AIR QUALITY APPENDIX

AIR-i

AIR QUALITY APPENDIX

This appendix contains the Air Quality Modeling 

Appendix included in the 2003 EIS (Air Quality 

Modeling Appendix – Part 1) and the Air Quality 

Modeling Report for the recently (2006) completed 

air modeling conducted for the SEIS (Air Quality 

Modeling Appendix - Part 2). The SEIS Air 

Modeling Appendix - Part 2 contains attachments for

information on Health Effects and Mitigation 

Measures.

 
AIR QUALITY APPENDIX

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AIR QUALITY APPENDIX

Part 1 AIR-1

AIR QUALITY MODELING APPENDIX – PART 1

QUANTITATIVE REVIEW OF 

AMBIENT AIR QUALITY IMPACTS

Final Oil and Gas Environmental Impact Statement and Amendment of the Powder River 

and Billings Resource Management Plans 

Prepared for 

U.S. BUREAU OF LAND MANAGEMENT

Miles City Field Office

111 Garryowen Road

Miles City, MT 59301-0940 

January 2003
AIR QUALITY APPENDIX

Part 1 AIR-2

2003 EIS AIR QUALITY MODELING APPENDIX

Air Quality Impact Technical 

Support Document

The following technical support document describes 

the processes used to conduct the air quality impact 

assessment, and provides summaries of relevant 

analysis data:

Argonne National Laboratory.

2002. Technical Support Document - Air Quality 

Impact Assessment for the Montana Statewide 

Final Oil and Gas EIS and Amendment of the 

Powder River and Billings Resource 

Management Plans and the Wyoming Final 

EIS and Planning Amendment for the Powder 

River Basin Oil and Gas Development 

Project. Prepared for the U.S. Department of 

the Interior, Bureau of Land Management, 

Montana and Wyoming State Offices, by the 

Environmental Assessment Division, Argonne 

National Laboratory. Argonne, Illinois.

Copies of this technical support document are available 

upon request from:

Scott Archer, Senior Air Resource Specialist

National Science and Technology Center (ST-133)

Denver Federal Center, Building 50

P.O. Box 25047

Denver, Colorado 80225-0047

303.236.6400 Voice

303.236.3508 Telefax

scott_archer@blm.gov

1.0 Introduction

Air pollution impacts are limited by local, state, tribal 

and federal air quality regulations, standards, and 

implementation plans established under the CAA and 

administered by the MDEQ and the EPA. Although not 

applicable to the proposed Alternatives, the WYDEQ 

has similar jurisdiction over potential air pollutant 

emission sources in Wyoming, which can have a 

cumulative impact with MDEQ approved sources. Air 

quality regulations require certain proposed new, or 

modified existing, air pollutant emission sources 

(including CBM compression facilities) undergo a 

permitting review before their construction can begin. 

Therefore, the applicable air quality regulatory 

agencies have the primary authority and responsibility 

to review permit applications and to require emission 

permits, fees and control devices, prior to construction 

and/or operation. 

Fugitive dust and exhaust from construction activities, 

along with air pollutants emitted during operation (i.e., 

well operations, field [booster] and sales [pipeline] 

compressor engines, etc.), are potential causes of air 

quality impacts. These issues are more likely to 

generate public concern where natural gas development 

activities occur near residential areas. The FS, NPS, 

and the FWS have also expressed concerns regarding 

potential atmospheric deposition (acid rain) and 

visibility impacts within distant downwind PSD Class I 

and PSD Class II areas under their administration, 

located throughout Montana, Wyoming, southwestern 

North Dakota, western South Dakota, and northwestern 

Nebraska.

2.0 Existing Air Quality

As described in Chapter 3 - Affected Environment 

(Air Quality), specific air quality monitoring is not 

conducted throughout most of the CBM emphasis area, 

but air quality conditions are likely to be very good, as 

characterized by limited air pollution emission sources 

(few industrial facilities and residential emissions in 

the relatively small communities and isolated ranches) 

and good atmospheric dispersion conditions, resulting 

in relatively low air pollutant concentrations. Air 

quality monitoring is the appropriate tool for 

determining compliance with the NAAQS for both 

particulate matter with an aerodynamic diameter equal 

to or less than ten microns in diameter (PM10) and 

nitrogen dioxide (NO2). As part of the Air Quality 

Impact Assessment prepared by Argonne National 

Laboratory (Argonne 2002), monitoring data measured 

throughout the southeastern Montana and northeastern 

Wyoming were assembled and reviewed. Although 

monitoring is primarily conducted in urban or 

industrial areas, the data selected are considered to be 

the best available representation of background air 

pollutant concentrations throughout the CBM emphasis 

area. Specific values presented in Table AQ-1 were 

used to define background conditions in the air quality 

impact analysis. The selected background pollutant 

concentrations are below applicable ambient air quality 

standards for all pollutants and averaging times. These 

National and Montana standards, and the PSD 

increment values, are also presented in Table AQ-1.
AIR QUALITY APPENDIX

Part 1 AIR-3

TABLE AQ-1

ASSUMED BACKGROUND CONCENTRATIONS, APPLICABLE AMBIENT AIR QUALITY 

STANDARDS, AND PSD INCREMENT VALUES (IN (μG/M3)

Pollutant

Averaging 

Time a

Background 

Concentration

National 

Ambient 

Air Quality 

Standards

Montana 

Ambient 

Air Quality 

Standards

PSD

Class I 

Increment

PSD

Class II 

Increment

Carbon Monoxide 1-hour

8-hours

15,000

6,600

40,000

10,000

40,000

10,000

N/A

N/A

N/A

N/A

Lead Quarterly N/A 1.5 1.5 N/A N/A

Nitrogen Dioxide 1-hour

Annual

117

11

N/A

100

566

100

N/A

2.5

N/A

25

Ozone 1-hour

8-hours

N/A

100

235

157

196

N/A

N/A

N/A

N/A

N/A

PM 2.5 24-hours

Annual

20

8

65

15

N/A

N/A

N/A

N/A

N/A

N/A

PM 10 24-hours

Annual

105

30

150

50

150

50

8

4

30

17

Sulfur Dioxide 1-hour

3-hours

24-hours

Annual

666

291

73

16

N/A

1,300

365

80

1,300

N/A

260

60

N/A

25

5

2

N/A

512

91

20

Source: Argonne (2002)

Notes:

μg/m3 - micrograms per cubic meter a Annual standards are not to be exceeded; short-term standards are not to be exceeded more than once per year.

N/A – data not available

Note that for evaluating consumption of the PM10 and 

NO2 increments in Montana and Wyoming, as well as 

on Indian Reservations, modeling performed by an 

air quality regulatory agency is the appropriate tool 

(emissions solely from surface coal mines being the 

only exception). It should be noted that the BLM 

model used to identify and analyze impacts in this 

EIS is not intended or designed to be a regulatory 

PSD increment consumption modeling process.

Monitoring should be used to supplement modeling 

efforts, to:

1. Determine if identified levels of concern are 

exceeded, triggering the need to implement 

additional mitigation measures in order to avoid 

regulatory action

2. Provide additional indication of the need for 

regulatory modeling to determine if increments 

are being exceeded and an updated State 

Implementation Plan needed

The States of Wyoming and Montana will work with 

EPA to develop monitoring plans, which will 

consider population areas, modeled hot spots and 

other potential areas of concern. EPA will work with 

the Crow Tribe and Northern Cheyenne Tribe to 

identify the need for and to deploy additional 

monitoring as needed. The EIS predicts that full 
AIR QUALITY APPENDIX

Part 1 AIR-4

development of the Coal Bed Methane resource in 

Montana, in culmination with non-project and RFFA

sources, may generate criteria air pollutants (PM, 

VOCs and NOx) in sufficient quantities to require 

regulatory action on the part of MDEQ to protect 

both the PSD increments and the Montana and 

National Ambient Air Quality Standards. MDEQ will 

need to accurately predict the impacts of proposed 

projects during the New Source Review process and 

assure that both the ambient standards and the 

increments are protected. Once projects are up and 

running MDEQ will also require ambient monitoring 

data from appropriately sited monitors to verify the 

permit analysis projections and provide a feedback 

loop of current ambient data to make sure that future 

permitting decisions continue to protect the standards 

and increments. MDEQ can and will require ambient 

monitoring as a permit condition for major sources.

Additionally, much of the permit analysis for sources 

of this nature requires good ambient data to 

accurately predict project impacts. Permitting sources 

of NO2 and Ozone (O3 ) precursors (VOCs)), requires 

representative monitoring data to adequately analyze 

the expected impact of new emissions. Prediction of 

NO2 is highly dependent on some knowledge of NO 

to NO2 conversion rates. This information is 

supposed to come from either an analysis of actual 

NO/NO2 ratios determined by monitoring results

(preferred method), the use of a default value (very 

conservative and has recently resulted in predicted 

violations of the annual standard), or by the use of 

ambient Ozone data to predict conversion rates. 

Permitting large VOC sources raises similar 

questions. Ozone analysis requires at least some 

knowledge of atmospheric chemistry conversion rates 

in the area of analysis. At this time MDEQ does not 

have reliable data on the actual chemistry that is 

occurring in the development area and doesn't have 

any reliable background Ozone values.

Therefore, MDEQ will need NO/NO2, O3 and PM 

data for the development area from a regionally 

scaled ambient monitoring station. MDEQ has 

reviewed the modeling done for the EIS and a 

monitor sited in the Birney/Ashland area would be 

the best choice. Provided that funds become 

available, MDEQ would establish and maintain a 

monitoring station in this area.

It is important that monitors be deployed before 

CBM development occurs, or as early in the 

development cycle as possible, in order to provide 

baseline information and trend data.

3.0 Regulatory Framework

The National and Montana ambient air quality 

standards set the absolute upper limits for specific air 

pollutant concentrations at all locations where the 

public has access. The analysis of the proposed 

Alternatives must demonstrate continued compliance 

with all applicable local, state, tribal and federal air 

quality standards. Existing air quality throughout 

most of the CBM emphasis area is in attainment with 

all ambient air quality standards, as demonstrated by 

the relatively low concentration levels presented in 

Table AQ-1. However, three areas have been 

designated as federal nonattainment areas where the 

applicable standards have been violated in the past: 

Lame Deer (PM10 - moderate) and Laurel (sulfur 

dioxide (SO2) - primary), Montana; and Sheridan, 

Wyoming (PM10 - moderate). Specific monitoring 

data collected by the Northern Cheyenne Tribe are 

presented in Table AQ-2.

Air quality regulations require certain proposed new, 

or modified existing, air pollutant emission sources 

(including CBM compression facilities) to undergo a 

permitting review before their construction can begin. 

Therefore, the applicable air quality regulatory 

agencies have the primary authority and 

responsibility to review permit applications and to 

require emission permits, fees and control devices, 

prior to construction and/or operation. In addition, the 

U.S. Congress (through the CAA Section 116) 

authorized local, state and tribal air quality regulatory 

agencies to establish air pollution control 

requirements more (but not less) stringent than 

federal requirements. Also, for resources discussed in 

this SEIS, the BLM will not authorize any activity 

that does not conform to all applicable local, state, 

tribal and federal air quality laws, regulations, 

standards, and implementation plans.

Given most the CBM emphasis area’s current 

attainment status, future development projects which 

have the potential to emit more than 250 tons per 

year of any criteria pollutant (or certain listed sources 

that have the potential to emit more than 100 tons per 

year) would be required to undergo a site-specific 

regulatory PSD Increment Consumption analysis 

under the federal New Source Review and permitting 

regulations. Development projects subject to the PSD 

regulations may also be required by the applicable air 

quality regulatory agencies to incorporate additional 

emission control measures (including a BACT 

analysis and determination) to ensure protection of 

air quality resources, and demonstrate that the 

combined impacts of all PSD sources will not exceed 
AIR QUALITY APPENDIX

Part 1 AIR-5

the allowable incremental air quality impacts for 

NO2, PM10, and SO2.

The NEPA analysis compares potential air quality 

impacts from the proposed alternatives to applicable 

ambient air quality standards and PSD increments, 

but comparisons to the PSD Class I and II increments 

are intended to evaluate a threshold of concern for 

potential impacts, and do not represent a regulatory 

PSD Increment Consumption Analysis. Even though 

most of the development activities would occur 

within areas designated PSD Class II, the potential 

impacts on regional Class I areas are to be evaluated. 

The Montana DEQ will perform the required 

regulatory PSD increment analysis during the new 

sources review process. This formal regulatory 

process will include analysis of impacts on Class I 

and II air quality areas by existing and proposed 

emission sources. The activities are not allowed to 

cause incremental effects greater than the stringent 

Class I thresholds to occur inside any PSD Class I 

Area. Stringent emission controls (BACT – Best 

Available Control Technology) and emission limits 

may be stipulated in air quality permits as a result of 

this review, or a permit could be denied.

Sources subject to the PSD permit review procedure 

are also required to demonstrate potential impacts to 

air quality related values (AQRV). These include 

visibility impacts, degradation of mountain lakes 

from atmospheric deposition (acid rain), and effects 

on sensitive flora and fauna in the Class I areas. The 

CAA also provides specific visibility protection 

procedures for the mandatory federal Class I areas 

designated by the U.S. Congress on August 7, 1977, 

which included wilderness areas greater than 

5,000 acres in size, and national parks and national 

memorial parks greater than 6,000 acres in size as of 

that date. The Fort Peck and Northern Cheyenne 

tribes have also designated their lands as PSD Class 

I, although the national visibility regulations do not 

apply in these areas. The allowable incremental 

impacts for NO2, PM10, and SO2 within these PSD

Class I areas are very limited. The remainder of the 

CBM emphasis area is designated PSD Class II with 

less stringent requirements.
AIR QUALITY APPENDIX

Part 1 AIR-6

TABLE AQ-2

AMBIENT AIR QUALITY MONITORING DATA COLLECTED BY THE NORTHERN CHEYENNE TRIBE (IN (μG/M3)

Pollutant

Averaging 

Time a Year Morningstar

Garfield 

Peak

Badger 

Peak

Lame Deer 

# 1

Lame Deer 

# 2

Lame Deer 

# 3

Lame Deer 

“PM10A”

Lame Deer 

“TEOM”

nitrogen 

dioxide

Annual 1996

1997

1998

1999

2000

5.7

5.7

5.7

5.7

5.7

5.7

5.7

5.7

5.7

5.7

5.7

5.7

5.7

5.7

5.7

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

PM10 Annual

24-hours

1996

1997

1998

1999

2000

2001

1996

1997

1998

1999

2000

2001

6

N/A

N/A

N/A

N/A

N/A

19

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

20

18

23

19

18

16

120

106

55

41

40

33

N/A

26

32

33

29

36

N/A

75

153

106

124

135

N/A

N/A

32

32

N/A

N/A

N/A

N/A

153

107

N/A

N/A

N/A

N/A

N/A

[22] b

17 b

N/A

N/A

N/A

N/A

[36] b

39 b

N/A

N/A

N/A

N/A

32 b

28 b

N/A

N/A

N/A

N/A

93 b

93 b

N/A
AIR QUALITY APPENDIX

Part 1 AIR-7

TABLE AQ-2

AMBIENT AIR QUALITY MONITORING DATA COLLECTED BY THE NORTHERN CHEYENNE TRIBE (IN (μG/M3)

Pollutant

Averaging 

Time a Year Morningstar

Garfield 

Peak

Badger 

Peak

Lame Deer 

# 1

Lame Deer 

# 2

Lame Deer 

# 3

Lame Deer 

“PM10A”

Lame Deer 

“TEOM”

sulfur 

dioxide

Annual

24-hours

3-hours

1996

1997

1998

1999

2000

1996

1997

1998

1999

2000

1996

1997

1998

1999

2000

2.7

2.7

2.7

2.7

2.7

5.7

5.7

5.7

5.7

5.7

5.2

5.2

10.4

7.8

5.2

2.7

2.7

2.7

2.7

2.7

5.7

5.7

5.7

5.7

5.7

7.8

7.8

10.4

7.8

5.2

2.7

2.7

2.7

2.7

2.7

5.7

5.7

5.7

5.7

5.7

5.2

5.2

10.4

5.2

5.2

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

Source: EPA (2002b)

Notes: μg/m3 - micrograms per cubic meter

N/A - data not available a Short-term averages are reported as the second maximum values. b Supplemental data provided by (Littlewolf 2002).

[data] - data in brackets are not reliable due to the small number of samples collected.
AIR QUALITY APPENDIX

Part 1 AIR-8

4.0 Agency Roles and 

Authorities

4.1 Environmental Protection 

Agency

The Environmental Protection Agency (EPA) 

administers the Federal Clean Air Act (CAA), 

(42 U.S.C. 7401 et seq.) to maintain the National 

Ambient Air Quality Standards (NAAQS) that protect 

human health and to preserve the rural air quality in the 

region by assuring the Prevention of Significant 

Deterioration Class I and Class II increments for SO2, 

NO2, and PM10, are not exceeded. EPA has delegated 

this CAA authority to the States of Montana and 

Wyoming. 

Until the Tribes have an EPA-approved Tribal 

program, EPA will administer air quality requirements 

within Indian country. EPA is responsible for assuring 

that NAAQS are attained and that the Triballydesignated Northern Cheyenne Class I sensitive airshed 

is protected, as well as the Class II increment limits 

that apply on the Crow Reservation. EPA will 

implement an air permitting program for major sources 

within Indian country, including BACT analysis, where 

appropriate. At this time, there is no federal minor 

source permitting program. Therefore, EPA cannot 

regulate minor sources in Indian country directly unless 

EPA decides to implement a Federal Implementation 

Plan (FIP). Mitigation of particulate emissions from 

unimproved roads in Indian country may be necessary 

to protect the Class I and Class II PM10 increments. 

4.2 Montana DEQ

The MDEQ has been delegated Federal Clean Air Act 

(CAA) authority from the United States Environmental 

Protection Agency (EPA) to manage the New Source 

Review—Prevention of Significant Deterioration 

(PSD) permit program for listed major sources with the 

potential to emit (PTE) greater than 100 tons per year 

(tpy) of any regulated pollutant and all other sources 

with a PTE greater than 250 tpy of any regulated 

pollutant. Further, the MDEQ, under the Clean Air Act 

of Montana (MCA 75-2-101 et seq.) and the 

Administrative Rules of Montana (ARM) administers a 

minor source air quality permitting program for sources 

with a PTE greater than 25 tons per year unless 

otherwise noted in the ARM. This program requires, 

among other things, that Best Available Control 

Technology (BACT) apply to regulated air pollutant 

emission sources. MDEQ also has delegated 

responsibility to operate an approved ambient air 

quality monitoring network for the purpose of 

demonstrating compliance with the National and 

Montana Ambient Air Quality Standards (NAAQS/ 

MAAQS). 

Currently, the MDEQ imposes a minor source permit 

limitation on gas compressor engines on a permit-bypermit basis for sources exceeding the Montana minor 

source permitting threshold (ARM Chapter 17.8, 

Subchapter 7). Under the authority of ARM 17.8.715, 

Emission Control Requirements, the MDEQ 

establishes BACT on a case-by-case basis for natural 

gas compressor engines, such as those sources 

indicated for coal bed methane (CBM) development. In 

general, the Department has required NO2 emission 

limits of around 2 grams per brake horsepower hour 

(g/bhp-hr), a CO emission limit of around 3 g/bhp-hr, 

and a volatile organic compound (VOC) emission limit 

of around 1 g/bhp-hr for these sources. Again, as part 

of the minor source permitting program, Montana 

applies pollutant specific BACT to compressor engines 

on a case-by-case basis with limits as described above. 

However, should future regulatory modeling indicate 

potential NAAQS/MAAQS or increment consumption 

exceedances, the MDEQ may require more stringent 

limits to protect applicable standards. 

In addition to the applicable point source BACT 

emission limits described above, under the authority of 

ARM 17.8.308, the MDEQ requires that a permitted 

source use reasonable precautions to limit fugitive 

particulate emissions from haul roads, access roads, 

parking lots, or the general plant property. In general, 

the MDEQ requires that a source have fresh water 

and/or chemical dust suppressant available on site and 

used as necessary to maintain compliance with 

applicable limits, including, but not limited to, the 

reasonable precautions and opacity limits. Further, the 

MDEQ could establish more stringent BACT limits for 

permitted sources and require that counties apply 

BACM to unimproved roads or other control measures 

sufficient to avoid exceeding applicable standards and 

the Class I and Class II increment limits for PM10. 

Further, the ARM establishes generally applicable air 

quality rules pertaining to all sources of air pollution, 

including sources not subject to air quality permitting. 

These rules include, but are not limited to, the 

requirements contained in ARM 17.8, Subchapter 1 

and ARM 17.8, Subchapter 3.

4.3 Bureau of Indian Affairs

BIA is responsible for approval of any lease, 

agreement, permit, or document that could encumber 

lands and minerals owned by either Tribes or allottees. 

Under the Indian Mineral Development Act (IMDA), 
AIR QUALITY APPENDIX

Part 1 AIR-9

the Secretary of Interior is responsible, based upon BIA 

recommendation, for approving any contractual 

arrangement to develop CBM resources. Specific 

discussion of tribal air quality management issues are 

addressed separately.

4.4 Bureau of Land Management

NEPA requires that federal agencies consider 

mitigation of direct and cumulative impacts during 

their preparation of an EIS. (BLM Land Use Planning 

Manual 1601.) Prior to approval of Resource 

Management Plans (RMPs) or Amendments to RMPs, 

the State Director is to submit any known 

inconsistencies with State Implementation Plan (SIP) 

to the Governor of that state. If the Governor of the 

State recommends changes in the proposed RMP or 

Amendment to meet SIP requirements, the State 

Director shall provide the public an opportunity to 

comment on those recommendations. (BLM Land Use 

Planning Manual at Section 1610.3-2.) 

4.5 Forest Service

The Forest Service administers nine wilderness areas 

(WAs) that could be affected by direct effects 

associated with project and non-project sources: 

Bridger WA; Fitzpatrick WA; North Absaroka, 

Absaroka-Beartooth, and Washakie WAs, next to 

Yellowstone NP; Teton WA; U.L. Bend WA; Cloud 

Peak WA; and Popo Agie WA with mandatory Class I 

designation. As federal land managers, the Forest 

Service could act in a consultative role to stipulate that 

the BLM modeling results, or any future EPA or Stateadministered PSD refined modeling results (if 

justified), triggers adverse impairment status. Should 

the Forest Service determine impairment of WAs, then 

BLM, the State, and/or EPA may need to mitigate this 

predicted adverse air quality effect. 

4.6 National Park Service

Three areas administered by the National Park 

Service—Yellowstone National Park, Devils Tower 

National Monument, and Bighorn Canyon National 

Recreation Area—could be affected by direct effects 

associated with project and non-project sources. (Note: 

Additional Park Service Class I and II areas may be 

impacted by the non-project sources evaluated, without 

significant impact from project sources.) As federal 

land managers, the Park Service could act in a 

consultative role to stipulate that the BLM modeling 

results, or any future EPA or State-administered PSD 

refined modeling results (if justified), triggers adverse 

impairment status. Should the Park Service determine 

impairment of NPS-administered Class I areas, then 

BLM, the State, and/or EPA may need to mitigate this 

predicted adverse air quality effect. 

5.0 Air Quality Management 

on Tribal Lands

The 1990 Clean Air Act (CAA) Amendments 

(Section 301(d)) provided tribes the authority to 

implement CAA programs for their reservations. The 

Tribal Authority Rule (TAR), promulgated February 

12, 1998, reiterates that tribes have direct 

implementation authority for the CAA. However, until 

such time as the tribe assumes such responsibility to 

implement its own program, EPA must implement 

Federal air quality laws for them. The TAR also 

requires under §49.11 that EPA promulgate a Federal 

Implementation Plan (FIP) as necessary or appropriate 

to protect air quality on the reservations. 

EPA has the authority to implement two permitting 

programs and three source specific programs. EPA has 

regulatory authority to issue pre-construction permits 

to major air pollution emissions sources under the 

Prevention of Significant Deterioration (PSD) program 

at 40 CFR part 52 and operating permits to major 

sources under the Title V program at 40 CFR part 71. 

The PSD program requires that subject sources conduct 

an air quality analysis to determine the impact on the 

National Ambient Air Quality Standards (NAAQS) and 

the PSD increments for NO2, SO2, and PM10 for three 

different area classifications (Class I, Class II, and 

Class III). Under the PSD program, Class I status was 

assigned to pristine areas, such as national parks and 

forest lands. Several tribes have been redesignated 

from a Class II status to a Class I status. The rest of the 

country is Class II and there are no Class III areas. 

EPA also has regulatory authority to implement the 

New Source Performance Standards (NSPS) at 40 CFR 

part 60, the National Emission Standards for 

Hazardous Air Pollutants (NESHAP) at 40 CFR part 

61, and the Maximum Achievable Control Technology 

(MACT) standards at 40 CFR part 63. 

EPA does not have a rule for a minor source preconstruction permitting program for permitting new 

and modified sources. A minor source rule is being 

addressed by the Agency, but such a rule will not be 

final for 2-3 years. A minor source rule could give EPA 

the authority to implement a minor source Best 

Available Control Technology (BACT) requirement for 

engines. Nor does EPA have a FIP in place for Indian

country to address measures for controlling fugitive 

dust or control technologies for engines.
AIR QUALITY APPENDIX

Part 1 AIR-10

In 1977, the Northern Cheyenne Indian Tribe’s 

Reservation was redesignated as a Class I airshed 

under the PSD program. The Tribe has implemented an 

air quality monitoring program, delivering air quality 

data to AIRS-AQS since 1981. Currently, the Tribe 

does not have any EPA approved CAA programs for 

issuing permits, nor is there a Tribal Implementation 

Plan (TIP) with general source or source specific 

requirements or any of the federal NSPS, MACT, or 

NESHAP standards. At this time, if permitting of 

major air pollution sources was required, EPA would 

be the permitting authority. 

The Crow Indian Reservation is a Class II airshed. 

Currently, the Tribe does not have any EPA approved 

CAA programs for issuing permits, nor is there a TIP 

with general source or source specific requirements, or 

any of the federal NSPS, MACT, or NESHAP 

standards. The Tribe was approved for a CAA Section 

103 grant in 2001 to conduct an emissions inventory of 

the sources on the Reservation. The Tribe is not 

currently implementing an air quality monitoring 

program. At this time, if permitting of major air 

pollution sources were required, EPA would be the 

permitting authority.

The preferred method to determine the mitigation 

required to prevent exceedances of ambient air quality 

standards and to prevent significant deterioration is 

modeling. EPA will work with the states of Wyoming 

and Montana along with the tribes to see that, wherever 

possible, tribal air quality issues are addressed in 

regional modeling efforts related to coal bed methane 

development. Additional modeling efforts addressing 

specific tribal concerns, as necessary, can be 

undertaken by EPA and the tribal air quality agencies.

Ambient air monitoring can be used to augment and 

validate modeled results. The Northern Cheyenne Tribe 

currently conducts ambient air PM10 and particulate 

matter with an aerodynamic diameter equal to or less 

than 2.5 microns (PM2.5) monitoring in the Lame Deer 

PM10 non-attainment area on the Northern Cheyenne 

Reservation. In order to track the impacts of nearby 

industrial activities on air quality, the tribe also 

conducts IMPROVE protocol speciated PM2.5

monitoring at the Morningstar site, and PM10, SO2 and 

NO2 monitoring at the Morningstar, Badger Peak and 

Garfield Peak monitoring stations. These monitoring 

stations also have collocated meteorological monitors. 

With updates to emission inventories as a result of coal 

bed methane development on or outside the Northern 

Cheyenne Reservation, the monitoring network may 

need revision or augmentation.

The Crow Tribe does not currently have an air 

monitoring program and has never had one that 

submitted data to AIRS-AQS. The Crow tribe has the 

same rights and potential capabilities as the Northern 

Cheyenne Tribe. If regional emission increases are 

sufficient to threaten the NAAQS or other relevant air 

quality standard on Crow lands, EPA would work with 

the tribe to encourage them to initiate monitoring 

activities. To this end, the Tribe can build the 

capability necessary to conduct ambient air quality 

monitoring. In the event the tribe chooses not to 

conduct monitoring, EPA can choose to conduct 

monitoring using either EPA personnel or contract 

assistance under Section 301 of the Clean Air Act. 

In addition to point source emissions, fugitive dust 

controls for coal bed methane sources will likely be 

needed for development on tribal lands. The Tribes can 

use contractual relationships with developers to require 

necessary construction phase dust controls on wells on 

Tribal lands. EPA will work with Tribal, BIA and 

county agencies as needed to develop and implement 

necessary mitigation on unpaved roads used for 

development related traffic.

6.0 Air Quality Impact 

Assessment

As described in Chapter 4, Environmental 

Consequences (Air Quality), an extensive air quality 

impact assessment technical support document was 

prepared by Argonne National Laboratory (Argonne 

2002) and is available for review. Argonne analyzed 

potential impacts from: individual proposed 

Alternatives A, B/C/E, and D (project sources); “Nonproject” emission sources (existing sources, RFFA and 

Wyoming PRBO&G Alternative 1; RFFA emissions 

from potential CBM development on the Northern 

Cheyenne and Crow Indian Reservations and the 

Ashland District of the Custer National Forest; and all 

sources cumulatively by Alternative. Since 

Alternatives B, C and E have very similar emission 

inventories, a single air quality impact analysis 

represents all of these three Alternatives. For example, 

under Alternative C the number of wells connected to a 

field (booster) compressor would not be limited but the 

number was assumed to be the same as in 

Alternative B, and under Alternative E electrical field 

(booster) compressors would be required where noise 

is an issue although all compressors were assumed to 

be gas-fired.

The air quality impact assessment was based on the 

best available engineering data and assumptions, 

meteorology data, and dispersion modeling procedures, 

as well as professional and scientific judgment. 

However, where specific data or procedures were not 
AIR QUALITY APPENDIX

Part 1 AIR-11

available, reasonable assumptions were made. Note 

that these assumptions could result in under or overestimates of impacts. It is difficult to ascertain the 

overall bias of the emission estimates and modeling; no 

sensitivity or probabilities of occurrence analyses were 

performed.

Air quality impacts for various air pollutants are 

determined by the use of air dispersion models using 

specific source emission rates. For natural gas 

compressors, the emissions of nitrogen oxides are 

determined by the assumed permitted emission rate 

allowed by the state. For fugitive dust impacts, 

emission rates are obtained from EPA’s AP-42 

document that is titled “Compilation of Air Pollutant 

Emission Factors”. An AP-42 emission factor is a 

representative value that attempts to relate the quantity 

of a pollutant released to the atmosphere with an 

activity associated with the release of that pollutant. 

Emission factors may be appropriate to use in a number 

of situations such as making source-specific emission 

estimates for area-wide inventories. These inventories 

have many purposes including ambient dispersion 

modeling and analysis, control strategy development, 

and in screening sources for compliance investigations. 

In most cases, these factors are simply averages of all 

available data of acceptable quality, and are generally 

assumed to be representative of long-term averages for 

all sources in a specific category. 

Potential air pollutant emissions from the proposed 

Alternatives emission sources (denoted as “project” 

sources) were calculated separately to determine 

potential impacts. These emissions were then combined 

with existing sources, proposed non-PRBO&G 

developments and reasonably foreseeable future 

actions (RFFA) emissions (denoted as “non-project” 

sources) and RFFA emissions from potential CBM 

development on the Northern Cheyenne and Crow 

Indian Reservations and the Ashland District of the 

Custer National Forest to determine the total potential 

cumulative air quality impacts. All of the tables in this 

Air Quality Modeling Appendix display impacts from: 

1) the project sources only; 2) the project sources 

combined with emissions from potential CBM 

development on the Northern Cheyenne and Crow 

Indian Reservations and the Ashland District of the 

Custer National Forest (denoted as “Project + RFFA 

Sources ); 3) the non-project sources; and 

4) cumulative totals.

The non-project sources include development 

permitted: 1) by the MDEQ; 2) by the WYDEQ; and 3) 

within the states of North Dakota, South Dakota, and 

Nebraska; and projections for the Wyoming Powder 

River Basin Oil and Gas Project DEIS Alternative 

sources (BLM 2002a); and other RFFA sources from 

states within the geographic area covered by the model. 

Potential direct, indirect and cumulative air quality 

impacts were analyzed and reported solely under the 

requirements of NEPA, in order to assess and disclose 

reasonably foreseeable impacts to both the public and 

the BLM decision maker before a Record of Decision 

is issued. Due to the preliminary nature of this NEPA 

analysis, it should be considered a reasonable estimate 

of predicted impacts. Actual impacts at the time of 

development (subject to air pollutant emission source 

permitting) could be different. To the extent that 

impacts are predicted to be greater than regulatory 

thresholds, appropriate mitigation efforts would be 

undertaken.

Given the lack of representative wind measurements 

throughout the CBM emphasis area, the EPA 

CALPUFF dispersion model was used with regional 

wind speed and direction values derived from the 1996 

MM5 (mesoscale model) and CALMET 

meteorological models (Argonne 2002). 

Meteorological information was assembled to 

characterize atmospheric transport and dispersion from

several 1996 data sources, including: 36 km gridded 

MM5 (mesoscale model) values with continuous fourdimensional data assimilation; and hourly surface 

observations (wind speed, wind direction, temperature, 

cloud cover, ceiling height, surface pressure, relative 

humidity, and precipitation.)

Potential air quality impacts were predicted using the 

EPA CALPUFF dispersion model. The meteorology 

data and air pollutant emission values were combined 

to predict maximum potential direct, indirect, and 

cumulative near-field air quality impacts in the vicinity 

of assumed well and compressor engine emission 

sources for comparison with applicable air quality 

standards and PSD Class II increments. Maximum 

potential near-field particulate matter emissions from 

traffic on unpaved roads and during well pad 

construction were used to predict the maximum annual 

and 24-hour average PM2.5, PM10, and SO2 impacts. 

Maximum air pollutant emissions from each CBM well 

would be temporary (i.e., occurring during a 12-day 

construction period) and would occur in isolation, 

without significantly interacting with adjacent well 

locations. Particulate matter emissions from well pad 

and resource road construction would be minimized by 

application of water and/or chemical dust suppressants. 

The control efficiency of these dust suppressants was 

computed at 50 per cent during construction. During 

well completion testing, natural gas could be burned 

(flared) up to 24 hours.
AIR QUALITY APPENDIX

Part 1 AIR-12

Air pollutant dispersion modeling was also performed 

to quantify CO, NO2, PM2.5, PM10, and HAP impacts 

during operation. Operation emissions would primarily 

occur due to increased compression requirements, 

including field (booster) and sales (pipeline) 

compressor stations. Since produced natural gas is 

nearly pure methane, with little or no liquid 

hydrocarbons or sulfur compounds, direct VOC 

emissions or objectionable odors are not likely to 

occur. HAP impacts were predicted based on an 

assumed 9,900 horsepower, six-unit, reciprocating 

compressor engine station operating at full load with 

emissions generated by a single stack.

The significance criteria for potential air quality 

impacts include local, state, tribal and federally 

enforced legal requirements to ensure air pollutant 

concentrations will remain within specific allowable 

levels. These requirements and legal limits were 

presented in Table AQ-1. Where legal limits have not 

been established, the BLM uses the best available 

scientific information to identify thresholds of 

significant adverse impacts. Thresholds have been 

identified for hazardous air pollutant (HAP) exposure, 

potential acid neutralizing capacity (ANC) changes to 

sensitive lake water chemistry, and a 1.0 dv “just 

noticeable change” in potential visibility impacts.

Since neither the MDEQ nor EPA have established 

HAP standards, predicted 8-hour HAP concentrations 

were compared to a range of 8-hour state maximum 

Acceptable Ambient Concentration Levels (EPA 

1997a). Pollutants which were predicted to exceed 

these state threshold levels were also analyzed to 

determine the possible incremental cancer-risk for a 

most likely exposure (MLE) to residents, and to a 

maximally exposed individual (MEI), such as 

compressor station workers. These cancer risks were 

calculated based on the maximum predicted annual 

concentrations, EPA’s unit risk factors for carcinogenic 

compounds (EPA 1997b), and an adjustment for time 

spent at home or on the job.

The EPA CALPUFF dispersion model was also used to 

determine maximum far-field ambient air quality 

impacts at downwind mandatory federal PSD Class I 

areas, and other sensitive receptors, to: 1) determine if 

the PSD Class I increments might be exceeded; 

2) calculate potential total sulfur and nitrogen 

deposition, and their related impacts to in sensitive 

lakes; and 3) predict potential visibility impacts 

(regional haze) within distant sensitive receptors.

Several lakes within five FS designated wilderness 

areas were identified as being sensitive to atmospheric 

deposition and for which the most recent and complete 

data have been collected. The FS (Fox et al. 1989) has 

identified the following total deposition (wet plus dry) 

thresholds below which no adverse impacts are likely: 

five kg/ha-yr for sulfur, and three kg/ha-yr for nitrogen. 

The FS (2000) has also developed a screening method 

which identifies the following Limit of Acceptable 

Change regarding potential changes in lake chemistry: 

no more than a ten per cent change in ANC for those 

water bodies where the existing ANC is at or above 

25 μeq/l, and no more than a one μeq/l change for 

those extremely sensitive water bodies where the 

existing ANC is below 25 μeq/l. No sensitive lakes 

were identified by either the NPS or FWS.

Since the potential air pollutant emission sources 

constitute many small sources spread out over a very 

large area, discrete visible plumes are not likely to 

impact the distant sensitive areas, but the potential for 

cumulative visibility impacts (increased regional haze) 

is a concern. Regional haze degradation is caused by 

fine particles and gases scattering and absorbing light.

Potential changes to regional haze are calculated in 

terms of a perceptible “just noticeable change” (1.0 dv) 

in visibility when compared to background conditions. 

A 1.0 dv change is considered potentially significant in 

mandatory federal PSD Class I areas as described in 

the EPA Regional Haze Regulations (40 CFR 51.300 et 

seq.), and as originally presented in Pitchford and 

Malm (1994). A 1.0 dv change is defined as about a ten 

per cent change in the extinction coefficient 

(corresponding to a two to five per cent change in 

contrast, for black target against a clear sky, at the most 

optically sensitive distance from an observer), which is 

a small but noticeable change in haziness under most 

circumstances when viewing scenes in mandatory 

federal Class I areas.

It should be noted that a 1.0 dv change is not a “just 

noticeable change” in all cases for all scenes. Visibility 

changes less than 1.0 dv are likely to be perceptible in 

some cases, especially where the scene being viewed is 

highly sensitive to small amounts of pollution, such as 

due to preferential forward light scattering. Under other 

view-specific conditions, such as where the sight path 

to a scenic feature is less than the maximum visual 

range, a change greater than 1.0 dv might be required 

to be a “just noticeable change.” However, this NEPA 

analysis is not designed to predict specific visibility 

impacts for specific views in specific mandatory 

federal PSD Class I areas based on specific project 

designs, but to characterize reasonably foreseeable 

visibility conditions that are representative of a fairly 

broad geographic region, based on reasonable emission 

source assumptions. This approach is consistent with 

both the nature of regional haze and the requirements 

of NEPA. At the time of a pre-construction air quality 

permit review, the applicable air quality regulatory 
AIR QUALITY APPENDIX

Part 1 AIR-13

agency may require a much more detailed visibility 

impact analysis. Factors such as the magnitude of 

change, frequency, time of the year, and the 

meteorological conditions during times when predicted 

visibility impacts are above the 1.0 dv threshold (as 

well as inherent conservatism in the modeling 

analyses) should all be considered when assessing the 

significance of predicted impacts.

The FS, NPS and FWS have published their “Final 

FLAG Phase I Report” (Federal Register, Vol. 66 

No. 2, dated January 3, 2001), providing “a consistent 

and predictable process for assessing the impacts of 

new and existing sources on AQRVs” including 

visibility. For example, the FLAG report states “A 

cumulative effects analysis of new growth (defined as 

all PSD increment-consuming sources) on visibility 

impairment should be performed,” and further, “If the 

visibility impairment from the proposed action, in 

combination with cumulative new source growth, is 

less than a change in extinction of 10% [1.0 dv] for all 

time periods, the Federal Land Managers (FLM) will 

not likely object to the proposed action.”

The FLAG report also recommends a two-step analysis 

process to evaluate potential visibility impacts from 

either a single proposed air pollutant emission source 

(the seasonal FLAG screening method) or potential 

cumulative visibility impacts from a group of air 

pollutant emission sources (the daily FLAG refined 

method). As described in Argonne (2002), this NEPA 

analysis first used the seasonal FLAG screening 

method (based on both the FLAG and WYDEQ-AQD 

“natural background” reference levels) to exclude those 

sensitive areas where visibility impacts were not likely 

to occur. Since no areas were excluded using the 

seasonal FLAG screening method, this NEPA analysis 

then applied the daily FLAG refined method (based on 

hourly background optical extinction and relative 

humidity values measured in both the Badlands and 

Bridger wilderness areas between 1989 and 1999) to 

determine the average number of days a 1.0 dv “just 

noticeable change” would be reached annually in each 

sensitive area. Although the use of observed hourly 

optical extinction and relative humidity values is 

appropriate in this NEPA analysis (where the potential 

visibility impacts are predicted to occur under the 

Alternatives based on the reasonably foreseeable 

background conditions), EPA’s Regional Haze 

Regulations are based on optical conditions 

reconstructed from PM2.5 and PM10 data collected 

every third day under the IMPROVE program.

7.0 Modeling Assumptions

When reviewing the predicted near- and far-field air 

quality impacts, it is important to understand that 

assumptions were made regarding development, 

emissions, meteorology, atmospheric transport and 

chemistry, and atmospheric deposition. For example, 

there is uncertainty regarding ultimate development 

(i.e., number of wells, equipment to be used, specific 

locations of wells, etc.). 

The following assumptions were used in the analysis:

• Total predicted short-term air pollutant impact 

concentrations were assumed to be the sum of the 

assumed background concentration, plus the 

predicted maximum cumulative modeled 

concentrations, which may occur under different 

meteorological conditions. 

• Assumed background air pollution concentrations 

were assumed to occur throughout the 20-year life 

of project (LOP) at all locations in the region, even 

though monitoring is primarily conducted in urban 

or industrial areas, rather than rural areas. The 

uniform background PM10 levels for each state are 

assumed to be representative of the background 

conditions for the entire modeled area of the PRB, 

based on monitoring data gathered throughout 

northeastern Wyoming and southeastern Montana.

• The maximum predicted air quality impacts occur 

only in the vicinity of the anticipated emission 

sources. Actual impacts would likely be less at 

distances beyond the predicted points of maximum 

impact.

• All emission sources were assumed to operate at 

their reasonably foreseeable maximum emission 

rates simultaneously throughout the LOP. Given 

the number of sources included in this analysis, the 

probability of such a scenario actually occurring 

over an entire year is small.

• In developing the emissions inventory and model, 

there is uncertainty regarding ultimate 

development (i.e., number of wells, equipment to 

be used, specific locations, etc.) Most (90 per cent) 

proposed CBM wells and 30 per cent of 

conventional wells were assumed to be fully 

operational and remain operating (no shut ins) 

throughout the LOP.

• The total proposed booster (field) and pipeline 

(sales) compression engines were assumed to 

operate at their rated capacities continuously 

throughout the LOP (no phased increases or 
AIR QUALITY APPENDIX

Part 1 AIR-14

reductions). In reality, compression equipment 

would be added or removed incrementally as 

required by the well field operation, compressor 

engines would operate below full horsepower 

ratings, and it is unlikely all compressor stations 

would operate at maximum levels simultaneously.

• The HAP analyses assumed a six-unit, 1,650 hp 

each, reciprocating compressor engine station 

would operate at full load and at maximum 

emission levels continuously throughout the LOP. 

• The emissions inventory and model use peak years 

of construction and peak years of operations, 

which would not occur throughout the entire 

development region at the same time. However, 

these conditions may occur in some areas.

• The emissions inventory and model assumed that a 

reasonably foreseeable emission rate for 

compressor engines of 1.5 g/hp-hr of nitrogen 

oxides (NOx)is achievable in Montana. Since 

BACT is decided on a case-by-case basis, actual 

emission rates could be decided to be less or more 

than this level by the Departments of 

Environmental Quality in Montana or Wyoming, 

and on Indian lands by EPA, for field and sales 

compressor engines. Reasonable NOx emission 

rates may range from 0.7 to 2 g/hp-hr.

• There are no applicable local, state, tribal or 

federal acid deposition standards. In the absence of 

applicable standards, the acid deposition analysis 

assumed that a “limit of acceptable change” is: a 

10 per cent change in acid neutralizing capacity 

(ANC) for lakes with a background ANC greater 

than 25 μeq/l; or a 1 μeq/l change in ANC for 

lakes with a background ANC less than 25 μeq/l, 

and would be a reasonably foreseeable significant 

adverse impact. Further, the atmospheric 

deposition impact analysis assumed no other 

ecosystem components would affect lake 

chemistry for a full year (assuming no chemical 

buffering due to interaction with vegetation or soil 

materials).

• The visibility impact analysis assumed that a 

1.0 dv “just noticeable change” would be a 

reasonably foreseeable significant adverse impact, 

although there are no applicable local, state, tribal 

or federal regulatory visibility standards. However, 

some FLMs are using 0.5 dv as a screening 

threshold for significance.

• Mitigation measures are included in the emissions 

inventory and model that may not be achievable in 

all circumstances. However, actual mitigation 

decided by the developers and local and state 

authorities may be greater or less than those 

assumed in the analysis. For example, maintaining 

a construction road speed limit of 15 mph may be 

reasonable in a construction zone but difficult to 

enforce elsewhere. Full (100%) mitigation of 

fugitive dust from disturbed lands may not be 

achievable. Further, 50% reduction in fugitive 

emissions is assumed based on construction road 

wetting on the unimproved access road to the pad 

and at the pad, but this level of effectiveness is 

characterized as the maximum possible. In the air 

quality modeling, no specific road wetting or other 

emissions controls were assumed to be used during 

the operations phase of the development (e.g., for 

maintenance vehicle traffic). However, during the 

review of proposed projects (Applications for 

Permit to Drill) the BLM would require specific 

mitigation measures in certain areas during the 

operational phase of development.

• Induced or secondary growth related to increases 

in vehicle miles traveled (VMT) (believed to be on 

the order of 10 per cent overall) is not included in 

the emissions inventory and model. Not all 

fugitive dust emissions (including county and 

other collector roads) have been included in the 

emissions inventory and model. 

• Fugitive dust emissions from roads are treated as 

area sources rather than line sources in the model, 

which may thereby reduce or increase the 

predicted ambient concentrations at maximum 

concentration receptor points near the source, 

depending on the inputs to the model 

(meteorology, terrain, etc.) By not placing 

modeled receptors close to emission sources (e.g. 

wells and roads), the model may not capture 

higher ambient concentrations near these sources. 

A more refined, regulatory model may yield higher 

concentrations at locations near fugitive dust 

sources.

• For comparisons to the PSD Class I and II 

increments, the emissions inventory and model 

included only CBM and RFFA sources. Other 

existing increment consuming sources such as 

Campbell County, Wyoming coal mines were not 

included in this comparison, as the air quality 

analysis does not represent a regulatory PSD 

increment consumption analysis. A regulatory 

PSD increment consumption analysis needs to 

identify and consider all PSD increment 

consuming sources to determine the level of PSD 

Class II increment consumption. Monitoring data 

in Wyoming has indicated an upward trend in PM 

concentrations in Campbell County since 1999, 
AIR QUALITY APPENDIX

Part 1 AIR-15

which coincides with CBM development but is 

also exacerbated by prolonged drought in the 

region.

It is important to note that before actual development 

could occur, the applicable air quality regulatory 

agencies (including the state, tribe or EPA) would 

review specific air pollutant emissions pre-construction 

permit applications that examine potential projectspecific air quality impacts for some source categories. 

As part of these permit reviews (depending on source 

size), the air quality regulatory agencies could require 

additional air quality impact analyses or mitigation 

measures. Thus, before development occurs, additional 

site-specific air quality analyses would be performed to 

ensure protection of air quality.

8.0 Modeling Results

The following Tables present the detailed atmospheric 

dispersion modeling results which are summarized in 

Chapter 4, Environmental Consequences (Air 

Quality).

TABLE AQ-3

PREDICTED HAZARDOUS AIR POLLUTANT IMPACTS AND SIGNIFICANCE THRESHOLDS (IN 

(μG/M3)

Pollutant

Averaging 

Time

Direct Modeled 

Impact

Range of State

Acceptable Ambient Concentration Levels

formaldehyde 8-hours 11.9 4.5 (FL07) - 71 (NV01)

n-hexane 8-hours 0.6 1,800 (FL07) - 36,000 (CT01)

benzene 8-hours 0.7 30 (FL04) - 714 (NV01)

toluene 8-hours 4.6 1,870 (IN03) - 8,930 (NV01)

ethyl benzene 8-hours < 0.1 4,340 (ND01) - 43,500 (VT01)

xylene 8-hours 0.2 2,170 (IN01) - 10,400 (NV01)

Source: Argonne (2002)

Agencies: CT01 - Connecticut Department of Environmental Protection; Air Compliance Unit

FL04 - Broward County Department of Natural Resource Protection (Florida)

FL07 - Pinellas County Air Pollution Control Board (Florida)

IN01 - Indiana Department of Environmental Management

IN03 - Indianapolis Air Pollution Control Division (Indiana)

ND01 - North Dakota Dept. of Health; Division of Environmental Engineering

NV01 - Nevada Division of Environmental Protection; Air Quality Control

VT01 - Vermont Dept. of Environmental Conservation; Air Pollution Control Division
Part 1 AIR-16

AIR QUALITY APPENDIX

TABLE AQ-4

ALTERNATIVE A—PREDICTED CRITERIA POLLUTANT IMPACTS AND APPLICABLE SIGNIFICANCE THRESHOLDS (IN (μG/M3)

Pollutant Avg Time a Location

PSD 

Increment

Alt A 

Project 

NonProject Cum Background Total NAAQS MAAQS

carbon monoxide 1-hour

8-hours

near-field

far-field 1

near-field

far-field 1

- - -

- - -

- - -

- - -

49

1

30

<1

540

100

311

52

540

100

314

52

15,000

15,000

6,600

6,600

15,540

15,100

6,914

6,652

40,000

40,000

10,000

10,000

26,000

26,000

10,000

10,000

nitrogen dioxide 1-hour

Annual

near-field

far-field 1

near-field

far-field 3

far-field 2

- - -

- - -

25

25

2.5

21

2.0

1.9

1.2

0.2

181

36

4.8

1.1

0.5

187

36

6.0

2.0

0.7

117

117

11

11

11

304

153

17

13

12

- - -

- - -

100

100

100

566

566

100

100

100

PM2.5 24-hours

Annual

near-field

far-field 4

near-field

far-field 4

- - -

- - -

- - -

- - -

1.0

0.1

0.3

0.0

44.1

12.7

5.6

1.2

44.4

12.7

5.8

1.2

20

20

8

8

64

33

14

9

65

65

15

15

- - -

- - -

- - -

- - -

PM10 24-hours

Annual

near-field

far-field 4

far-field 2

far-field 5

near-field

far-field 4

30 b

30

8 b

8

17

17

1.8

0.1

0.5

0.2

0.5

0.0

104 b

29.7

8.4 b

7.2

13.1

2.7

105 b

29.7

8.7 b

7.4

13.4

2.7

105

105

105

105

30

30

210 c

135

114

112

43

33

150 c

150

150

150

50

50

150 c

150

150

150

50

50

sulfur dioxide 1-hour

3-hours

24-hours

Annual

near-field

far-field 3

near-field

far-field 3

near-field

far-field 3

near-field

far-field 3

- - -

- - -

512

512

91

91

20

20

1.9

1.2

1.5

1.0

0.9

0.6

0.3

0.2

27.4

29.6

22.6

17.1

9.8

5.3

1.0

0.4

28.0

29.6

23.3

17.1

10.2

5.3

1.1

0.4

666

666

291

291

73

73

16

16

694

696

314

308

83

78

17

16

- - -

- - -

1,300

1,300

365

365

80

80

1,300

1,300

- - -

- - -

260

260

60

60
AIR QUALITY APPENDIX

Part 1 AIR-17

TABLE AQ-4

ALTERNATIVE A—PREDICTED CRITERIA POLLUTANT IMPACTS AND APPLICABLE SIGNIFICANCE THRESHOLDS (IN (μG/M3)

Pollutant Avg Time a Location

PSD 

Increment

Alt A 

Project 

NonProject Cum Background Total NAAQS MAAQS

Source: Argonne (2002)

Notes:

a Annual impacts are the first maximum value; short-term impacts are the second maximum value. There are uncertainties, unquantified at this point, associated 

with the modeled values. Actual maximum impacts may be larger or smaller than those shown. 

b It is possible that Non-Project and Cum emission sources could exceed the PSD Class I increment on the Northern Cheyenne Indian Reservation, as well as the 

PSD Class II increment near the maximum assumed development; a regulatory “PSD Increment Consumption Analysis” should be conducted during permitting

by the appropriate air quality regulatory agency.

c Two receptor locations just south of the Spring Creek Coal Mine when combined with an assumed background concentration of 105 μg/m2 were predicted to 

exceed the National and Montana ambient air quality standards due to Non-Project and Cum emission sources.

Alt A Project - Direct modeled Alternative A project sources impacts. 

Non-Project - Direct modeled non-project source impacts. The impact from all air pollutant emission sources not included in Alt A, including the Wyoming 

“Powder River Basin Oil and Gas Project” DEIS Alternative 1 sources. Potential impacts from Wyoming Alternatives 2A, 2B and 3 would be less.

Cum - Cumulative modeled impacts. Since these values represent the maximum cumulative impact location, they may not be a simple sum of the maximum 

direct Alt A Project and Non-Project impacts, which can occur at different locations.

Total - The sum of the cumulative modeled impact and the assumed background concentration.

NAAQS - Applicable National Ambient Air Quality Standard.

MAAQS - Applicable Montana Ambient Air Quality Standard.

Locations: 

1 – Absaroka-Beartooth Wilderness Area

2 – Northern Cheyenne Indian Reservation 

3 – Crow Indian Reservation

4 – Fort Belknap Indian Reservation 

5 – Washakie Wilderness Area
AIR QUALITY APPENDIX

Part 1 AIR-18

TABLE AQ-5

ALTERNATIVE A - PREDICTED ATMOSPHERIC DEPOSITION IMPACTS AND APPLICABLE SIGNIFICANCE THRESHOLDS

Total Sulfur Deposition

(kg/ha-yr)

Total Nitrogen Deposition

(kg/ha-yr)

Acid Neutralizing Capacity

(per cent)

Location

PSD 

Class Lake

Alt A 

Project

NonProject Cum ThldAlt A 

Project

NonProject Cum Thld

Bkgd 

(μeq/l)

Alt A 

Project

NonProject Cum Thld

Bridger WA

I

Black Joe

Deep

Hobbs

Upper Frozen

<0.01

<0.01

<0.01

<0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

5

5

5

5

<0.01

<0.01

<0.01

<0.01

0.03

0.03

0.02

0.03

0.03

0.03

0.02

0.03

3

3

3

3

69.0

61.0

68.0

5.8

0.1

0.1

<0.1

<0.1 a

2.2

2.5

1.2

1.6 a

2.3

2.6

1.3

1.6 a

10

10

10

1 a

Fitzpatrick WA I Ross <0.01 0.01 0.01 5 <0.01 0.02 0.02 3 61.4 0.1 1.7 1.7 10

AbsarokaBeartooth WA II Stepping Stone

Twin Island

<0.01

<0.01

0.02

0.01

0.02

0.02

5

5

<0.01

<0.01

0.02

0.02

0.03

0.03

3

3

27.0

36.0

0.1

0.1

2.0

1.4

2.1

1.5

10

10

Cloud Peak WA

II

Emerald

Florence

<0.01

<0.01

0.03

0.03

0.03

0.03

5

5

<0.01

<0.01

0.07

0.08

0.08

0.08

3

3

53.3

32.7

0.2

0.3

4.4

8.1

4.6

8.4

10

10

Popo Agie WA II Lower Saddlebag <0.01 0.01 0.01 5 <0.01 0.03 0.03 3 55.5 0.1 3.2 3.2 10

Source: Argonne (2002)

Notes: Alt A Project - Direct modeled Alternative A impacts.

Non-Project - Direct modeled non-project source impacts. The impact from all air pollutant emission sources not included in Alt A, including the Wyoming “Powder River 

Basin Oil and Gas Project” DEIS Alternative 1 sources. Potential impacts from Wyoming Alternatives 2A, 2B and 3 would be less.

Cum – Cumulative modeled impacts. Since these values represent the maximum cumulative impact at a specific location, they are the sum of the maximum direct Alt A Project

and Non-Project impacts. There are uncertainties, unquantified at this point, associated with the modeled values. Actual maximum impacts may be larger or smaller than those 

shown.

Thld – Impact threshold. Total sulfur and nitrogen thresholds from Fox, et al. (1989); acid neutralizing capacity thresholds from FS (2000).

WA – Wilderness Area.

a - Since the background acid neutralizing capacity at Upper Frozen Lake is less than 25 μeq/l, the applicable significance threshold is less than a 1 μeq/l change. This threshold 

is exceeded by Non-Project and Cum emission sources. However, the background concentration is based on only six samples taken on four days between 1997 and 2001.
AIR QUALITY APPENDIX

Part 1 AIR-19

TABLE AQ-6

ALTERNATIVE A—DAILY FLAG REFINED METHOD—VISIBILITY IMPACT ANALYSIS 

(NUMBER OF DAYS Δ1.0 DV PER YEAR)

Sensitive Location PSD Classification Alt A Project Non-Project Cum

Badlands WA mandatory federal Class I 0 17 to 25 18 to 25

Bridger WA mandatory federal Class I 0 8 to 10 8 to 10

Fitzpatrick WA mandatory federal Class I 0 7 to 9 8 to 10

Gates of the Mountains WA mandatory federal Class I 0 3 to 4 3 to 4

Grand Teton NP mandatory federal Class I 0 4 to 6 4 to 6

North Absaroka WA mandatory federal Class I 0 10 to 12 11 to 12

Red Rock Lakes WA mandatory federal Class I 0 0 to 1 0 to 1

Scapegoat WA mandatory federal Class I 0 2 to 2 2 to 3

Teton WA mandatory federal Class I 0 7 to 9 7 to 10

Theodore Roosevelt NP (North Unit) mandatory federal Class I 0 1 to 2 1 to 2

Theodore Roosevelt NP (South Unit) mandatory federal Class I 0 2 to 4 2 to 4

U.L. Bend WA mandatory federal Class I 0 5 to 5 5 to 6

Washakie WA mandatory federal Class I 0 11 to 14 12 to 15

Wind Cave NP mandatory federal Class I 0 21 to 27 22 to 28

Yellowstone NP mandatory federal Class I 0 9 to 11 9 to 11

Fort Peck IR Tribal designated Class I 0 1 to 2 2 to 2

Northern Cheyenne IR Tribal designated Class I 0 30 to 38 33 to 42

Absaroka-Beartooth WA federal Class II 0 28 to 29 28 to 30

Agate Fossil Beds NM federal Class II 0 10 to 15 10 to 15

Bighorn Canyon NRA federal Class II 0 19 to 21 19 to 23

Black Elk WA federal Class II 0 20 to 26 20 to 26

Cloud Peak WA federal Class II 0 21 to 28 23 to 30

Crow IR federal Class II 2 56 to 61 65 to 69

Devils Tower NM federal Class II 0 24 to 38 26 to 39

Fort Belknap IR federal Class II 0 60 to 61 61 to 61

Fort Laramie NHS federal Class II 0 13 to 17 13 to 17
AIR QUALITY APPENDIX

Part 1 AIR-20

TABLE AQ-6

ALTERNATIVE A—DAILY FLAG REFINED METHOD—VISIBILITY IMPACT ANALYSIS 

(NUMBER OF DAYS Δ1.0 DV PER YEAR)

Sensitive Location PSD Classification Alt A Project Non-Project Cum

Jewel Cave NM federal Class II 0 24 to 31 24 to 32

Mount Rushmore NMem federal Class II 0 17 to 22 17 to 22

Popo Agie WA federal Class II 0 8 to 10 8 to 10

Soldier Creek WA federal Class II 0 13 to 18 13 to 18

Source: Argonne (2002)

Notes: Alt A Project - Direct modeled Alternative 1 impacts.

Non-Project - Direct modeled non-project source impacts. The impact from all air pollutant emission sources not 

included in Alt A, including the Wyoming “Powder River Basin Oil and Gas Project” DEIS sources. The range of 

values corresponds to including Wyoming Alternative 3 (low) to Wyoming Alternative 1 (high).

Cum - Cumulative modeled impacts. Since these values represent the maximum visibility impact anywhere within 

the sensitive location, they may not be a simple sum of the maximum direct Alt A Project and Non-Project 

impacts, which can occur at different locations. There are uncertainties, unquantified at this point, associated with 

the modeled values. Actual maximum impacts may be larger or smaller than those shown.

Locations: 

IR - Indian Reservation. NHS - National Historic Site. NM - National Monument

NMem - National Memorial. NP - National Park. NRA - National Recreation Area

WA - Wilderness Area.
Part 1 AIR-21

AIR QUALITY APPENDIX

TABLE AQ-7

ALTERNATIVES B/C/E - PREDICTED CRITERIA POLLUTANT IMPACTS AND

APPLICABLE SIGNIFICANCE THRESHOLDS (IN (μG/M3)

Pollutant Avg Time a Location

PSD 

Increment

Alts

B/C/E 

Project

Alts 

B/C/E 

Project + 

RFFA

NonProject CumBackground Total NAAQS MAAQS

carbon monoxide

1-hour

8-hours

near-field

far-field 1

near-field

far-field 2

- - -

- - -

- - -

- - -

109

6

74

56

112.6

7.3

77.2

57.8

540.0

100.0

311.3

28.9

548.2

100.0

337.2

78.0

15,000

15,000

6,600

6,600

15,548

15,100

6,937

6,677

40,000

40,000

10,000

10,000

26,000

26,000

10,000

10,000

nitrogen dioxide

1-hour

Annual

near-field

far-field 3

near-field

far-field 3

far-field 2

- - -

- - -

25

25

2.5c

100

58

9.1

3.9

1.9

102.3

60.1

9.4

4.7

3.7c

181.0

27.5

4.8

1.1

0.5

207.3

73.3

10.7

5.4

4.2c

117

117

11

11

11

324.3

190.3

21.7

16.4

15.2

- - -

- - -

100

100

100

566

566

100

100

100

PM2.5

24-hours

Annual

near-field

far-field 3

near-field

far-field 3

- - -

- - -

- - -

- - -

6.2

4.2

1.4

0.7

6.9

5.1

1.5

0.8

44.1

10.6

5.6

0.5

45.9 

14.7

6.3

1.2

20

20

8

8

65.9 b

34.7

14.3

9.2

65 b

65

15

15

- - -

- - -

- - -

- - -

PM10

24-hours

Annual

near-field

far-field 4

far-field 2

far-field 5

near-field

far-field 4

30 c

30

8 c

8 c

17

17

12.1

0.3

4.2

1.4

3.6

<0.1

13.1

0.4

5.9

2.0

3.7

<0.1

103.8 c

29.7

8.4 c

7.2

13.1

2.7

107.1 c

29.7

12.8 c

9.2 c

14.3

2.7

105

105

105

105

30

30

212.1 d

134.7

117.8

114.2

44.3

32.7

150 d

150

150

150

50

50

150 d

150

150

150

50

50

sulfur dioxide

1-hour

3-hours

24-hours

Annual

near-field

far-field 3

near-field

far-field 3

near-field

far-field 3

near-field

far-field 3

- - -

- - -

512

512

91

91

20

20

4.6

2.2

3.5

1.7

2.1

1.0

0.7

0.3

4.6

2.2

3.5

1.8

2.1

1.1

0.7

0.3

27.4

29.6

22.6

17.1

9.8

5.3

1.0

0.4

28.2

29.6

23.6

17.1

10.5

5.3

1.2

0.4

666

666

291

291

73

73

16

16

694.2

695.6

314.6

308.1

83.5

78.3

17.2

16.4

- - -

- - -

1,300

1,300

365

365

80

80

1,300

1,300

- - -

- - -

260

260

60

60
AIR QUALITY APPENDIX

Part 1 AIR-22

TABLE AQ-7

ALTERNATIVES B/C/E - PREDICTED CRITERIA POLLUTANT IMPACTS AND

APPLICABLE SIGNIFICANCE THRESHOLDS (IN (μG/M3)

Pollutant Avg Time a Location

PSD 

Increment

Alts

B/C/E 

Project

Alts 

B/C/E 

Project + 

RFFA

NonProject CumBackground Total NAAQS MAAQS

Source: Argonne (2002)

Notes:

a Annual impacts are the first maximum value; short-term impacts are the second maximum value. There are uncertainties, unquantified at this point, 

associated with the modeled values. Actual maximum impacts may be larger or smaller than those shown.

b Two receptor locations just south of the Spring Creek Coal Mine when combined with an assumed background concentration of 20 μg/m2 were predicted to 

exceed the National ambient air quality standards due to Cum emission sources. c It is possible that Alts B/C/E Project + RFFA, Non-Project and/or Cum emission sources could exceed the PSD Class I increment on the Northern 

Cheyenne Indian Reservation and the Washakie Wilderness Area, as well as the PSD Class II increment near the maximum assumed development; a 

regulatory “PSD Increment Consumption Analysis” should be conducted during permitting by the appropriate air quality regulatory agency. d Two receptor locations just south of the Spring Creek Coal Mine when combined with an assumed background concentration of 105 μg/m2 were predicted to 

exceed the National and Montana ambient air quality standards due to Non-Project and Cum emission sources.

Alts B/C/E Project - Direct modeled Alternatives’ B/C/E impacts. 

Alts B/C/E Project + RFFA - Direct modeled Alternatives’ B/C/E impacts combined with emissions from potential CBM development on the Northern 

Cheyenne and Crow Indian Reservations and the Ashland District of the Custer National Forest.

Non-Project – Direct modeled non-project source impacts. The impact from all air pollutant emission sources not included in Alts B/C/E, including the 

Wyoming “Powder River Basin Oil and Gas Project” DEIS Alternative 1 sources. Potential impacts from Wyoming Alternatives 2A, 2B and 3 would be less.

Cum – Cumulative modeled impacts. Since these values represent the maximum cumulative impact location, they may not be a simple sum of the maximum 

direct Alts B/C/E Project and Non-Project impacts, which can occur at different locations.

Total - The sum of the cumulative modeled impact and the assumed background concentration.

NAAQS – Applicable National Ambient Air Quality Standard.

MAAQS – Applicable Montana Ambient Air Quality Standard.

Locations: 

1 – Absaroka-Beartooth Wilderness Area

2 – Northern Cheyenne Indian Reservation 

3 – Crow Indian Reservation

4 – Fort Belknap Indian Reservation 

5 – Washakie Wilderness Area
AIR QUALITY APPENDIX

Part 1 AIR-23

TABLE AQ-8

ALTERNATIVES B/C/E - PREDICTED ATMOSPHERIC DEPOSITION IMPACTS AND APPLICABLE SIGNIFICANCE THRESHOLDS

Total Sulfur Deposition

(kg/ha-yr)

Total Nitrogen Deposition

(kg/ha-yr)

Acid Neutralizing Capacity

(per cent)

Location

PSD 

Class Lake

Alts 

B/C/E 

Project

Alts 

B/C/E 

Project 

+ 

RFFA

NonProject Cum Thld

Alts 

B/C/E 

Project

Alts 

B/C/E 

Project 

+ 

RFFA

NonProject Cum Thld

Bkgd 

(μeq/l)

Alts 

B/C/E 

Project

Alts 

B/C/E 

Project 

+ 

RFFA

NonProject Cum Thld

Bridger WA I Black Joe

Deep

Hobbs

Upper Frozen

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

5

5

5

5

<0.01

<0.01

<0.01

<0.01

0.01

0.01

<0.01

0.01

0.03

0.03

0.02

0.03

0.03

0.03

0.02

0.03

3

3

3

3

69.0

61.0

68.0

5.8

0.3

0.3

0.2

0.2 a

0.4

0.4

0.3

0.25 a

2.2

2.5

1.2

1.6 a

2.6

2.9

1.5

1.8 a

10

10

10

1 a

Fitzpatrick 

WA

I Ross <0.01 <0.01 0.01 0.01 5 <0.01 0.01 0.02 0.02 3 61.4 0.3 0.4 1.7 2.1 10

AbsarokaBeartooth 

WA

II Stepping Stone

Twin Island

<0.01

<0.01

<0.01

<0.01

0.02

0.01

0.02

0.02

5

5

0.01

0.01

0.01

0.01

0.02

0.02

0.03

0.03

3

3

27.0

36.0

0.4

0.3

0.6

0.4

2.0

1.4

2.5

1.8

10

10

Cloud Peak 

WA

II Emerald

Florence

<0.01

<0.01

<0.01

<0.01

0.03

0.03

0.03

0.03

5

5

0.02

0.02

0.03

0.03

0.07

0.08

0.10

0.11

3

3

53.3

32.7

1.1

1.7

1.4

2.3

4.4

8.1

5.9

10.4b

10

10b

Popo Agie 

WA

II Lower 

Saddlebag

<0.01 <0.01 0.01 0.01 5 <0.01 0.01 0.03 0.04 3 55.5 0.3 0.5 3.2 3.6 10

Source: Argonne (2002)

Notes: Alts B/C/E Project - Direct modeled Alternatives’ B/C/E impacts.

Alts B/C/E Project + RFFA - Direct modeled Alternatives’ B/C/E impacts combined with emissions from potential CBM development on the Northern Cheyenne and Crow 

Indian Reservations and the Ashland District of the Custer National Forest

Non-Project - Direct modeled non-project source impacts. The impact from all air pollutant emission sources not included in Alts B/C/E, including the Wyoming “Powder 

River Basin Oil and Gas Project” DEIS Alternative 1 sources. Potential impacts from Wyoming Alternatives 2A, 2B and 3 would be less.

Cum - Cumulative modeled impacts. Since these values represent the maximum cumulative impact at a specific location, they are the sum of the maximum direct Alts 

B/C/E Project and Non-Project impacts. There are uncertainties, unquantified at this point, associated with the modeled values. Actual maximum impacts may be larger or 

smaller than those shown.

Thld - Impact threshold. Total sulfur and nitrogen thresholds from Fox, et al. (1989); acid neutralizing capacity thresholds from FS (2000).

WA - Wilderness Area.

a - Since the background acid neutralizing capacity at Upper Frozen Lake is less than 25 μeq/l, the applicable significance threshold is less than a 1 μeq/l change. This 

threshold is exceeded by Non-Project and Cum emission sources. However, the background concentration is based on only six samples taken on four days between 1997 and 

2001.

b – The potential cumulative impact of 10.4 μeq/l change would exceed the threshold level of 10 μeq/l for Florence Lake.
AIR QUALITY APPENDIX

Part 1 AIR-24

TABLE AQ-9

ALTERNATIVES B/C/E - DAILY FLAG REFINED METHOD - VISIBILITY IMPACT ANALYSIS 

(NUMBER OF DAYS Δ1.0 DV PER YEAR)

Sensitive Location PSD Classification

Alts B/C/E 

Project

Alts B/C/E 

Project + 

RFFA Non-Project Cum

Badlands WA mandatory federal Class I 0 0 17 to 25 21 to 28

Bridger WA mandatory federal Class I 2 3 8 to 10 10 to 12

Fitzpatrick WA mandatory federal Class I 2 3 7 to 9 10 to 12

Gates of the Mountains 

WA

mandatory federal Class I 0 0 3 to 4 4 to 4

Grand Teton NP mandatory federal Class I 0 0 4 to 6 6 to 8

North Absaroka WA mandatory federal Class I 2 4 10 to 12 13 to 15

Red Rock Lakes WA mandatory federal Class I 0 0 0 to 1 2 to 3

Scapegoat WA mandatory federal Class I 0 0 2 to 2 3 to 3

Teton WA mandatory federal Class I 1 3 7 to 9 10 to 11

Theodore Roosevelt NP 

(North Unit)

mandatory federal Class I 0 0 1 to 2 2 to 3

Theodore Roosevelt NP 

(South Unit)

mandatory federal Class I 0 1 2 to 4 4 to 7

U.L. Bend WA mandatory federal Class I 1 1 5 to 5 6 to 8

Washakie WA mandatory federal Class I 3 5 11 to 14 16 to 18

Wind Cave NP mandatory federal Class I 0 0 21 to 27 25 to 32

Yellowstone NP mandatory federal Class I 1 3 9 to 11 12 to 13

Fort Peck IR Tribal designated Class I 0 1 1 to 2 4 to 5

Northern Cheyenne IR Tribal designated Class I 33 60 30 to 38 87 to 92

Absaroka-Beartooth WA federal Class II 2 4 28 to 29 32 to 33

Agate Fossil Beds NM federal Class II 0 0 10 to 15 14 to 19

Bighorn Canyon NRA federal Class II 9 17 19 to 21 32 to 34

Black Elk WA federal Class II 0 1 20 to 26 24 to 31

Cloud Peak WA federal Class II 6 10 21 to 28 35 to 39

Crow IR federal Class II 61 75 56 to 61 113 to 116
AIR QUALITY APPENDIX

Part 1 AIR-25

TABLE AQ-9

ALTERNATIVES B/C/E - DAILY FLAG REFINED METHOD - VISIBILITY IMPACT ANALYSIS 

(NUMBER OF DAYS Δ1.0 DV PER YEAR)

Sensitive Location PSD Classification

Alts B/C/E 

Project

Alts B/C/E 

Project + 

RFFA Non-Project Cum

Devils Tower NM federal Class II 1 3 24 to 38 34 to 47

Fort Belknap IR federal Class II 1 1 60 to 61 61 to 62

Fort Laramie NHS federal Class II 0 1 13 to 17 16 to 20

Jewel Cave NM federal Class II 0 0 24 to 31 28 to 36

Mount Rushmore NMem federal Class II 0 0 17 to 22 20 to 26

Popo Agie WA federal Class II 2 3 8 to 10 11 to 13

Soldier Creek WA federal Class II 0 0 13 to 18 16 to 21

Source: Argonne (2002)

Notes: Alts B/C/E Project - Direct modeled Alternatives’ B/C/E impacts.

Alts B/C/E Project + RFFA - Direct modeled Alternatives’ B/C/E impacts combined with emissions from potential 

CBM development on the Northern Cheyenne and Crow Indian Reservations and the Ashland District of the Custer 

National Forest.

Non-Project - Direct modeled non-project source impacts. The impact from all air pollutant emission sources not 

included in Alts B/C/E, including the Wyoming “Powder River Basin Oil and Gas Project” DEIS sources. The range of 

values corresponds to including Wyoming Alternative 3 (low) to Wyoming Alternative 1 (high).Cum - Cumulative 

modeled impacts. Since these values represent the maximum visibility impact anywhere within the sensitive location, 

they may not be a simple sum of the maximum direct Alts B/C/E Project and Non-Project impacts, which can occur at 

different locations. There are uncertainties, unquantified at this point, associated with the modeled values. Actual 

maximum impacts may be larger or smaller than those shown.

Locations: 

IR - Indian Reservation. NHS - National Historic Site. NM - National Monument

NMem - National Memorial. NP - National Park. NRA - National Recreation Area 

WA - Wilderness Area.
Part 1 AIR-26

AIR QUALITY APPENDIX

TABLE AQ-10

ALTERNATIVE D - PREDICTED CRITERIA POLLUTANT IMPACTS AND APPLICABLE SIGNIFICANCE THRESHOLDS (IN (μG/M3)

Pollutant Avg Time a Location

PSD 

Increment

Alt D 

Project

Alt D 

Project 

+ RFFA

NonProject CumBackground Total NAAQS MAAQS

carbon monoxide 1-hour

8-hours

near-field

far-field 1

near-field

far-field 1

- - -

- - -

- - -

- - -

48

2

29

1

47.7

2.2

29.6

1.8

540

100

311.3

52

540.8

100.0

319.8

51.8

15,000

15,000

6,600

6,600

15,541

15,100

6,920

6,652

40,000

40,000

10,000

10,000

26,000

26,000

10,000

10,000

nitrogen dioxide 1-hour

Annual

near-field

far-field 3

near-field

far-field 3

far-field 2

- - -

- - -

25

25

2.5

50

33

6.4

2.4

1.1

59.6

32.7

6.5

2.8

2.0

181

27.5

4.8

1.1

0.5

195.1

43.9

7.8

3.5

2.5e

117

117

11

11

11

312.1

160.1

18.814.

5

13.5

- - -

- - -

100

100

100

566

566

100

100

100

PM2.5 24-hours

Annual

near-field

far-field 3

near-field

far-field 4

- - -

- - -

- - -

- - -

4.3

2.6

1.2

<0.1

4.7

2.9

1.2

<0.1

44.1

10.6

5.6

1.2

45.3

12.8

6.0

1.2

20

20

8

8

65.3 b

32.8

14.0

9.2

65 b

65

15

15

- - -

- - -

- - -

- - -

PM10 24-hours

Annual

near-field

far-field 4

far-field 2

far-field 5

near-field

far-field 4

30 c

30

8 c

8 c

17

17

10.8

0.1

3.3

0.6

3.3

<0.1

11.5

0.2

4.4

0.9

3.4

<0.1

103.8 c

29.7

8.4 c

7.2

13.1

2.7

106.5 c

29.7

11.1 c

8.1 c

14.1

2.7

105

105

105

105

30

30

211.5 d

134.7

116.1

113.1

44.1

32.7

150 d

150

150

150

50

50

150 d

150

150

150

50

50

sulfur dioxide 1-hour

3-hours

24-hours

Annual

near-field

far-field 3

near-field

far-field 3

near-field

far-field 3

near-field

far-field 3

- - -

- - -

512

512

91

91

20

20

4.5

2.2

3.5

1.7

2.1

1.0

0.7

0.3

4.5

2.2

3.5

1.8

2.1

1.1

0.7

0.3

27.4

29.6

22.6

17.1

9.8

5.3

1.0

0.4

28.2

29.6

23.6

17.1

10.5

5.3

1.2

0.4

666

666

291

291

73

73

16

16

694.2

695.6

314.6

308.1

83.5

78.3

17.1

16.4

- - -

- - -

1,300

1,300

365

365

80

80

1,300

1,300

- - -

- - -

260

260

60

60
AIR QUALITY APPENDIX

Part 1 AIR-27

Source: Argonne (2002)

Notes: a Annual impacts are the first maximum value; short-term impacts are the second maximum value. There are uncertainties, unquantified at this point, 

associated with the modeled values. Actual maximum impacts may be larger or smaller than those shown.

b Two receptor locations just south of the Spring Creek Coal Mine when combined with an assumed background concentration of 20 μg/m2 were predicted to 

exceed the National ambient air quality standards due to Cum emission sources.

c It is possible that Non-Project and/or Cum emission sources could exceed the PSD Class I increment on the Northern Cheyenne Indian Reservation and 

Washakie Wilderness Area, as well as the PSD Class II increment near the maximum assumed development; a regulatory “PSD Increment Consumption 

Analysis” should be conducted during permitting by the appropriate air quality regulatory agency.

d Two receptor locations just south of the Spring Creek Coal Mine when combined with an assumed background concentration of 105 μg/m2 were predicted 

to exceed the National and Montana ambient air quality standards due to Cum emission sources.

e Actual model results equal to 2.45 μg/m3

. See Argonne (2002) Appendix C, Table C.1.2.3.

Alt D Project - Direct modeled Alternative D impacts.

Alts D Project + RFFA - Direct modeled Alternatives’ D impacts combined with emissions from potential CBM development on the Northern Cheyenne 

and Crow Indian Reservations and the Ashland District of the Custer National Forest.

Non-Project - Direct modeled non-project source impacts. The impact from all air pollutant emission sources not included in Alt D, including the Wyoming 

“Powder River Basin Oil and Gas Project” DEIS Alternative 1 sources. Potential impacts from Wyoming Alternatives 2A, 2B and 3 would be less.

Cum – Cumulative modeled impacts. Since these values represent the maximum cumulative impact location, they may not be a simple sum of the maximum 

direct Alt D Project and Non-Project impacts, which can occur at different locations.

Total - The sum of the cumulative modeled impact and the assumed background concentration.

NAAAQS - Applicable National Ambient Air Quality Standard.

MAAQS - Applicable Montana Ambient Air Quality Standard.

Locations: 

1 – Absaroka-Beartooth Wilderness Area

2 – Northern Cheyenne Indian Reservation 

3 – Crow Indian Reservation

4 – Fort Belknap Indian Reservation 

5 – Washakie Wilderness Area
Part 1 AIR-28

AIR QUALITY APPENDIX

TABLE AQ-11

ALTERNATIVE D - PREDICTED ATMOSPHERIC DEPOSITION IMPACTS AND APPLICABLE SIGNIFICANCE THRESHOLDS

Total Sulfur Deposition

(kg/ha-yr)

Total Nitrogen Deposition

(kg/ha-yr)

Acid Neutralizing Capacity

(per cent)

Location

PSD 

Class Lake

Alt D 

Projec

t

Alt D 

Project 

+ 

RFFA

NonProjec

t Cum Thld

Alt D 

Projec

t

Alt D 

Project 

+ 

RFFA

NonProjec

t Cum Thld

Bkgd 

(μeq/l)

Alt D 

Projec

t

Alt D 

Project 

+ 

RFFA

NonProjec

t Cum Thld

Bridger WA I Black Joe

Deep

Hobbs

Upper Frozen

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

5

5

5

5

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

0.03

0.03

0.02

0.03

0.03

0.03

0.02

0.03

3

3

3

3

69.0

61.0

68.0

5.8

0.2

0.2

0.1

0.1 a

0.2

0.2

0.2

0.13 a

2.2

2.5

1.2

1.6 a

2.4

2.7

1.4

1.7 a

10

10

10

1 a

Fitzpatrick 

WA

I Ross <0.01 <0.01 0.01 0.01 5 <0.01 <0.01 0.02 0.02 3 61.4 0.2 0.2 1.7 1.9 10

AbsarokaBeartooth WA

II Stepping Stone

Twin Island

<0.01

<0.01

<0.01

<0.01

0.02

0.01

0.02

0.02

5

5

<0.01

<0.01

0.01

0.01

0.02

0.02

0.03

0.03

3

3

27.0

36.0

0.3

0.2

0.3

0.2

2.0

1.4

2.3

1.6

10

10

Cloud Peak 

WA

II Emerald

Florence

<0.01

<0.01

<0.01

<0.01

0.03

0.03

0.03

0.03

5

5

0.01

0.01

0.02

0.02

0.07

0.08

0.09

0.09

3

3

53.3

32.7

0.6

0.9

0.7

1.1

4.4

8.1

5.2

9.2

10

10

Popo Agie WA II Lower 

Saddlebag

<0.01 <0.01 0.01 0.01 5 <0.01 <0.01 0.03 0.03 3 55.5 0.2 0.2 3.2 3.4 10

Source: Argonne (2002)

Notes: Alt D Project - Direct modeled Alternative D impacts.

Alts D Project + RFFA - Direct modeled Alternatives’ D impacts combined with emissions from potential CBM development on the Northern Cheyenne and Crow 

Indian Reservations and the Ashland District of the Custer National Forest.

Non-Project - Direct modeled non-project source impacts. The impact from all air pollutant emission sources not included in Alt D, including the Wyoming “Powder 

River Basin Oil and Gas Project” DEIS Alternative 1 sources. Potential impacts from Wyoming Alternatives 2A, 2B and 3 would be less.

Cum - Cumulative modeled impacts. Since these values represent the maximum cumulative impact at a specific location, they are the sum of the maximum direct Alt D

Project and Non-Project impacts. There are uncertainties, unquantified at this point, associated with the modeled values. Actual maximum impacts may be larger or 

smaller than those shown.

Thld - Impact threshold. Total sulfur and nitrogen thresholds from Fox, et al. (1989); acid neutralizing capacity thresholds from FS (2000).

WA - Wilderness Area.

a - Since the background acid neutralizing capacity at Upper Frozen Lake is less than 25 μeq/l, the applicable significance threshold is less than a 1 μeq/l change. This 

threshold is exceeded by Non-Project and Cum emission sources. However, the background concentration is based on only six samples taken on four days between 1997 

and 2001.
AIR QUALITY APPENDIX

Part 1 AIR-29

TABLE AQ-12

ALTERNATIVE D - DAILY FLAG REFINED METHOD - VISIBILITY IMPACT ANALYSIS (NUMBER OF 

DAYS >1.0 DV PER YEAR)

Sensitive Location PSD Classification

Alt D 

Project

Alt D Project 

+ RFFA Non-Project Cum

Badlands WA mandatory federal Class I 0 0 17 to 25 20 to 26

Bridger WA mandatory federal Class I 0 1 8 to 10 9 to 11

Fitzpatrick WA mandatory federal Class I 0 0 7 to 9 8 to 10

Gates of the Mountains 

WA

mandatory federal Class I 0 0 3 to 4 3 to 4

Grand Teton NP mandatory federal Class I 0 0 4 to 6 5 to 7

North Absaroka WA mandatory federal Class I 0 1 10 to 12 12 to 14

Red Rock Lakes WA mandatory federal Class I 0 0 0 to 1 1 to 2

Scapegoat WA mandatory federal Class I 0 0 2 to 2 2 to 3

Teton WA mandatory federal Class I 0 0 7 to 9 9 to 10

Theodore Roosevelt NP 

(North Unit)

mandatory federal Class I 0 0 1 to 2 1 to 2

Theodore Roosevelt NP 

(South Unit)

mandatory federal Class I 0 0 2 to 4 3 to 5

U.L. Bend WA mandatory federal Class I 0 0 5 to 5 5 to 6

Washakie WA mandatory federal Class I 1 1 11 to 14 14 to 16

Wind Cave NP mandatory federal Class I 0 0 21 to 27 23 to 29

Yellowstone NP mandatory federal Class I 0 0 9 to 11 11 to 12

Fort Peck IR Tribal designated Class I 0 0 1 to 2 2 to 3

Northern Cheyenne IR Tribal designated Class I 17 38 30 to 38 70 to 76

Absaroka-Beartooth WA federal Class II 0 1 28 to 29 30 to 31

Agate Fossil Beds NM federal Class II 0 0 10 to 15 12 to 17

Bighorn Canyon NRA federal Class II 3 7 19 to 21 2 to 28

Black Elk WA federal Class II 0 0 20 to 26 22 to 28

Cloud Peak WA federal Class II 1 2 21 to 28 28 to 35

Crow IR federal Class II 42 56 56 to 61 102 to 105

Devils Tower NM federal Class II 0 0 24 to 38 29 to 42
AIR QUALITY APPENDIX

Part 1 AIR-30

TABLE AQ-12

ALTERNATIVE D - DAILY FLAG REFINED METHOD - VISIBILITY IMPACT ANALYSIS (NUMBER OF 

DAYS >1.0 DV PER YEAR)

Sensitive Location PSD Classification

Alt D 

Project

Alt D Project 

+ RFFA Non-Project Cum

Fort Belknap IR federal Class II 0 0 60 to 61 61 to 61

Fort Laramie NHS federal Class II 0 0 13 to 17 15 to 18

Jewel Cave NM federal Class II 0 0 24 to 31 26 to 34

Mount Rushmore NMem federal Class II 0 0 17 to 22 18 to 23

Popo Agie WA federal Class II 0 1 8 to 10 9 to 11

Soldier Creek WA federal Class II 0 0 13 to 18 14 to 20

Source: Argonne (2002)

Notes: Alt D Project - Direct modeled Alternative D impacts. 

Alts D Project + RFFA - Direct modeled Alternatives’ D impacts combined with emissions from potential CBM 

development on the Northern Cheyenne and Crow Indian Reservations and the Ashland District of the Custer National 

Forest.

Non-Project - Direct modeled non-project source impacts. The impact from all air pollutant emission sources not 

included in Alt D, including the Wyoming “Powder River Basin Oil and Gas Project” DEIS sources. The range of values 

corresponds to including Wyoming Alternative 3 (low) to Wyoming Alternative 1 (high).

Cum - Cumulative modeled impacts. Since these values represent the maximum visibility impact anywhere within the 

sensitive location, they may not be a simple sum of the maximum direct Alt D Project and Non-Project impacts, which 

can occur at different locations. There are uncertainties, unquantified at this point, associated with the modeled values. 

Actual maximum impacts may be larger or smaller than those shown.

Locations: 

IR - Indian Reservation. NHS - National Historic Site. NM - National Monument

NMem - National Memorial. NP - National Park. NRA - National Recreation Area 

WA - Wilderness Area.
AIR QUALITY APPENDIX

Part 1 AIR-31

9.0 Thresholds For 

Triggering Mitigation

9.1 Clean Air Act Regulatory 

Thresholds

For Prevention of Significant Deterioration 

(PSD) of air quality, modeled and monitored 

results for PM10and NO2 will be evaluated 

against the Class I and Class II increments to 

determine if additional mitigation will be 

required (see Table AQ-1). 

Monitoring data only will be used to determine if 

the NAAQS PM10 and NO2 standards (see Table 

AQ-1) have been exceeded. For federal lands 

with Class I areas, the Clean Air Act sets a 60-

year goal of clear vistas. Clear vistas are defined 

as reduction in visibility not to exceed 1.0 

deciview/year for more than 1 day. Where this

threshold is exceeded from a single project, this 

could be the basis for the federal land managers’ 

designation of visibility impairment. Such a 

designation could necessitate mitigation. Where 

the threshold is exceeded based on cumulative 

actions (i.e. RFFA), this also could be the basis 

for the federal land managers’ designation of 

visibility impairment. In this instance, Congress 

directed federal land managers to implement 

mitigation pursuant to the Regional Haze Rule, 

in a manner that results in a 25% reduction in 

impairment every 15-year period to meet the 60-

year clear vistas goal.

In order to prevent violations of national and 

local air quality standards, emission controls 

need to be implemented before standards are 

violated. For an analytic approach,

implementation of control adequate to lead to no 

predicted cumulative violations are adequate, 

since all known and anticipated emissions will 

presumably be modeled within model 

uncertainties. NO2 modeling of this well 

understood gas should be accurate enough to 

base mitigation decisions. 

9.2 “Levels of Concern”

If mitigation measures are not fully implemented 

until regulatory thresholds are exceeded, then a 

regulatory process is triggered to resolve the 

exceedances. Such a process may be lengthy, 

costly and administratively burdensome. 

Agencies may wish to avoid such a process by 

establishing a “level of concern” short of 

regulatory thresholds, which would trigger 

implementation of control measures of a type 

and quantity sufficient to avoid reaching 

regulatory thresholds.

Where predictive capability is well-developed, as 

is the case with modeling of NO2, an LOC might 

more closely approach the regulatory threshold. 

However, with a pollutant such as PM10, greater 

uncertainties exist in the prediction of ambient 

concentrations due to such factors as differential 

particle settling. In such a case, an LOC may 

need to be established at a lower level to achieve 

the objective of avoiding regulatory 

exceedances. 

9.3 Mitigation Measures

If air quality mitigation applied by all parties in 

the Powder River Basin are proven to be 

inadequate, cumulatively, to maintain these 

Class I and Class II increment limits based on 

regulatory air quality modeling or monitored 

conditions, Montana, Wyoming, or the Tribes 

may impose either a State or Tribal 

Implementation Plan (SIP or TIP) to assure 

preservation of the rural air quality. EPA may 

itself impose a Federal Implementation Plan 

(FIP) to obtain controls on all regulated pollutant 

emission sources in order to assure preservation 

of the rural air quality.

9.4 Mitigation 

Tables AQ-13 and AQ-14 include the array of 

measures available to mitigate potential PM10

and NOx impacts and the effectiveness of each 

measure.
AIR QUALITY APPENDIX

Part 1 AIR-32

TABLE AQ-13

FUGITIVE DUST MITIGATION MEASURES (PM10), EFFECTIVENESS AND COST

Dust Sources

Disturbed 

Areas Unpaved Roads1

Mitigation 

Options

Establish 

plant cover 

for all 

disturbed 

lands by 

certain time 

(revegetation)

Water roads 

to attain 

certain 

percent 

moisture 

Apply soil 

stabilizer

Set and 

enforce speed 

limit

Gravel roads Pave road

Effectiveness Level 

proportional 

to percentage 

of land cover

0 – 50% 

reduction in 

uncontrolled 

dust 

emissions

33 to 100% 

control 

efficiency

80% for 

15 mph

65% for 

20 mph

25% for 

30 mph 2

30% 

reduction

90% 

reduction

Estimated 

Cost

$/acre $4000/mile $2,000 to 

$4,000/mile 

per year

Unknown $9,000/mile $11,000 to 

$60,000/mile

1Improved and County roads

2

Reductions assume 40 mile per hour base speed.

TABLE AQ-14

NITROGEN OXIDES (NOX) MITIGATION MEASURES EFFICIENCY

Nox Emissions Sources1

Field Compressors Sales Compressors

Temporary Diesel 

Generators 2 Heavy Equipment

Mitigation 

Options/Efficiency

Implement Best 

Available Control 

Technology

Typically results in a 

NOx emission rate of 

about 1 g/bhp-hr

Implement Best 

Available Control 

Technology

Typically results in a 

NOx emission rate of 

about 1 g/bhp-hr

Register with State; 

will regulate as 

appropriate

Voluntary use of 

diesel engines

1 Using electric – powered compressor motors in place of the typical natural-gas fired compressor engines could 

eliminate direct NOx emissions from compressor station locations.

2Wyoming is currently registering these generators to determine if Nox emissions are significant.
AIR QUALITY APPENDIX

Part 2 AIR-i

AIR QUALITY MODELING APPENDIX – PART 2

QUANTITATIVE REVIEW OF 

AMBIENT AIR QUALITY IMPACTS

Supplement to Statewide Final Oil and Gas Environmental Impact Statement and 

Amendment of the Powder River and Billings Resource Management Plans 

Prepared for 

U.S. BUREAU OF LAND MANAGEMENT

Miles City Field Office

111 Garryowen Road

Miles City, MT 59301-0940 

November 2006
AIR QUALITY APPENDIX

Part 2 AIR-ii

This Page Intentionally Left Blank
AIR QUALITY APPENDIX

Part 2 AIR-iii

TABLE OF CONTENTS

1.0 INTRODUCTION AND BACKGROUND............................................................................................1

Oil and Gas EIS.................................................................................................................................2

Coal Review......................................................................................................................................3

Objective of This Study.....................................................................................................................5

Key Issues .........................................................................................................................................6

2.0 TECHNICAL APPROACH ...................................................................................................................9

Overview of Assessment Approach ..................................................................................................9

Air Quality Modeling ........................................................................................................................9

Receptor Grids and Analyses............................................................................................................9

Emissions Input Data ......................................................................................................................10

Emissions Source Groups .................................................................................................11

Base Year Selection ..........................................................................................................11

Alternative Development Year .........................................................................................11

Emissions by Source Group..............................................................................................11

Ambient Air Quality During the Base Year....................................................................................18

PM10 ..................................................................................................................................18

PM2.5 .................................................................................................................................18

NO2 ...................................................................................................................................18

SO2 ....................................................................................................................................19

3.0 MODELED RESULTS FOR BASE YEAR AND ALTERNATIVE 

DEVELOPMENT SCENARIOS.........................................................................................................20

Impacts on Ambient Air Quality .....................................................................................................20

Impacts at Near-field Receptors in Montana.....................................................................21

Impacts at Near-field Receptors in Wyoming...................................................................21

Air Quality Impacts at Class I Area Receptors................................................................................21

Air Quality Impacts at Sensitive Class II Area Receptors...............................................................22

Impacts on Visibility .......................................................................................................................22

Impacts on Acid Deposition ............................................................................................................23

Impacts on Sensitive Lake Acid Neutralizing Capacity ..................................................................23

Analysis of Hazardous Air Pollutant Impacts .................................................................................24

4.0 SUPPLEMENTAL AIR QUALITY ANALYSIS (SAQA) MITIGATION SCENARIOS ..............54

Overview of SAQA Assessment Approach.....................................................................................54

Results of SAQA Mitigation Assessment .......................................................................................56

Direct Project Impacts (RFD)..........................................................................................................56

Alt. H Revised...................................................................................................................56

Scenario 1..........................................................................................................................60

Scenario 1A.......................................................................................................................60

Scenario 2..........................................................................................................................60

Scenario 2A.......................................................................................................................60

Potential Visibility Impacts.............................................................................................................60

Method Two......................................................................................................................60

Acid Deposition Impacts.................................................................................................................63

Cumulative Impacts (Existing Sources+RFD + RFFA Sources).....................................................63

Alt. H Revised...................................................................................................................63

Scenario 1..........................................................................................................................63

Scenario 1A.......................................................................................................................63

Scenario 2..........................................................................................................................64

Scenario 2A.......................................................................................................................64

Tongue River Railroad ....................................................................................................................64
AIR QUALITY APPENDIX

Part 2 AIR-iv

TABLES

2-1 Total Annual Emissions for Alternatives Under Consideration..........................................................12

2-2 Summary of Total Emissions by Watershed Year 20 of Development..................................................14

2-3 Base Year 2004 Alternative Production Year (Year 20) Emissions MT Conventional O&G........................17

3-1 Summary of Modeled Air Quality Impacts ...................................................................................20

3-2 Visibility – Method 6 and Monthly f(RH) Values – Base Year..........................................................25

3-3 Visibility – Method 6 and Monthly f(RH) Values – Future Alternatives..............................................................26

3-4 Modeled Deposition for Nitrogen and Sulfur – Base Year....................................................................29

3-5 Maximum Deposition for Alternate Development Scenarios................................................................................30

3-6 Modeled Impacts on Acid Sensitive Lakes – Alternative Development Scenarios...............................................31

3-7 Modeled Acute Concentrations of Hazardous Air Pollutants (HAPs) All Production Scenarios..........................31

3-8 Modeled Annual Concentrations of Hazardous Air Pollutants (HAPs) All Production Scenarios........................32

FIGURES

1-1 Montana Statewide Oil and Gas EIS Study Receptor Grids and Modeling Domain................................. .07

1-2 Coal Review Receptor Grids and Modeling Domain......................................................................08

2-1 Projected Coal Development for PRB Wyoming...........................................................................16

3-1 Change in Modeled Concentrations of NO2, SO2, PM10, PM2.5, Montana Near-field Receptors ......................33

3-2 Change in Modeled Concentrations of NO2, SO2, PM10, PM2.5, Wyoming Near-field Receptors......................35

3-3 Change in Modeled Concentrations of NO2, SO2, PM10, PM2.5, Northern Cheyenne Indian Reservation..........37

3-4 Change in Modeled Concentrations of NO2, SO2, PM10, PM2.5, Theodore Roosevelt National Park...............40

3-5 Change in Modeled Concentrations of NO2, SO2, PM10, PM2.5, Wind Cave National Park...............................42

3-6 Change in Modeled Concentrations of NO2, SO2, PM10, PM2.5, Crow Indian Reservation................................44

3-7 Change in Modeled Concentrations of NO2, SO2, PM10, PM2.5, Cloud Peak Wilderness................................47

3-8 Change in Modeled Concentrations of NO2, SO2, PM10, PM2.5, Bighorn Canyon NRA.....................................49

3-9 Change in Modeled Concentrations of NO2, SO2, PM10, PM2.5, Wind River Indian Reservation.......................52

Attachment B – Review of Mitigation Effects

Attachments

Attachment A – Review of Information on Health Effects 
AIR QUALITY APPENDIX

Part 2 AIR-1

1.0 INTRODUCTION AND 

BACKGROUND

The Powder River Basin (PRB) of Montana and 

Wyoming is a major coal resource region in the 

United States. It has also produced large quantities of 

natural gas and oil, and has experienced significant 

development of coal bed natural gas from its coal 

seams. The region also has a diverse set of 

environmental values, including proximity to some of 

the most pristine areas in the United States. Sensitive 

areas that were evaluated include the identified Class 

I areas, for air quality regulatory purposes, and other 

selected Class II sensitive areas, based on previous 

studies of coal development and coal bed natural gas 

development in the region. 

A Montana Statewide Oil and Gas Environmental 

Impact Statement (EIS) had been developed in 

January 2003. This report provides a supplemental 

analysis of potential impacts related to air quality for 

Coal Bed Natural Gas Development in the Powder 

River Basin area. The potential air quality impacts

have recently been analyzed as part of two different

studies: 

• Final Statewide Oil and Gas Environmental 

Impact Statement and Proposed Amendment of 

the Powder River and Billings Resource 

Management Plans, prepared by the Bureau of 

Land Management Miles City Field Office and 

the Billings Field Office, and the State of 

Montana Board of Oil and Gas Conservation and 

the Montana Department of Environmental 

Quality (BLM and Montana, 2003); The bulk of 

the technical review was based on data included 

in the Technical Support Document (Argonne 

2002) that was applied to both the Montana 

Statewide Oil and Gas EIS, and;

• Task 1A and 3A Reports for the Powder River 

Basin Coal Review, Cumulative Air Quality 

Effects, prepared for the BLM Casper Field 

Office, and the Wyoming State Office (ENSR

2005a, b). 

A series of dispersion modeling exercises were 

conducted for each of the cited studies and analyses. 

In this report, the studies will be referred to as the Oil 

and Gas EIS and the Coal Review, respectively. 

Additional impact analyses have been carried out for 

the Tongue River Railroad expansion and the 

Proposed Roundup Power Plant in Musselshell 

County, Montana. The results of these proposed 

projects are also incorporated into this report.

This study provides a further evaluation of the air 

quality-related environmental impacts of continued 

development of coal bed natural gas resources in the 

region. The evaluation includes estimating emissions 

and potential impacts for a base year (2004), and 

estimating comparative potential impacts for peak 

development for three separate development 

scenarios. This report describes the emissions 

development, summarizes those data, discusses the 

modeling efforts, and presents results for the base 

year and alternative development plans. 

The purpose of the study was to evaluate the regional 

changes in air quality potential impacts resulting 

from three separate development scenarios. The study 

is not designed to provide specific air permitting data 

for a specific project. The focus is on potential 

impacts in the Powder River Basin “region,” which is 

characterized as the near-field grid, and on the 

sensitive receptor groups surrounding the region. 

Details of the analysis are provided for all groups, but 

emphasized for the near-field and for the sensitive 

areas that have the highest modeled potential impacts

from the sources in the region. 

Finally, a word should be said regarding dispersion 

modeling analyses and their use in planning and 

decision-making. All dispersion models, regardless of 

their level of complexity, are mathematical 

approximations (based largely on fluid dynamics) of 

the behavior of the atmosphere. Therefore, 

particularly given the uncertain nature of the number 

and placement of the RFD Alternative sources used 

in this analysis, the results need to be viewed 

appropriately as estimates of possible future 

concentrations and not exact predictions in time and 

space. 

Because of this, dispersion modeling is generally 

conducted in a somewhat conservative manner, 

attempting to insure that the final results do not 

underestimate the actual or future impacts, so that 

appropriate planning decisions can be made. For 

example, sources may be assumed to operate for 

longer times or emit more pollutants than might be 

reasonable to expect to insure that health-based air 

standards are protected. On the other hand, analyses 

are not conducted assuming the worst-case conditions 

across the board, which could lead to a “falsepositive” result. Hence, dispersion modeling analyses 

are a balancing act, using the best available 

information and methods (EPA-approved models, 

emission factors, etc.) when possible, and the best 

scientific and professional judgment otherwise, trying 

to shade the analysis so that the final results do not 

under-predict the actual concentrations.
AIR QUALITY APPENDIX

Part 2 AIR-2

Oil and Gas EIS 

The Oil and Gas EIS included evaluations of the full 

range of environmental issues for development in the 

Montana and Wyoming Project Areas. Figure 1-1 

depicts the EIS study area and the receptor grids. For 

comparison to this study, the EIS included three 

separate model runs to address potential impacts on 

air quality for several development alternatives that 

included no action, a preferred development 

alternative, and three other alternatives that addressed 

varying development limitations or emphases. The 

study addressed potential impacts from project 

sources and from non-project sources in a five-state 

region. It predicted potential impacts on ambient air 

quality standards (NO2, SO2, PM10, PM2.5, and CO), 

PSD Class I increments, sulfur and nitrogen 

deposition, visibility in Class I areas, and potential 

impacts on sensitive lakes. 

Among the analyzed alternatives, the common 

cumulative impacts for all alternatives included 

potential exceedances of the 24-hour PM10 standard 

in the near-field receptors in Montana. The 

exceedances were generally due to PM10 sources near 

mining operations; however, the method of analysis 

was not sufficiently detailed to provide a regulatory 

estimate of actual exceedances. The EIS analysis also 

reviewed PSD increments and noted potential 

impacts above the PSD levels, but did not specifically 

sort PSD increment consuming sources into their 

specific potential impacts The EIS noted that 

potential impacts among the alternatives are 

generally similar (Alternatives B, C, and E were 

stated to have similar potential impacts). The 

potential impacts of the alternatives under 

consideration were generally below applicable 

standards and increments, as well as having minimal 

potential impacts on visibility and acid deposition. 

The potential impacts of concern resulted from 

cumulative impacts of non-project sources that were 

analyzed in the study. All alternatives cumulative 

modeling showed visibility impacts at Class I areas, 

with the greatest potential impacts at the Northern 

Cheyenne Indian Reservation. Among the Class II 

areas reviewed, greatest potential impacts were at the 

Crow Indian Reservation, just west of the Northern 

Cheyenne Indian Reservation. 

The Oil and Gas EIS identified existing air quality 

conditions in the region at the Morningstar, Badger 

Peak, and Lame Deer monitoring sites. The summary 

stated that The Oil and Gas EIS first identified 

existing air quality conditions in the region at the 

Morningstar, Badger Peak, and Lame Deer 

monitoring sites. The summary further stated that one 

monitor has shown that some 24-hour PM10 potential 

impacts exceed the ambient air quality standard of 

150 μg/m3, specifically at the Lame Deer monitoring 

site on the Northern Cheyenne Indian Reservation. 

Additionally, modeled near-field potential impacts in 

Wyoming showed the possibility of exceedances of 

the 24-hour PM10 standard and Class II PSD 

increments. Air quality levels of NO2 and SO2 were

well below the ambient standards at all monitoring 

sites in the region. 

The key emissions input data were based on 

emissions from the proposed alternatives along with 

other selected non-alternative sources in the region. 

A review of the database used in the study prepared 

by Argonne National Labs (Argonne 2002) indicated 

that actual emissions data that were modeled 

included: those sources operating after the 

monitoring period used to establish baseline air 

quality conditions; the changes in emission rates for 

some existing projects associated with the period of 

development of any of the alternatives; and project 

RFD scenarios and reasonably foreseeable future 

actions. Only those sources with changes in 

emissions, as reported by regulatory agencies, 

including WDEQ were included in the modeling. As 

a result, the modeling effort focused on potential 

impacts from new and altered permitted sources in 

the region. A series of alternatives was evaluated 

including Alternative A (which projected limited

development under existing management 

prescriptions) and Alternatives B and D, which 

addressed various development scenarios and 

different measures that would influence air quality 

emissions. Other un-modified sources or potential 

emission rates were not modeled. The potential 

impacts from these sources were addressed by adding 

a background concentration to any analyses of the 

ambient air quality impacts for comparison to 

National and Montana Ambient Air Quality 

Standards. 

Montana Near-field Receptors: For Alternative A, 

the projected potential impacts were modeled to be 

below the associated ambient air quality standards for 

all criteria pollutants except for the cumulative 

analysis of potential impacts on the 24-hour PM10

standard. The cumulative impact on the annual PM10

standard was estimated to be about 86 percent of the 

applicable standard (50 μg/m3) for near-field and 66 

percent at far-field receptors. Potential impacts from 

other pollutants were evaluated to be only a few 

percent of the applicable ambient standard, and 

potential impacts from the proposed development 

were also well below the applicable Class II PSD 

increments. The potential impacts from Alternatives 
AIR QUALITY APPENDIX

Part 2 AIR-3

B-D showed slight increases in the PM10 impacts, but 

did not change the fact that the predicted 24-hour 

PM10 impact was above the established national and 

state ambient air quality standards. The potential 

impacts of other pollutants increased slightly, but did 

not exceed the ambient standards. Those impacts 

remained at just a few percent of the established 

standards. 

Class I and Class II Sensitive Receptor Areas:

The Oil and Gas EIS also addressed potential impacts

on the Class I – Air Quality Related Values (AQRVs)

including visibility, acid deposition, and acid 

neutralizing capacity at sensitive lakes. Potential 

impacts on visibility were evaluated in accord with 

the FLAG (2000) method which tabulated the 

number of days in which increased visibility 

impairment was greater than 10 percent of the 

background value at each receptor group. The results 

for Alternative A showed almost no impact from 

project development sources only; however potential

impacts associated with non-project sources and 

cumulative impacts led to modeled impacts up to 25 

and 28 days per year at Class I receptors to the east 

(predominately downwind) of the project area 

(Badlands National Park and Wind Cave National 

Park, respectively). Although the Northern Cheyenne 

Indian Reservation is designated as Class I for air 

quality, national visibility regulations do not apply to 

the Northern Cheyenne Indian Reservation Class I 

area because such regulations only apply to 

mandatory Class I areas. The maximum potential 

impacts on visibility show up to 42 days in which 

potential impacts were modeled at the Northern 

Cheyenne Indian Reservation. Among the Class II

areas evaluated, the maximum potential impacts were 

noted for up to 69 days or more at the Crow Indian 

Reservation and up to 61 days at the Fort Belknap 

Indian Reservation. 

The results for the other full development alternatives 

show modeled potential impacts at mandatory Class I 

areas for only 0-4 more days per year when emissions 

from all sources are considered. Potential impacts at 

the Northern Cheyenne Indian Reservation are up to 

92 days per year and up to 116 days per year at the 

Crow Indian Reservation. 

 

The Oil and Gas EIS evaluated air quality potential 

impacts from criteria pollutants in the Class I and 

Class II areas with national and state ambient air 

quality standards and PSD increments. The results for 

Alternative A showed cumulative potential impacts

exceeding the 24-hour PM10 ambient air quality 

standard in the near-field and the PSD increments in

the near-field Crow Indian Reservation Class II area

and the Northern Cheyenne Indian Reservation Class 

I area. The cumulative potential impacts from 

Alternatives B-D indicated similar exceedances of 

the 24-hour PM10 ambient air quality standard in 

near-field and PSD increment in near-field and 

Northern Cheyenne Indian Reservation receptors and 

the Washakie WSA. However, under Alternatives B 

and C, cumulative potential impacts were also 

predicted to exceed the annual NO2 PSD increment 

on Northern Cheyenne Indian Reservation receptors. 

The air quality analysis does not represent a 

regulatory PSD increment consumption analysis. 

Acid Deposition

Coal Review 

: The Oil and Gas EIS evaluated 

potential impacts at identified sensitive lakes. The 

acid neutralizing capacity of each of the lakes was 

tabulated, and the predicted deposition of nitrogen 

and sulfur compounds was used to evaluate changes 

in acid neutralizing capacity at each lake. The 

guideline indicates that if the acid neutralizing 

capacity of a lake is above 25 micro-equivalents per 

liter (μeq/L) then a 10 percent change in acid 

neutralizing capacity is considered significant

(USDA 2000, Fox et al. 1989). For lakes with lower 

acid neutralizing capacity a change of 1 μeq/L is 

considered significant. 

Results showed that potential impacts were below the 

established thresholds for all lakes except Upper 

Frozen Lake in the Bridger Wilderness Area for all 

alternatives considered. For this lake, whose acid 

neutralizing capacity is less than 25 μeq/L, each 

alternative led to an increase of more than 1 μeq/L. 

For other lakes only Florence Lake in the Cloud Peak 

Wilderness Area showed a potential impact that was 

above the 10 percent change. Under Alternative B, C, 

and E, a cumulative increase of 10.4% was indicated. 

As noted above, the Coal Review documented the air 

quality impacts of operations for coal development in 

the same region along with technical analyses of 

water and socioeconomic studies for potential coal 

development in the Montana and Wyoming Powder 

River Basin area. Figure 1-2 provides a depiction of 

the coal review study area and the associated receptor 

grids. Modeling results were presented for a base 

year (2002), using actual emissions and estimates of 

actual emissions and operations for that year. 

Modeling results were also presented for upper and 

lower reasonably foreseeable development scenarios, 

projected for 2010; and qualitative estimates of 

potential impacts were provided for 2015 based on 

expected development of specified source groupings. 

The analyses evaluated potential impacts both within 

the PRB itself and at selected sensitive areas 
AIR QUALITY APPENDIX

Part 2 AIR-4

surrounding the region. The analysis specifically 

looked at potential impacts of coal mines, power 

plants, coal-bed methane development, and other 

activities. Results were provided for both Montana 

and Wyoming source groups and receptors. 

The study area covers the CBNG development region

in Montana. The technical air quality analysis effort 

focused on coal development, with additional 

assessment of CBNG development in Wyoming. 

For the base year, results were provided as maximum 

potential impacts for receptor groups, including the 

near-field grid receptors, separately in Montana and 

Wyoming, and at the sensitive Class I and Class II 

receptor groups. This analysis provided the basis for 

making estimates of changes in future impacts. The 

analysis also provided potential impacts of acid 

deposition and visibility in the sensitive receptor 

areas, as well as assessment of changes in acid 

neutralizing capacity at identified sensitive lakes. 

In general, the air quality in the region is very good, 

as demonstrated by measured levels of NO2, SO2, and 

PM10 with the exception of PM10 concentrations near 

coal mine operations. Both the monitored data and 

the modeled results for the base year study showed 

that there was a concern about ambient 

concentrations of PM10, particularly for the 24-hour 

standard in the near-field receptor grid at receptors 

near coal mine operations in both Wyoming and 

Montana. This result was consistent with the modeled 

concentrations, which showed potential exceedances 

of the 24-hour PM10 standard for the base year. The 

Class I area potential impacts were evaluated to 

compare potential impacts to PSD increments as a 

threshold of concern and do not represent a 

regulatory PSD Increment Consumption Analysis.

At the Wyoming near-field receptors, the maximum 

potential impacts were associated with coal-related 

operations in Wyoming. Potential impacts of NO2

and SO2 were well below the ambient air quality 

standards for all receptors. For PM10 the analysis 

predicted potential impacts above the 24-hour PM10

National and Wyoming Ambient Air Quality 

Standard of 150 μg/m3 at a few receptors near the 

mining operations. The base year maximum annual 

potential impacts were predicted to be below the 

annual PM10 standard of 50 μg/m3. The maximum 

potential impacts were restricted to a few receptors 

near the mining operations, however. 

Similar to the near-field in Wyoming, the projected 

potential impacts on NO2 and SO2 levels in Montana 

were well below the applicable state and federal 

standards. The predicted impacts on 24-hour PM10

levels were above the standard of 150 μg/m3 at a few 

points near mining operations. The annual PM10

impact was predicted to be below the annual 

standards. 

Of all the Class I areas that were analyzed, the 

maximum potential impacts were predicted to occur 

at the Northern Cheyenne Indian Reservation in 

Montana. The bulk of the potential impacts for all 

three criteria pollutants at Class I areas were caused 

by coal-related sources in Montana, and the bulk of 

the SO2 impacts occurred from power plant 

emissions. All potential impacts were predicted to be 

below the ambient standards at all receptors for the 

base year. Of all the Class I areas that were analyzed, 

the maximum potential impacts were predicted to 

occur at the Northern Cheyenne Indian Reservation 

in Montana. Potential impacts at other Class I areas 

were also tabulated, but showed still lower impacts. 

At the nearest areas (Washakie Wilderness Area and 

Wind Cave National Park) impacts were generally a 

few percent of the ambient standards. 

Among the sensitive Class II areas, the maximum 

potential impacts occurred at the Crow Indian 

Reservation in Montana. Potential impacts of NO2

and SO2 at sensitive Class II areas were again well 

below the ambient standards, but PM10 impacts were 

20 percent of the 24-hour ambient standard and 6 

percent of the annual PM10 standard. Among the 

sensitive Class II areas, the maximum potential 

impacts occurred at the Crow Indian Reservation in 

Montana.

Visibility potential impacts were analyzed for the 

indicated Class I and Class II areas. Using the 

CALPUFF modeling system, potential impacts were 

analyzed using the Method 6 approach, which uses 

monthly relative humidity values for each of the 

receptor groups. Potential impacts were assessed 

using the highest 24-hour calculated extinction within 

each receptor group, and were calculated as a percent 

change in extinction from a background value. The 

study tabulated the reduced visibility at the maximum 

impact receptor in each of the Class I and Class II 

groups. Results were presented as the number of days 

of annual visibility reduction of 5 percent and 10 

percent of the background value. Maximum potential 

impacts were observed at Class I areas adjacent to the 

source area (the Northern Cheyenne Indian 

Reservation) and to the east of the PRB, specifically 

the Badlands National Park and the Wind Cave 

National Park. These receptor groups had maximum 

modeled impacts above 10 percent degradation for 

200 days or more per year. 
AIR QUALITY APPENDIX

Part 2 AIR-5

Acid deposition potential impacts were analyzed for 

nitrogen and sulfur compounds for all the indicated 

Class I areas. For all areas, the combined deposition 

rates did not exceed the established thresholds of 3 

kilograms per hectare per year (kg/ha-yr) for nitrogen 

compounds and 5 kg/ha-yr for sulfur compounds. 

The maximum deposition rates were observed at the 

Wind Cave National Park but all potential impacts

were less than 10 percent of the established 

thresholds. 

Eight separate lakes were identified as sensitive to 

acid deposition impacts, and were analyzed in accord 

with the screening methodology as provided by the 

US Forest Service. Data for lake acid neutralizing 

capacity were taken from the FS web site, which 

provides data for the 10 percent ANC values for the 

individual lakes. The threshold for significance was 

established at a change of 10 percent reduction for 

lakes with an acid neutralizing capacity of 25 microequivalents per liter (μeq/L) or more and a change of 

1 μeq/L for lakes with less than 25 μeq/L acid 

neutralizing capacity. For the base year, all potential 

impacts were below the established thresholds, but 

were close to the established thresholds for Upper 

Frozen Lake in the Bridger Wilderness Area and at 

Florence Lake in the Cloud Peak Wilderness Area. 

The Task 3A report for the Coal Review provided a 

modeling assessment of projected coal-related growth 

for 2010. Both a projected lower development 

scenario and an upper development scenario were 

analyzed. For coal-related sources, the overall 

projected growth in operations (and emissions) for 

the lower development scenario was about 13 percent 

in both Wyoming and Montana. For the upper 

development scenario, the projected growth from the 

base year was about 32 percent in Wyoming and 41 

percent in Montana. The analyses included the 

foreseeable growth in power plant emissions, as a 

result of foreseeable additions to power generation. 

The Roundup Power Plant was not included directly 

in this analysis (although a separate evaluation of this 

individual source was conducted with the same 

modeling effort).

In comparison to the base year results discussed 

above, the following conclusions were made: For the 

near-field receptor grids, air quality modeling results 

showed that the predicted development continued to 

exacerbate the predicted air quality impacts for 24-

hour PM10 and that the impacts on annual PM10 levels 

in Wyoming only would exceed the PM10 standard of 

50 μg/m3 at a few receptor points under the 2010 

upper development scenario. Potential impacts of 

other pollutants increased with increased 

development, but the modeled impacts remained well 

below the ambient air quality standards. 

The major potential impacts on Class I areas 

continued to occur at the Northern Cheyenne Indian 

Reservation. Predicted impacts were well below the 

ambient standards, but were above the PSD 

increments. At other Class I areas, only the 24-hour 

PM10 impacts were modeled to be above the PSD 

increments for the base year and for the 2010 upper 

and lower development scenarios. 

At the modeled Class II receptor areas, the maximum 

potential impacts occurred at the Crow Indian 

Reservation. Predicted 24-hour PM10 impacts were 

above the PSD Class II increments (30.5 to 36.7 

μg/m3 versus a standard of 30 μg/m3). Impacts at 

other Class II areas were below the established Class 

II increments. 

At the identified Class I areas, the analysis identified 

the modeled increase in the number of days where 

potential impacts exceeded a 10 percent reduction in 

visibility. The major potential impacts occurred at 

Class I areas to the east of the PRB area, including, 

for the 2010 upper development scenario, an increase 

of 26 days per year at Badlands National Park, 22 

days per year at Theodore Roosevelt National Park, 

and 15 days per year at Wind Cave National Park. 

For sensitive lake impacts, modeled results showed 

changes in acid neutralizing capacity above 10 

percent at Florence Lake for each of the 2010 

scenarios, and an increase of more than 1 μeq/L at 

Upper Frozen Lake. These findings are consistent 

with the Oil and Gas EIS and with the base year Coal 

Review analysis. In general impacts at other lakes are 

well below the thresholds for significant impact.

Objective of This Study 

The main objective of this study is to identify the 

changes in air quality impact resulting from the 

projected alternatives of development. Potential 

impacts are assessed at “near-field receptor grids” in 

both Wyoming and Montana and at the individual 

sensitive receptor areas as well. The impacts were 

evaluated for the same receptor set that was used in 

the Coal Review, using the same dispersion model 

and the receptor data. The near-field potential 

impacts refer to receptors in the Powder River Basin, 

near the projected development. Generally those 

receptors are within 50km of the development area. 

The assessment included evaluation of potential 

impacts at all receptor groups on ambient air levels of 

nitrogen dioxide (NO2), sulfur dioxide (SO2), 
AIR QUALITY APPENDIX

Part 2 AIR-6

particulate matter with aerodynamic diameter of 10 

microns or less (PM10), and selected hazardous air 

pollutants (HAPs). The HAPs were evaluated at the 

near-field receptors in Montana and Wyoming, but 

not at the sensitive receptor areas. At the sensitive 

receptor areas, potential impacts on visibility and 

acid deposition were also evaluated. The study 

evaluates the changes in potential impacts for each of 

these fields for the expected levels of development. 

The study includes evaluation of potential impacts at 

identified sensitive lakes in the region.

The study included development of emission rates 

and emission factors, or increases in emissions, for 

each of the source groups. Emission rates for CBNG 

development and conventional oil and gas 

development were based on data developed for the 

2003 final EIS (Argonne 2002). Information from 

state agencies was utilized for development of the 

baseline year emissions from non-project sources.

Key Issues 

Similar to the Coal Review, the key issues include 

the following: 

• Characterizing emissions and controls. The 

emission source groups that were developed for 

the Coal Review form the basis for developing 

emission rates for this study, based on the 

changes in expected production for those source 

groups. 

• Using representative meteorological data. 

Modeling was conducted using three years of 

gridded meteorological data, using the 

CALPUFF modeling system. The potential 

impacts of base year operations were modeled 

with all three years, and the year with the 

maximum impact was chosen for further 

modeling addressing the alternate development 

scenarios. 

• Assessing nearby impacts. The evaluation of 

potential impacts in the PRB, using a “near-field 

receptor grid” is similar to the Coal Review Task 

1A study. The study does not address the type of 

impact analyses that would be provided for 

obtaining an air permit for a specific facility. The 

focus is to provide a general depiction of overall 

potential impacts in the region.

• Assessing potential impacts on Class I and 

sensitive Class II areas. Class I sensitive areas 

require enhanced protection, based on federal 

law. The study evaluates potential impacts on 

ambient air quality standards, acid deposition, 

visibility, and identified sensitive lakes. The PSD 

increment consuming sources are not identified 

or modeled separately in this study. Therefore 

while the results are compared to the Class I and 

Class II PSD increments, no formal PSD 

evaluation is made.
AIR QUALITY APPENDIX

Part 2 AIR-7

Figure 1-1

Montana Statewide Oil and Gas EIS Study Receptor Grids and Modeling Domain 
AIR QUALITY APPENDIX

Part 2 AIR-8

Figure 1-2 

Coal Review Receptor Grids and Modeling Domain 
AIR QUALITY APPENDIX

Part 2 AIR-9

2.0 TECHNICAL 

APPROACH

Overview of Assessment 

Approach

The objective of the study is to evaluate potential 

impacts over a wide range of receptors centered over 

the PRB study area. The evaluation covers receptors 

within the PRB in both Montana and Wyoming, and 

it includes individual sensitive receptor groups in the 

region surrounding the PRB study area. Key aspects 

of the assessment include the selection of air 

emissions within the study area, the selection of a 

modeling system to conduct that evaluation, the 

selection of a receptor set (within the model system) 

to be used for evaluating those potential impacts, and 

the selection of criteria for evaluation of those 

potential impacts. 

This study addressed the impact of changes in 

emissions from a base year for three separate 

development scenarios. The assessment evaluated 

changes in air quality levels for NO2, SO2, PM10 and

PM2.5 at the identified receptors. The potential 

impacts from the development scenarios were 

assessed at all receptor groups. The study analyzed 

the potential impacts from identified separate source 

groups, which allowed a characterization of potential 

impacts from the individual groups. 

This section provides a detailed review of the 

modeling system, the emissions characterization, the 

receptor grids that were used, and the assessment 

criteria that were used for evaluation of potential 

impacts. 

Air Quality Modeling 

To conduct a formal modeling of those potential 

impacts, the USEPA guideline model CALPUFF 

(Scire, et al. 2000) was used to estimate potential 

impacts in both the PRB receptors and the sensitive 

surrounding areas. The CALPUFF modeling system 

was recommended for a refined modeling analysis of 

the region in order to assess potential impacts over 

near-field and distant receptor areas. The CALPUFF 

modeling system has three main components:

• CALMET (a diagnostic three-dimensional 

meteorological model, which develops the 

meteorological data for modeling input);

• CALPUFF (the transport and dispersion model 

that carries out calculations of dispersion);

• CALPOST (a post processing package that is 

used to depict overall concentrations and 

potential impacts). 

The CALPUFF modeling system is designed to treat 

the time-varying point and area source emissions, 

model domains at distances from tens of meters to 

hundreds of kilometers from the sources; predict 

averaging times from 1 hour to 1 year; predict 

impacts for inert pollutants that are not chemically 

changed in the atmosphere; predict potential impacts

of pollutants that may be subject to removal and 

chemical conversion mechanisms; and be applied to 

rough terrain situations. Given these strengths and the

objectives of the study, the CALPUFF model is aptly 

suited to carrying out the required atmospheric 

dispersion modeling. 

The CALPUFF modeling domain for the PRB Coal 

Study was established to be identical to that used in 

the PRB Oil and Gas Final EIS (BLM 2003d) and the 

base year study that is part of the overall coal review 

(ENSR 2005a,b). A depiction of the CALPUFF 

modeling domain, along with the depiction of the 

study area and sensitive receptors, is provided in 

Figure 1-2.

The CALMET input files were developed from the 

regional MM5 data base for 2001, 2002, and 2003. 

All three years were used to develop the potential 

impacts for the base year (2004 emissions). The study 

first analyzed the potential impacts for all three years 

for the base year, focusing on potential impacts in the 

near-field. A comparison of the potential impacts

from those three years concluded that the year 2002 

would provide the highest potential impacts in the 

near-field. For each of the development scenarios, the 

potential impacts were then analyzed using only 2002 

meteorological data. 

Receptor Grids and Analyses 

Receptor grids were established for both near-field 

and far-field areas (sensitive Class I and Class II 

areas of concern). These included the near-field

receptors in both states, which cover the study area in 

each state. The receptor grids are the same as those in 

the Coal Review, as shown in Figure 1-2. The nearfield grid receptors cover grid points within the 

boundaries of the PRB development area. Near-field 

receptors were arranged to obtain the maximum 

estimated concentrations that result from 

development within the PRB. 
AIR QUALITY APPENDIX

Part 2 AIR-10

The purpose of establishing the near-field receptors is 

to characterize the overall air quality conditions in 

the PRB as a result of this development, but not to 

focus on potential impacts from any one individual 

source. This approach does NOT address the 

modeling that would be needed for assessing 

potential impacts at any facility fence lines, which is 

generally required for obtaining an air permit from a 

regulatory agency. Consequently, all near-field 

receptors that were located within 1 km of a modeled 

source were removed from the near-field grid. 

Overall the near-field receptor grid points were 

spaced at 1-km intervals over the study area. The 

elevation of each receptor was obtained from the 

USGS Digital Elevation Model data for the 

1:250,000 quads with 90-meter horizontal resolution. 

Receptors spaced at 1-km intervals were located 

along boundaries of Class I and Class II areas and 

receptors spaced at 2-km intervals were located 

within each of the following Class I and specified 

Class II sensitive areas of concern within the 

modeling domain: 

• Badlands National Park 

• Wind Cave National Park 

• Bridger Wilderness Area 

• Fitzpatrick Wilderness Area 

• Washakie Wilderness Area 

• North Absaroka Wilderness Area 

• Northern Cheyenne Indian Reservation (Class 1, 

Northern Cheyenne Tribal Council)

• Devils Tower National Monument 

• Mount Rushmore National Memorial 

• Jewel Cave National Monument 

• Agate Fossil Beds National Monument 

• Fort Laramie National Historic Site 

• Black Elk Wilderness Area 

• Soldier Creek Wilderness Area 

• Cloud Peak Wilderness Area 

• Yellowstone National Park 

• Grand Teton National Park 

• Teton Wilderness Area 

• Absaroka Beartooth Wilderness Area 

• Bighorn Canyon National Recreation Area 

• Popo Agie Wilderness Area 

• Crow Indian Reservation (Class II, Crow Tribal 

Council) 

• Theodore Roosevelt National Park 

The following areas are near the edge of the 

modeling domain. Modeled impacts at receptors 

within these areas near the edge of the modeling 

domain might be associated with model inaccuracies 

and uncertainties due to edge effects of the modeling. 

Therefore, estimates of potential impacts to these 

areas near the edge of the modeling domain were 

made by placing representative receptors no nearer 

than 25 km from the edge of the modeling domain: 

• Bob Marshall Wilderness Area 

• Gates of the Mountains Wilderness Area 

• Lee Metcalf Wilderness Area, Spanish Peaks 

Unit

• Lee Metcalf Wilderness Area, Taylor Hillgard 

Unit

• Red Rock Lakes Wilderness Area 

• Jedediah Smith Wilderness Area

• Mount Naomi Wilderness Area 

• Wellsville Mountain Wilderness Area 

• U.L. Bend Wilderness Area 

• Fort Peck Indian Reservation (Class I, Fort Peck 

Tribal Council)

• Scapegoat Wilderness Area 

• Fort Belknap Indian Reservation. 

These locations as well as other sensitive receptors, 

such as lakes are indicated in Figure 1-2. The 

receptors were spaced with sufficient density to 

assure that the maximum potential air quality impacts 

are evaluated. All sensitive receptors were identified 

and reviewed in the modeling protocol by the 

stakeholder group, prior to initiating the modeling. 

Emissions Input Data 

Source characterization and emissions data are key 

inputs to conducting a successful modeling analysis. 

The bulk of the emissions data were provided by the 

regulatory agencies (Wyoming Department of 

Environmental Quality, or WDEQ, and the Montana 

Department of Environmental Quality, or MDEQ). 

Emissions data for major sources in nearby states, 

which are also within the model grid, were obtained 

from the individual state regulatory agencies (Idaho, 

Utah, Nebraska, South Dakota, and North Dakota). 
AIR QUALITY APPENDIX

Part 2 AIR-11

Emissions Source Groups 

Similar to the Coal Review, the emission sources for 

the study were separated into various emission source 

groups, which were analyzed separately. The 

emission source groups that were analyzed focused 

on certain air pollutant emissions including SO2, 

NOx, and PM10. The emission source groups that 

were analyzed also focused on certain hazardous air 

pollutant (HAP) emissions including benzene, nhexane, toluene, ethyl-benzene, xylene and 

formaldehyde. The study also included a group of 

major sources that were identified by the 

Environmental Defense Fund (and others) in 

response to the analyses in the Montana Statewide 

EIS. The following emission source groups were 

analyzed as part of this study: 

• All sources combined; 

• CBNG sources;

 CBNG production, separately for each state 

 CBNG operation, separately for each state;

• Conventional oil and gas sources; 

• Coal-related sources (from both states, including 

power plants and conversion facilities) ;

• Coal mines (in both states) ;

• Montana sources (all sources located in Montana

not otherwise identified);

• Wyoming sources (all sources located in 

Wyoming not otherwise identified);

• Non-coal sources (roads, railroads, urban areas, 

miscellaneous sources, all sources in ID, UT, 

NE, SD, ND) ; 

• Environmental Defense Fund (EDF) identified 

sources; and 

• Power plants (includes coal- and gas-fired power 

plants in Wyoming and Montana). 

Base Year Selection 

At the start of the project the year 2004 was selected 

as a base year for determining current emissions and 

potential impacts. The 2004 data were readily 

available, and the year coincided with the emissions 

inventory being collected by the Western Regional 

Air Partnership (WRAP). Emission rates for 2004 

were calculated in different manners for each 

emission source group. Emission rates for the 

projected development scenarios were estimated for 

the year with the expected maximum emissions from 

the development scenarios. For this effort, the 20th

year of projected development was used, as discussed 

below. The methodology used to calculate emission 

rates for each emission source group is as follows.

Alternative Development Year 

The purpose of this effort is to characterize maximum 

emissions from selected alternate development 

scenarios over an extended period in the future, and 

to evaluate the comparative potential impacts from 

the emissions associated with each alternate

development scenario when considering approval of 

any of those alternatives. This study will use 

projected emissions for each scenario as input into 

the dispersion model. The alternative development 

year (ADY) that was used for evaluation of 

alternatives was selected based on the total maximum 

emissions from the Montana CBM construction and 

operation combined for each of the alternatives over 

a 20 year span. 

Data shown in Table 2-1 provide the total emissions 

from well construction and operations, and total 

emissions from the combined sources for each 

alternative. The table shows the maximum potential 

impacts are likely to occur in year 20 or 21 of this 

analysis (2026 or 2027) for all alternatives. 

Construction emissions peak in Year 4, but 

operational emissions are much larger and therefore 

dominate the emission pattern. Details of the total 

emissions are provided in the Air Quality Modeling 

Technical Support Document (ALL 2006). Based on 

the emissions data presented in Table 2-1, Year 20

was selected as the ADY for which potential impacts

are modeled in this report. For the base year (2004)

and the ADY (Year 20), a set of emission factors and 

emission rates for each of the identified source 

groups was developed, as described below. 

Emissions by Source Group 

This section summarizes the calculation of emissions 

for each source group identified above. Both the base 

year and ADY are included in this discussion. 

Coal Bed Natural Gas Sources 

As shown in Table 2-1, the coal bed natural gas

(CBNG) production sources form the basis for 

conducting the evaluation. For this study, projected 

CBNG development was provided for the Montana 

area study by watershed area. Each of the watersheds 

was identified and a level of CBNG development was 

assigned to each watershed, including both well 

development/construction and well operation in year 

20. Emissions from the well development and 

operation were calculated based on the number of 
AIR QUALITY APPENDIX

Part 2 AIR-12

wells in each category, using emission factors that 

were developed for Table 2-1.

A total of 15 separate watersheds are included in this 

analysis, for each of the three alternative 

development scenarios that are under consideration. 

Table 2-2 lists each alternative, along with projected 

development and associated emission rates for each 

watershed. The total wells and emissions are also 

provided for each alternative. 

Among the alternatives, there are different 

development rates in several of the watersheds. In the 

Rosebud watershed, the maximum operation wells

occur in Alternative E, with less in Alternatives F and 

H respectively. The Lower Yellowstone Sunday and 

Upper Yellowstone Lake B combined had greater 

development in Alternative E than in any of the other 

alternatives. 

Overall Alternative E had greater development in 

terms of operational wells, but the least in terms of 

wells under construction. In general the development 

from Alternative E through Alternative H showed an 

increase in the number of wells under construction. 

Other relevant development data is presented in

Tables 2-1 and 2-2.

To conduct the modeling, the emissions from each 

watershed were assigned to 5 separate point sources 

within each watershed, using representative stack 

parameters for oil and gas development. 

Table 2-1 

Total Annual Emissions for Alternatives Under Consideration 

Alternative E Alternative F Alternative H

Year

Sum Total 

Emissions 

Oper (Tons)

Sum Total 

Emissions 

Const (Tons)

Sum Total 

Emissions 

All (Tons)

Sum Total 

Emissions 

Oper (Tons)

Sum Total 

Emissions 

Const (Tons)

Sum Total 

Emissions 

All (Tons)

Sum Total 

Emissions 

Oper (Tons)

Sum Total 

Emissions 

Const (Tons)

Sum Total 

Emissions 

All (Tons)

1 536 1917 2454 357 1277 1634 357 1276 1633

2 1717 2303 4021 1250 1915 3166 1250 1914 3164

3 3543 4220 7762 2419 2261 4679 2419 2263 4681

4 6009 4596 10605 3740 2461 6201 3744 2473 6217

5 8476 4220 12696 5080 2260 7340 5069 2263 7332

6 10516 3070 13586 6255 1914 8169 6261 1999 8260

7 12126 2684 14810 7333 1916 9249 7356 1914 9271

8 13413 1917 15331 8412 1914 10326 8428 1914 10342

9 14486 1918 16404 9490 1914 11404 9499 1914 11413

10 15452 1532 16984 10568 1914 12482 10570 1914 12485

11 16202 1151 17353 11644 1915 13559 11642 1914 13556

12 16846 1151 17998 12713 1905 14618 12713 1914 14627

13 17490 1150 18641 13731 1734 15465 13784 1914 15699

14 18134 1150 19285 14702 1735 16437 14856 1914 16770

15 18778 1151 19929 15673 1735 17407 15927 1914 17842

16 19368 959 20327 16573 1482 18055 16998 1914 18913

17 19905 957 20862 17401 1479 18880 18040 1809 19850

18 20441 960 21400 18200 1377 19578 19018 1683 20701

19 20924 766 21690 18906 1143 20049 19930 1578 21508

20 21457 571 22028 19487 935 20422 20754 1367 22122

21 0 0 0 19691 1070 20761 21071 575 21646

22 0 0 0 19032 1043 20075 0 0 0

23 0 0 0 17198 1049 18247 0 0 0
AIR QUALITY APPENDIX

Part 2 AIR-13

Coal Production Related Sources

For coal production related sources, which included 

mines, mine roads, railroads, and coal conversion 

sources, the base year data (2004) was used to 

establish the baseline emissions. Coal production 

estimates were obtained from analyses of the Coal 

Review, and those estimates were used to change 

total coal-related mining sources Total coal 

development was based on the Coal Review. 

Emissions for the ADY were based on coal 

development projections and applied to both 

Montana and Wyoming. 

Figure 2-1 provides a graphical representation of the 

expected changes in coal production over the next 

two decades. The Coal Review provided an updated 

coal production scenario for 2004 and 2020. The coal 

average values of the coal production increase from 

380 million tons/year in 2004 to 580 million 

tons/year in 2020. This ratio (1.53) was applied to 

coal development in Wyoming and Montana from the 

base year to the ADY. 

Conventional Oil & Gas Sources

For conventional oil and gas sources, the baseline 

year data (2004) was used to establish the baseline 

emissions. The number of operating wells and the 

number of conventional oil and gas production levels 

for the base year and for the ADY were obtained 

from available data (MBOGC 2006). Emissions 

estimates include both operating wells and well 

construction as indicated in the Table 2-3. The 

emission factors shown in Table 2-3 were developed 

from a combination of data sources, and the factors 

represent the emissions in ton/year that would be 

emitted by either well construction or well operation. 

For the ADY, the total number of wells, including 

operation and construction are also indicated. The 

table shows the dramatic increase in the number of 

operating wells, but a slight reduction in the number 

of wells being constructed. Overall, emissions of 

NOX from this source group would decline about 109 

ton/year from the base year to the ADY. Emissions of 

PM10 would increase slightly and emissions of SO2

would decrease slightly from the base year. 

To conduct the modeling effort, the locations of the 

emissions sources were assigned to five separate 

point sources within each of the indicated counties. 

No specific site location data were available, and 

therefore this approach represented a suitable 

approximation for the modeling effort.

Power Plant Sources

For coal-fired power plants, the projected ADY 

emission rates for power plants that were not 

operational in 2004 but are expected to be operational 

in the ADY were derived from the actual power plant 

permit applications or the power plant permits from 

the specified facility. This should allow for a 

conservative estimate since the permitted emission 

rates will be the allowable emission rates, and actual 

emission rates from these new power plants could be 

less than the allowable emissions but cannot be 

higher. Where stack parameters were available, those 

data were used for input into the modeling. Emissions 

of NOx, SO2, and PM10 from the power plant permits 

were determined from expected levels of best 

available control technology (BACT) that would be 

applied to those sources. If a coal-fired plant permit 

application or permit was not obtainable, emissions 

from a coal-fired plant of the equivalent size was 

used to estimate emissions. The coal-fired power 

plants for which emissions were estimated for the 

ADY include the following:

• WYGEN2

• Two Elk Unit 1 

• Basin Electric / Gillette

• Hardin Generating Station

• Roundup Power Plant 

• Great Falls Power Plant 

These coal-fired power plants are included as 

individual sources, in addition to the existing coalfired facilities which were also analyzed. For existing 

coal-fired power plant sources that were operational 

in 2004, to account for a possible increase in capacity 

between the baseline year to ADY, a scaling factor 

was used to increase the capacity of these sources 

from 88% capacity factor in 2004 to a 90% capacity 

factor in the ADY.
AIR QUALITY APPENDIX

Part 2 AIR-14

Table 2-2

Summary of Total Emissions by Watershed 

Year 20 of Development 

Alternative E

Watersheds

Operational 

Wells

Construction 

Wells

NOx 

Emissions 

(Tons)

PM10 

Emissions 

(Tons)

SO2 

Emissions 

(Tons)

VOC 

Emissions 

(Tons)

Upper Tongue 5024 0 1930 424 37 2141

Lower Tongue 4503 0 1730 380 33 1919

Middle Powder 2741 0 1053 231 20 1168

Little Powder 261 0 100 22 2 111

Rosebud 4698 0 1805 396 35 2003

Mizpah 163 0 63 14 1 70

Clarks Fork Yellowstone 587 0 226 50 4 250

Lower Yellowstone Sunday 2219 0 852 187 16 946

Upper Yellowstone Lake B 1045 93 490 121 14 453

Little Bighorn 881 100 433 110 13 384

Lower Bighorn 1043 121 516 131 15 455

Middle Musselshell 131 9 59 14 2 57

Upper Yellowstone Pompeys 262 35 133 34 4 114

Stillwater 131 23 72 19 2 57

Upper Musselshell 98 13 50 13 2 43

TOTAL 23787 394 9511 2145 201 10170

Alternative F

Watersheds

Operational 

Wells

Construction 

Wells

NOx 

Emissions 

(Tons)

PM10 

Emissions 

(Tons)

SO2 

Emissions 

(Tons)

VOC 

Emissions 

(Tons)

Upper Tongue 5024 0 1930 424 37 2141

Lower Tongue 4440 139 1838 424 42 1904

Middle Powder 2638 122 1129 266 27 1134

Little Powder 261 0 100 22 2 111

Rosebud 4515 198 1923 451 46 1941

Mizpah 164 0 63 14 1 70

Clarks Fork Yellowstone 653 0 251 55 5 278

Lower Yellowstone Sunday 1565 49 648 149 15 671

Upper Yellowstone Lake B 687 57 318 78 9 298

Little Bighorn 582 20 242 56 6 250

Lower Bighorn 663 35 288 68 7 286

Middle Musselshell 89 3 37 9 1 38

Upper Yellowstone Pompeys 173 12 77 19 2 75

Stillwater 85 6 38 9 1 37

Upper Musselshell 63 4 28 7 1 27

TOTAL 21602 645 8911 2050 201 9260
AIR QUALITY APPENDIX

Part 2 AIR-15

Table 2-2 (Continued)

Alternative H

Watersheds

Operational 

Wells

Construction 

Wells

NOx 

Emissions 

(Tons)

PM10 

Emissions 

(Tons)

SO2 

Emissions 

(Tons)

VOC 

Emissions 

(Tons)

Upper Tongue 5024 0 1930 424 37 2142

Lower Tongue 4502 0 1730 380 33 1919

Middle Powder 2741 0 1053 231 20 1168

Little Powder 261 0 100 22 2 111

Rosebud 4263 322 1944 474 52 1843

Mizpah 164 0 63 14 1 70

Clarks Fork 

Yellowstone 587 0 226 50 4 250

Lower Yellowstone 

Sunday 2219 0 852 187 16 946

Upper Yellowstone 

Lake B 841 303 611 179 26 383

Little Bighorn 882 0 339 74 7 376

Lower Bighorn 1044 0 401 88 8 445

Middle Musselshell 86 100 128 43 7 45

Upper Yellowstone 

Pompeys 163 218 270 91 15 87

Stillwater 131 0 50 11 1 56

Upper Musselshell 99 0 38 8 1 42

TOTAL 23007 943 9734 2275 231 9882
AIR QUALITY APPENDIX

Part 2 AIR-16

 

350

400

450

500

550

600

650

2002 2004 2006 2008 2010 2012 2014 2016 2018 2020 2022

Year

Total Production (mmtpy)

Forecasted Upper Production Forecasted Lower Production

Figure 2-1

Projected Coal Development for PRB Wyoming 
AIR QUALITY APPENDIX

Part 2 AIR-17

Table 2-3

Base Year 2004 and Alternative Production Year (Year 20) Emissions 

Montana Conventional Oil and Gas Operation and Construction 

Base

Year County

Wells 

Oper

Wells 

Const

NOx Emissions 

Oper (Tons)

NOx 

Emissions 

Const (Tons)

PM10 

Emissions 

Oper (Tons)

PM10 

Emissions 

Const 

(Tons)

SO2 

Emissions 

Oper (Tons)

SO2 

Emissions 

Const 

(Tons)

2004Big Horn 46 2 1.22 18.99 0.99 1.67 0.09 2.34

2004Carbon 99 7 2.62 66.47 2.14 5.85 0.19 8.20

2004Custer 4 0 0.11 0.00 0.09 0.00 0.01 0.00

2004 Golden Valley 2 0 0.05 0.00 0.04 0.00 0.00 0.00

2004Musselshell 74 20 1.96 189.90 1.60 16.70 0.14 23.42

2004 Powder River 57 5 1.51 47.48 1.23 4.18 0.11 5.86

2004Rosebud 96 10 2.54 94.95 2.07 8.35 0.19 11.71

2004 Stillwater 16 9 0.42 85.46 0.35 7.52 0.03 10.54

2004 Sweetgrass 5 3 0.13 28.49 0.11 2.51 0.01 3.51

2004 Yellowstone 28 5 0.74 47.48 0.60 4.18 0.05 5.86

2004Carter 0 12 0.00 113.94 0.00 10.02 0.00 14.05

2004Wheatland 0 0 0.00 0.00 0.00 0.00 0.00 0.00

2004Treasure 0 0 0.00 0.00 0.00 0.00 0.00 0.00

TOTAL 427 73 11.30 693.15 9.21 60.96 0.82 85.49

Emission Factors 0.0264573 9.4951754 0.0215694 0.8350877 0.0019282 1.1710526

ADY1

20 Big Horn 230 6 6.08 60.64 4.96 5.33 0.44 7.48

20 Carbon 230 6 6.08 60.64 4.96 5.33 0.44 7.48

20 Carter 115 3 3.04 30.32 2.48 2.67 0.22 3.74

20 Custer 69 2 1.82 18.19 1.49 1.60 0.13 2.24

20 Golden Valley 34 1 0.91 9.10 0.74 0.80 0.07 1.12

20 Musselshell 402 11 10.65 106.12 8.68 9.33 0.78 13.09

20 Powder River 345 10 9.12 90.96 7.44 8.00 0.67 11.22

20 Rosebud 345 10 9.12 90.96 7.44 8.00 0.67 11.22

20 Stillwater 115 3 3.04 30.32 2.48 2.67 0.22 3.74

20 Sweetgrass 23 1 0.61 6.06 0.50 0.53 0.04 0.75

20 Treasure 11 0 0.30 3.03 0.25 0.27 0.02 0.37

20 Wheatland 17 0 0.46 4.55 0.37 0.40 0.03 0.56

20 Yellowstone 115 3 3.04 30.32 2.48 2.67 0.22 3.74

TOTAL 2052 57 54.29 541.23 44.26 47.60 3.96 66.75

NET CHANGE 1625 -16 42.99 -151.92 35.05 -13.36 3.13 -18.74

1 – ADY – Alternative Development Year
AIR QUALITY APPENDIX

Part 2 AIR-18

Other Major Sources

This analysis included emissions from other major 

sources in both Montana and Wyoming as wells as 

nearby states, which are located within the modeling 

domain as presented above. Each regulatory agency 

in Idaho, Utah, Nebraska, South Dakota, and North 

Dakota were contacted to obtain emissions data for 

sources with major operating permits (as required 

under Title V of the Clean Air Act Amendments of 

1990). Locations and stack parameters were taken 

from available source data. Emissions data for 2004 

were used for most cases, but for some instances, the 

potential emissions were used. In addition for some 

sources with multiple emission sources, the total 

source emissions were characterized as a single point 

for the whole facility. These sources were all over 

400 km from the near-field grids in Montana and 

Wyoming, and such characterizations would not 

affect the potential impacts at these distant receptors. 

The other sources included all the sources in the 

domain that were identified by the Environmental 

Defense Fund in its comments on the Montana 

Statewide Oil and Gas EIS. 

As a convenience in interpreting the modeling, 

source potential impacts were grouped in several 

components, including all Montana sources, all 

Wyoming sources, railroad data, etc. In addition the 

Tongue River Railroad projected emissions were 

included. Emissions were developed for points along 

the segments of the railroad, with emission rates per 

mile developed from the Tongue River Railroad EIS. 

For these other sources there was no adjustment to 

the emission rates from the baseline year to the

alternative development year (ADY). The modeled 

location for the projections did not change from the 

baseline modeling for any sources except for the 

CBM development, conventional oil and gas 

development and new power plants. 

Ambient Air Quality During the 

Base Year 

Ambient air quality conditions in Montana for 2004 

were generally very good. Reported data as provided 

on the USEPA AIRS data base 

(www.epa.gov/air/data/reports.html) for 2004 were 

downloaded and are summarized for each pollutant 

below. 

PM10

A total of 40 separate PM10 monitors were installed 

and operated in Montana in 2004. The applicable 

standards are 150 μg/m3 for the second-highest 24-

hour level and 50 μg/m3 for the annual average. 

In Big Horn County 8 separate monitors operated, 

with the highest second-highest 24-hour PM10 level 

of 82 μg/m3 at Decker Coal #1 and the highest annual 

level of 25 μg/m3 at Decker Coal #7. For background 

concentrations, the 4th highest 24-hour level was 28 

μg/m3 at Decker Coal #5 and the lowest annual 

average was 14 μg/m3 at two sites. 

In Rosebud County, one station operated at Lame 

Deer (intersection of Highways 212 and 39). The 

second highest 24-hour PM10 level was 48 μg/m3, 

with an annual average of 22 μg/m3. 

In Yellowstone County (Billings) there were two 

operating PM10 monitoring sites. At these two sites, 

second highest 24-hour monitored level was 38 

μg/m3 and the annual averages were 16 and 21 μg/m3

respectively. 

PM2.5

A total of 21 separate PM2.5 monitoring sites were 

installed and operating in 2004, with two at Lame 

Deer and one in Billings (in the study area). The 24-

hour standard is met by evaluating the 98th percentile 

of the highest concentrations for all the collected 24-

hour samples. At Lame Deer Site 1, there were 114 

observations and the 98th percentile value would be 

the 111th (fourth highest) reading. The fourth-highest 

24-hour PM2.5 level at that site was 16 μg/m3

compared to a standard of 65 μg/m3 (proposed to be 

35 μg/m3). At the second Lame Deer Site, there were 

25 readings taken, and the second highest reading 

(98th percentile) was 11 μg/m3. In Billings there were 

116 observations, and the fourth-highest 24-hour 

reading was 19 μg/m3. The annual average PM2.5

levels were 5.8 and 5.9 μg/m3 at the two Lame Deer 

sites, and 8.2 μg/m3 in Billings, versus an annual 

arithmetic average standard of 15 μg/m3. 

NO2

NO2 was measured at three sites in Montana in 2004, 

with all three sites in Rosebud County. The Montana 

1-hour standards (not to be exceeded more than once 

per year) is 0.5 ppm, and the actual readings were 

0.027, 0.027, and 0.029 ppm at the three sites. The 

Montana and federal ambient standard is 0.053 ppm 

and the measurements for annual average at all three 
AIR QUALITY APPENDIX

Part 2 AIR-19

Rosebud County sites was 0.003 ppm. Ambient 

levels are well below the applicable standards. The 

annual average reading is about 6 percent of the 

annual standard. 

SO2

A total of 13 SO2 monitoring stations operated in 

Montana in 2004. Three were in Rosebud County and 

nine were in Yellowstone County. The Yellowstone 

observations are not discussed here, because they 

reflect impacts of nearby major SO2 sources 

(although all readings are below applicable ambient 

standards). In Rosebud County, the highest secondhighest 1-hour SO2 readings are 0.007, 0.013, and 

0.016 ppm respectively, against a Montana-only 1-

hour standard of 0.5 ppm. The highest second-highest 

3-hour values are 0.003, 0.006 and 0.007 ppm 

respectively compared to a standard of 0.5 ppm. The 

highest second-highest 24-hour averages are 0.002, 

0.003, and 0.004 ppm respectively, compared to an 

ambient standard of 0.14 ppm. For the annual 

average, all Rosebud measurements are 0.001 ppm, 

compared to an annual average standard of 0.03 ppm. 

Results show that for the Rosebud County area, the 

actual levels are about 3 percent of the standards or 

less. Current SO2 conditions in the study area are 

very clean. 
AIR QUALITY APPENDIX

Part 2 AIR-20

3.0 MODELED RESULTS 

FOR BASE YEAR AND 

ALTERNATIVE 

DEVELOPMENT 

SCENARIOS

Using the model and source groups discussed in 

Chapter 2, the modeling effort evaluated the three 

meteorological years (2001, 2002, and 2003) by 

modeling potential impacts of each of the source 

groups for the base year (2004). Potential impacts

from the base year study showed that maximum 

potential impacts occurred with the 2002 

meteorological data. Further analyses for the three

development alternatives then used the 2002 

meteorological data only for assessing potential 

impacts. 

A summary of the key findings for each of the air 

quality components is provided in Table 3-1. The 

detailed analysis for each of the components is

provided in this Chapter. In general the results of this 

modeling study are consistent with the findings of the 

Coal Review and the Oil and Gas EIS. 

Impacts on Ambient Air Quality 

Using the receptor grids identified in Chapter 2 along 

with the source groupings, the model was used to 

predict the potential impacts at each receptor point in 

the receptor grid. For this analysis, the results are 

provided for the maximum receptor in each group, 

which may not be the same receptor in each of the 

modeling scenarios. Potential impacts may occur at 

different receptors for each of the modeling 

scenarios, but those changes in maximum receptor 

are not identified in these results. 

The analysis does not separate the sources into PSD 

increment-consuming and non PSD increment 

consuming sources. Therefore the results cannot be 

used to develop a pattern of increment consumption 

for a particular site. The PSD comparisons are for 

disclosure of potential impacts and identification of 

potential areas of concern only and do not constitute 

a regulatory PSD increment consumption analysis, 

which may be required for specific projects by air 

permitting authorities. 

The model results are also limited by certain 

assumptions regarding sources and receptors. The 

source characterizations are based on available data, 

and do not represent specific stacks or sources of 

fugitive emissions. The modeling sources are 

generally provided by area or volume, to represent 

multiple sources within each specified unit. The 

specific fence lines or exclusion areas around a 

modeled source are also not specifically identified in 

this study. The results cannot, therefore, be 

interpreted as evaluating maximum potential impacts

that might occur at the boundary or fence line of a 

specific source. The receptors in the near-field grid in 

both states were removed from modeling if their 

location was within 1 km of any source.

Table 3-1

Summary of Modeled Air Quality Impacts

Air Quality Component

Alternate Development Year Impacts 

(includes modeled base year emissions)

Concentrations Criteria Below NAAQS and state AAQS, except near-field 

PM10 

HAPs Less than RELs and RfCs, except for benzene

Visibility Far-field Class I areas have greater than 200 days with 

greater than 1 dv, maximum impacts not affected 

by scenarios E, F and H. 

Atmospheric Deposition Sulfur LOC Below 5 kg/hectare-year

Atmospheric Deposition Nitrogen LOC Below 3 kg/hectare-year

Atmospheric Deposition Lake Chemistry ANC Development raises impacts above LAC for two 

lakes. 
AIR QUALITY APPENDIX

Part 2 AIR-21

Impacts at Near-field Receptors in 

Montana 

Results are provided for the near-field receptor grid 

for Montana in Figure 3-1. The figure shows the 

potential impacts at the maximum receptor for each 

modeling scenario: the base year, and the maximum

potential impact for each of the alternative scenarios. 

The potential impacts on that receptor group are 

depicted for all sources and the potential impacts that 

result from the individual source groups are identified 

in Figure 3-1. Data are provided for each ambient 

standard and PSD increment for NOx, SO2 and PM10. 

Specific data are provided in The Air Quality Model 

Technical Support Document (ALL 2006), for air 

quality impacts at all receptor groups. In this 

presentation, the impact from one source group 

would not likely be at the same receptor as that of the 

other source group; therefore the results for each 

group are not

Figure 3-1 also provides results for PM10, PM2.5 and 

SO2. The results show a relatively high impact from 

the Tongue River Railroad and from MT CBM 

operations but all potential impacts are well below 

any standards. The NO2 potential impacts would be 

the major concern regarding the development of the 

alternatives, on the Montana near-field grid. 

arithmetically additive to obtain an 

overall impact. 

The results show a predicted impact from the Tongue 

River Railroad emissions for the 1-hour Montana 

NO2 standard, about 50 percent of that standard. This 

result may be due partially to the relationship 

between the source characterization and the receptor 

grid. The Tongue River Railroad is presumed to 

operate in the ADY. 

The potential impacts from all sources on the nearfield receptor grid do increase over the base year, but 

overall the NOx emissions from the alternatives show 

a higher impact for Alternative E than for the other 

alternatives for the one-hour standard. When 

evaluating the potential impacts of the alternatives 

alone, the emissions do not lead to substantial 

differences among them for the annual or 1-hour NO2 

potential impacts. This discrepancy can be explained 

by the areal distribution of potential impacts, which

for Alternative E would include areas already 

impacted by existing sources. 

For the annual NO2 potential impacts in Montana the 

Tongue River Railroad and the CBNG operation play 

the major role, but are clearly well below the 

NAAQS and even the comparative PSD annual NO2

increment. These data are provided for comparison 

only and do not represent a regulatory PSD 

Increment Consumption Analysis.

Impacts at Near-field Receptors in 

Wyoming 

Results for the Wyoming near-field receptors are 

provided in Figure 3-2. In Wyoming the coal 

operations led to modeled impacts on PM10 levels 

that are above the NAAQS for the 24-hour period 

(150 μg/m3), for the base year as well as for ADY. 

The modeled impacts are nearly double the standard 

for the base year scenario. The remaining data show 

that potential impacts are well below the ambient air 

quality standards. The Wyoming coal operations are 

largely responsible for the predicted impacts for all 

scenarios, although non-coal sources do contribute a 

notable portion of the impact. 

The potential impacts of NO2 are generally about 40 

percent of the annual standard, with no real 

difference for the alternatives analyzed in the ADY. 

The coal sources are the largest contributor to the 

maximum NO2 potential impacts, however, CBNG 

and non-coal sources also have contributions. 

Potential impacts of NO2 are above the Class II PSD 

increment at the maximum receptors in Wyoming. 

The potential impacts of SO2 emissions are well 

below the ambient standards and PSD increments for 

all scenarios. The potential impacts from power 

plants do, however, show substantial increases in 

impacts at the maximum power plant receptor. Those 

potential impacts are, however, still well below the 

ambient standards and PSD increments. These data 

are provided for comparison only and do not 

represent a regulatory PSD Increment Consumption 

Analysis.

Air Quality Impacts at Class I Area 

Receptors 

As discussed in Chapter 2, the potential impacts at 

Class I areas were also modeled, with separate 

assessments for each Class I receptor group. The 

Class I area with the highest potential impacts was 

the Northern Cheyenne Indian Reservation in 

Montana. Those results are provided in Figure 3-3. 

The potential impacts are all well below the ambient 

standards, and also are less than the respective PSD 

increments. 

Data for two other Class I areas are also presented

(the Theodore Roosevelt National Park in Figure 3-4
AIR QUALITY APPENDIX

Part 2 AIR-22

and the Wind Cave National Park in Figure 3-5) as 

these two Class I areas represent the closest Class I 

areas east of the development area, and should 

provide a representative depiction of potential 

impacts at the Class I areas in western North Dakota 

and western South Dakota. For all areas, all potential 

impacts are well below the ambient standards, and 

are also well below the PSD increments for all 

pollutants modeled. It is also important to note that 

the comparative impacts for the ADY show little 

differentiation in potential impacts among the 

alternatives. The base year 24-hour PM10 impact at 

Theodore Roosevelt was 5.2 μg/m3, and the impact at 

Wind Cave was 6.4 μg/m3, against a Class I PSD 

increment of 8 μg/m3. These data are provided for 

comparison only and do not represent a regulatory 

PSD Increment Consumption Analysis. 

Air Quality Impacts at Sensitive 

Class II Area Receptors 

Potential impacts at the Crow Indian Reservation are 

higher than potential impacts at the other identified 

Class II area receptor groups for all scenarios. Figure 

3-6 provides a depiction of results similar to those 

provided above. For this receptor group, modeled 

impacts are all well below the ambient standards and 

they are below the established Class II PSD 

increments, except for potential impacts on the 24-

hour PM10 levels. Again, there is little difference in 

impact among the proposed alternative development 

scenarios. 

The other nearby Class II receptor group is the Cloud 

Peak Wilderness Area in north Central Wyoming, 

just west of the PRB. Results for this receptor group 

are shown in Figure 3-7. All potential impacts are 

well below applicable standards for all scenarios, and 

potential impacts are less than the Class II PSD 

increments for all scenarios. The 24-hour PM10

potential impacts reach 5 μg/m3 for the base year, but 

this is less than the comparable PSD increment of 30 

μg/m3. The greatest percentage increases arise from 

coal and power plant operations, but these increases 

still do not exceed ambient standards or PSD 

increments. Data is also presented for the Bighorn 

Canyon National Recreation Area (Figure 3-8) and 

the Wind River Indian Reservation (Figure (3-9). For 

both of these Class II areas, potential impacts are 

well below applicable standards for all scenarios, and 

potential impacts are less than the Class II PSD 

increments for all scenarios. These data are provided 

for comparison only and do not represent a regulatory 

PSD Increment Consumption Analysis.

Impacts on Visibility 

Under the Clean Air Act, visibility has been 

established as a critical resource for identified Class I 

areas. The study provides an analysis of potential 

impacts at the Class I areas and at sensitive Class II 

areas in the region. Under the guidance of the Federal 

Land Managers Air Quality Workgroup (FLAG), the 

potential impacts were provided using the CALPUFF 

modeling system and the Method 6 approach, which 

uses monthly relative humidity values for 

representative receptor groups. 

Visibility potential impacts are based on the highest 

24-hour calculated extinction at the indicated source 

receptors. Potential impacts are based on a presumed 

pristine background and calculated as a percent 

increase in extinction (reduced visibility) from that 

background value. The study tabulated the reduced 

visibility at the maximum impact receptor in each of 

the Class I and Class II groups in terms of the 

maximum reduction on any one 24-hour period, the 

number of days annually that showed visibility 

reductions of 5 percent and 10 percent. These 

reductions are indicated as reductions in deciviews 

(0.5 and 1 deciview respectively). A significance 

threshold of 10 percent has been used in this analysis 

to evaluate the impact from the source groups. 

Table 3-2 provides a listing of potential visibility 

impacts for the base year for each of the analyzed 

areas with source contributions provided for all 

sources combined, all Montana sources, the listed 

CBM operation and construction potential impacts, 

and potential impacts from Montana oil and gas 

operations. More detailed data for contributions from 

other source groups are provided in Attachment A. 

For the Class I areas, the maximum potential impacts

were determined at the North Cheyenne Indian 

Reservation, the Wind Cave National Park, and the 

Badlands National Park in South Dakota. Both of the 

South Dakota areas are downwind (prevailing wind 

direction from the west) from the PRB and the 

sources analyzed in this study. In the base year, 

model results showed more than 200 days of 

potential impacts with a change of 10 percent or more 

in extinction at each of these locations. All Class I 

areas showed some impact with no fewer than 21 

days of impact greater than 1 deciview.

For the Class II areas, the maximum potential 

impacts were at the Crow Indian Reservation in 

Montana. Nine other Class II areas showed potential 

impacts of 1 deciview or more for 200 days or more 

per year, and these areas also were east (downwind in 

the prevailing wind direction) of the PRB. The results 
AIR QUALITY APPENDIX

Part 2 AIR-23

showed that there was at least some impact on each 

of the receptor groups from each of the source 

groups. Coal operations dominated the potential 

impacts at the Class II areas, and the potential 

impacts on the Class I areas were noted for all the 

source groups. 

The results also show that the Montana Oil and Gas 

operations and construction do not play a significant 

role in potential visibility impacts at either Class I or 

sensitive Class II areas. For the base year there are 

only a few days with visibility potential impacts

above 5 deciviews at the Crow Indian Reservation 

and at the Northern Cheyenne Indian Reservation.

Table 3-3 provides a depiction of the potential 

impacts of all sources for each of the proposed 

alternatives. Data are provided for all receptor areas 

for all sources for each of the alternatives. For most 

areas, there is no change in impact among the 

alternatives. For example, at the areas with high 

potential impacts (Badlands and Theodore Roosevelt 

National Parks) there is no overall difference among 

the alternatives. At the Northern Cheyenne Indian 

Reservation, there is a change of 3 and 8 days 

respectively (for all sources combined) when 

comparing the potential impacts of Alternative E to 

Alternatives F and Alternative H respectively. At the 

Crow Indian Reservation, a maximum of 365 days 

per year are impacted for all scenarios. When 

examining the visibility potential impacts of all 

Montana sources for each alternative, there is only a 

change of one or two days of impact above 1.0 

deciviews when comparing the potential impacts of 

these alternatives. The Northern Cheyenne Indian 

Reservation would see a slight increase in the number 

of days with potential impacts above 1.0 deciviews 

(from Alternative E through Alternative H), and the 

Crow Indian Reservation would continue to see 365 

days/year impacted by a 1.0 deciview level. Other 

visibility impact data are provided in detail in 

Appendix A.

Impacts on Acid Deposition 

Emissions of NOX and SO2 can lead to increasing 

potential impacts of acidic deposition in the region. 

This analysis evaluates the potential increase in acid 

deposition as a result of the increased production 

activity noted above. The base year analysis showed 

that potential impacts for all listed Class I and Class 

II areas were below the established thresholds for 

sulfur and nitrogen deposition, which are 5 kilograms 

per hectare per year (kg/ha-yr) for sulfur compounds 

and 3 kg/ha-yr for nitrogen compounds. Table 3-3 

provides a summary of base year deposition levels at 

the sensitive receptor areas. The highest modeled 

impacts are at the Northern Cheyenne Indian 

Reservation with nitrogen deposition reaching 0.292 

kg/ha-yr, or about 10 percent of the threshold. 

Maximum sulfur deposition is approximately 0.39 

kg/ha-yr at the Northern Cheyenne Indian 

Reservation, or about 8 percent of the threshold. The 

table also shows that the contributions from base year 

CBM and Montana oil and gas operations and 

construction are minimal at any of the receptor areas. 

Additional data are provided for other source groups 

in Appendix A. Relatively higher deposition rates 

were noted to the east of the PRB, as a result of the 

prevailing wind direction in the region. For all 

receptors and for both sulfur and nitrogen 

compounds, the combined deposition rates do not 

exceed the thresholds given in these tables. 

For the ADY, potential impacts on acid deposition 

were calculated for each alternative. Table 3-4 

provides a summary listing of potential impacts for 

each alternative, for all source groups combined. The 

results show that potential impacts are slightly higher 

than in the base year, but all potential impacts remain 

well below the deposition threshold. Potential 

impacts continue to be highest at the Northern 

Cheyenne Indian Reservation, with little difference 

among the alternatives. Total nitrogen potential 

impacts approach 2 kg/hectare-year, or about twothirds of the threshold value. Sulfur deposition 

potential impacts also show little difference among 

the scenarios, and they approach approximately 10 

percent of the threshold value. 

Impacts on Sensitive Lake Acid 

Neutralizing Capacity 

The analysis of potential impacts of deposition of 

acidic substances was carried out in accordance with 

the screening methodology as provided by the US 

Forest Service (USFS 2000). Data for lake 

neutralizing capacity were obtained from the USFS 

web site, which provides data for the 10 percent ANC 

values for the individual lakes that were evaluated. 

The threshold is intended to account for sensitive 

conditions that may occur with an episodic or 

seasonal basis. Input data to the analysis include the 

deposition rates that were modeled for the base year, 

and the development scenarios analyzed herein. 

The input data are provided in Table 3-5 for the 

analyzed lakes. Results are provided for the base year 

analysis as well as the predicted development 

scenarios. The threshold for significance is based on 
AIR QUALITY APPENDIX

Part 2 AIR-24

a 10 percent change in ANC for lakes with an ANC 

of 25 micro equivalents per liter (ueq/L) and a 1 

ueq/L threshold change for lakes with an ANC value 

of less than 25 ueq/L. 

Data on the modeled potential impacts for the lakes 

analyzed is provided in Table 3-6. All lakes except 

the Upper Frozen Lake in the Bridger WA have 10 

percent ANC values of 25 ueq/L or more, and 

therefore Upper Frozen Lake is discussed separately 

below. For the other lakes the modeled percent ANC 

change is 10 percent or less at all lakes except 

Florence Lake. For that lake, the analyzed base year 

impact is 11.7 percent and the predicted impact for 

the ADY is 12.9 percent for all alternative 

development scenarios. There is no difference among 

the scenarios for potential impacts on these pristine 

lakes. 

At Upper Frozen Lake, the base year impact was 2.4 

ueq/L, which is more than the threshold value of 1 

ueq/L threshold that is established for such lakes. The 

modeled results for each of the development 

scenarios show an impact of 2.6 ueq/L for Upper 

Frozen Lake, a change of only 0.2 ueq/L for that lake. 

The results show a minimal impact, and no difference 

in impact, among the alternatives considered for this 

evaluation. 

Analysis of Hazardous Air 

Pollutant Impacts 

The modeling study also addressed HAP potential 

impacts from sources in the study area. Since the 

potential impacts were greatest in the near-field

receptor grids of both states, only those areas were 

analyzed for HAP potential impacts. The model was 

used to develop both 1-hour and annual potential 

impacts for these emissions. Results of the 1-hour 

modeled impacts for these modeling efforts were 

compared to the RELs (USEPA 2005a). Table 3-6

provides an analysis of the short term potential 

impacts for the six analyzed compounds (benzene, 

ethyl benzene, formaldehyde, n-hexane, toluene, and 

xylene) compared to the RELs. Results show that all 

potential impacts are below the RELs except for 

formaldehyde in the Wyoming near-field receptor 

grid. Potential impacts are about 70 percent greater 

than the established REL for formaldehyde. 

The potential impacts for chronic and carcinogenic 

risks are provided in Table 3-7 for the Montana and 

Wyoming near-field receptor grids. All potential 

impacts are well below the non-carcinogenic RfCs, 

with the maximum comparative impact for 

formaldehyde at the Wyoming near-field receptors, 

where those potential impacts are about 66 percent of 

the established RfC. The potential impacts for 

carcinogenic risk are also provided in Table 3-8. All 

potential impacts are well below the 1 in 1 million 

risk, except for benzene potential impacts in 

Wyoming, where the potential impacts are about 1.0 

to 1.3 X 10-5 for the various scenarios. This impact is 

evident in the base year as well as each of the 

development scenarios.
AIR QUALITY APPENDIX

Part 2 AIR-25

ALL SOURCES ALL MT MT CBM Construction MT CBM Operation MT OIL & GAS

Number of 

Days > N% 

Change in 

B ext

Number of 

Days > N% 

Change in 

B ext

Number of 

Days > N% 

Change in 

B ext

Number of 

Days > N% 

Change in 

B ext

Number of 

Days > N% 

Change in 

B ext

5% 10% 5% 10% 5% 10% 5% 10% 5% 10%

CLASS I AREAS

Badlands NP Class I 272 206 219 118 53 20 25 14 0 0 0.1 0.1 0 0 0.1 0.1 0 0 0.7 0.5

Bob Marshall W Class I 28 21 48 30 20 10 34 17 0 0 0.0 0.0 0 0 0.0 0.0 0 0 0.2 0.1

Bridger W Class I 230 152 437 156 38 19 40 18 0 0 0.2 0.0 0 0 0.2 0.1 0 0 0.9 0.2

Fitzpatrick W Class I 157 105 291 129 35 17 58 23 0 0 0.1 0.0 0 0 0.2 0.1 0 0 0.8 0.2

Fort Peck IR Class I 120 79 168 77 55 25 26 17 0 0 0.1 0.0 0 0 0.2 0.1 0 0 2.9 1.0

Gates of the Mountain W Class I 85 52 113 52 66 39 60 34 0 0 0.0 0.0 0 0 0.0 0.0 0 0 0.4 0.2

Grand Teton NP Class I 163 90 180 71 45 19 31 13 0 0 0.1 0.0 0 0 0.1 0.0 0 0 0.5 0.1

North Absaorka W Class I 149 85 229 110 90 41 66 37 0 0 0.2 0.0 0 0 0.3 0.1 0 0 1.1 0.5

North Cheyenne IR Class I 299 234 313 122 192 97 79 33 1 0 6.8 2.2 2 0 9.5 3.1 0 0 2.5 1.3

Red Rock Lakes Class I 96 48 87 49 49 20 41 16 0 0 0.0 0.0 0 0 0.1 0.0 0 0 0.4 0.1

Scapegoat W Class I 47 29 78 48 36 20 52 37 0 0 0.0 0.0 0 0 0.0 0.0 0 0 0.3 0.1

Teton W Class I 149 87 247 108 53 21 64 23 0 0 0.1 0.0 0 0 0.2 0.0 0 0 0.7 0.2

Theodore Roosevelt NP Class I 213 153 356 131 74 33 57 26 0 0 0.2 0.1 0 0 0.3 0.1 0 0 4.6 1.3

UL Bend W Class I 125 62 140 48 79 27 43 21 0 0 0.1 0.0 0 0 0.1 0.0 0 0 0.5 0.4

Washakie W Class I 169 110 335 144 75 38 85 43 0 0 0.2 0.0 0 0 0.3 0.1 0 0 1.1 0.4

Wind Cave NP Class I 320 247 265 147 69 22 24 16 0 0 0.2 0.1 0 0 0.2 0.1 0 0 2.0 0.8

Yellowstone NP Class I 188 102 207 91 102 45 64 30 0 0 0.2 0.0 0 0 0.2 0.0 0 0 1.1 0.2

SENSITIVE CLASS II AREAS

Absaorka Beartooth W Class II 201 131 266 109 170 100 135 45 0 0 0.4 0.1 0 0 0.6 0.1 0 0 2.1 0.5

Agate Fossil Beds NM Class II 295 225 401 130 54 14 21 14 0 0 0.1 0.1 0 0 0.2 0.1 0 0 1.0 0.3

Big Horn Canyon NRA Class II 356 295 376 154 200 122 143 63 0 0 1.2 0.6 0 0 1.9 0.9 10 2 24.6 5.8

Black Elk W Class II 306 214 252 144 67 23 22 15 0 0 0.1 0.1 0 0 0.2 0.1 0 0 2.3 0.6

Cloud Peak Class II 201 136 232 162 92 44 34 24 0 0 3.1 0.3 0 0 4.5 0.4 0 0 1.8 0.7

Crow IR Class II 365 360 428 266 365 350 401 165 1 0 5.2 2.6 5 0 7.2 3.4 14 2 18.1 6.7

Devils Tower NM Class II 324 260 268 130 82 29 29 17 0 0 0.2 0.1 0 0 0.3 0.2 0 0 2.2 0.9

Fort Belknap IR Class II 100 52 131 45 56 21 44 26 0 0 0.1 0.0 0 0 0.1 0.0 0 0 0.5 0.3

Fort Laramie NHS Class II 288 244 514 145 48 10 21 13 0 0 0.1 0.0 0 0 0.1 0.1 0 0 1.0 0.4

Jedediah Smith W Class II 167 94 172 59 45 22 31 14 0 0 0.0 0.0 0 0 0.1 0.0 0 0 0 0

Jewel Cave NM Class II 309 238 271 140 65 24 22 14 0 0 0.2 0.1 0 0 0.4 0.1 0 0 2 1

Lee Metcalf W Class II 165 107 138 55 140 87 89 40 0 0 0.1 0.0 0 0 0.1 0.0 0 0 1 0

Mt Naomi W Class II 78 51 195 70 4 1 12 3 0 0 0.0 0.0 0 0 0.1 0.0 0 0 0 0

Mt Rushmore Class II 297 202 248 140 61 23 22 15 0 0 0.1 0.1 0 0 0.2 0.1 0 0 2 1

Popo Agie W Class II 207 136 485 166 37 17 38 17 0 0 0.2 0.1 0 0 0.2 0.1 0 0 1 0

Soldier Creek WA Class II 297 240 396 119 59 18 20 15 0 0 0.1 0.1 0 0 0.2 0.1 0 0 1 0

Wellsville Mountain W Class II 62 36 157 54 1 0 8 2 0 0 0.0 0.0 0 0 0.1 0.0 0 0 0 0

Wind River IR Class II 305 235 546 224 97 44 88 39 0 0 3 2 1 0 5 1 0 0 1 0

Receptor Set 8th Highest 

% Change 

in B ext

8th Highest 

% Change 

in B ext

Table 3-2

Visibility - Method 6 and Monthly f(RH) values - Base Year

Maximum 

% Change 

in B ext

Maximum 

% Change 

in B ext

Maximum 

% Change 

in B ext

8th Highest 

% Change 

in B ext

8th Highest 

% Change 

in B ext

8th Highest 

% Change 

in B ext

Maximum 

% Change 

in B ext

Maximum 

% Change 

in B ext
AIR QUALITY APPENDIX

Part 2 AIR-26

Number of 

Days > N% 

Change in 

Bext

Number of 

Days > N% 

Change in 

Bext

Number of 

Days > N% 

Change in 

Bext

5% 10% 5% 10% 5% 10%

CLASS I AREAS

Badlands NP Class I 283 219 230 125 283 219 230 125 283 219 230 125

Bob Marshall W Class I 46 28 60 42 46 28 60 42 46 28 60 42

Bridger W Class I 225 146 456 152 225 146 456 152 225 147 456 152

Fitzpatrick W Class I 157 109 318 128 157 109 318 128 157 109 318 128

Fort Peck IR Class I 154 92 169 82 154 91 169 82 154 92 169 82

Gates of the Mountain W Class I 103 69 118 92 103 69 118 91 103 69 118 92

Grand Teton NP Class I 165 92 182 77 165 92 182 76 165 93 182 77

North Absaorka W Class I 161 90 256 129 161 90 255 129 161 90 256 129

North Cheyenne IR Class I 361 325 338 175 362 328 338 178 362 333 339 180

Red Rock Lakes Class I 99 50 94 53 99 50 94 53 99 50 94 53

Scapegoat W Class I 68 48 113 68 68 48 113 68 68 48 113 68

Teton W Class I 154 92 268 120 154 92 267 119 154 92 268 120

Theodore Roosevelt NP Class I 232 172 356 136 232 172 356 136 232 172 356 136

UL Bend W Class I 176 99 154 60 176 97 153 60 176 99 154 60

Washakie W Class I 178 115 368 152 177 115 368 152 178 115 369 152

Wind Cave NP Class I 325 262 275 147 325 262 275 147 325 262 276 147

Yellowstone NP Class I 193 105 226 97 193 105 225 97 193 105 226 97

SENSITIVE CLASS II AREAS

Absaorka Beartooth W Class II 213 137 303 127 213 137 302 126 213 137 303 128

Agate Fossil Beds NM Class II 297 237 399 133 297 237 399 133 297 237 399 134

Big Horn Canyon NRA Class II 356 298 411 185 356 298 409 185 356 298 410 185

Black Elk W Class II 318 233 270 150 318 233 270 150 318 233 270 150

Cloud Peak Class II 216 147 239 177 216 146 239 176 216 147 239 177

Crow IR Class II 365 365 578 259 365 365 577 253 365 365 578 257

Devils Tower NM Class II 328 279 278 135 328 279 278 134 328 279 278 135

Fort Belknap IR Class II 173 92 143 54 172 92 143 54 173 92 143 54

Fort Laramie NHS Class II 296 249 537 151 296 249 537 150 296 249 537 151

Jedediah Smith W Class II 169 96 174 66 169 95 174 66 169 96 174 66

Jewel Cave NM Class II 320 252 293 142 320 252 293 142 320 252 293 142

Lee Metcalf W Class II 175 114 153 62 175 114 152 62 175 114 153 62

Mt Naomi W Class II 80 52 198 70 80 52 198 70 80 52 198 70

Mt Rushmore Class II 312 221 262 147 311 221 262 147 312 221 262 147

Popo Agie W Class II 211 137 502 164 211 137 502 164 211 138 502 165

Soldier Creek WA Class II 299 245 396 126 299 245 396 126 299 245 396 126

Wellsville Mountain W Class II 64 40 161 57 64 40 161 57 64 40 161 57

Wind River IR Class II 310 243 566 214 310 243 565 214 311 243 566 214

Visibility - Method 6 and Monthly f(RH) values - Future Alternatives

Table 3-3

Maximum 

% Change 

in Bext

8th Highest 

% Change 

in Bext

Maximum 

% Change 

in Bext

8th Highest 

% Change 

in Bext

Receptor Set

ALL SOURCES - ALT E ALL SOURCES - ALT F ALL SOURCES - ALT H

Maximum 

% Change 

in Bext

8th Highest 

% Change 

in Bext
AIR QUALITY APPENDIX

Part 2 AIR-27

Number of 

Days > N% 

Change in 

Bext

Number of 

Days > N% 

Change in 

Bext

Number of 

Days > N% 

Change in 

Bext

5% 10% 5% 10% 5% 10%

CLASS I AREAS

Badlands NP Class I 0 0 0.2 0.1 0 0 0.4 0.3 0 0 0.5 0.3

Bob Marshall W Class I 0 0 0.1 0.0 0 0 0.1 0.0 0 0 0.3 0.1

Bridger W Class I 0 0 0.3 0.1 0 0 0.5 0.2 0 0 0.6 0.3

Fitzpatrick W Class I 0 0 0.3 0.1 0 0 0.5 0.2 0 0 0.9 0.4

Fort Peck IR Class I 0 0 0.7 0.2 0 0 0.7 0.3 0 0 2.6 0.6

Gates of the Mountain W Class I 0 0 0.4 0.1 0 0 0.3 0.1 0 0 1.4 0.4

Grand Teton NP Class I 0 0 0.2 0.1 0 0 0.3 0.1 0 0 0.4 0.2

North Absaorka W Class I 0 0 0.5 0.3 0 0 1.1 0.4 0 0 1.3 0.8

North Cheyenne IR Class I 0 0 2.4 0.9 50 8 19.1 10.0 122 26 30.8 16.1

Red Rock Lakes Class I 0 0 0.3 0.1 0 0 0.3 0.1 0 0 1.1 0.2

Scapegoat W Class I 0 0 0.2 0.1 0 0 0.2 0.1 0 0 0.7 0.2

Teton W Class I 0 0 0.3 0.1 0 0 0.5 0.1 0 0 0.9 0.3

Theodore Roosevelt NP Class I 0 0 0.8 0.3 0 0 1.1 0.5 0 0 2.1 0.8

UL Bend W Class I 0 0 0.3 0.2 0 0 0.7 0.2 0 0 0.9 0.7

Washakie W Class I 0 0 0.7 0.3 0 0 0.7 0.4 0 0 1.5 0.7

Wind Cave NP Class I 0 0 0.3 0.1 0 0 0.6 0.3 0 0 0.6 0.3

Yellowstone NP Class I 0 0 0.4 0.2 0 0 0.9 0.2 0 0 1.6 0.6

SENSITIVE CLASS II AREAS

Absaorka Beartooth W Class II 0 0 3.6 1.2 0 0 2.0 0.6 0 0 2.5 1.4

Agate Fossil Beds NM Class II 0 0 0.2 0.1 0 0 0.4 0.2 0 0 0.4 0.2

Big Horn Canyon NRA Class II 1 0 8.4 3.4 0 0 3.1 1.4 0 0 2.0 1.0

Black Elk W Class II 0 0 0.3 0.1 0 0 0.3 0.1 0 0 0.3 0.1

Cloud Peak Class II 0 0 1.3 0.4 0 0 2.5 0.6 0 0 2.2 0.5

Crow IR Class II 166 117 110.0 60.7 106 34 31.5 17.4 64 21 29.4 19.5

Devils Tower NM Class II 0 0 0.6 0.2 0 0 0.9 0.5 0 0 0.8 0.4

Fort Belknap IR Class II 0 0 0.3 0.1 0 0 0.6 0.2 0 0 1.0 0.5

Fort Laramie NHS Class II 0 0 0.2 0.1 0 0 0.4 0.2 0 0 0.4 0.2

Jedediah Smith W Class II 0 0 0.1 0.1 0 0 0.3 0.1 0 0 0.3 0.1

Jewel Cave NM Class II 0 0 0.3 0.1 0 0 0.6 0.3 0 0 0.5 0.3

Lee Metcalf W Class II 0 0 0.5 0.2 0 0 0.6 0.2 0 0 1.9 0.6

Mt Naomi W Class II 0 0 0.1 0.0 0 0 0.1 0.0 0 0 0.2 0.0

Mt Rushmore Class II 0 0 0.3 0.1 0 0 0.6 0.3 0 0 0.6 0.3

Popo Agie W Class II 0 0 0.3 0.2 0 0 0.5 0.2 0 0 0.6 0.3

Soldier Creek WA Class II 0 0 0.2 0.1 0 0 0.5 0.2 0 0 0.4 0.3

Wellsville Mountain W Class II 0 0 0.1 0.0 0 0 0.1 0.0 0 0 0.2 0.0

Wind River IR Class II 0 0 0.5 0.3 0 0 0.8 0.4 0 0 1.5 0.7

Maximum % 

Change in 

Bext

8th Highest 

% Change in 

Bext

Maximum % 

Change in 

Bext

8th Highest 

% Change in 

Bext

MT CBM Construction - ALT E MT CBM Construction - ALT F

Table 3-3 (continued)

Visibility - Method 6 and Monthly f(RH) values - Future Alternatives

Receptor Set

MT CBM Construction - ALT H

Maximum % 

Change in 

Bext

8th 

Highest % 

Change in 

Bext
AIR QUALITY APPENDIX

Part 2 AIR-28

Number of 

Days > N% 

Change in 

Bext

Number of 

Days > N% 

Change in 

Bext

Number of 

Days > N% 

Change in 

Bext

5% 10% 5% 10% 5% 10%

CLASS I AREAS CLASS I AREAS

Badlands NP Class I 2 0 6.7 3.9 1 0 6.4 3.7 2 0 6.5 3.7

Bob Marshall W Class I 0 0 1.5 0.5 0 0 1.4 0.4 0 0 1.5 0.5

Bridger W Class I 2 0 8.3 2.7 2 0 7.8 2.4 2 0 8.2 2.6

Fitzpatrick W Class I 2 0 8.9 2.4 2 0 8.2 2.2 2 0 8.7 2.3

Fort Peck IR Class I 7 1 10.2 5.0 6 0 9.4 4.1 6 0 9.9 4.9

Gates of the Mountain W Class I 0 0 4.1 1.1 0 0 3.5 0.9 0 0 3.9 1.0

Grand Teton NP Class I 0 0 4.9 1.2 0 0 4.5 1.1 0 0 4.8 1.2

North Absaorka W Class I 8 3 14.9 5.7 8 3 14.1 5.1 8 3 14.5 5.5

North Cheyenne IR Class I 296 215 130.4 61.8 294 206 125.3 59.0 328 240 118.7 63.5

Red Rock Lakes Class I 0 0 4.1 0.9 0 0 3.9 0.8 0 0 4.1 0.8

Scapegoat W Class I 0 0 2.5 0.8 0 0 2.2 0.7 0 0 2.4 0.8

Teton W Class I 3 0 7.8 2.2 3 0 7.2 2.0 3 0 7.6 2.1

Theodore Roosevelt NP Class I 11 2 15.0 6.8 11 2 13.6 6.2 11 2 14.4 6.6

UL Bend W Class I 6 1 10.2 3.5 6 0 9.6 3.2 6 0 9.9 3.4

Washakie W Class I 10 3 12.0 5.8 8 3 11.1 5.4 9 3 11.7 5.5

Wind Cave NP Class I 8 0 8.4 5.0 6 0 7.8 4.5 7 0 8.3 4.8

Yellowstone NP Class I 5 1 13.2 2.5 3 1 12.3 2.2 4 1 12.9 2.3

SENSITIVE CLASS II AREAS

Absaorka Beartooth W Class II 12 4 33.1 6.5 10 3 31.3 6.0 12 4 32.1 6.2

Agate Fossil Beds NM Class II 1 0 5.3 2.5 0 0 4.9 2.3 1 0 5.2 2.4

Big Horn Canyon NRA Class II 45 24 34.8 17.7 37 18 30.5 14.2 52 27 33.9 18.2

Black Elk W Class II 6 0 8.8 4.7 4 0 8.2 4.4 6 0 8.7 4.5

Cloud Peak Class II 22 9 71.1 10.9 21 8 68.4 10.2 21 9 70.2 10.6

Crow IR Class II 228 131 133.6 54.1 205 115 129.2 46.0 331 257 240.5 128.8

Devils Tower NM Class II 11 2 10.9 6.7 11 1 10.2 6.3 11 1 10.6 6.6

Fort Belknap IR Class II 3 0 8.5 2.9 3 0 7.9 2.5 3 0 8.2 2.9

Fort Laramie NHS Class II 3 0 5.8 2.7 1 0 5.5 2.4 2 0 5.7 2.6

Jedediah Smith W Class II 0 0 3.9 1.3 0 0 3.7 1.2 0 0 3.9 1.3

Jewel Cave NM Class II 6 0 9.4 4.0 6 0 8.7 3.6 6 0 9.2 3.9

Lee Metcalf W Class II 1 0 8.2 1.9 1 0 7.6 1.5 1 0 8.0 1.8

Mt Naomi W Class II 0 0 2.3 0.4 0 0 2.1 0.4 0 0 2.2 0.4

Mt Rushmore Class II 6 0 8.5 4.5 4 0 7.9 4.3 6 0 8.3 4.4

Popo Agie W Class II 4 0 9.1 3.3 3 0 8.5 3.0 4 0 9.0 3.2

Soldier Creek WA Class II 1 0 6.2 3.1 1 0 5.8 2.9 1 0 6.1 3.0

Wellsville Mountain W Class II 0 0 2.1 0.3 0 0 2.0 0.3 0 0 2.1 0.3

Wind River IR Class II 9 4 13.2 6.0 9 3 12.2 5.6 9 4 12.9 5.8

MT CBM Operation - ALT E

Table 3-3 (continued)

Visibility - Method 6 and Monthly f(RH) values - Future Alternatives

Maximum % 

Change in 

Bext

8th Highest 

% Change in 

Bext

Maximum % 

Change in 

Bext

8th Highest 

% Change in 

Bext

MT CBM Operation - ALT F MT CBM Operation - ALT H

Receptor Set Maximum % 

Change in 

Bext

8th Highest 

% Change 

in Bext
AIR QUALITY APPENDIX

Part 2 AIR-29

Note: Bold type indicate a modeled impact that is above the Comparative Deposition Value

ALL 

SOURCES

MT CBM 

Construction

MT CBM 

Operation

MT 

Oil & Gas Threshold 

Nitrogen 1.13E-01 2.75E-05 4.49E-05 5.05E-04 3

Sulfur 1.63E-01 5.03E-06 2.37E-06 1.30E-05 5

Nitrogen 1.18E-02 1.57E-06 2.54E-06 4.20E-05 3

Sulfur 1.96E-02 3.22E-07 1.53E-07 1.03E-06 5

Nitrogen 1.17E-01 7.53E-06 1.26E-05 4.67E-05 3

Sulfur 2.09E-01 1.84E-06 8.70E-07 1.10E-06 5

Nitrogen 1.29E-01 7.41E-06 1.23E-05 6.05E-05 3

Sulfur 1.72E-01 1.58E-06 7.51E-07 1.37E-06 5

Nitrogen 7.10E-02 1.52E-05 2.49E-05 6.00E-03 3

Sulfur 1.33E-01 2.36E-06 1.12E-06 2.31E-05 5

Nitrogen 6.70E-02 4.46E-06 7.22E-06 1.48E-04 3

Sulfur 8.11E-02 7.96E-07 3.79E-07 2.69E-06 5

Nitrogen 6.36E-02 5.46E-06 8.94E-06 4.47E-05 3

Sulfur 1.69E-01 8.99E-07 4.27E-07 9.17E-07 5

Nitrogen 1.21E-01 1.51E-05 2.50E-05 3.31E-04 3

Sulfur 1.97E-01 2.73E-06 1.28E-06 2.97E-06 5

Nitrogen 2.92E-01 4.29E-03 7.15E-03 5.48E-03 3

Sulfur 3.91E-01 3.76E-04 1.78E-04 2.92E-05 5

Nitrogen 4.36E-02 2.76E-06 4.52E-06 3.59E-05 3

Sulfur 6.13E-02 4.27E-07 2.03E-07 6.39E-07 5

Nitrogen 2.76E-02 3.08E-06 4.95E-06 2.62E-04 3

Sulfur 4.44E-02 5.69E-07 2.70E-07 2.23E-06 5

Nitrogen 7.98E-02 7.92E-06 1.31E-05 9.97E-05 3

Sulfur 1.51E-01 1.51E-06 7.13E-07 1.51E-06 5

Nitrogen 2.50E-01 2.79E-05 4.60E-05 2.89E-03 3

Sulfur 3.39E-01 4.42E-06 2.10E-06 5.01E-05 5

Nitrogen 6.46E-02 1.19E-05 1.92E-05 3.86E-04 3

Sulfur 9.09E-02 2.10E-06 9.98E-07 6.00E-06 5

Nitrogen 1.17E-01 1.12E-05 1.86E-05 2.19E-04 3

Sulfur 2.18E-01 2.15E-06 1.01E-06 2.44E-06 5

Nitrogen 1.96E-01 3.71E-05 6.40E-05 5.21E-04 3

Sulfur 3.21E-01 7.02E-06 3.33E-06 1.37E-05 5

Nitrogen 8.02E-02 1.39E-05 2.30E-05 1.26E-04 3

Sulfur 1.28E-01 2.16E-06 1.01E-06 1.85E-06 5

Nitrogen 9.64E-02 7.41E-06 1.24E-05 4.44E-05 3

Sulfur 1.90E-01 1.81E-06 8.59E-07 1.08E-06 5

Nitrogen 9.87E-02 7.25E-06 1.21E-05 4.32E-05 3

Sulfur 1.91E-01 1.78E-06 8.42E-07 1.06E-06 5

Nitrogen 1.52E-01 1.45E-04 2.60E-04 4.33E-04 3

Sulfur 2.08E-01 2.07E-05 9.82E-06 6.16E-06 5

Nitrogen 1.58E-01 1.37E-04 2.52E-04 4.27E-04 3

Sulfur 2.16E-01 2.10E-05 9.95E-06 6.39E-06 5

Nitrogen 8.95E-02 5.54E-06 9.13E-06 3.68E-05 3

Sulfur 1.69E-01 1.23E-06 5.83E-07 9.59E-07 5

Nitrogen 1.16E-01 8.05E-06 1.36E-05 4.48E-05 3

Sulfur 2.21E-01 1.96E-06 9.27E-07 1.07E-06 5

Nitrogen 8.88E-02 6.93E-06 1.14E-05 5.09E-05 3

Sulfur 1.64E-01 1.40E-06 6.63E-07 1.19E-06 5

Nitrogen 1.04E-01 7.18E-06 1.20E-05 4.20E-05 3

Sulfur 1.97E-01 1.76E-06 8.34E-07 1.03E-06 5

Receptor Set

Theodore Roosevelt NP Class I Area

CLASS I AREAS

POLLUTANT

Fitzpatrick W Class I Area

North Cheyenne IR Class I Area

Bridger W Class I Area

Badlands NP Class I Area

Fort Peck IR Class I Area

Gates of the Mountain W Class I Area

Scapegoat W Class I Area

UL Bend W Class I Area

Washakie W Class I Area

North Absaorka W Class I Are

Upper Frozen Lake, Bridger WA

Hobbs Lake, Bridger WA

Deep Lake, Bridger WA

Black Joe Lake, Bridger WA

Lower Saddlebag, Popo Agie WA

Ross Lake, Cloud Peak WA

Emerald Lake, Cloud Peak WA

Florence, Cloud Peak WA,

CLASS I / CLASS II SENSITIVE LAKES

Table 3-4

Modeled Deposition for Nitrogen and Sulfur - Base Year

Maximum Deposition (kg/ha - yr)

Yellowstone NP Class I Area

Red Rock Lakes Class I Area

Grand Teton NP Class I Area

Teton W Class I Area

Bob Marshall W Class I Area

Wind Cave NP Class I Area
AIR QUALITY APPENDIX

Part 2 AIR-30

ALL 

SOURCES - 

Alternative E

ALL 

SOURCES - 

Alternative F

ALL 

SOURCES - 

Alternative H

Threshold 

Nitrogen 1.20E-01 1.20E-01 1.20E-01 3

Sulfur 1.83E-01 1.83E-01 1.83E-01 5

Nitrogen 1.79E-02 1.78E-02 1.79E-02 3

Sulfur 2.70E-02 2.70E-02 2.70E-02 5

Nitrogen 1.14E-01 1.14E-01 1.14E-01 3

Sulfur 2.38E-01 2.38E-01 2.38E-01 5

Nitrogen 1.30E-01 1.30E-01 1.30E-01 3

Sulfur 1.87E-01 1.87E-01 1.88E-01 5

Nitrogen 7.93E-02 7.90E-02 7.95E-02 3

Sulfur 1.46E-01 1.46E-01 1.46E-01 5

Nitrogen 9.39E-02 9.37E-02 9.39E-02 3

Sulfur 1.11E-01 1.11E-01 1.11E-01 5

Nitrogen 6.53E-02 6.53E-02 6.53E-02 3

Sulfur 1.78E-01 1.78E-01 1.78E-01 5

Nitrogen 1.30E-01 1.30E-01 1.30E-01 3

Sulfur 2.13E-01 2.13E-01 2.13E-01 5

Nitrogen 1.87E+00 1.97E+00 1.99E+00 3

Sulfur 4.88E-01 4.89E-01 4.92E-01 5

Nitrogen 4.55E-02 4.55E-02 4.56E-02 3

Sulfur 6.52E-02 6.52E-02 6.52E-02 5

Nitrogen 4.13E-02 4.12E-02 4.14E-02 3

Sulfur 6.12E-02 6.12E-02 6.12E-02 5

Nitrogen 8.36E-02 8.34E-02 8.36E-02 3

Sulfur 1.61E-01 1.61E-01 1.61E-01 5

Nitrogen 2.58E-01 2.58E-01 2.58E-01 3

Sulfur 3.53E-01 3.53E-01 3.53E-01 5

Nitrogen 9.11E-02 9.07E-02 9.15E-02 3

Sulfur 1.23E-01 1.23E-01 1.23E-01 5

Nitrogen 1.25E-01 1.24E-01 1.25E-01 3

Sulfur 2.37E-01 2.37E-01 2.38E-01 5

Nitrogen 2.07E-01 2.07E-01 2.07E-01 3

Sulfur 3.58E-01 3.58E-01 3.58E-01 5

Nitrogen 8.58E-02 8.56E-02 8.58E-02 3

Sulfur 1.36E-01 1.36E-01 1.36E-01 5

Nitrogen 9.63E-02 9.62E-02 9.63E-02 3

Sulfur 2.15E-01 2.15E-01 2.15E-01 5

Nitrogen 9.81E-02 9.81E-02 9.82E-02 3

Sulfur 2.16E-01 2.16E-01 2.16E-01 5

Nitrogen 1.65E-01 1.64E-01 1.65E-01 3

Sulfur 2.34E-01 2.34E-01 2.34E-01 5

Nitrogen 1.70E-01 1.69E-01 1.70E-01 3

Sulfur 2.43E-01 2.43E-01 2.43E-01 5

Nitrogen 8.83E-02 8.82E-02 8.83E-02 3

Sulfur 1.82E-01 1.82E-01 1.82E-01 5

Nitrogen 1.15E-01 1.15E-01 1.15E-01 3

Sulfur 2.55E-01 2.55E-01 2.55E-01 5

Nitrogen 8.94E-02 8.94E-02 8.95E-02 3

Sulfur 1.76E-01 1.76E-01 1.76E-01 5

Nitrogen 1.03E-01 1.03E-01 1.03E-01 3

Sulfur 2.22E-01 2.22E-01 2.22E-01 5

Table 3-5

Maximum Deposition for Alternate Development Scenarios 

Hobbs Lake, Bridger WA

Lower Saddlebag, Popo Agie WA

Washakie W Class I Area

Wind Cave NP Class I Area

Yellowstone NP Class I Area

CLASS I / CLASS II SENSITIVE LAKES

Scapegoat W Class I Area

Teton W Class I Area

Ross Lake, Cloud Peak WA

Upper Frozen Lake, Bridger WA

Black Joe Lake, Bridger WA

Deep Lake, Bridger WA

Emerald Lake, Cloud Peak WA

Florence, Cloud Peak WA,

Theodore Roosevelt NP Class I Area

UL Bend W Class I Area

Grand Teton NP Class I Area

North Absaorka W Class I Are

North Cheyenne IR Class I Area

Red Rock Lakes Class I Area

Bob Marshall W Class I Area

Fitzpatrick W Class I Area

Fort Peck IR Class I Area

Gates of the Mountain W Class I Area

Badlands NP Class I Area

Bridger W Class I Area

Maximum Deposition (kg/ha - yr)

CLASS I AREAS

Receptor Set POLLUTANT
AIR QUALITY APPENDIX

Part 2 AIR-31

Wilderness Area

Background 

ANC

Number of 

Samples 

Watershed 

Area

Annual 

Precipitation ANC(o) %ANC Hdep %ANC Hdep %ANC Hdep %ANC Hdep

Lake (ueq/l) (ha) (meter) (eq) change ueq/l change ueq/l change ueq/l change ueq/l

Bridger

Black Joe 67 43 890 0.97 397109 4.2 2.9 4.6 3.1 4.6 3.1 4.6 3.1

Deep 60 61 205 0.97 80864 4.8 2.9 5.2 3.2 5.2 3.2 5.2 3.2

Hobbs 70 68 293 0.76 101715 4.9 3.3 5.1 3.5 5.1 3.5 5.1 3.5

Upper Frozen 5 (NA) 64.8 1.22 1033 123.9 2.4 133.1 2.6 133.1 2.6 133.1 2.6

Cloud Peak

Emerald 55.3 9 293 0.97 104776 6.7 3.7 7.4 4.1 7.4 4.1 7.4 4.1

Florence 32.7 10 417 0.97 88177 11.7 3.8 12.9 4.2 12.9 4.2 12.9 4.2

Fitzpatrick

Ross 53.5 35 4455 0.97 1768834 4.2 2.6 4.4 2.7 4.4 2.7 4.4 2.7

Popo Agie

Lower Saddlebag 55.5 34 155 0.97 55628 6.2 3.4 6.7 3.7 6.7 3.7 6.7 3.7

Table 3-6

Modeled Impacts on Acid Sensitive Lakes -Alternate Development Scenarios

Base Year Alternative E Alternative F Alternative H 

Table 3-7

Modeled Acute Concentrations of Hazardous Air Pollutants (HAPs) 

All Production Scenarios - All Sources 

Receptor Set Pollutant

Averag

ing 

Period

RANK Base 

Year 

ALT E 

Total 

Impact 

ALT F 

Total 

Impact 

ALT H 

Total 

Impact

REL 

(μg/m3)

Near Field 

Receptors 

All Data in 

μg/m3

Montana Near 

Field Receptors

Benzene 1-hour 1ST 

HIGH 0.29 0.36 0.31 0.30 1,300

Ethyl Benzene 1-hour 1ST 

HIGH 0.01 0.01 0.01 0.01 35,000

Formaldehyde 1-hour 1ST 

HIGH 13.3 16.6 14.2 13.8 94

n-Hexane 1-hour 1ST 

HIGH 4.44 207.00 207.00 207.00 39,000

Toluene 1-hour 1ST 

HIGH 0.2 0.3 0.3 0.3 37,000

Xylene 1-hour 1ST 

HIGH 0.1 0.1 0.1 0.1 22,000

Wyoming Near 

Field Receptors

Benzene 1-hour 1ST 

HIGH 1.9 1.0 1.0 1.0 1,300

Ethyl Benzene 1-hour 1ST 

HIGH 0.1 0.04 0.0 0.0 35,000

Formaldehyde 1-hour 1ST 

HIGH 86.2 46.5 46.5 46.5 94

n-Hexane 1-hour 1ST 

HIGH 3.1 12.8 12.8 12.8 39,000

Toluene 1-hour 1ST 

HIGH 1.0 0.5 0.5 0.5 37,000

Xylene 1-hour 1ST HIGH 0.4 0.2 0.2 0.2 22,000
AIR QUALITY APPENDIX

Part 2 AIR-32

Table 3-8

Modeled Annual Concentrations of Hazardous Air Pollutants (HAPs) - All Production Scenarios 

All Sources 

Receptor Set Pollutant Averaging 

Period* RANK Base Year 

ALT E 

Total 

Impact 

ALT F 

Total 

Impact 

ALT H 

Total 

Impact

NonCarcinogenic 

RfCs

Near Field Receptors - Non-Carcinogenic Impacts All Data in μg/m3

Montana Near 

Field Receptors

Benzene Annual 1ST HIGH 0.0026 0.0031 0.0031 0.0032 30

Ethyl Benzene Annual 1ST HIGH 0.0001 0.0001 0.0001 0.0001 1,000

Formaldehyde Annual 1ST HIGH 0.1210 0.1400 0.1400 0.1400 9.8

n-Hexane Annual 1ST HIGH 0.1250 1.6000 1.6000 1.6000 200

Toluene Annual 1ST HIGH 0.0001 0.0034 0.0034 0.0034 400

Xylene Annual 1ST HIGH 0.0006 0.0006 0.0006 0.0006 100

Wyoming Near 

Field Receptors

Benzene Annual 1ST HIGH 0.0093 0.0055 0.0055 0.0055 30

Ethyl Benzene Annual 1ST HIGH 0.0004 0.0003 0.0003 0.0003 1,000

Formaldehyde Annual 1ST HIGH 0.4270 0.2390 0.2390 0.2390 9.8

n-Hexane Annual 1ST HIGH 0.0562 0.0826 0.0826 0.0826 200

Toluene Annual 1ST HIGH 0.0049 0.0028 0.0028 0.0028 400

Xylene Annual 1ST HIGH 0.0020 0.0011 0.0011 0.0011 100

Near Field Receptors - Carcinogenic Risk Evaluation* Risk Evaluation X 10-6

Montana Benzene Annual 1ST HIGH 0.015 0.017 0.017 0.017

Formaldehyde Annual 1ST HIGH 0.000 0.001 0.001 0.001

Wyoming Benzene Annual 1ST HIGH 0.052 0.030 0.030 0.030

Formaldehyde Annual 1ST HIGH 0.002 0.001 0.001 0.001

*Benzene Concentrations multiplied by risk factor: 7.8 X 10-6 X 0.71)

*Formaldehyde Concentrations multiplied by risk factor: 5.5 X 10-9 X 

0.71)
AIR QUALITY APPENDIX

Part 2 AIR-33

Applicable Standards 

(ug/m3) 

MAAQS: 565 

NAAQS: 100

NAAQS: 150

NAAQS: 50

NAAQS: 35

Figure 3-1

Change in Modeled Concentrations of NO2, SO2, PM10, PM2.5

Montana Near-field Receptors 

0

1

2

3

4

5

Annual NO2 (ug/m3)

0

10

20

30

40

50

24-Hr PM10 (ug/m3)

0

0.5

1

1.5

2

2.5

3

3.5

4

Annual PM10 (ug/m3)

0

1

2

3

4

5

6

7

8

24-Hr PM2.5 (ug/m3)

0

100

200

300

400

500

600

1-Hr NO2 (ug/m3)

Base Year Alt. E Alt. F Alt. H
AIR QUALITY APPENDIX

Part 2 AIR-34

Applicable Standards 

(ug/m3) 

NAAQS: 15

NAAQS: 1300

NAAQS: 1300

NAAQS: 260

NAAQS: 60

Change in Modeled Concentrations of NO2, SO2, PM10, PM2.5

Montana Near-field Receptors 

Figure 3-1 (continued)

0

0.2

0.4

0.6

0.8

1

Annual PM2.5 (ug/m3)

0

20

40

60

80

100

120

140

160

1-Hr SO2 (ug/m3)

0

5

10

15

20

25

30

35

40

45

50

3-Hr SO2 (ug/m3)

0

2

4

6

8

10

12

14

16

24-Hr SO2 (ug/m3)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

Tongue

River

Railroad

ALL MT

MT CBM

Operation

WY CBM

Construction

WY Oil &

Gas Cons

WY OTHER

Annual SO2 (ug/m3)
AIR QUALITY APPENDIX

Part 2 AIR-35

Applicable Standards 

(ug/m3) 

NAAQS: 100 

NAAQS: 150

NAAQS: 50

NAAQS: 35

Figure 3-2

Change in Modeled Concentrations of NO2, SO2, PM10, PM2.5

Wyoming Near-field Receptors 

0

10

20

30

40

50

Annual NO2 (ug/m3)

Base Year Alt. E Alt. F Alt. H

0

20

40

60

80

100

120

24-Hr PM10 (ug/m3)

0

5

10

15

20

25

Annual PM10 (ug/m3)

0

2

4

6

8

10

12

14

24-Hr PM2.5 (ug/m3)
AIR QUALITY APPENDIX

Part 2 AIR-36

Applicable Standards 

(ug/m3) 

NAAQS: 15

NAAQS: 1300

NAAQS: 260

NAAQS: 60

Change in Modeled Concentrations of NO2, SO2, PM10, PM2.5

Wyoming Near-field Receptors 

Figure 3-2 (continued)

0

0.5

1

1.5

2

2.5

3

Annual PM2.5 (ug/m3)

Base Year Alt. E Alt. F Alt. H

0

100

200

300

400

500

600

3-Hr SO2 (ug/m3)

0

20

40

60

80

100

120

140

24-Hr SO2 (ug/m3)

0

2

4

6

8

10

12

14

16

18

20

Tongue

River

Railroad

ALL MT

MT CBM

Operation

WY CBM

Construction

WY Oil &

Gas Cons

WY OTHER

Annual SO2 (ug/m3)
AIR QUALITY APPENDIX

Part 2 AIR-37

Applicable Standards 

(ug/m3) 

MAAQS: 565 

NAAQS: 100

NAAQS: 150

NAAQS: 50

Figure 3-3

Change in Modeled Concentrations of NO2, SO2, PM10, PM2.5

Northern Cheyenne Indian Reservation 

0

100

200

300

400

500

1-Hr NO2 (ug/m3)

Base Year Alt. E Alt. F Alt. H

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

Annual NO2 (ug/m3)

0

2

4

6

8

10

12

14

24-Hr PM10 (ug/m3)

0

0.5

1

1.5

2

2.5

Annual PM10 (ug/m3)
AIR QUALITY APPENDIX

Part 2 AIR-38

Applicable Standards 

(ug/m3) 

NAAQS: 35

NAAQS: 15

NAAQS: 1300

Figure 3-3 (continued)

Change in Modeled Concentrations of NO2, SO2, PM10, PM2.5

Northern Cheyenne Indian Reservation 

0

1

2

3

4

5

6

24-Hr PM2.5 (ug/m3)

Base Year Alt. E Alt. F Alt. H

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Annual PM2.5 (ug/m3)

0

5

10

15

20

25

30

35

40

45

1-Hr SO2 (ug/m3)
AIR QUALITY APPENDIX

Part 2 AIR-39

Applicable Standards 

(ug/m3) 

NAAQS: 1300

NAAQS: 260

NAAQS: 60

Change in Modeled Concentrations of NO2, SO2, PM10, PM2.5

Northern Cheyenne Indian Reservation 

Figure 3-3 (continued)

0

2

4

6

8

10

12

14

3-Hr SO2 (ug/m3)

Base Year Alt. E Alt. F Alt. H

0

1

2

3

4

5

24-Hr SO2 (ug/m3)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Tongue

River

Railroad

ALL MT

MT CBM

Operation

WY CBM

Construction

WY Oil &

Gas Cons

WY OTHER

Annual SO2 (ug/m3)
AIR QUALITY APPENDIX

Part 2 AIR-40

Applicable Standards 

(ug/m3) 

NAAQS: 100 

NAAQS: 150

NAAQS: 50

NAAQS: 35

Figure 3-4

Change in Modeled Concentrations of NO2, SO2, PM10, PM2.5

Theodore Roosevelt National Park

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Annual NO2 (ug/m3)

Base Year Alt. E Alt. F Alt. H

0

1

2

3

4

5

6

24-Hr PM10 (ug/m3)

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

Annual PM10 (ug/m3)

0

1

2

3

4

5

6

24-Hr PM2.5 (ug/m3)
AIR QUALITY APPENDIX

Part 2 AIR-41

Applicable Standards 

(ug/m3) 

NAAQS: 15

NAAQS: 1300

NAAQS: 260

NAAQS: 60

Change in Modeled Concentrations of NO2, SO2, PM10, PM2.5

Theodore Roosevelt National Park

Figure 3-4 (continued)

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

Annual PM2.5 (ug/m3)

Base Year Alt. E Alt. F Alt. H

0

5

10

15

20

25

30

35

40

3-Hr SO2 (ug/m3)

0

2

4

6

8

10

12

24-Hr SO2 (ug/m3)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Tongue

River

Railroad

ALL MT

MT CBM

Operation

WY CBM

Construction

WY Oil &

Gas Cons

WY OTHER

Annual SO2 (ug/m3)
AIR QUALITY APPENDIX

Part 2 AIR-42

Applicable Standards 

(ug/m3) 

NAAQS: 100 

NAAQS: 150

NAAQS: 50

NAAQS: 35

Figure 3-5

Change in Modeled Concentrations of NO2, SO2, PM10, PM2.5

Wind Cave National Park

0

0.05

0.1

0.15

0.2

Annual NO2 (ug/m3)

Base Year Alt. E Alt. F Alt. H

0

1

2

3

4

5

6

7

8

24-Hr PM10 (ug/m3)

0

0.2

0.4

0.6

0.8

1

Annual PM10 (ug/m3)

0

1

2

3

4

5

6

24-Hr PM2.5 (ug/m3)
AIR QUALITY APPENDIX

Part 2 AIR-43

Applicable Standards 

(ug/m3) 

NAAQS: 15

NAAQS: 1300

NAAQS: 260

NAAQS: 60

Change in Modeled Concentrations of NO2, SO2, PM10, PM2.5

Wind Cave National Park

Figure 3-5 (continued)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Annual PM2.5 (ug/m3)

Base Year Alt. E Alt. F Alt. H

0

2

4

6

8

10

3-Hr SO2 (ug/m3)

0

0.5

1

1.5

2

2.5

3

3.5

4

24-Hr SO2 (ug/m3)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Tongue

River

Railroad

ALL MT

MT CBM

Operation

WY CBM

Construction

WY Oil &

Gas Cons

WY OTHER

Annual SO2 (ug/m3)
AIR QUALITY APPENDIX

Part 2 AIR-44

Applicable Standards 

(ug/m3) 

MAAQS: 565 

NAAQS: 100

NAAQS: 150

NAAQS: 50

Figure 3-6

Change in Modeled Concentrations of NO2, SO2, PM10, PM2.5

Crow Indian Reservation

0

100

200

300

400

500

1-Hr NO2 (ug/m3)

Base Year Alt. E Alt. F Alt. H

0

1

2

3

4

5

6

Annual NO2 (ug/m3)

0

10

20

30

40

50

24-Hr PM10 (ug/m3)

0

1

2

3

4

5

Annual PM10 (ug/m3)
AIR QUALITY APPENDIX

Part 2 AIR-45

Applicable Standards 

(ug/m3) 

NAAQS: 35

NAAQS: 15

NAAQS: 1300

Figure 3-6 (continued)

Change in Modeled Concentrations of NO2, SO2, PM10, PM2.5

Crow Indian Reservation

0

1

2

3

4

5

6

7

8

24-Hr PM2.5 (ug/m3)

Base Year Alt. E Alt. F Alt. H

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Annual PM2.5 (ug/m3)

0

20

40

60

80

100

120

140

160

1-Hr SO2 (ug/m3)
AIR QUALITY APPENDIX

Part 2 AIR-46

Applicable Standards 

(ug/m3) 

NAAQS: 1300

NAAQS: 260

NAAQS: 60

Change in Modeled Concentrations of NO2, SO2, PM10, PM2.5

Crow Indian Reservation

Figure 3-6 (continued)

0

10

20

30

40

50

60

70

3-Hr SO2 (ug/m3)

Base Year Alt. E Alt. F Alt. H

0

5

10

15

20

25

24-Hr SO2 (ug/m3)

0

0.5

1

1.5

2

2.5

3

Tongue

River

Railroad

ALL MT

MT CBM

Operation

WY CBM

Construction

WY Oil &

Gas Cons

WY OTHER

Annual SO2 (ug/m3)
AIR QUALITY APPENDIX

Part 2 AIR-47

Applicable Standards 

(ug/m3) 

NAAQS: 100 

NAAQS: 150

NAAQS: 50

NAAQS: 35

Figure 3-7

Change in Modeled Concentrations of NO2, SO2, PM10, PM2.5

Cloud Peak Wilderness 

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

Annual NO2 (ug/m3)

Base Year Alt. E Alt. F Alt. H

0

1

2

3

4

5

6

7

8

24-Hr PM10 (ug/m3)

0

0.1

0.2

0.3

0.4

0.5

0.6

Annual PM10 (ug/m3)

0

1

2

3

4

5

6

24-Hr PM2.5 (ug/m3)
AIR QUALITY APPENDIX

Part 2 AIR-48

Applicable Standards 

(ug/m3) 

NAAQS: 15

NAAQS: 1300

NAAQS: 260

NAAQS: 60

Figure 3-7 (continued)

Change in Modeled Concentrations of NO2, SO2, PM10, PM2.5

Cloud Peak Wilderness 

0

0.1

0.2

0.3

0.4

0.5

Annual PM2.5 (ug/m3)

Base Year Alt. E Alt. F Alt. H

0

1

2

3

4

5

6

3-Hr SO2 (ug/m3)

0

0.5

1

1.5

2

2.5

3

24-Hr SO2 (ug/m3)

0

0.05

0.1

0.15

0.2

0.25

0.3

Tongue

River

Railroad

ALL MT

MT CBM

Operation

WY CBM

Construction

WY Oil &

Gas Cons

WY OTHER

Annual SO2 (ug/m3)
AIR QUALITY APPENDIX

Part 2 AIR-49

Applicable Standards 

(ug/m3) 

MAAQS: 565 

NAAQS: 100

NAAQS: 150

NAAQS: 50

Figure 3-8

Change in Modeled Concentrations of NO2, SO2, PM10, PM2.5

Bighorn Canyon NRA

0

10

20

30

40

50

60

70

1-Hr NO2 (ug/m3)

Base Year Alt. E Alt. F Alt. H

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Annual NO2 (ug/m3)

0

2

4

6

8

10

12

14

16

18

24-Hr PM10 (ug/m3)

0

0.5

1

1.5

2

Annual PM10 (ug/m3)
AIR QUALITY APPENDIX

Part 2 AIR-50

Applicable Standards 

(ug/m3) 

NAAQS: 35

NAAQS: 15

NAAQS: 1300

Change in Modeled Concentrations of NO2, SO2, PM10, PM2.5

Bighorn Canyon NRA

Figure 3-8 (continued)

0

1

2

3

4

5

6

7

24-Hr PM2.5 (ug/m3)

Base Year Alt. E Alt. F Alt. H

0

0.1

0.2

0.3

0.4

0.5

0.6

Annual PM2.5 (ug/m3)

0

5

10

15

20

25

30

1-Hr SO2 (ug/m3)
AIR QUALITY APPENDIX

Part 2 AIR-51

Applicable Standards 

(ug/m3) 

NAAQS: 1300

NAAQS: 260

NAAQS: 60

Bighorn Canyon NRA

Figure 3-8 (continued)

Change in Modeled Concentrations of NO2, SO2, PM10, PM2.5

0

2

4

6

8

10

12

14

3-Hr SO2 (ug/m3)

Base Year Alt. E Alt. F Alt. H

0

1

2

3

4

5

6

24-Hr SO2 (ug/m3)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Tongue

River

Railroad

ALL MT

MT CBM

Operation

WY CBM

Construction

WY Oil &

Gas Cons

WY OTHER

Annual SO2 (ug/m3)
AIR QUALITY APPENDIX

Part 2 AIR-52

Applicable Standards 

(ug/m3) 

NAAQS: 100 

NAAQS: 150

NAAQS: 50

NAAQS: 35

Figure 3-9

Change in Modeled Concentrations of NO2, SO2, PM10, PM2.5

Wind River Indian Reservation

0

0.2

0.4

0.6

0.8

1

1.2

Annual NO2 (ug/m3)

Base Year Alt. E Alt. F Alt. H

0

2

4

6

8

10

12

14

24-Hr PM10 (ug/m3)

0

0.2

0.4

0.6

0.8

1

1.2

Annual PM10 (ug/m3)

0

2

4

6

8

10

12

24-Hr PM2.5 (ug/m3)
AIR QUALITY APPENDIX

Part 2 AIR-53

Applicable Standards 

(ug/m3) 

NAAQS: 15

NAAQS: 1300

NAAQS: 260

NAAQS: 60

Change in Modeled Concentrations of NO2, SO2, PM10, PM2.5

Wind River Indian Reservation

Figure 3-9 (continued)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Annual PM2.5 (ug/m3)

Base Year Alt. E Alt. F Alt. H

0

50

100

150

200

3-Hr SO2 (ug/m3)

0

5

10

15

20

25

30

35

40

24-Hr SO2 (ug/m3)

0

1

2

3

4

5

6

Tongue

River

Railroad

ALL MT

MT CBM

Operation

WY CBM

Construction

WY Oil &

Gas Cons

WY OTHER

Annual SO2 (ug/m3)
AIR QUALITY APPENDIX

Part 2 AIR-54

4.0 SUPPLEMENTAL AIR QUALITY ANALYSIS (SAQA)

MITIGATION SCENARIOS

Overview of SAQA Assessment 

Approach

The Supplemental Air Quality Analysis (SAQA) 

supplements the Air Quality Technical Support 

Document (AQTSD) (ALL revised 2007) that was 

prepared in support of the Draft Supplemental 

Environmental Impact Statement (DSEIS). The 

DSEIS evaluated potential emissions from Coal Bed 

Natural Gas (CBNG) related activities by combining 

project related CBNG development, as outlined in the 

Reasonably Foreseeable Development (RFD) 

scenario, with non-project related CBNG 

development on the Crow and Northern Cheyenne 

Reservations, as outlined in the Reasonably 

Foreseeable Future Actions (RFFA) scenario, into 

one emissions source group. The SAQA evaluates 

these two emissions groups separately to allow for 

the determination of potential air quality impacts that 

result directly from project related CBNG activities. 

Also included are potential air quality impacts from 

emission sources in Montana (All Montana Source 

Group), which includes project related CNBG 

emissions, and cumulative emissions (All Source 

Group) which includes all emissions sources both 

project related and non-project related. Information 

on the potential air quality impacts from specific 

source groups is contained within Appendix C of the 

SAQA document. Additionally, emission points 

representing potential emissions from CBNG 

construction, operations, and maintenance activities 

were decentralized within each watershed to better 

represent actual development conditions (locations 

shown on Figure 4-1). The adjustments to emission 

point locations and the separation of RFD and RFFA 

CBNG wells were applied to each of the 

supplemental scenarios analyzed which are described 

below. Emission factors used were derived from the 

air quality modeling analyses conducted for the 2003 

Final Environmental Impact Statement (FEIS) 

conducted by Argonne National Laboratories 

(Argonne 2002). The air modeling analysis was 

conducted to separate project RFD emissions from 

non-project RFFA emissions; decentralize the project 

RFD and non-project RFFA emission source points;

and utilize a well to field compressor to sales 

compressor ratio of 240 wells connected to 10 field 

compressors connected to 1 sales compressor 

(240:10:1) with a NOX emissions factor for 

compressors of 1.5 grams per brake horsepower-hour 

(1.5 g/bhp-hr). This scenario is referred to in the 

SAQA document as Alternative H Revised.

Current CBNG development within the Montana 

portion of the Powder River Basin (PRB) is 

conducted using a ratio of 200 wells connected to 5 

field compressors connected to 1 sales compressor. 

The SAQA includes an air modeling analysis scenario 

which uses this ratio of 200:5:1 and a NOX emissions 

factor for compressors of 1.5 g/bhp-hr for project 

RFD wells; the well to field compressor to sales 

compressor ratio for non-project RFFA wells was not 

adjusted. This scenario is referred to as Scenario 1.

The SAQA also evaluates an air modeling analysis 

scenario (Scenario 2) using the 200:5:1 well to field 

compressor to sales compressor ratio and the NOX

emissions factor of 1.0 g/bhp-hr for project RFD 

wells; the NOX emissions factor for non-project 

RFFA wells was not adjusted. The 1.0 g/bhp-hr NOx 

emission factor was selected for Scenario 2 to reflect 

the emission level currently being permitted by the 

Montana Department of Environmental Quality 

(MDEQ) for CBNG compressors within the PRB. 

Scenarios 1 and 2 utilize the same number of 

operating CBNG wells but would have varying 

compressor and horsepower requirements and 

subsequent emissions output related to compressor 

operations. The lowering of the NOX emissions factor 

to reflect current MDEQ permitting levels for 

Scenario 2 would further reduce the emissions 

associated with Scenario 1.

The SAQA evaluates a mitigation scenario (Scenario 

1A) which assumes a 50% reduction applied to 

Scenario 1 compressor horsepower requirements. 

This scenario reduces compressor operations 

emissions and associated maintenance emissions by 

50% but leaves all other emissions the same as 

previously modeled for Scenario 1. The effect of this 

assumption reduces calculated compressor emissions 

by 50% for NOX, SO2, PM10, and PM2.5.

The SAQA evaluates a second air quality mitigation 

scenario (Scenario 2A) which assumes a 50% 

reduction applied to the Scenario 2 compressor
Part 2 AIR-55

AIR QUALITY APPENDIX

Figure 4-1
AIR QUALITY APPENDIX

Part 2 AIR-56

horsepower requirements. This scenario reduces 

compressor operations emissions and associated 

maintenance emissions by 50% but leaves all other 

emissions the same as previously modeled for 

Scenario 2. The effect of this assumption reduces 

calculated compressor emissions by 50% for NOX, 

SO2, PM10, and PM2.5.

The SAQA also includes revised emissions data for 

the Tongue River Railroad (TRR) which was 

reconfigured to better simulate a linear emission 

source. The total emissions for the TRR were kept 

constant and are the same as presented in the 

AQTSD; however, the number of emission points 

representing the TRR alignment was increased 

from 20 to 96.

Project related emissions include emissions from 

CBNG construction and operations activities in 

Montana. The scenarios presented within the SAQA

were analyzed to assess project related versus nonproject related CBNG emissions under Revised 

Alternative H, assess emissions associated with 

compressor operations utilizing different NOX

emissions factors and adjusting well to field to sales 

compressor ratios to more accurately represents 

current practice within the Montana portion of the 

PRB under Scenarios 1 and 2, and assess at what 

level project related CBNG emissions would need 

to be reduced to achieve zero days of impacts to 

visibility at the Prevention of Significant 

Deterioration (PSD) Class I areas under Scenarios 

1A and 2A.

Results of SAQA Mitigation 

Assessment 

For each of the five CBNG development scenarios, 

the projected impacts on air quality were 

determined for each receptor group. The analyses 

for the key receptor groups are provided in Table 4-

1 for the Montana near-field receptor grid, in Table 

4-2 for the Northern Cheyenne Indian Reservation, 

and in Table 4-3 for the Crow Indian Reservation. 

The project CBNG impacts for construction and 

operation activities have been combined in Tables 

4-1, 4-2, and 4-3 to provide a conservative estimate 

of total project impacts. In actuality, the impacts 

from different source groups are not arithmetically 

additive, as maximum impacts may occur at 

different receptors and/or at different times. 

Changes from these scenarios at other receptors 

were generally very minor or not detectable, but are 

provided in Appendix C in the SAQA document. 

Direct Project Impacts (RFD)

This section describes the CALPUFF model 

predicted direct project impacts for reasonably 

foreseeable development of CBNG within the 

project planning area. Only impacts directly 

attributed to project CBNG construction and 

operations are discussed in this section. 

Comprehensive details of modeled emission 

impacts are provided in Appendix C.

This section also provides discussion of potential 

project related CBNG impacts as they pertain to 

Potential for Significant Deterioration (PSD) 

increment thresholds within the Class I and Class II 

areas located in the model domain. All National 

Environmental Policy Act (NEPA) analysis 

comparisons to the PSD increments are intended to 

evaluate a threshold of concern and do not 

represent a regulatory PSD increment consumption 

analysis.

Alt. H Revised 

Under the Alternative H Revised modeling, 

potential direct project CBNG impacts for both 

operation and construction activities are below 

applicable standards for NO2, SO2, PM10 and PM2.5

at the Montana Near-Field receptor grid. Similar 

results are seen at the Northern Cheyenne Indian 

Reservation and the Crow Indian Reservation. The 

predicted project CBNG operation impacts at the 

Crow Indian Reservation indicate a 1-hour NO2

ambient concentration of 425 micrograms per cubic 

meter (μg/m3) in comparison with a NAAQS 

standard of 565 μg/m3. Combined project CBNG 

construction and operation impacts shown in Tables 

4-1 through 4-3 as “Project CBNG” indicate that 

Class I PSD increment levels for at the Northern 

Cheyenne Indian Reservation and Class II PSD 

increment levels at the Montana near-field and 

Crow Indian Reservation receptors would not be 

exceeded. Combined project CBNG impacts would 

not exceed the MAAQS as well.
AIR QUALITY APPENDIX

Part 2 AIR-57

Table 4-1 Potential Modeled Concentrations of NO2, PM10, PM2.5 and SO2 for Montana Near-Field Grid

Pollutant Avg. Time

Project 

CBNG 

Impact Alt 

H Revised 

(μg/m3)

Project 

CBNG 

Impact 

Scenario 

1 

(μg/m3)

Project 

CBNG 

Impact 

Scenario 

1A 

(μg/m3)

Project 

CBNG 

Impact 

Scenario 

2 

(μg/m3)

Project 

CBNG 

Impact 

Scenario 2A 

(μg/m3)

Project 

CBNG 

Sources 

Montana

Base Year 

(μg/m3)

MT

Background 

Values 

(μg/m3)

PSD1

Increment 

Class II 

(μg/m3)

NAAQS /2

MAAQS 

(μg/m3)

NO2 Annual 2.41 1.93 1.09 1.39 0.81 1 6 25 100

1-Hour 354 284 158 203 118 322 75 n/a 565

SO2 Annual 0.03 0.03 0.03 0.03 0.03 0 3 20 60

24-Hour 0.21 0.21 0.19 0.21 0.19 0 11 91 260

3-Hour 1.21 1.22 1.08 1.22 1.08 4 26 512 1,300

1-Hour 4.09 4.12 3.65 4.12 3.65 16 35 n/a 1,300

PM10 Annual 0.64 0.61 0.40 0.59 0.39 0 8 17 50

24-Hour 4.33 4.03 2.58 3.75 2.44 6 30 30 150

PM2.5 Annual 0.30 0.26 0.16 0.24 0.14 0 3.4 n/a 15

24-Hour 2.18 1.86 1.11 1.60 0.98 1 17.2 n/a 35

Pollutant Avg. Time

ALL 

Sources 

Impact Alt 

H Revised 

(μg/m3)

ALL 

Sources 

Impact 

Scenario 

1 (μg/m3)

ALL 

Sources 

Impact 

Scenario 

1A 

(μg/m3)

ALL 

Sources 

Impact 

Scenario 

2 (μg/m3)

ALL 

Sources 

Impact 

Scenario 2A 

(μg/m3)

ALL 

Sources 

Montana

Base Year 

(μg/m3)

MT

Background 

Values 

(μg/m3)

PSD1

Increment 

Class II 

(μg/m3)

NAAQS /2

MAAQS 

(μg/m3)

NO2 Annual 3.5 3.32 3.00 3.11 2.90 3.91 6 25 100

1-Hour 540 540 539 540 539 428 75 n/a 565

SO2 Annual 1.79 1.79 1.79 1.79 1.79 1.71 3 20 60

24-Hour 15.1 15.1 15.1 15.1 15.1 15.1 11 91 260

3-Hour 43.9 43.9 43.9 43.9 43.9 43.9 26 512 1,300

1-Hour 140 140 140 140 140 140 35 n/a 1,300

PM10 Annual 2.88 2.88 2.87 2.87 2.86 3.52 8 17 50

24-Hour 46.9 46.9 46.8 46.8 46.8 30.6 30 30 150

PM2.5 Annual 0.89 0.85 0.78 0.83 0.77 0.88 3.4 n/a 15

24-Hour 7.01 6.95 6.77 6.90 6.72 6.83 17.2 n/a 35 1

PSD Increment is to be compared directly to the modeled impact

2

Background should be added to modeled impact for comparison to AAQS

n/a – not applicable
AIR QUALITY APPENDIX

Part 2 AIR-58

Table 4-2 Potential Modeled Concentrations of NO2, PM10, PM2.5 and SO2 for Northern Cheyenne Indian Reservation 

Pollutant Avg. Time

Project 

CBNG 

Impact Alt 

H Revised 

(μg/m3)

Project 

CBNG 

Impact 

Scenario 

1 

(μg/m3)

Project 

CBNG 

Impact 

Scenario 

1A 

(μg/m3)

Project 

CBNG 

Impact 

Scenario 

2 

(μg/m3)

Project 

CBNG 

Impact 

Scenario 2A

(μg/m3)

Project 

CBNG 

Sources 

Montana

Base Year 

(μg/m3)

MT

Background 

Values 

(μg/m3)

PSD1

Increment 

Class I 

(μg/m3)

NAAQS /2

MAAQS 

(μg/m3)

NO2 Annual 0.65 0.52 0.29 0.37 0.22 0.03 6 2.5 100

1-Hour 125 100 56 71.7 42 10.4 75 n/a 565

SO2 Annual 0.01 0.01 0.01 0.01 0.01 0 3 2 60

24-Hour 0.07 0.08 0.07 0.08 0.07 0.03 11 5 260

3-Hour 0.49 0.50 0.44 0.50 0.44 0.18 26 25 1,300

1-Hour 1.50 1.52 1.34 1.52 1.34 0.6 35 n/a 1,300

PM10 Annual 0.20 0.19 0.12 0.18 0.12 0.02 8 4 50

24-Hour 1.55 1.48 0.95 1.42 0.92 0.43 30 8 150

PM2.5 Annual 0.10 0.09 0.05 0.08 0.05 0 3.4 n/a 15

24-Hour 0.76 0.64 0.38 0.57 0.34 0.16 17.2 n/a 35

Pollutant Avg. Time

ALL 

Sources 

Impact Alt 

H Revised 

(μg/m3)

ALL 

Sources 

Impact 

Scenario 

1 

(μg/m3)

ALL 

Sources 

Impact 

Scenario 

1A 

(μg/m3)

ALL 

Sources 

Impact 

Scenario 

2 

(μg/m3)

ALL 

Sources 

Impact 

Scenario 2A 

(μg/m3)

ALL 

Sources 

Montana

Base Year 

(μg/m3)

MT

Background 

Values 

(μg/m3)

PSD1

Increment 

Class I 

(μg/m3)

NAAQS /2

MAAQS 

(μg/m3)

NO2 Annual 2.27 2.15 1.84 2.0 1.85 0.30 6 2.5 100

1-Hour 428 428 428 428 428 13.1 75 n/a 565

SO2 Annual 0.72 0.72 0.72 0.72 0.72 0.55 3 2 60

24-Hour 4.70 4.70 4.70 4.70 4.70 4.46 11 5 260

3-Hour 10.5 10.5 10.5 10.5 10.5 10.0 26 25 1,300

1-Hour 30.7 30.7 30.7 30.7 30.7 160 35 n/a 1,300

PM10 Annual 1.32 1.31 1.24 1.30 1.23 0.84 8 4 50

24-Hour 8.46 8.40 8.25 8.34 8.22 7.33 30 8 150

PM2.5 Annual 0.72 0.70 0.67 0.69 0.66 0.48 3.4 n/a 15

24-Hour 6.02 5.97 5.85 5.92 5.82 5.69 17.2 n/a 35 1

PSD Increment is to be compared directly to the modeled impact

2

Background should be added to modeled impact for comparison to AAQS

n/a – not applicable
AIR QUALITY APPENDIX

Part 2 AIR-59

Table 4-3 Potential Modeled Concentrations of NO2, PM10, PM2.5 and SO2 for Crow Indian Reservation 

Pollutant Avg. Time

Project 

CBNG 

Impact Alt 

H Revised 

(μg/m3)

Project 

CBNG 

Impact 

Scenario 

1 

(μg/m3)

Project 

CBNG 

Impact 

Scenario 

1A 

(μg/m3)

Project 

CBNG 

Impact 

Scenario 

2 

(μg/m3)

Project 

CBNG 

Impact 

Scenario 2A 

(μg/m3)

Project 

CBNG 

Sources 

Montana

Base Year 

(μg/m3)

MT

Background 

Values 

(μg/m3)

PSD1

Increment 

Class II 

(μg/m3)

NAAQS /2

MAAQS 

(μg/m3)

NO2 Annual 1.18 0.94 0.53 0.67 0.39 0.03 6 25 100

1-Hour 469 376 210 269 157 36.5 75 n/a 565

SO2 Annual 0.02 0.02 0.01 0.02 0.01 0 3 20 60

24-Hour 0.17 0.17 0.15 0.17 0.15 0.06 11 91 260

3-Hour 1.28 1.29 1.14 1.29 1.14 0.43 26 512 1,300

1-Hour 5.42 5.46 4.84 5.46 4.84 1.94 35 n/a 1,300

PM10 Annual 0.33 0.31 0.20 0.29 0.19 0.02 8 17 50

24-Hour 3.52 3.39 2.19 3.30 2.14 0.74 30 30 150

PM2.5 Annual 0.16 0.14 0.08 0.12 0.07 0 3.4 n/a 15

24-Hour 1.49 1.29 0.79 1.29 0.75 0.16 17.2 n/a 35

Pollutant Avg. Time

ALL 

Sources 

Impact Alt 

H Revised 

(μg/m3)

ALL 

Sources 

Impact 

Scenario 

1 

(μg/m3)

ALL 

Sources 

Impact 

Scenario 

1A 

(μg/m3)

ALL 

Sources 

Impact 

Scenario 

2 

(μg/m3)

ALL 

Sources 

Impact 

Scenario 2A

(μg/m3)

ALL 

Sources 

Montana

Base Year 

(μg/m3)

MT

Background 

Values 

(μg/m3)

PSD1

Increment 

Class II 

(μg/m3)

NAAQS /2

MAAQS 

(μg/m3)

NO2 Annual 2.78 2.63 2.36 2.46 2.27 1.83 6 25 100

1-Hour 469 376 322 322 322 461.7 75 n/a 565

SO2 Annual 2.62 2.62 2.62 2.62 2.62 2.57 3 20 60

24-Hour 14.7 14.7 14.7 14.7 14.7 14.7 11 91 260

3-Hour 61.2 61.2 61.2 61.2 61.2 61.1 26 512 1,300

1-Hour 151 151 151 151 151 150.6 35 n/a 1,300

PM10 Annual 4.29 4.28 4.27 4.28 4.27 4.38 8 17 50

24-Hour 45.6 45.6 45.6 45.6 45.6 46.7 30 30 150

PM2.5 Annual 0.83 0.81 0.77 0.798 0.76 0.68 3.4 n/a 15

24-Hour 6.59 6.51 6.38 6.47 6.38 7.11 17.2 n/a 35 1

PSD Increment is to be compared directly to the modeled impact 2

Background should be added to modeled impact for comparison to AAQS

n/a – not applicable
AIR QUALITY APPENDIX

Part 2 AIR-60

Scenario 1 

Potential direct project CBNG impacts for both 

operation and construction activities are below 

applicable standards for NO2, SO2, PM10 and PM2.5

at the Montana Near-Field, Northern Cheyenne 

Indian Reservation, and Crow Indian Reservation 

receptor grids under Scenario 1. Impacts for SO2

would be unchanged from the Alternative H 

Revised Scenario at the Montana Near-Field and 

Crow Indian Reservation receptors, and only 

slightly increased at the Northern Cheyenne Indian 

Reservation receptors. Combined project CBNG 

impacts shown in Tables 4-1 through 4-3 as 

“Project CBNG” are decreased from the Alternative 

H Revised Scenario, and are still below both Class I 

and Class II PSD increment levels at all receptors.

Scenario 1A 

Scenario 1A potential impacts are less than the 

impacts described in the Alternative H Revised 

Scenario, Scenario 1, and Scenario 2 for SO2, but 

are the same as the SO2 impacts predicted by 

Scenario 2A. Similar results are seen with other 

modeled pollutants, with the exception that impacts 

are slightly higher than those predicted by Scenario 

2A. Combined project CBNG impacts shown in 

Tables 4-1 through 4-3 as “Project CBNG” are 

decreased from the Alternative H Revised Scenario, 

Scenario 1, and Scenario 2. Combined project 

CBNG impacts are still below both Class I and 

Class II PSD increment levels at all receptors.

Scenario 2 

The model predicted potential impacts under 

Scenario 2 are less than those of Scenario 1, with 

direct project CBNG construction and operation 

impacts well below any applicable standard for 

NO2, SO2, PM10 and PM2.5 at the Montana NearField receptor grid and on the Northern Cheyenne 

Indian Reservation and the Crow Indian 

Reservation. The predicted impacts at the Crow 

Indian Reservation indicate a decrease in the 1-hour 

NO2 ambient concentration from the project CBNG 

operation source to 225 μg/m3. Combined project 

CBNG impacts shown in Tables 4-1 through 4-3 as 

“Project CBNG” are still below both Class I and 

Class II PSD increment levels at all receptors.

Scenario 2A 

As would be anticipated with a 50 percent 

reduction in Scenario 2 emissions from the CBNG 

field and sales compressor operation and 

maintenance emissions, potential direct impacts at 

the Montana Near-Field receptor grid and on the 

Northern Cheyenne Indian Reservation and the 

Crow Indian Reservation are further reduced from 

Scenario 2. The predicted impacts at the Crow 

Indian Reservation indicate a further reduction in 

the 1-hour NO2 ambient concentration to 113 

μg/m3. Combined project CBNG impacts shown in 

Tables 4-1 through 4-3 as “Project CBNG” are still 

below both Class I and Class II PSD increment 

levels at all receptors.

Potential Visibility Impacts

Table 4-4 shows the impacts at the Northern 

Cheyenne Indian Reservation and the Crow Indian 

Reservation for the base year, and each of the 

modeled Alternative H scenarios. Results are 

provided separately for the Montana project CBNG 

construction and operation, as well as combined 

Montana project CBNG construction and operation 

with RFFA sources as the All Montana and All 

Sources source groups. Comprehensive details of 

the modeling results are given in Appendix C of the 

SAQA document. The key impacts under the 

Method 2 approach are summarized in Table 4-4. 

Visibility impacts for each of the scenarios for the 

Northern Cheyenne Indian Reservation and Crow 

Indian Reservation under Method 6 are summarized 

in Table 4-5.

Method Two

Potential impacts at Northern Cheyenne Indian 

Reservation from project CBNG construction are 

reduced slightly from the Alternative H Revised 

Scenario through Scenario 2A. For this construction 

source group there are no days with impacts greater 

than 10% of the background at either the Northern 

Cheyenne Indian Reservation or the Crow Indian 

Reservation. For project CBNG operation there are 

35 days per year with impacts above 10% at the 

Northern Cheyenne Indian Reservation for Scenario 

1, but the number of days drops to 2 per year for 

Scenario 2A. The number of days with impacts 

above 10% at the Crow Indian Reservation drops 

from 87 days per year for Scenario 1 to 11 days per 

year under.
Part 2 AIR-61

AIR QUALITY APPENDIX

Table 4-4 Potential Visibility Impacts – Method 2 - Summary

Receptor Set

Project CBNG Construction Project CBNG Operation ALL MT ALL SOURCES

Number of 

Days>N% 

Change in 

Bext

Maximum % 

Change in 

Bext

Number of 

Days>N% 

Change in Bext

Maximum % 

Change in 

Bext

Number of 

Days>N% 

Change in Bext

Maximum % 

Change in 

Bext

Number of 

Days>N% 

Change in Bext

Maximum % 

Change in 

Bext

5% 10% 5% 10% 5% 10% 5% 10%

Northern Cheyenne Indian Reservation Class I

Base Year 3 1 10 7 1 15 209 113 90 298 235 416

Revised Alternative H 2 0 6.0 129 55 51.1 357 279 186.0 363 314 467.7

Scenario 1 2 0 6.0 108 35 40.1 357 276 177.6 363 314 461.6

Scenario 1A 1 0 5.0 35 7 20.1 353 269 170.3 363 310 450.8

Scenario 2 2 0 6.0 67 17 27.7 354 273 173.0 363 312 454.8

Scenario 2A 1 0 5.0 17 2 13.9 353 264 168.0 363 308 447.4

Crow Indian Reservation Class II

Base Year 3 0 8.3 8 2 12.1 365 350 710 365 358 714

Revised Alternative H 17 0 8.3 195 107 71.0 365 365 >1,000 365 365 >1,000

Scenario 1 17 0 8.3 169 87 55.6 365 365 >1,000 365 365 >1,000

Scenario 1A 7 0 6.7 87 29 28 365 365 >1,000 365 365 >1,000

Scenario 2 17 0 8.3 141 60 38.1 365 365 >1,000 365 365 >1,000

Scenario 2A 7 0 6.7 60 11 19.1 365 365 >1,000 365 365 >1,000
AIR-62

AIR QUALITY APPENDIX

Part 2 AIR-62

Table 4-5 Potential Visibility Impacts – Method 6 and Monthly f(RH) values - Scenarios

Receptor Set

Project CBNG Construction Project CBNG Operation ALL MT ALL SOURCES

Number of 

Days>N% 

Change in 

Bext

Maximum 

% 

Change 

in Bext

8th Highest 

% Change 

in Bext

Number of 

Days>N% 

Change in Bext

Maximum 

% Change 

in Bext

8th Highest 

% Change 

in Bext

Number of 

Days>N% 

Change in Bext

Maximum 

% Change 

in Bext

8th Highest 

% Change 

in Bext

Number of 

Days>N% 

Change in 

Bext

Maximum 

% Change 

in Bext

8th Highest 

% Change 

in Bext

5% 10% 5% 10% 5% 10% 5% 10%

Northern Cheyenne Indian Reservation Class I

Base Year 1 0 5.0 2.2 2 0 9.5 3.1 192 97 79 33 299 234 312 121

Revised 

Alternative H 1 0 5.0 2.6 97 24 21.9 17.1 356 271 140.7 68.5 364 316 339.2 156.1

Scenario 1 1 0 5.0 2.6 64 19 18.6 13.9 356 268 139.1 65.7 364 314 337.89 154.4

Scenario 1A 1 0 5.0 2.6 20 0 9.3 7.0 355 261 136.1 61.2 364 311 335.4 151.5

Scenario 2 1 0 5.0 2.6 37 7 15.2 9.9 355 264 137.3 62.6 364 312 336.40 152.6

Scenario 2A 1 0 5.0 2.6 7 0 7.6 4.9 354 257 135.2 60.8 364 310 334.68 150.5

Crow Indian Reservation Class II

Base Year 1 0 5.2 2.6 5 0 7.2 3.4 365 352 659 430 365 360 664 441

Revised 

Alternative H 7 0 6.7 4.9 173 82 36.7 27.7 365 365 999.5 651.9 365 365 >1,000 666.9

Scenario 1 7 0 6.7 4.9 146 61 29.2 22.0 365 365 999.5 651.3 365 365 >1,000 664.8

Scenario 1A 7 0 6.7 4.9 61 11 14.6 11.0 365 365 999.5 650.3 365 365 >1,000 663.2

Scenario 2 7 0 6.7 4.9 118 38 21.1 16.6 365 365 999.5 650.7 365 365 >1,000 663.6

Scenario 2A 7 0 6.7 4.9 38 3 10.6 8.3 365 365 999.5 649.9 365 365 >1,000 662.8
AIR QUALITY APPENDIX

Part 2 AIR-63

Acid Deposition Impacts

The acid deposition rates for nitrogen and sulfur 

compounds from project CBNG operation and 

construction are below established thresholds 

which are 3 kilograms per hectare per year 

(kg/ha-year) for nitrogen compounds and 5 

kg/ha-year for sulfur compounds (Fox, et. Al. 

1989). Complete results are provided in 

Appendix C of the SAQA document, with the 

base year summary in Table 4-4 of the SAQA 

document. A careful examination of those results 

shows that there are no exceedances of 

applicable regulatory thresholds for any of the 

modeled scenarios. 

Cumulative Impacts (Existing 

Sources + RFD + RFFA 

Sources)

The cumulative impacts analysis discussion 

which follows describes the combined effects of 

project CBNG development sources with 

reasonably foreseeable future action sources and 

existing sources which may contribute to 

potential air quality impacts within the project 

planning area (Additional detail on potential 

modeled emissions is provided within the tables 

in Appendix C of the SAQA document). Model 

results indicate the potential for impact to the 

Class II PSD increment for 24-hour PM10 on the 

Crow Indian Reservation. The Montana NearField shows a potential to exceed the Class II 

PSD increment for 24-hour PM10 and the 1-hour 

NO2 ambient air quality standard. There is also a 

potential to exceed the Class I PSD increment for 

24-hour PM10 at the Northern Cheyenne Indian 

Reservation. Cumulative impacts to the key 

receptors from the All Montana source group and 

the All Sources source group are very similar 

between all modeled scenarios. This indicates 

that there is most likely a dominant emission 

source in the RFFA which affects the impacts at 

a given receptor.

Alt. H Revised 

The cumulative impacts under the Alternative H 

Revised scenario for the Montana Near-Field 

receptor grid indicate that there are no 

exceedances of air quality standards predicted 

(Additional detail on potential modeled 

emissions is provided within the tables in 

Appendix C of the SAQA document). The 1-

hour NO2 ambient concentration for the All 

Montana source group is 539 μg/m3 and for the 

All Sources source group is 540 μg/m3. Thus, 

while the standard is not exceeded, the model 

predicts that there is a potential for impact to this 

standard. Cumulative impacts at the Northern 

Cheyenne Indian Reservation are all predicted to 

be below any applicable air quality standards. On 

the Crow Indian Reservation cumulative impacts 

to the 1-hour NO2 standard is not predicted to be 

exceeded in the All Montana and All Source 

group categories. The 1-hour NO2 is 469 μg/m3

for both of these source groups in comparison 

with a standard of 565 μg/m3. The Base Year 

impacts for the All Montana and All Sources 

source groups for 1-hour NO2 is 461.7 μg/m3

indicating an increase of 7.3 μg/m3, and the 24-

hour PM10 is 45.6 μg/m3 for both source groups 

indicating an decrease of 1 μg/m3 from the Base 

Year for the All Sources source group and the 

All Montana source group. While the direct 

modeled impacts for the All Montana and All 

Sources source groups are above the Class II 

PSD increment, the model indicates a reduction 

in 24-hour PM10 from the Base Year and is 

below the PSD increment of 30 μg/m3 for Class 

II areas. All other impacts are below any 

applicable air quality standard.

Scenario 1 

Cumulative impacts under Scenario 1 for the 

Montana near field receptor grid indicate that 

there are no exceedances of air quality standards 

predicted (Additional detail on potential modeled 

emissions is provided within the tables in 

Appendix C of the SAQA document). The 1-

hour NO2 ambient concentration for the All 

Montana source group is 539 μg/m3 and for the 

All Sources source group is 540 μg/m3. While 

the standard is not exceeded, the model predicts 

that there is a potential for impact to this 

standard. At the Northern Cheyenne Indian 

Reservation cumulative impacts are all predicted 

to be below any applicable air quality standards. 

All impacts on the Crow Indian Reservation are 

predicted to be below any applicable air quality 

standard.

Scenario 1A 

There is a small difference between Scenario 1A 

and Scenario 1 cumulative impacts at the 

Montana near field receptor grid. The 1-hour 

NO2 ambient concentration for the All Montana 

source group and All Sources source group is 
AIR QUALITY APPENDIX

Part 2 AIR-64

539 μg/m3. Cumulative impacts to the Northern 

Cheyenne Indian Reservation are all predicted to 

be below any applicable air quality standards. 

All impacts on the Crow Indian Reservation are 

predicted to be below any applicable air quality 

standard.

Scenario 2 

There is no difference between Scenario 2 and 

Scenario 1 cumulative impacts at the Montana 

near field receptor grid. This indicates that there 

is most likely a dominant emission source in the 

RFFA source emissions or other existing 

emission sources (such as the Colstrip power 

plants and coal mine) which affects the impacts 

at a specific receptor. Cumulative impacts to the 

Northern Cheyenne Indian Reservation are all 

predicted to be below any applicable air quality 

standards. The same results for cumulative 

impacts on the Crow Indian Reservation are 

predicted under Scenario 2 as in Scenario 1. All 

impacts are below any applicable air quality 

standard.

Scenario 2A 

There is a small difference between Scenario 2A 

and Scenario 2 cumulative impacts at the 

Montana near field receptor grid. The 1-hour 

NO2 ambient concentration for the All Montana 

source group and All Sources source group is 

539 μg/m3. Cumulative impacts to the Northern 

Cheyenne Indian Reservation are all predicted to 

be below any applicable air quality standards. 

The same results for cumulative impacts on the 

Crow Indian Reservation are predicted under 

Scenario 2 as in Scenario 1. All impacts are 

below any applicable air quality standard.

Tongue River Railroad

The results from the revised modeling effort 

shown in Table 4-6 indicate that the reductions 

in emissions and the reconfiguration of the 

Tongue River Railroad sources led to reductions 

in visibility impacts at nearby sensitive Class I 

and Class II areas, and no notable reductions in 

impacts at the more distant sensitive area 

receptors. 

In the original configuration, the Tongue River 

Railroad emissions led to measurable impacts on 

the Northern Cheyenne Indian Reservation and 

on the Crow Indian Reservation. Originally, the 

number of days with impacts above 1.0 deciview 

was 23 days for the Northern Cheyenne Indian 

Reservation, 8 days for the Crow Indian 

Reservation, and 2 days at the Cloud Peak 

Wilderness area. As a result of modifying the 

source configuration, those numbers dropped to 

one day at the Northern Cheyenne and zero days 

at the Crow Indian Reservation and Cloud Peak 

Wilderness area. The reconfiguration of the 

emission source points demonstrates that the 

Tongue River Railroad by itself does not have 

the potential to cause any impacts on visibility at 

any mandatory Class I or Class II areas.
AIR QUALITY APPENDIX

Part 2 AIR-65

Table 4-6

Visibility Impacts of Original Versus Revised Tongue River Railroad Source

Receptor Set

Original Analysis 

Tongue River Railroad

Revised Source Configuration

Tongue River Railroad

Number of Days 

> N% Change in 

Bext

Maximum 

% Change 

in Bext

8th Highest 

% Change 

in Bext

Number of Days 

> N% Change in 

Bext

Maximum 

% Change 

in Bext

8th Highest 

% Change 

in Bext

5% 10% 5% 10%

CLASS I AREAS

Badlands NP Class I 0 0 1 1 0 0 0 0

Bob Marshall W Class I 0 0 0 0 0 0 0 0

Bridger W Class I 0 0 2 1 0 0 0 0

Fitzpatrick W Class I 0 0 2 0 0 0 0 0

Fort Peck IR Class I 0 0 2 1 0 0 0 0

Gates of the Mountain W Class I 0 0 1 0 0 0 0 0

Grand Teton NP Class I 0 0 1 0 0 0 0 0

North Absaroka W Class I 0 0 3 1 0 0 1 0

North Cheyenne IR Class I 71 23 27 14 1 0 7 3

Red Rock Lakes Class I 0 0 1 0 0 0 0 0

Scapegoat W Class I 0 0 1 0 0 0 0 0

Teton W Class I 0 0 1 0 0 0 0 0

Theodore Roosevelt NP Class I 0 0 3 1 0 0 1 0

UL Bend W Class I 0 0 2 1 0 0 0 0

Washakie W Class I 0 0 2 1 0 0 0 0

Wind Cave NP Class I 0 0 2 1 0 0 0 0

Yellowstone NP Class I 0 0 3 0 0 0 1 0

SENSITIVE CLASS II AREAS

Absaroka Beartooth W Class II 1 0 6 1 0 0 1 0

Agate Fossil Beds NM Class II 0 0 1 1 0 0 0 0

Big Horn Canyon NRA Class II 3 0 7 3 0 0 1 1

Black Elk W Class II 0 0 2 1 0 0 0 0

Cloud Peak Class II 4 2 24 3 0 0 5 1

Crow IR Class II 27 8 21 11 0 0 5 2

Devils Tower NM Class II 0 0 2 1 0 0 0 0

Fort Belknap IR Class II 0 0 2 1 0 0 0 0

Fort Laramie NHS Class II 0 0 1 1 0 0 0 0

Jedediah Smith W Class II 0 0 1 0 0 0 0 0

Jewel Cave NM Class II 0 0 2 1 0 0 0 0

Lee Metcalf W Class II 0 0 2 0 0 0 0 0

Mt Naomi W Class II 0 0 1 0 0 0 0 0

Mt Rushmore Class II 0 0 2 1 0 0 0 0

Popo Agie W Class II 0 0 2 1 0 0 0 0

Soldier Creek WA Class II 0 0 1 1 0 0 0 0

Wellsville Mountain W Class II 0 0 0 0 0 0 0 0

Wind River IR Class II 0 0 3 1 0 0 1 0
AIR QUALITY APPENDIX

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ATTACHMENT A

Review of Information on Health Effects
AIR QUALITY APPENDIX

Attachment A - Review of Applicable Health Effects

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Attachment A - Review of Applicable Health Effects

Part 2 AIR-69

REVIEW OF INFORMATION ON HEALTH EFFECTS 

Introduction 

In response to the findings of ambient air quality potential 

impacts in the Powder River Basin of Montana and 

Wyoming, resulting from current and projected 

development, this Attachment contains a summary of 

published information regarding potential health effects

from Particulate Matter (PM). The modeled impacts 

showed the potential for PM10 concentrations to exceed 

the 24-hour ambient standards. The modeled exceedances 

were confined to a small number of receptors generally 

near major source development, such as coal fired power 

plants and coal mines. 

Air monitoring station data collected for 2004 in Montana 

showed no exceedances of the 24-hour PM10 standard. 

PM10 Health Effects: 

The potential impacts of PM concentrations are focused 

on sensitive populations, including those with existing 

cardiopulmonary disease. Nine percent of adults and 

eleven percent of children are diagnosed with asthma. 

There is some evidence that socioeconomic status also 

plays a role in predicting exposure and impact of PM 

levels of concern. 

The study concludes that “Of concentration–response 

functions for PM-related effects, it can generally be said 

that the effect estimates are small in magnitude. In 

historical episodes with very high air pollution levels, 

risks on the order of a four-fold increase in mortality were 

estimated, but much smaller risk estimates have been 

reported from recent studies at current pollution levels.” 

“Relative risk estimates for total mortality from the 

prospective cohort studies fall in the range of 7 to 13 

percent increase per 10 μg/m3 increase in PM2.5; there are 

no significant associations with long-term exposure to 

PM10-2.5. Risk estimates from the short-term exposure 

studies are considerably smaller in magnitude, on the 

order of 2 to 6 percent increase in mortality per 25 μg/m3

increase in PM2.5 and PM10-2.5.”

“Effect estimates for morbidity responses to short-term 

changes in PM tend to be larger in magnitude that those 

for mortality; those for hospitalization generally range 

from 4-10 percent increases for cardiovascular diseases 

and 5-15 percent increases for respiratory diseases per 25 

μg/m3 increase in PM2.5 and PM10-2.5. From the more 

recent studies on visits to the emergency department or 

physicians’ offices for respiratory conditions, effect 

estimate sizes have been somewhat larger, ranging up to 

about 35 percent per 25 μg/m3 increase in PM2.5.” 

The health effects of short-term 

particulate concentrations on the public health have been 

reviewed in great detail, and were again reviewed as a 

part of the EPA-mandated evaluation of current ambient 

air quality standards. The most recent review (EPA 

2004a: Air Quality Criteria for Particulate Matter,

EPA/600-P-99/002aF, October 2004) focuses on the 

establishment of the alternate PM2.5 standards and 

discussed PM levels in general. The study summarizes 

both morbidity and mortality of potential impacts for both 

short term and long term exposures. The current standards 

for PM10 (150 μg/m3 for 24 hours and 50 μg/m3 for 

annual standards) are focused on protecting against 

morbidity and mortality effects. The study re-iterates a 

previous conclusion that “Efforts to quantify the number 

of deaths attributable to, and the years of life lost to, 

ambient PM exposures are currently subject to much 

uncertainty.”

Recently a new PM standard (PM2.5) has been 

promulgated, and state regulatory agencies are currently 

implementing programs to address those standards. PM2.5

levels are being measured at Lame Deer in the study area, 

and results show that those levels are below the 

established ambient standards. 

As is indicated in the referenced EPA study, the 

predictive impact of these studies on individual small 

communities is subject to much uncertainty. However, 

given the fact that predicted impacts that exceed the 24-

hour ambient air quality standard for PM10 are in remote, 

generally unpopulated areas, and that sensitive 

populations would generally not be confined to these 

areas, it is unlikely that the modeled impacts of PM10

levels would lead to any actual increase in morbidity or 

mortality of specific receptor populations. 
AIR QUALITY APPENDIX

Attachment A - Review of Applicable Health Effects

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ATTACHMENT B

Review of Mitigation Measures
AIR QUALITY APPENDIX

Attachment B - Review of Mitigation Effects

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REVIEW OF MITIGATION MEASURES

Model results have indicated the potential for PM10 to 

exceed the 24-hour regulatory standard. In addition, both 

PM10 and NOx have the potential to impact visibility 

within PSD Class I and Class II areas. The following 

mitigation measures for PM and NOx are those that are 

commonly employed to control air emissions. Other 

mitigation measures could be employed to achieve a 

desired control, including in tribal designated Class I 

areas, such as the Northern Cheyenne Indian Reservation. 

Additionally, through the air permitting process 

regulatory agencies may require specific controls based 

on the volume and type of emissions or the location of the 

emission source.

Mitigation of PM:

(1) Surface exposure. When vegetation is 

removed from the right-of-ways for hauling 

or construction activities, applicants shall 

clear the smallest possible amount of cover 

to minimize the impact of wind erosion and 

fugitive dust. 

 Emissions of PM2.5 and PM10 from 

industrial operations can be subjected to a wide range of 

mitigation activities or controls. Emissions of these 

pollutants from industrial sources, including stacks or 

vents, are often controlled satisfactorily by employing bag 

filters or electrostatic precipitators. Emissions of PM2.5

and PM10 from these sources is generally subjected to 

review by air permitting agencies, because the nature of 

the source would trigger the need to obtain an air permit 

to construct such a facility. Any modifications to those 

facilities would also trigger the need to obtain such a 

permit. As a part of the review of those permits, agencies 

ensure that emissions are controlled and that impacts are 

with acceptable concentrations. 

The PM2.5 and PM10 emissions from fugitive sources, 

such as material stockpiles, construction operations, and 

material handling operations are also subject to potential 

mitigating controls. As impacts are identified, any 

impacts of concern can be addressed by imposing the 

related mitigation measures. 

In general the mitigation measures that can be employed 

for materials handling, construction, hauling operations, 

and storage activities can be summarized as in the list of 

activities below. 

(2) Revegetation. Where vegetation has been 

removed, and soils exposed, begin 

revegetation as soon as possible, and 

enhance revegetation with mulching or 

matting to stabilize the surface and promote 

plant growth. 

(3) Construction or soil excavation. For exposed 

active construction surfaces and related 

stockpiles, include dust suppression 

activities such as surface watering or 

stabilization with chemical surfactants. 

(4) Construction and handling during windy 

periods. Restrict construction or material 

handling operations during periods with 

high winds, such as a threshold of 30 miles 

per hour. Enhance surface water sprays as an 

option. 

(5) Hauling operations. Maintain all haul roads 

that are continually active by surface 

watering, chemical stabilization, restricted 

vehicle speeds, and removal of all spillage 

onto the roadway surface. Cover and 

maintain the roadways with dust-inhibiting 

material to include gravel or small rocks.

(6) Construction equipment operations. Require 

the use of high quality (low sulfur) diesel 

fuel in all diesel-fired construction or 

operational engines. Maintain all engines in 

satisfactory operating conditions. 

Mitigation of NOx: NOx, which includes nitrogen oxide 

(NO) and nitrogen dioxide (NO2), is produced as a 

byproduct of combustion. Efforts aimed at controlling 

NOx emissions and ambient air impacts can be focused on 

either decreasing the emissions or increasing the 

dispersion. 

The EPA has researched mechanisms that govern the 

formation of NOx during combustion as a basis for 

reducing NOx emissions from combustion sources. EPA's 

early efforts focused on the prevention of NOx through 

modification of the combustion process, since this 

approach held the promise of higher emissions reductions 

and greater economic efficiency than the use of flue gas 

treatment for NOx control. There have been significant 

advances in combustion technology which can reduce the 

primary production of NO2 at the combustion source. 

Control of NOx is a complex process affected by the 

nitrogen content of the fuel, the amount and distribution 

of air in the combustion process, temperature, unit load, 

and burner design, among other factors. Therefore, NOx 

emissions can vary significantly with changes in 

temperature and air/fuel mixing, and are controlled 

primarily by modifying the basic combustion process, 

with the result that combustion modification NOx controls 

directly affect not only emissions, but often the efficiency 

and operability of the unit as well.
AIR QUALITY APPENDIX

Part 2 AIR-74

Flue gas control of NOx consists of adding secondary 

control systems to the exhaust gas from a combustion 

process. Types of secondary control systems include 

selective catalytic systems, non-selective catalytic 

systems, chemical scrubbers, and wet scrubbers. In most 

cases, these types of control systems require periodic 

replacement, regeneration, or disposal of wastes resulting 

from their actions, which leads to increased costs for 

operation. 

Another alternative for NOx emissions control is to 

eliminate the combustion source and replace it with an 

electric process. Electric motors can be used to replace 

combustion driven engines. 

Increased dispersion of NOx emissions does not reduce 

emissions at the source, but acts to reduce near field 

impacts by spreading the emissions over a larger area. 

Enhanced dispersion can be achieved by increasing the 

buoyancy of the emissions or increasing the height of the 

emissions release in relation to the topographic 

surroundings. Buoyancy can be increased by increasing 

the temperature of the exhaust or by increasing the 

exhaust flow velocity. Release height is governed by good 

engineering practices, which limits the actual stack height 

allowed in relation to existing surrounding features, or a 

maximum allowable height, whichever is less. 

Another mitigation alternative includes the regulatory 

permitting process, which would act to protect ambient air 

quality by preventing the issuance of permits in areas that 

would experience significant impacts from additional 

permitted sources.

The following mitigation measure are commonly

employed to prevent potential impacts from NOx which 

could lead to exceedances of federal or state ambient air 

quality standards:

(1) Implement Best Available Control 

Technology (BACT) for the emissions unit. 

For compressor engines, this can result in 

NOx emission rate of 1 g/bhp-hr, which is 

lower than the 1.5 g/bhp-hr rate used in the 

modeling.

(2) Utilize electric powered compressor engines 

in place of fuel combustion sources. Using 

electric-powered compressor motors in place 

of the typical natural gas-fired compressor 

engines could eliminate primary NOx

emissions from compressor stations. 

(3) Use alternative fuels, which have lower fuel 

nitrogen content. Natural gas-fired 

compressor engines typically have lower 

NOx emissions than diesel-fired engines.

(4) Increase dispersion of NOx emissions to 

reduce near field impacts by spreading 

emissions over a larger area.

(5) Use of regulatory permitting to prevent new 

or additional sources into areas where their 

emissions would cause significant impacts 

to ambient air quality identified through the 

permitting process.