Standards, Research 

Triangle Park, North Carolina. Available at: http://www.epa.gov/ttn/scram/Additional_Clarifications_

AppendixW_Hourly-NO2-NAAQS_FINAL_03-01-2011.pdf [Accessed 05/15/2012].

US EPA 2012a. Source Specific Federal Implementation Plan for Implementing Best Available 

Retrofit Technology for Four Corners Power Plant (77 FR 51620). (August 24, 2012).

US EPA 2012b. Haul Road Workgroup Final Report. (March 2, 2012) 

http://www.epa.gov/ttn/scram/reports/Haul_Road_Workgroup-Final_Report_Package-20120302.pdf 

[Accessed 05/15/2012].

US EPA 2013a. Next Steps for Area Designations and Implementation of the Sulfur Dioxide NAAQS 

(February 6, 2013) http://www.epa.gov/airquality/sulfurdioxide/pdfs/20130207SO2StrategyPaper.pdf, 

(pages 2-3). [Accessed 02/08/2013].

US EPA 2013b. EPA Air Data website, (http://www.epa.gov/airdata/ad_rep_mon.html). [Accessed 

09/15/2012].

US Geological Survey (USGS) 1992. USGS Landcover Institute. 

http://edcftp.cr.usgs.gov/pub/data/landcover/states/. [Accessed 07/15/2012].

USGS. National Elevation Dataset. http://seamless.usgs.gov/. [Accessed 07/15/2012].
AECOM Environment

Four Corners Power Plant and Navajo Mine EIS / NAAQS Modeling Study April 2013

Appendix A

Emissions Calculations
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Four Corners Power Plant and Navajo Mine EIS / NAAQS Modeling Study April 2013

Appendix B

2011 EPA Clarification Memo
On pages 8 and 9 of the memo, EPA notes the following regarding intermittent sources such as emergency 

generators:

“...the intermittent nature of the actual emissions associated with emergency generators and 

startup/shutdown in many cases, when coupled with the probabilistic form of the standard, could result in 

modeled impacts being significantly higher than actual impacts would realistically be expected to be for 

these emission scenarios. The potential overestimation in these cases results from the implicit assumption 

that worst-case emissions will coincide with worst-case meteorological conditions based on the specific 

hours on specific days of each of the years associated with the modeled design value based on the form of 

the hourly standard. In fact, the probabilistic form of the standard is explicitly intended to provide a more 

stable metric for characterizing ambient air quality levels by mitigating the impact that outliers in the 

distribution might have on the design value. The February 9, 2010, preamble to the rule promulgating the 

new 1-hour NO2 standard stated that “it is desirable from a public health perspective to have a form that is 

reasonably stable and insulated from the impacts of extreme meteorological events.” 75 FR 6492. Also, the 

Clean Air Science Advisory Committee (CASAC) “recommended a 98 th -percentile form averaged over 3 

years for such a standard, given the potential for instability in the higher percentile concentrations around 

major roadways.” 75 FR 6493. 

To illustrate the importance of this point, consider the following example. Under a deterministic 1-hour 

standard, where the modeled design value would be based on the highest of the second-highest hourly 

impacts (allowing one exceedance per year), a single emission episode lasting 2 hours for an emergency 

generator or other intermittent emission scenario could determine the modeled design value if that episode 

coincided with worst-case meteorological conditions. While the probability of a particular 2-hour emission 

episode actually coinciding with the worst-case meteorological conditions is relatively low, there is 

nonetheless a clear linkage between a specific emission episode and the modeled design value. By 

contrast, under the form of the 1-hour NO2 NAAQS, only one hour from that emission episode could 

contribute to the modeled design value, i.e., the daily maximum 1-hour value. However, by assuming 

continuous operation of intermittent emissions the modeled design value for the 1-hour NO2 NAAQS 

effectively assumes that the intermittent emission scenario occurs on the specific hours of the specific days 

for each of the specific years of meteorological data included in the analysis which factor into the multiyear 

average of the 98th-percentile of the annual distribution of daily maximum 1-hour values. The probability of 

the controlling emission episode occurring on this particular temporal schedule to determine the design 

value under the probabilistic standard is significantly smaller than the probability of occurrence under the 

deterministic standard; thereby increasing the likelihood that impact estimates based on assuming 

continuous emissions would significantly overestimate actual impacts for these sources. 

Given the implications of the probabilistic form of the 1-hour NO2 NAAQS discussed above, we are 

concerned that assuming continuous operations for intermittent emissions would effectively impose an 

additional level of stringency beyond that intended by the level of the standard itself. As a result, we feel that 

it would be inappropriate to implement the 1-hour NO2 standard in such a manner and recommend that 

compliance demonstrations for the 1-hour NO2 NAAQS be based on emission scenarios that can logically 

be assumed to be relatively continuous or which occur frequently enough to contribute significantly to the 

annual distribution of daily maximum 1-hour concentrations. EPA believes that existing modeling guidelines 

provide sufficient discretion for reviewing authorities to exclude certain types of intermittent emissions from 

compliance demonstrations for the 1-hour NO2 standard under these circumstances.”
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Four Corners Power Plant and Navajo Mine EIS / NAAQS Modeling Study April 2013

Appendix C

Alternative Model Justification 

for Low Wind Speed 

Beta Options: 

AERMET and AERMOD
Appendix C

Alternative Model Justification for Low Wind Speed Beta Options: AERMET and 

AERMOD

Appendix W, Section 3.2.2 provides an approach for approval of an alternative model to 

determine whether it is more appropriate for the modeling application discussed here (Four 

Corners Power Plant and mine sources). These sources involve both low-level non-buoyant 

sources as well as tall stack buoyant releases.

EPA indicates that for this purpose, an alternative refined model may be used provided that: 

i. The model has received a scientific peer review; 

ii. The model can be demonstrated to be applicable to the problem on a theoretical basis; 

iii. The data bases which are necessary to perform the analysis are available and adequate; 

iv. Appropriate performance evaluations of the model have shown that the model is not biased 

toward underestimates; and 

v. A protocol on methods and procedures to be followed has been established.

These five points are discussed below.

The model selected for this modeling application is the low wind beta option provided in

AERMET, while using the default option in AERMOD. One update done for the beta AERMET 

application was the scaling of theta-star as the AERMOD formulation document1 specifies. This 

update is also discussed in an evaluation paper2 given at an Air & Waste Management 

Association specialty conference in March 2013.

i. The model has received a scientific peer review

The AERMET beta changes3 reference a Boundary-Layer Meteorology peer-reviewed paper4

that is the source of the AERMET formulation for changes in the friction velocity computation for 

low wind speeds.

ii. The model can be demonstrated to be applicable to the problem on a theoretical basis.

 1 http://www.epa.gov/ttn/scram/7thconf/aermod/aermod_mfd.pdf, page 20.

2 Connors. J. A., R. J. Paine, and S. R. Hanna, 2013. AERMOD Low Wind Speed Issues: 

Review of New Model Release. Control #3, presented at the Air & Waste Management Association specialty 

conference, Raleigh, NC, March 19-21, 2013.

3 http://www.epa.gov/ttn/scram/models/aermod/aermod_mcb8.txt. 

4 Qian, W., and A. Venkatram. Performance of Steady-State Dispersion Models Under Low Wind-Speed Conditions. 

Boundary-Layer Meteorology 138:475–491. (2011)
There is no known limitation to the AERMET changes – they are generally applicable. 

iii. The data bases which are necessary to perform the analysis are available and adequate.

Routine meteorological databases are sufficient for exercising this meteorological option.

iv. Appropriate performance evaluations of the model have shown that the model is not biased 

toward underestimates.

The two studies2,4 cited above provide this demonstration.

v. A protocol on methods and procedures to be followed has been established.

This report documents the methods and procedures followed.
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Appendix D

AERSURFACE Comparison
Navajo Met I - Average Conditions

Albedo

Bowen 

Ratio

Roughness 

Length Albedo

Bowen 

Ratio

Roughness 

Length Albedo

Bowen 

Ratio

Roughness 

Length

1 1 0.22 2.45 0.063 0.22 2.46 0.064 0 0.01 0.001

1 2 0.22 2.45 0.111 0.22 2.46 0.111 0 0.01 0

1 3 0.22 2.45 0.183 0.22 2.46 0.183 0 0.01 0

2 1 0.22 2.45 0.063 0.22 2.46 0.064 0 0.01 0.001

2 2 0.22 2.45 0.111 0.22 2.46 0.111 0 0.01 0

2 3 0.22 2.45 0.183 0.22 2.46 0.183 0 0.01 0

3 1 0.22 2.45 0.063 0.22 2.46 0.064 0 0.01 0.001

3 2 0.22 2.45 0.111 0.22 2.46 0.111 0 0.01 0

3 3 0.22 2.45 0.183 0.22 2.46 0.183 0 0.01 0

4 1 0.21 1.27 0.085 0.21 1.28 0.085 0 0.01 0

4 2 0.21 1.27 0.131 0.21 1.28 0.131 0 0.01 0

4 3 0.21 1.27 0.203 0.21 1.28 0.203 0 0.01 0

5 1 0.21 1.27 0.085 0.21 1.28 0.085 0 0.01 0

5 2 0.21 1.27 0.131 0.21 1.28 0.131 0 0.01 0

5 3 0.21 1.27 0.203 0.21 1.28 0.203 0 0.01 0

6 1 0.22 1.79 0.144 0.22 1.8 0.144 0 0.01 0

6 2 0.22 1.79 0.145 0.22 1.8 0.145 0 0.01 0

6 3 0.22 1.79 0.212 0.22 1.8 0.212 0 0.01 0

7 1 0.22 1.79 0.144 0.22 1.8 0.144 0 0.01 0

7 2 0.22 1.79 0.145 0.22 1.8 0.145 0 0.01 0

7 3 0.22 1.79 0.212 0.22 1.8 0.212 0 0.01 0

8 1 0.22 1.79 0.144 0.22 1.8 0.144 0 0.01 0

8 2 0.22 1.79 0.145 0.22 1.8 0.145 0 0.01 0

8 3 0.22 1.79 0.212 0.22 1.8 0.212 0 0.01 0

9 1 0.22 1.79 0.144 0.22 1.8 0.144 0 0.01 0

9 2 0.22 1.79 0.145 0.22 1.8 0.145 0 0.01 0

9 3 0.22 1.79 0.212 0.22 1.8 0.212 0 0.01 0

10 1 0.22 2.45 0.144 0.22 2.46 0.144 0 0.01 0

10 2 0.22 2.45 0.145 0.22 2.46 0.145 0 0.01 0

10 3 0.22 2.45 0.212 0.22 2.46 0.212 0 0.01 0

11 1 0.22 2.45 0.144 0.22 2.46 0.144 0 0.01 0

11 2 0.22 2.45 0.145 0.22 2.46 0.145 0 0.01 0

11 3 0.22 2.45 0.212 0.22 2.46 0.212 0 0.01 0

12 1 0.22 2.45 0.063 0.22 2.46 0.064 0 0.01 0.001

12 2 0.22 2.45 0.111 0.22 2.46 0.111 0 0.01 0

12 3 0.22 2.45 0.183 0.22 2.46 0.183 0 0.01 0

Month Sector

AERSURFACE (Version 08009) AERSURFACE (Version 13016) Difference
Navajo Met I - Wet Conditions

Albedo

Bowen 

Ratio

Roughness 

Length Albedo

Bowen 

Ratio

Roughness 

Length Albedo

Bowen 

Ratio

Roughness 

Length

1 1 0.220 1.050 0.063 0.220 1.050 0.064 -- -- 0.001

1 2 0.220 1.050 0.111 0.220 1.050 0.111 -- -- --

1 3 0.220 1.050 0.183 0.220 1.050 0.183 -- -- --

2 1 0.220 1.050 0.063 0.220 1.050 0.064 -- -- 0.001

2 2 0.220 1.050 0.111 0.220 1.050 0.111 -- -- --

2 3 0.220 1.050 0.183 0.220 1.050 0.183 -- -- --

3 1 0.220 1.050 0.063 0.220 1.050 0.064 -- -- 0.001

3 2 0.220 1.050 0.111 0.220 1.050 0.111 -- -- --

3 3 0.220 1.050 0.183 0.220 1.050 0.183 -- -- --

4 1 0.210 0.590 0.085 0.210 0.600 0.085 -- 0.010 --

4 2 0.210 0.590 0.131 0.210 0.600 0.131 -- 0.010 --

4 3 0.210 0.590 0.203 0.210 0.600 0.203 -- 0.010 --

5 1 0.210 0.590 0.085 0.210 0.600 0.085 -- 0.010 --

5 2 0.210 0.590 0.131 0.210 0.600 0.131 -- 0.010 --

5 3 0.210 0.590 0.203 0.210 0.600 0.203 -- 0.010 --

6 1 0.220 0.830 0.144 0.220 0.830 0.144 -- -- --

6 2 0.220 0.830 0.145 0.220 0.830 0.145 -- -- --

6 3 0.220 0.830 0.212 0.220 0.830 0.212 -- -- --

7 1 0.220 0.830 0.144 0.220 0.830 0.144 -- -- --

7 2 0.220 0.830 0.145 0.220 0.830 0.145 -- -- --

7 3 0.220 0.830 0.212 0.220 0.830 0.212 -- -- --

8 1 0.220 0.830 0.144 0.220 0.830 0.144 -- -- --

8 2 0.220 0.830 0.145 0.220 0.830 0.145 -- -- --

8 3 0.220 0.830 0.212 0.220 0.830 0.212 -- -- --

9 1 0.220 0.830 0.144 0.220 0.830 0.144 -- -- --

9 2 0.220 0.830 0.145 0.220 0.830 0.145 -- -- --

9 3 0.220 0.830 0.212 0.220 0.830 0.212 -- -- --

10 1 0.220 1.050 0.144 0.220 1.050 0.144 -- -- --

10 2 0.220 1.050 0.145 0.220 1.050 0.145 -- -- --

10 3 0.220 1.050 0.212 0.220 1.050 0.212 -- -- --

11 1 0.220 1.050 0.144 0.220 1.050 0.144 -- -- --

11 2 0.220 1.050 0.145 0.220 1.050 0.145 -- -- --

11 3 0.220 1.050 0.212 0.220 1.050 0.212 -- -- --

12 1 0.220 1.050 0.063 0.220 1.050 0.064 -- -- 0.001

12 2 0.220 1.050 0.111 0.220 1.050 0.111 -- -- --

12 3 0.220 1.050 0.183 0.220 1.050 0.183 -- -- --

Month Sector

AERSURFACE (Version 08009) AERSURFACE (Version 13016) Difference
Navajo Met I - Dry Conditions

Albedo

Bowen 

Ratio

Roughness 

Length Albedo

Bowen 

Ratio

Roughness 

Length Albedo

Bowen 

Ratio

Roughness 

Length

1 1 0.22 4.43 0.063 0.22 4.44 0.064 -- 0.010 0.001

1 2 0.22 4.43 0.111 0.22 4.44 0.111 -- 0.010 --

1 3 0.22 4.43 0.183 0.22 4.44 0.183 -- 0.010 --

2 1 0.22 4.43 0.063 0.22 4.44 0.064 -- 0.010 0.001

2 2 0.22 4.43 0.111 0.22 4.44 0.111 -- 0.010 --

2 3 0.22 4.43 0.183 0.22 4.44 0.183 -- 0.010 --

3 1 0.22 4.43 0.063 0.22 4.44 0.064 -- 0.010 0.001

3 2 0.22 4.43 0.111 0.22 4.44 0.111 -- 0.010 --

3 3 0.22 4.43 0.183 0.22 4.44 0.183 -- 0.010 --

4 1 0.21 2.43 0.085 0.21 2.44 0.085 -- 0.010 --

4 2 0.21 2.43 0.131 0.21 2.44 0.131 -- 0.010 --

4 3 0.21 2.43 0.203 0.21 2.44 0.203 -- 0.010 --

5 1 0.21 2.43 0.085 0.21 2.44 0.085 -- 0.010 --

5 2 0.21 2.43 0.131 0.21 2.44 0.131 -- 0.010 --

5 3 0.21 2.43 0.203 0.21 2.44 0.203 -- 0.010 --

6 1 0.22 3.19 0.144 0.22 3.2 0.144 -- 0.010 --

6 2 0.22 3.19 0.145 0.22 3.2 0.145 -- 0.010 --

6 3 0.22 3.19 0.212 0.22 3.2 0.212 -- 0.010 --

7 1 0.22 3.19 0.144 0.22 3.2 0.144 -- 0.010 --

7 2 0.22 3.19 0.145 0.22 3.2 0.145 -- 0.010 --

7 3 0.22 3.19 0.212 0.22 3.2 0.212 -- 0.010 --

8 1 0.22 3.19 0.144 0.22 3.2 0.144 -- 0.010 --

8 2 0.22 3.19 0.145 0.22 3.2 0.145 -- 0.010 --

8 3 0.22 3.19 0.212 0.22 3.2 0.212 -- 0.010 --

9 1 0.22 3.19 0.144 0.22 3.2 0.144 -- 0.010 --

9 2 0.22 3.19 0.145 0.22 3.2 0.145 -- 0.010 --

9 3 0.22 3.19 0.212 0.22 3.2 0.212 -- 0.010 --

10 1 0.22 4.43 0.144 0.22 4.44 0.144 -- 0.010 --

10 2 0.22 4.43 0.145 0.22 4.44 0.145 -- 0.010 --

10 3 0.22 4.43 0.212 0.22 4.44 0.212 -- 0.010 --

11 1 0.22 4.43 0.144 0.22 4.44 0.144 -- 0.010 --

11 2 0.22 4.43 0.145 0.22 4.44 0.145 -- 0.010 --

11 3 0.22 4.43 0.212 0.22 4.44 0.212 -- 0.010 --

12 1 0.22 4.43 0.063 0.22 4.44 0.064 -- 0.010 0.001

12 2 0.22 4.43 0.111 0.22 4.44 0.111 -- 0.010 --

12 3 0.22 4.43 0.183 0.22 4.44 0.183 -- 0.010 --

AERSURFACE (Version 08009) AERSURFACE (Version 13016) Difference

Month Sector
Navajo Met III - Average Conditions

Albedo

Bowen 

Ratio

Roughness 

Length Albedo

Bowen 

Ratio

Roughness 

Length Albedo

Bowen 

Ratio

Rou

Le

1 1 0.24 4.55 0.148 0.24 4.55 0.148 -- -- 1 2 0.24 4.55 0.107 0.24 4.55 0.107 -- -- 2 1 0.24 4.55 0.148 0.24 4.55 0.148 -- -- 2 2 0.24 4.55 0.107 0.24 4.55 0.107 -- -- 3 1 0.24 4.55 0.148 0.24 4.55 0.148 -- -- 3 2 0.24 4.55 0.107 0.24 4.55 0.107 -- -- 4 1 0.24 2.25 0.149 0.24 2.25 0.149 -- -- 4 2 0.24 2.25 0.131 0.24 2.25 0.131 -- -- 5 1 0.24 2.25 0.149 0.24 2.25 0.149 -- -- 5 2 0.24 2.25 0.131 0.24 2.25 0.131 -- -- 6 1 0.24 3.14 0.15 0.24 3.13 0.15 -- -0.01 6 2 0.24 3.14 0.143 0.24 3.13 0.143 -- -0.01 7 1 0.24 3.14 0.15 0.24 3.13 0.15 -- -0.01 7 2 0.24 3.14 0.143 0.24 3.13 0.143 -- -0.01 8 1 0.24 3.14 0.15 0.24 3.13 0.15 -- -0.01 8 2 0.24 3.14 0.143 0.24 3.13 0.143 -- -0.01 9 1 0.24 3.14 0.15 0.24 3.13 0.15 -- -0.01 9 2 0.24 3.14 0.143 0.24 3.13 0.143 -- -0.01 

10 1 0.24 4.55 0.15 0.24 4.55 0.15 -- -- 10 2 0.24 4.55 0.143 0.24 4.55 0.143 -- -- 11 1 0.24 4.55 0.15 0.24 4.55 0.15 -- -- 11 2 0.24 4.55 0.143 0.24 4.55 0.143 -- -- 12 1 0.24 4.55 0.148 0.24 4.55 0.148 -- -- 12 2 0.24 4.55 0.107 0.24 4.55 0.107 -- -- 

Month Sector

AERSURFACE (Version 08009) AERSURFACE (Version 13016) Difference
Navajo Met III - Wet Conditions

Albedo

Bowen 

Ratio

Roughness 

Length Albedo

Bowen 

Ratio

Roughness 

Length Albedo

Bowen 

Ratio

Rough

Len

1 1 0.24 1.63 0.148 0.24 1.63 0.148 -- -- -1 2 0.24 1.63 0.107 0.24 1.63 0.107 -- -- -2 1 0.24 1.63 0.148 0.24 1.63 0.148 -- -- -2 2 0.24 1.63 0.107 0.24 1.63 0.107 -- -- -3 1 0.24 1.63 0.148 0.24 1.63 0.148 -- -- -3 2 0.24 1.63 0.107 0.24 1.63 0.107 -- -- -4 1 0.24 0.85 0.149 0.24 0.85 0.149 -- -- -4 2 0.24 0.85 0.131 0.24 0.85 0.131 -- -- -5 1 0.24 0.85 0.149 0.24 0.85 0.149 -- -- -5 2 0.24 0.85 0.131 0.24 0.85 0.131 -- -- -6 1 0.24 1.24 0.15 0.24 1.24 0.15 -- -- -6 2 0.24 1.24 0.143 0.24 1.24 0.143 -- -- -7 1 0.24 1.24 0.15 0.24 1.24 0.15 -- -- -7 2 0.24 1.24 0.143 0.24 1.24 0.143 -- -- -8 1 0.24 1.24 0.15 0.24 1.24 0.15 -- -- -8 2 0.24 1.24 0.143 0.24 1.24 0.143 -- -- -9 1 0.24 1.24 0.15 0.24 1.24 0.15 -- -- -9 2 0.24 1.24 0.143 0.24 1.24 0.143 -- -- -

10 1 0.24 1.63 0.15 0.24 1.63 0.15 -- -- -10 2 0.24 1.63 0.143 0.24 1.63 0.143 -- -- -11 1 0.24 1.63 0.15 0.24 1.63 0.15 -- -- -11 2 0.24 1.63 0.143 0.24 1.63 0.143 -- -- -12 1 0.24 1.63 0.148 0.24 1.63 0.148 -- -- -12 2 0.24 1.63 0.107 0.24 1.63 0.107 -- -- -

Month Sector

AERSURFACE (Version 08009) AERSURFACE (Version 13016) Difference
Navajo Met III - Dry Conditions

Albedo

Bowen 

Ratio

Roughness 

Length Albedo

Bowen 

Ratio

Roughness 

Length Albedo

Bowen 

Ratio

Roughness 

Length

1 1 0.24 7.79 0.148 0.24 7.79 0.148 -- -- -- 1 2 0.24 7.79 0.107 0.24 7.79 0.107 -- -- -- 2 1 0.24 7.79 0.148 0.24 7.79 0.148 -- -- -- 2 2 0.24 7.79 0.107 0.24 7.79 0.107 -- -- -- 3 1 0.24 7.79 0.148 0.24 7.79 0.148 -- -- -- 3 2 0.24 7.79 0.107 0.24 7.79 0.107 -- -- -- 4 1 0.24 3.97 0.149 0.24 3.97 0.149 -- -- -- 4 2 0.24 3.97 0.131 0.24 3.97 0.131 -- -- -- 5 1 0.24 3.97 0.149 0.24 3.97 0.149 -- -- -- 5 2 0.24 3.97 0.131 0.24 3.97 0.131 -- -- -- 6 1 0.24 5.08 0.15 0.24 5.08 0.15 -- -- -- 6 2 0.24 5.08 0.143 0.24 5.08 0.143 -- -- -- 7 1 0.24 5.08 0.15 0.24 5.08 0.15 -- -- -- 7 2 0.24 5.08 0.143 0.24 5.08 0.143 -- -- -- 8 1 0.24 5.08 0.15 0.24 5.08 0.15 -- -- -- 8 2 0.24 5.08 0.143 0.24 5.08 0.143 -- -- -- 9 1 0.24 5.08 0.15 0.24 5.08 0.15 -- -- -- 9 2 0.24 5.08 0.143 0.24 5.08 0.143 -- -- -- 10 1 0.24 7.79 0.15 0.24 7.79 0.15 -- -- -- 10 2 0.24 7.79 0.143 0.24 7.79 0.143 -- -- -- 11 1 0.24 7.79 0.15 0.24 7.79 0.15 -- -- -- 11 2 0.24 7.79 0.143 0.24 7.79 0.143 -- -- -- 12 1 0.24 7.79 0.148 0.24 7.79 0.148 -- -- -- 12 2 0.24 7.79 0.107 0.24 7.79 0.107 -- -- -- AERSURFACE (Version 08009) AERSURFACE (Version 13016) Difference Month Sector
AECOM Environment

Four Corners Power Plant and Navajo Mine EIS / NAAQS Modeling Study April 2013

Appendix E

30-Year Precipitation Data
AERSURFACE Worksheet - 2006

Monthly Averaged Precipitation (inches) at Farmington, NV

Values are specified by the user.

Jan Feb Mar Apr May Jun Jul Aug Sep Oct N

1977 1.67 0.08 0.04 0.01 1.17 0.08 0.58 1.58 0.51 0.63 0

1978 0.71 0.69 1.12 0.62 0.74 0.00 0.08 0.18 1.48 1.49 11979 0.23 0.23 0.04 0.33 0.58 0.46 1.00 0.40 0.05 1.10 01980 1.06 1.16 0.56 0.06 0.18 0.00 0.09 0.77 0.89 0.81 01981 0.00 0.14 1.28 0.37 1.04 0.47 0.71 0.23 0.75 0.92 01982 0.22 0.83 0.79 0.59 0.72 0.00 0.96 3.69 1.25 0.11 01983 0.94 0.58 1.58 0.30 0.04 0.34 1.63 0.63 0.51 0.30 11984 0.03 0.31 0.28 0.68 0.05 0.50 0.47 0.26 0.10 2.00 01985 0.30 0.20 1.70 1.90 0.40 0.00 1.20 0.40 1.30 1.20 01986 0.10 0.70 0.60 1.00 0.20 1.00 3.50 0.90 2.00 0.90 21987 0.20 1.70 0.70 0.20 0.60 0.00 0.20 1.10 0.20 1.00 11988 0.60 0.90 0.10 0.60 1.00 0.30 0.40 1.80 0.20 0.20 01989 1.00 0.40 0.00 0.00 0.00 0.00 2.10 0.50 0.10 0.40 01990 0.60 0.50 0.60 0.70 0.90 0.10 0.30 1.10 1.80 1.00 01991 0.50 0.50 0.50 0.00 0.50 0.40 0.30 0.50 1.30 0.20 21992 0.20 0.40 0.80 0.20 1.60 0.00 0.90 1.20 0.80 0.40 01993 2.20 1.20 0.70 0.30 0.40 0.00 0.00 2.10 0.70 1.20 01994 0.10 0.50 0.20 0.50 1.20 0.00 0.20 0.60 1.60 0.50 01995 0.40 0.00 1.10 1.10 0.60 0.00 0.00 1.80 2.00 0.00 01996 0.30 0.20 0.10 0.10 0.00 0.50 0.40 1.00 0.60 2.10 01997 1.00 0.50 0.00 2.50 0.00 0.40 1.20 1.60 1.80 0.20 01998 0.20 0.60 0.20 0.00 0.00 0.00 1.10 1.50 0.60 2.00 11999 0.10 0.10 0.00 1.10 1.20 0.20 2.10 0.80 0.20 0.00 02000 0.60 0.20 2.10 0.10 0.00 0.30 1.00 1.10 0.40 1.80 02001 0.40 0.90 1.30 0.60 0.90 0.00 0.70 0.70 0.00 0.20 02002 0.00 0.00 0.10 0.30 0.00 0.00 0.40 0.20 3.20 0.00 02003 0.20 1.10 0.00 0.10 0.10 0.00 0.10 1.00 0.80 0.70 02004 0.40 0.80 0.00 2.00 0.00 0.20 0.20 0.10 2.70 0.40 02005 1.33 1.70 0.40 0.80 0.50 0.00 0.50 1.70 0.40 0.90 02006 0.46 0.01 0.84 0.60 0.03 0.12 2.41 0.18 1.68 2.61 0

30-Year Avg 0.54 0.57 0.59 0.59 0.49 0.18 0.82 0.99 1.00 0.84 0

2006 0.46 0.01 0.84 0.60 0.03 0.12 2.41 0.18 1.68 2.61 030th percentile 0.20 0.22 0.10 0.20 0.05 0.00 0.30 0.50 0.48 0.37 070th percentile 0.60 0.73 0.79 0.64 0.73 0.30 1.00 1.13 1.35 1.03 0Average / Dry / Wet ? Average Dry Wet Average Dry Average Wet Dry Wet Wet D

Year Precipitation (inches) Amount for 30 Years at APS Four Corners Power Plant, NV - 2006
AERSURFACE Worksheet - 2007

Monthly Averaged Precipitation (inches) at Farmington, NV

Values are specified by the user.

Jan Feb Mar Apr May Jun Jul Aug Sep Oct N

1978 0.71 0.69 1.12 0.62 0.74 0.00 0.08 0.18 1.48 1.49 1

1979 0.23 0.23 0.04 0.33 0.58 0.46 1.00 0.40 0.05 1.10 01980 1.06 1.16 0.56 0.06 0.18 0.00 0.09 0.77 0.89 0.81 01981 0.00 0.14 1.28 0.37 1.04 0.47 0.71 0.23 0.75 0.92 01982 0.22 0.83 0.79 0.59 0.72 0.00 0.96 3.69 1.25 0.11 01983 0.94 0.58 1.58 0.30 0.04 0.34 1.63 0.63 0.51 0.30 11984 0.03 0.31 0.28 0.68 0.05 0.50 0.47 0.26 0.10 2.00 01985 0.30 0.20 1.70 1.90 0.40 0.00 1.20 0.40 1.30 1.20 01986 0.10 0.70 0.60 1.00 0.20 1.00 3.50 0.90 2.00 0.90 21987 0.20 1.70 0.70 0.20 0.60 0.00 0.20 1.10 0.20 1.00 11988 0.60 0.90 0.10 0.60 1.00 0.30 0.40 1.80 0.20 0.20 01989 1.00 0.40 0.00 0.00 0.00 0.00 2.10 0.50 0.10 0.40 01990 0.60 0.50 0.60 0.70 0.90 0.10 0.30 1.10 1.80 1.00 01991 0.50 0.50 0.50 0.00 0.50 0.40 0.30 0.50 1.30 0.20 21992 0.20 0.40 0.80 0.20 1.60 0.00 0.90 1.20 0.80 0.40 01993 2.20 1.20 0.70 0.30 0.40 0.00 0.00 2.10 0.70 1.20 01994 0.10 0.50 0.20 0.50 1.20 0.00 0.20 0.60 1.60 0.50 01995 0.40 0.00 1.10 1.10 0.60 0.00 0.00 1.80 2.00 0.00 01996 0.30 0.20 0.10 0.10 0.00 0.50 0.40 1.00 0.60 2.10 01997 1.00 0.50 0.00 2.50 0.00 0.40 1.20 1.60 1.80 0.20 01998 0.20 0.60 0.20 0.00 0.00 0.00 1.10 1.50 0.60 2.00 11999 0.10 0.10 0.00 1.10 1.20 0.20 2.10 0.80 0.20 0.00 02000 0.60 0.20 2.10 0.10 0.00 0.30 1.00 1.10 0.40 1.80 02001 0.40 0.90 1.30 0.60 0.90 0.00 0.70 0.70 0.00 0.20 02002 0.00 0.00 0.10 0.30 0.00 0.00 0.40 0.20 3.20 0.00 02003 0.20 1.10 0.00 0.10 0.10 0.00 0.10 1.00 0.80 0.70 02004 0.40 0.80 0.00 2.00 0.00 0.20 0.20 0.10 2.70 0.40 02005 1.33 1.70 0.40 0.80 0.50 0.00 0.50 1.70 0.40 0.90 02006 0.46 0.01 0.84 0.60 0.03 0.12 2.41 0.18 1.68 2.61 02007 0.44 0.81 0.83 0.34 1.31 0.12 0.45 0.93 0.47 0.23 0

30-Year Avg 0.49 0.60 0.62 0.60 0.49 0.18 0.82 0.97 1.00 0.83 0

2007 0.44 0.81 0.83 0.34 1.31 0.12 0.45 0.93 0.47 0.23 030th percentile 0.20 0.29 0.17 0.27 0.05 0.00 0.30 0.50 0.45 0.28 070th percentile 0.60 0.80 0.81 0.64 0.73 0.30 1.00 1.10 1.35 1.03 0Average / Dry / Wet ? Average Wet Wet Average Wet Average Average Average Average Dry D

Year Precipitation (inches) Amount for 30 Years at APS Four Corners Power Plant, NV - 2007
AERSURFACE Worksheet - 2008

Monthly Averaged Precipitation (inches) at Farmington, NV

Values are specified by the user.

Jan Feb Mar Apr May Jun Jul Aug Sep Oct N

1979 0.23 0.23 0.04 0.33 0.58 0.46 1.00 0.40 0.05 1.10 0

1980 1.06 1.16 0.56 0.06 0.18 0.00 0.09 0.77 0.89 0.81 01981 0.00 0.14 1.28 0.37 1.04 0.47 0.71 0.23 0.75 0.92 01982 0.22 0.83 0.79 0.59 0.72 0.00 0.96 3.69 1.25 0.11 01983 0.94 0.58 1.58 0.30 0.04 0.34 1.63 0.63 0.51 0.30 11984 0.03 0.31 0.28 0.68 0.05 0.50 0.47 0.26 0.10 2.00 01985 0.30 0.20 1.70 1.90 0.40 0.00 1.20 0.40 1.30 1.20 01986 0.10 0.70 0.60 1.00 0.20 1.00 3.50 0.90 2.00 0.90 21987 0.20 1.70 0.70 0.20 0.60 0.00 0.20 1.10 0.20 1.00 11988 0.60 0.90 0.10 0.60 1.00 0.30 0.40 1.80 0.20 0.20 01989 1.00 0.40 0.00 0.00 0.00 0.00 2.10 0.50 0.10 0.40 01990 0.60 0.50 0.60 0.70 0.90 0.10 0.30 1.10 1.80 1.00 01991 0.50 0.50 0.50 0.00 0.50 0.40 0.30 0.50 1.30 0.20 21992 0.20 0.40 0.80 0.20 1.60 0.00 0.90 1.20 0.80 0.40 01993 2.20 1.20 0.70 0.30 0.40 0.00 0.00 2.10 0.70 1.20 01994 0.10 0.50 0.20 0.50 1.20 0.00 0.20 0.60 1.60 0.50 01995 0.40 0.00 1.10 1.10 0.60 0.00 0.00 1.80 2.00 0.00 01996 0.30 0.20 0.10 0.10 0.00 0.50 0.40 1.00 0.60 2.10 01997 1.00 0.50 0.00 2.50 0.00 0.40 1.20 1.60 1.80 0.20 01998 0.20 0.60 0.20 0.00 0.00 0.00 1.10 1.50 0.60 2.00 11999 0.10 0.10 0.00 1.10 1.20 0.20 2.10 0.80 0.20 0.00 02000 0.60 0.20 2.10 0.10 0.00 0.30 1.00 1.10 0.40 1.80 02001 0.40 0.90 1.30 0.60 0.90 0.00 0.70 0.70 0.00 0.20 02002 0.00 0.00 0.10 0.30 0.00 0.00 0.40 0.20 3.20 0.00 02003 0.20 1.10 0.00 0.10 0.10 0.00 0.10 1.00 0.80 0.70 02004 0.40 0.80 0.00 2.00 0.00 0.20 0.20 0.10 2.70 0.40 02005 1.33 1.70 0.40 0.80 0.50 0.00 0.50 1.70 0.40 0.90 02006 0.46 0.01 0.84 0.60 0.03 0.12 2.41 0.18 1.68 2.61 02007 0.44 0.81 0.83 0.34 1.31 0.12 0.45 0.93 0.47 0.23 02008 1.60 0.77 0.07 0.12 0.55 0.02 0.13 1.34 0.33 0.57 0

30-Year Avg 0.52 0.60 0.58 0.58 0.49 0.18 0.82 1.00 0.96 0.80 0

2008 1.60 0.77 0.07 0.12 0.55 0.02 0.13 1.34 0.33 0.57 030th percentile 0.20 0.29 0.10 0.20 0.05 0.00 0.30 0.57 0.40 0.28 070th percentile 0.60 0.80 0.79 0.62 0.64 0.30 1.00 1.13 1.30 1.00 0Average / Dry / Wet ? Wet Average Dry Dry Average Average Dry Wet Dry Average Ave

Year Precipitation (inches) Amount for 30 Years at APS Four Corners Power Plant, NV - 2008
AERSURFACE Worksheet - 2009

Monthly Averaged Precipitation (inches) at Farmington, NV

Values are specified by the user.

. Feb Mar Apr May Jun Jul Aug Sep Oct N

1980 1.06 1.16 0.56 0.06 0.18 0.00 0.09 0.77 0.89 0.81 0

1981 0.00 0.14 1.28 0.37 1.04 0.47 0.71 0.23 0.75 0.92 01982 0.22 0.83 0.79 0.59 0.72 0.00 0.96 3.69 1.25 0.11 01983 0.94 0.58 1.58 0.30 0.04 0.34 1.63 0.63 0.51 0.30 11984 0.03 0.31 0.28 0.68 0.05 0.50 0.47 0.26 0.10 2.00 01985 0.30 0.20 1.70 1.90 0.40 0.00 1.20 0.40 1.30 1.20 01986 0.10 0.70 0.60 1.00 0.20 1.00 3.50 0.90 2.00 0.90 21987 0.20 1.70 0.70 0.20 0.60 0.00 0.20 1.10 0.20 1.00 11988 0.60 0.90 0.10 0.60 1.00 0.30 0.40 1.80 0.20 0.20 01989 1.00 0.40 0.00 0.00 0.00 0.00 2.10 0.50 0.10 0.40 01990 0.60 0.50 0.60 0.70 0.90 0.10 0.30 1.10 1.80 1.00 01991 0.50 0.50 0.50 0.00 0.50 0.40 0.30 0.50 1.30 0.20 21992 0.20 0.40 0.80 0.20 1.60 0.00 0.90 1.20 0.80 0.40 01993 2.20 1.20 0.70 0.30 0.40 0.00 0.00 2.10 0.70 1.20 01994 0.10 0.50 0.20 0.50 1.20 0.00 0.20 0.60 1.60 0.50 01995 0.40 0.00 1.10 1.10 0.60 0.00 0.00 1.80 2.00 0.00 01996 0.30 0.20 0.10 0.10 0.00 0.50 0.40 1.00 0.60 2.10 01997 1.00 0.50 0.00 2.50 0.00 0.40 1.20 1.60 1.80 0.20 01998 0.20 0.60 0.20 0.00 0.00 0.00 1.10 1.50 0.60 2.00 11999 0.10 0.10 0.00 1.10 1.20 0.20 2.10 0.80 0.20 0.00 02000 0.60 0.20 2.10 0.10 0.00 0.30 1.00 1.10 0.40 1.80 02001 0.40 0.90 1.30 0.60 0.90 0.00 0.70 0.70 0.00 0.20 02002 0.00 0.00 0.10 0.30 0.00 0.00 0.40 0.20 3.20 0.00 02003 0.20 1.10 0.00 0.10 0.10 0.00 0.10 1.00 0.80 0.70 02004 0.40 0.80 0.00 2.00 0.00 0.20 0.20 0.10 2.70 0.40 02005 1.33 1.70 0.40 0.80 0.50 0.00 0.50 1.70 0.40 0.90 02006 0.46 0.01 0.84 0.60 0.03 0.12 2.41 0.18 1.68 2.61 02007 0.44 0.81 0.83 0.34 1.31 0.12 0.45 0.93 0.47 0.23 02008 1.60 0.77 0.07 0.12 0.55 0.02 0.13 1.34 0.33 0.57 02009 0.37 0.56 0.29 0.38 0.93 0.46 0.04 0.17 0.08 0.81 0

30-Year Avg 0.53 0.61 0.59 0.58 0.50 0.18 0.79 1.00 0.96 0.79 0

2009 0.37 0.56 0.29 0.38 0.93 0.46 0.04 0.17 0.08 0.81 030th percentile 0.20 0.37 0.17 0.20 0.05 0.00 0.27 0.57 0.40 0.28 070th percentile 0.60 0.80 0.79 0.62 0.77 0.30 0.97 1.13 1.30 0.94 0Average / Dry / Wet ? Average Average Average Average Wet Wet Dry Dry Dry Average Ave

Year Precipitation (inches) Amount for 30 Years at APS Four Corners Power Plant, NV - 2009
AERSURFACE Worksheet - 2010

Monthly Averaged Precipitation (inches) at Farmington, NV

Values are specified by the user.

Jan Feb Mar Apr May Jun Jul Aug Sep Oct N

1981 0.00 0.14 1.28 0.37 1.04 0.47 0.71 0.23 0.75 0.92 0

1982 0.22 0.83 0.79 0.59 0.72 0.00 0.96 3.69 1.25 0.11 01983 0.94 0.58 1.58 0.30 0.04 0.34 1.63 0.63 0.51 0.30 11984 0.03 0.31 0.28 0.68 0.05 0.50 0.47 0.26 0.10 2.00 01985 0.30 0.20 1.70 1.90 0.40 0.00 1.20 0.40 1.30 1.20 01986 0.10 0.70 0.60 1.00 0.20 1.00 3.50 0.90 2.00 0.90 21987 0.20 1.70 0.70 0.20 0.60 0.00 0.20 1.10 0.20 1.00 11988 0.60 0.90 0.10 0.60 1.00 0.30 0.40 1.80 0.20 0.20 01989 1.00 0.40 0.00 0.00 0.00 0.00 2.10 0.50 0.10 0.40 01990 0.60 0.50 0.60 0.70 0.90 0.10 0.30 1.10 1.80 1.00 01991 0.50 0.50 0.50 0.00 0.50 0.40 0.30 0.50 1.30 0.20 21992 0.20 0.40 0.80 0.20 1.60 0.00 0.90 1.20 0.80 0.40 01993 2.20 1.20 0.70 0.30 0.40 0.00 0.00 2.10 0.70 1.20 01994 0.10 0.50 0.20 0.50 1.20 0.00 0.20 0.60 1.60 0.50 01995 0.40 0.00 1.10 1.10 0.60 0.00 0.00 1.80 2.00 0.00 01996 0.30 0.20 0.10 0.10 0.00 0.50 0.40 1.00 0.60 2.10 01997 1.00 0.50 0.00 2.50 0.00 0.40 1.20 1.60 1.80 0.20 01998 0.20 0.60 0.20 0.00 0.00 0.00 1.10 1.50 0.60 2.00 11999 0.10 0.10 0.00 1.10 1.20 0.20 2.10 0.80 0.20 0.00 02000 0.60 0.20 2.10 0.10 0.00 0.30 1.00 1.10 0.40 1.80 02001 0.40 0.90 1.30 0.60 0.90 0.00 0.70 0.70 0.00 0.20 02002 0.00 0.00 0.10 0.30 0.00 0.00 0.40 0.20 3.20 0.00 02003 0.20 1.10 0.00 0.10 0.10 0.00 0.10 1.00 0.80 0.70 02004 0.40 0.80 0.00 2.00 0.00 0.20 0.20 0.10 2.70 0.40 02005 1.33 1.70 0.40 0.80 0.50 0.00 0.50 1.70 0.40 0.90 02006 0.46 0.01 0.84 0.60 0.03 0.12 2.41 0.18 1.68 2.61 02007 0.44 0.81 0.83 0.34 1.31 0.12 0.45 0.93 0.47 0.23 02008 1.60 0.77 0.07 0.12 0.55 0.02 0.13 1.34 0.33 0.57 02009 0.37 0.56 0.29 0.38 0.93 0.46 0.04 0.17 0.08 0.81 02010 1.93 1.08 0.76 0.40 0.03 0.19 0.66 1.97 0.99 0.91 0

30-Year Avg 0.56 0.61 0.60 0.60 0.49 0.19 0.81 1.04 0.96 0.79 0

2010 1.93 1.08 0.76 0.40 0.03 0.19 0.66 1.97 0.99 0.91 030th percentile 0.20 0.37 0.17 0.27 0.04 0.00 0.30 0.57 0.40 0.28 070th percentile 0.60 0.80 0.79 0.62 0.77 0.30 0.97 1.24 1.30 0.94 0Average / Dry / Wet ? Wet Wet Average Average Dry Average Average Wet Average Average D

Year Precipitation (inches) Amount for 30 Years at APS Four Corners Power Plant, NV - 2010
AERSURFACE Worksheet - 2011

Monthly Averaged Precipitation (inches) at Farmington, NV

Values are specified by the user.

Jan Feb Mar Apr May Jun Jul Aug Sep Oct N

1982 0.22 0.83 0.79 0.59 0.72 0.00 0.96 3.69 1.25 0.11 0

1983 0.94 0.58 1.58 0.30 0.04 0.34 1.63 0.63 0.51 0.30 11984 0.03 0.31 0.28 0.68 0.05 0.50 0.47 0.26 0.10 2.00 01985 0.30 0.20 1.70 1.90 0.40 0.00 1.20 0.40 1.30 1.20 01986 0.10 0.70 0.60 1.00 0.20 1.00 3.50 0.90 2.00 0.90 21987 0.20 1.70 0.70 0.20 0.60 0.00 0.20 1.10 0.20 1.00 11988 0.60 0.90 0.10 0.60 1.00 0.30 0.40 1.80 0.20 0.20 01989 1.00 0.40 0.00 0.00 0.00 0.00 2.10 0.50 0.10 0.40 01990 0.60 0.50 0.60 0.70 0.90 0.10 0.30 1.10 1.80 1.00 01991 0.50 0.50 0.50 0.00 0.50 0.40 0.30 0.50 1.30 0.20 21992 0.20 0.40 0.80 0.20 1.60 0.00 0.90 1.20 0.80 0.40 01993 2.20 1.20 0.70 0.30 0.40 0.00 0.00 2.10 0.70 1.20 01994 0.10 0.50 0.20 0.50 1.20 0.00 0.20 0.60 1.60 0.50 01995 0.40 0.00 1.10 1.10 0.60 0.00 0.00 1.80 2.00 0.00 01996 0.30 0.20 0.10 0.10 0.00 0.50 0.40 1.00 0.60 2.10 01997 1.00 0.50 0.00 2.50 0.00 0.40 1.20 1.60 1.80 0.20 01998 0.20 0.60 0.20 0.00 0.00 0.00 1.10 1.50 0.60 2.00 11999 0.10 0.10 0.00 1.10 1.20 0.20 2.10 0.80 0.20 0.00 02000 0.60 0.20 2.10 0.10 0.00 0.30 1.00 1.10 0.40 1.80 02001 0.40 0.90 1.30 0.60 0.90 0.00 0.70 0.70 0.00 0.20 02002 0.00 0.00 0.10 0.30 0.00 0.00 0.40 0.20 3.20 0.00 02003 0.20 1.10 0.00 0.10 0.10 0.00 0.10 1.00 0.80 0.70 02004 0.40 0.80 0.00 2.00 0.00 0.20 0.20 0.10 2.70 0.40 02005 1.33 1.70 0.40 0.80 0.50 0.00 0.50 1.70 0.40 0.90 02006 0.46 0.01 0.84 0.60 0.03 0.12 2.41 0.18 1.68 2.61 02007 0.44 0.81 0.83 0.34 1.31 0.12 0.45 0.93 0.47 0.23 02008 1.60 0.77 0.07 0.12 0.55 0.02 0.13 1.34 0.33 0.57 02009 0.37 0.56 0.29 0.38 0.93 0.46 0.04 0.17 0.08 0.81 02010 1.93 1.08 0.76 0.40 0.03 0.19 0.66 1.97 0.99 0.91 02011 0.03 0.18 0.36 1.20 0.72 0.00 0.51 0.14 2.39 1.72 0

30-Year Avg 0.56 0.61 0.57 0.62 0.48 0.17 0.80 1.03 1.02 0.82 0

2011 0.03 0.18 0.36 1.20 0.72 0.00 0.51 0.14 2.39 1.72 030th percentile 0.20 0.37 0.17 0.27 0.04 0.00 0.30 0.57 0.40 0.28 070th percentile 0.60 0.80 0.77 0.69 0.72 0.23 0.97 1.24 1.39 1.00 0Average / Dry / Wet ? Dry Dry Average Wet Average Average Average Dry Wet Wet W

Year Precipitation (inches) Amount for 30 Years at APS Four Corners Power Plant, NV - 2011
AECOM Environment

Four Corners Power Plant and Navajo Mine EIS / NAAQS Modeling Study April 2013

Appendix F

Significant Concentration 

Gradient Analysis
AECOM Environment

Four Corners Power Plant and Navajo Mine EIS / NAAQS Modeling Study April 2013

In recent memoranda24, EPA has acknowledged its 1990 Draft New Source Review Workshop 

Manual procedures25 for determining background sources to include in NAAQS demonstrations may 

be overly conservative, especially regarding the short-term 1-hour SO2 and NO2 standards. For 

example, EPA states in its March 1, 2011 memo that : “Our main concern is that following such 

procedures in a literal and uncritical manner may in many cases result in cumulative assessments 

that are overly conservative and could unnecessarily complicate the permitting process in some 

cases.” 

EPA’s modeling guidance in Appendix W identifies “a significant concentration gradient in the vicinity” 

of a source as the main criterion for determining which nearby sources should be included in a 

cumulative impact assessment. An analysis of concentration gradients can help evaluate the 

potential for monitored background concentrations to double count the impacts from background 

sources that are explicitly modeled. 

The EPA March 1, 2011 memo states that: “a general “rule of thumb” for estimating the distance to 

maximum 1-hour impact and the region of significant concentration gradients that may apply in 

relatively flat terrain is approximately 10 times the source release height. Even accounting for some 

terrain influences on the location and gradients of maximum 1-hour concentrations, these 

considerations suggest that the emphasis on determining which nearby sources to include in the 

modeling analysis should focus on the area within about 10 kilometers of the project location in most 

cases. The routine inclusion of all sources within 50 kilometers of the project location, the nominal 

distance for which AERMOD is applicable, is likely to produce an overly conservative result in most 

cases.” 

To determine if there is a significant concentration gradient beyond the FCPP location from San Juan 

Generating Station (SJGS) emissions, AECOM modeled impacts from that facility along a line of 

receptors passing through and beyond the Four Corners Power Plant (assuming a perfect 1-hour 

line-up) with a unit emission rate as shown in Figure F-1. The peak impact from the FCPP is located 

in a direction that does not line up with SJGS, as discussed in the modeling results in Section 5. 

The uncertainty in monitored concentrations is +/- 15% (a total uncertainty of 30%), based upon

EPA’s monitoring guidelines26. That means that for identical actual concentrations along a path (no 

concentration gradient), there could be a change in the reported values from monitors of up to 30% 

which would still within monitoring tolerance. This indicates that a change in concentration of 30% is 

within the “noise” level of valid instruments, and cannot be distinguished from an actual zero gradient. 

Therefore, relative concentration changes up to 30% should be regarded as insignificant.

 

24 For example, the March 1, 2011 Clarification Memo available at 

http://www.epa.gov/ttn/scram/guidance/clarification/Additional_Clarifications_AppendixW_Hourly-NO2-

NAAQS_FINAL_03-01-2011.pdf. 

25 http://www.epa.gov/NSR/ttnnsr01/gen/wkshpman.pdf 

26 EPA 2008b. (http://www.epa.gov/ttnamti1/files/ambient/pm25/qa/QA-Handbook-Vol-II.pdf; see Table 10-3 for audit 

tolerances)
AECOM Environment

Four Corners Power Plant and Navajo Mine EIS / NAAQS Modeling Study April 2013

Figure F-1 SJGS Unit Emission Modeling Set Up
AECOM Environment

Four Corners Power Plant and Navajo Mine EIS / NAAQS Modeling Study April 2013

We determined the change in predicted concentrations from SJGS along a line with a perfect lineup 

through the location of FCPP from the location of FCPP to its peak impact along this line in a 

direction away from SJGS. The “chi/q” (concentration per emission unit) modeled impact of SJGS at 

FCPP is 0.211 μg/m3/g/s. The relative change in the predicted concentration from SJGS along this 

line segment is a small fraction of 30% between FCPP and the distance to the FCPP modeled 

maximum concentration for 1-hour SO2 (1,100 meters) as shown in Figure F-2. Therefore, SJGS 

does not have a detectable (significant) concentration gradient in the vicinity of FCPP, and was not 

included in the modeling. Its impact is accounted for in the monitored background.

Figure F-2 SJGS Unit Emission Rate Modeled Impacts
AECOM Environment

Four Corners Power Plant and Navajo Mine EIS / NAAQS Modeling Study April 2013

Appendix G 

Modeling Archive