Document ID: EPA-R08-OAR-2018-0109-0001
Agency: epa
Document Type: Proposed Rule
Title: Air Quality State Implementation Plans; Approvals and Promulgations: Colorado, Montana, North Dakota, South Dakota and Wyoming; Interstate Transport Prongs 1 and 2 for the 2010 Sulfur Dioxide Standard
Posted Date: 2018-06-04T04:00Z

[Federal Register Volume 83, Number 107 (Monday, June 4, 2018)]
[Proposed Rules]
[Pages 25617-25632]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2018-11846]

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 52

[EPA-R08-OAR-2018-0109; FRL-9978-72-Region 8]

Interstate Transport Prongs 1 and 2 for the 2010 Sulfur Dioxide 
(SO2) Standard for Colorado, Montana, North Dakota, South Dakota and 
Wyoming

AGENCY: Environmental Protection Agency (EPA).

ACTION: Proposed rule.

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SUMMARY: The Environmental Protection Agency (EPA) is proposing to 
approve portions of State Implementation Plan (SIP) submissions from 
Colorado, Montana, North Dakota, South Dakota and Wyoming addressing 
the Clean Air Act (CAA or Act) interstate transport SIP requirements 
for the 2010 Sulfur Dioxide (SO2) National Ambient Air 
Quality Standards (NAAQS). These submissions address the requirement 
that each SIP contain adequate provisions prohibiting air emissions 
that will have certain adverse air quality effects in other states. The 
EPA is proposing to approve portions of these infrastructure SIPs for 
the aforementioned states as containing adequate provisions to ensure 
that air emissions in the states will not significantly contribute to 
nonattainment or interfere with maintenance of the 2010 SO2 
NAAQS in any other state.

DATES: Comments must be received on or before July 5, 2018.

ADDRESSES: Submit your comments, identified by Docket ID No EPA-R08-
OAR-2018-0109 at http://www.regulations.gov. Follow the online 
instructions for submitting comments. Once submitted, comments cannot 
be edited or removed from www.regulations.gov. The EPA may publish any 
comment received to its public docket. Do not submit electronically any 
information you consider to be Confidential Business Information (CBI) 
or other information whose disclosure is restricted by statute. 
Multimedia submissions (audio, video, etc.) must be accompanied by a 
written comment. The written comment is considered the official comment 
and should include discussion of all points you wish to make. The EPA 
will generally not consider comments or comment contents located 
outside of the primary submission (i.e., on the web, cloud, or other 
file sharing system). For additional submission methods, the full EPA 
public comment policy, information about CBI or multimedia submissions 
and general guidance on making effective comments, please visit http://www2.epa.gov/dockets/commenting-epa-dockets.

FOR FURTHER INFORMATION CONTACT: Adam Clark, Air Program, U.S. EPA 
Region 8, (303) 312-7104, [email protected].

SUPPLEMENTARY INFORMATION:
I. Background
II. Relevant Factors To Evaluate 2010 SO2 Interstate 
Transport SIPs
III. States' Submissions and EPA's Analysis
    A. Colorado
    1. State's Analysis
    2. EPA's Prong 1 Evaluation
    3. EPA's Prong 2 Evaluation
    B. Montana
    1. State's Analysis
    2. EPA's Prong 1 Evaluation
    3. EPA's Prong 2 Evaluation
    C. North Dakota
    1. State's Analysis
    2. EPA's Prong 1 Evaluation
    3. EPA's Prong 2 Evaluation
    D. South Dakota
    1. State's Analysis
    2. EPA's Prong 1 Evaluation
    3. EPA's Prong 2 Evaluation
    E. Wyoming
    1. State's Analysis
    2. EPA's Prong 1 Evaluation
    3. EPA's Prong 2 Evaluation
IV. Proposed Action
V. Statutory and Executive Order Reviews

I. Background

    On June 2, 2010, the EPA established a new primary 1-hour 
SO2 NAAQS of 75 parts per billion (ppb), based on a 3-year 
average of the annual 99th percentile of 1-hour daily maximum 
concentrations.\1\ The CAA requires states to submit, within 3 years 
after promulgation of a new or revised NAAQS, SIPs meeting the 
applicable ``infrastructure'' elements of sections 110(a)(1) and (2). 
One of these applicable infrastructure elements, CAA section 
110(a)(2)(D)(i), requires SIPs to contain ``good neighbor'' provisions 
to prohibit certain adverse air quality effects on neighboring states 
due to interstate transport of pollution.
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    \1\ 75 FR 35520 (June 22, 2010).
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    Section 110(a)(2)(D)(i) includes four distinct components, commonly 
referred to as ``prongs,'' that must be addressed in infrastructure SIP 
submissions. The first two prongs, which are codified in section 
110(a)(2)(D)(i)(I), require SIPs to contain adequate provisions that 
prohibit any source or other type of emissions activity in one state 
from contributing significantly to nonattainment of the NAAQS in 
another state (prong 1) and from interfering with maintenance of the 
NAAQS in another state (prong 2). The third and fourth prongs, which 
are codified in section 110(a)(2)(D)(i)(II), require SIPs to contain 
adequate provisions that prohibit emissions activity in one state from 
interfering with measures required to prevent significant deterioration 
of air quality in another state (prong 3) or from interfering with 
measures to protect visibility in another state (prong 4).
    In this action, the EPA is proposing to approve the prong 1 and 
prong 2 portions of infrastructure SIP submissions submitted by: 
Colorado on July 17, 2013 and February 16, 2018; Montana on July 15, 
2013; North Dakota on March 7, 2013; South Dakota on December 20, 2013; 
and Wyoming on March 6, 2015, as containing adequate provisions to 
ensure that air emissions in these states will not significantly 
contribute to nonattainment or interfere with maintenance of the 2010 
SO2 NAAQS in any other state. All other applicable 
infrastructure SIP requirements for these SIP submissions have been 
addressed in separate rulemakings.

[[Page 25618]]

II. Relevant Factors To Evaluate 2010 SO2 Interstate Transport SIPs

    Although SO2 is emitted from a similar universe of point 
and nonpoint sources, interstate transport of SO2 is unlike 
the transport of fine particulate matter (PM2.5) or ozone, 
in that SO2 is not a regional pollutant and does not 
commonly contribute to widespread nonattainment over a large (and often 
multi-state) area. The transport of SO2 is more analogous to 
the transport of lead (Pb) because its physical properties result in 
localized pollutant impacts very near the emissions source. However, 
ambient concentrations of SO2 do not decrease as quickly 
with distance from the source as Pb because of the physical properties 
and typical release heights of SO2. Emissions of 
SO2 travel farther and have wider ranging impacts than 
emissions of Pb, but do not travel far enough to be treated in a manner 
similar to ozone or PM2.5. The approaches that the EPA has 
adopted for ozone or PM2.5 transport are too regionally 
focused and the approach for Pb transport is too tightly circumscribed 
to the source. SO2 transport is therefore a unique case and 
requires a different approach.
    Given the physical properties of SO2, the EPA selected 
the ``urban scale''--a spatial scale with dimensions from 4 to 50 
kilometers (km) from point sources--given the usefulness of that range 
in assessing trends in both area-wide air quality and the effectiveness 
of large-scale pollution control strategies at such point sources.\2\ 
As such, the EPA utilized an assessment up to 50 km from point sources 
in order to assess trends in area-wide air quality that might impact 
downwind states.
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    \2\ For the definition of spatial scales for SO2, 
please see 40 CFR part 58, Appendix D, section 4.4 (``Sulfur Dioxide 
(SO2) Design Criteria''). For further discussion on how 
the EPA is applying these definitions with respect to interstate 
transport of SO2, see the EPA's proposal on Connecticut's 
SO2 transport SIP. 82 FR 21351, 21352, 21354 (May 8, 
2017).
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    As discussed in Section III of this proposed action, the EPA first 
reviewed each state's analysis to assess how the state evaluated the 
transport of SO2 to other states, the types of information 
used in the analysis and the conclusions drawn by the state. The EPA 
then conducted a weight of evidence analysis, including review of each 
state's submission and other available information, including air 
quality, emission sources and emission trends within the state and in 
neighboring states to which it could potentially contribute or 
interfere.\3\
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    \3\ This proposed approval action is based on the information 
contained in the administrative record for this action, and does not 
prejudge any other future EPA action that may make other 
determinations regarding any of the subject state's air quality 
status. Any such future actions, such as area designations under any 
NAAQS, will be based on their own administrative records and the 
EPA's analyses of information that becomes available at those times. 
Future available information may include, and is not limited to, 
monitoring data and modeling analyses conducted pursuant to the 
EPA's SO2 Data Requirements Rule (80 FR 51052, August 21, 
2015) and information submitted to the EPA by states, air agencies, 
and third party stakeholders such as citizen groups and industry 
representatives.
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III. States' Submissions and EPA's Analysis

    In this section, we provide an overview of each state's 2010 
SO2 transport analysis, as well as the EPA's evaluation of 
prongs 1 and 2 for each state. Table 1, below, shows emission trends 
for the five states addressed in this notice along with their 
neighboring states. The table will be referenced as part of the EPA's 
analysis for each state.\4\
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    \4\ This emissions trends information was derived from EPA's 
webpage https://www.epa.gov/air-emissions-inventories/air-pollutant-emissions-trends-data.

                                          Table 1--SO2 Emission Trends
----------------------------------------------------------------------------------------------------------------
                                                                                                  SO2 reduction,
              State                    2000            2005            2010            2016        2000-2016 (%)
 
----------------------------------------------------------------------------------------------------------------
Arizona.........................         118,528          90,577          73,075          38,089              68
Colorado........................         115,122          80,468          60,459          20,626              82
Idaho...........................          34,525          35,451          14,774          10,051              70
Iowa............................         265,005         222,419         142,738          48,776              81
Kansas..........................         148,416         199,006          80,267          16,054              89
Minnesota.......................         148,899         156,468          85,254          34,219              77
Montana.........................          57,517          42,085          26,869          12,379              78
Nebraska........................          86,894         121,785          77,898          40,964              52
New Mexico......................         164,631          47,671          23,651          15,529              90
North Dakota....................         275,138         159,221         199,322         152,505              44
Oklahoma........................         145,862         169,464         136,348          73,006              50
South Dakota....................          41,120          28,579          16,202           2,642              93
Utah............................          58,040          52,998          29,776          15,226              73
Wyoming.........................         141,439         122,453          91,022          57,313              59
----------------------------------------------------------------------------------------------------------------

A. Colorado

1. State's Analysis
    Colorado conducted a weight of evidence analysis to examine whether 
SO2 emissions from Colorado adversely affect attainment or 
maintenance of the 2010 SO2 NAAQS in downwind states. 
Colorado evaluated potential air quality impacts on areas outside the 
State through an assessment of whether SO2 emissions from 
sources located within 50 km of Colorado's borders may have associated 
interstate transport impacts. Colorado's analysis included 
SO2 emissions information in the State, with specific focus 
on sources and counties located within 50 km of Colorado's borders. 
Among these sources, Colorado provided an in-depth analysis of the two 
sources emitting over 100 tons per year (tpy) of SO2; the 
Nucla Generating Station (47 km east of Utah border) and Rawhide Energy 
Station (15 km south of Wyoming border). Colorado also reviewed 
meteorological conditions at SO2 sources within 50 km of the 
State's border, and the distances from identified SO2 
sources in Colorado to the nearest area that is not attaining the NAAQS 
or may have trouble maintaining the NAAQS in another state. Finally, 
Colorado reviewed mobile source emissions data from highway and off-
highway vehicles in all of the Colorado counties which border other 
states. Based on this weight of evidence analysis, Colorado concluded 
that emissions within the State will not contribute to nonattainment or 
interfere with maintenance of the 2010 SO2 NAAQS in 
neighboring states.

[[Page 25619]]

2. EPA's Prong 1 Evaluation
    The EPA proposes to find that Colorado's SIP meets the interstate 
transport requirements of CAA section 110(a)(2)(D)(i)(I), prong 1 for 
the 2010 SO2 NAAQS, as discussed below. We have analyzed the 
air quality, emission sources and emission trends in Colorado and 
neighboring states, i.e., Arizona, Kansas, Nebraska, New Mexico, 
Oklahoma, Utah and Wyoming. Based on that analysis, we propose to find 
that Colorado will not significantly contribute to nonattainment of the 
2010 SO2 NAAQS in any other state.
    We reviewed 2014-2016 SO2 design value concentrations at 
monitors with data sufficient to produce valid 1-hour SO2 
design values for Colorado and neighboring states.\5\ In Table 2, 
below, we have included monitoring data from four scenarios: (1) All of 
the monitor data from Colorado; (2) the monitor with the highest 
SO2 level in each neighboring state; (3) the monitor in each 
neighboring state located closest to the Colorado border; and (4) all 
monitors in each neighboring state within 50 km of the border.
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    \5\ Data retrieved from EPA's https://www.epa.gov/air-trends/air-quality-design-values#report.

                         Table 2--SO2 Monitor Values in Colorado and Neighboring States
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                                                                                    Distance to      2014-2016
                   State/area                        Scenario         Site ID        Colorado      Design value
                                                                                   border (km) *     (ppb) \6\
----------------------------------------------------------------------------------------------------------------
Arizona/Miami...................................               3       040070009             432             146
Arizona/Hayden..................................               2       040071001             470             280
Colorado/Denver.................................               1       080013001             127              18
Colorado/Denver.................................               1       080310002             138              12
Colorado/Denver.................................               1       080310026             135              14
Colorado/Colorado Springs.......................               1       080410015             203              52
Kansas/Trego County.............................               3       201950001             198               5
Kansas/Kansas City..............................               2       202090021             640              34
Nebraska/Omaha..................................               2       310550053             515              59
Nebraska/Omaha..................................               3       310550019             676              27
New Mexico/Fruitland............................               4       350450009              28               3
New Mexico/Waterflow............................         2, 3, 4       350451005              22               8
Oklahoma/Muskogee...............................               2       401010167             618              44
Oklahoma/Oklahoma City..........................               3       401091037             437               3
Wyoming/Cheyenne................................            3, 4       560210100              20               9
Wyoming/Casper..................................               2       560252601             206              25
----------------------------------------------------------------------------------------------------------------
* All distances throughout this notice are approximations.

    The EPA reviewed ambient air quality data in Colorado and 
neighboring states to see whether there were any monitoring sites, 
particularly near the Colorado border, with elevated SO2 
concentrations that might warrant further investigation with respect to 
interstate transport of SO2 from emission sources near any 
given monitor. As shown, there are no violating design values in 
Colorado or neighboring states apart from in the Hayden, Arizona and 
Miami, Arizona areas. In Colorado's analysis, the state reviewed its 
potential impact on the Hayden and Miami, Arizona 2010 SO2 
nonattainment areas, which are the only areas designated nonattainment 
in states bordering Colorado. Colorado noted the significant distance 
between its border and these nonattainment areas, as well as the larger 
distance between the nonattainment areas to the nearest major 
SO2 source in Colorado (Nucla Generating Station--582 km).
---------------------------------------------------------------------------

    \6\ Id.
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    The data presented in Table 2, above, show that Colorado's network 
of SO2 monitors with data sufficient to produce valid 1-hour 
SO2 design values indicates that monitored 1-hour 
SO2 levels in Colorado are between 16% and 69% of the 75 ppb 
level of the NAAQS. As shown, there are no Colorado monitors located 
within 50 km of a neighboring state's border. Three monitors in 
neighboring states are located within 50 km of the Colorado border, and 
these monitors recorded SO2 design values ranging between 4% 
and 12% of the 2010 SO2 NAAQS. Thus, these air quality data 
do not, by themselves, indicate any particular location that would 
warrant further investigation with respect to SO2 emission 
sources that might significantly contribute to nonattainment in the 
neighboring states. However, because the monitoring network is not 
necessarily designed to find all locations of high SO2 
concentrations, this observation indicates an absence of evidence of 
impact at these locations but is not sufficient evidence by itself of 
an absence of impact at all locations in the neighboring states. We 
have therefore also conducted a source-oriented analysis.
    As noted, the EPA finds that it is appropriate to examine the 
impacts of emissions from stationary sources in Colorado in distances 
ranging from 0 km to 50 km from the facility, based on the ``urban 
scale'' definition contained in Appendix D to 40 CFR part 58, Section 
4.4. Colorado assessed point sources up to 50 km from state borders to 
evaluate trends and SO2 concentrations in area-wide air 
quality. The list of sources of 100 tpy \7\ or more of SO2 
within 50 km from state borders, provided by Colorado, is shown in 
Table 3 below.
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    \7\ Colorado limited its analysis to Colorado sources of 
SO2 emitting at least 100 tpy. We agree with Colorado's 
choice to limit its analysis in this way, because in the absence of 
special factors, for example the presence of a nearby larger source 
or unusual physical factors, Colorado sources emitting less than 100 
tpy can appropriately be presumed to not be causing or contributing 
to SO2 concentrations above the NAAQS.

[[Page 25620]]

                              Table 3--Colorado SO2 Sources Near Neighboring States
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                                                                                                    Neighboring
                                           2016 SO2       Distance to      Distance to nearest     state source
            Colorado source                emissions       Colorado       neighboring state SO2   2016 emissions
                                           (tons) *       border (km)          source (km)            (tons)
----------------------------------------------------------------------------------------------------------------
Nucla Generating Station..............             439              47  68 (Lisbon Natural Gas               499
                                                                         Processing Plant--San
                                                                         Juan County, Utah).
Rawhide Energy Station................             878              15  35 (Frontier Petroleum               311
                                                                         Refinery--Cheyenne,
                                                                         Wyoming).
----------------------------------------------------------------------------------------------------------------
* Emissions data throughout this document were obtained using EPA's Emissions Inventory System (EIS) Gateway.

    Table 3 shows the distance from the sources listed therein to the 
nearest out-of-state source emitting above 100 tpy of SO2, 
because elevated levels of SO2, to which SO2 
emitted in Colorado may have a downwind impact, are most likely to be 
found near such sources. In the case of the Nucla Generating Station, 
the distance between this source and the Colorado-Utah state border (47 
km) and the nearest major SO2 source in neighboring state 
Utah (68 km), indicate that emissions from Colorado are very unlikely 
to contribute significantly to problems with attainment of the 2010 
SO2 NAAQS in Utah. The EPA notes that Colorado recently 
revised the Nucla Generating Station NOX reasonable progress 
determination in its regional haze SIP to require the source to shut 
down before December 31, 2022, and the EPA has proposed approval of 
this SIP revision. See 83 FR 18244 (April 26, 2018).
    With regard to the Rawhide Energy Station, because it is located 
within 50 km of the Frontier Petroleum Refinery in Cheyenne, Wyoming, 
the EPA has assessed potential SO2 impacts from the Rawhide 
Energy Station on the Cheyenne area. First, the EPA reviewed available 
monitoring data in Cheyenne, Wyoming, 6 km northeast of the Frontier 
Petroleum Refinery. The 2014-2016 SO2 design value for this 
monitor (Site ID 560210100--See Table 2) was 9 ppb. The maximum 1-hour 
SO2 value measured at this monitor from January 1, 2011, 
(when it began operation) to December 31, 2017, was 31 ppb. A second 
monitor not listed in Table 2, located 3 km east of the Frontier 
Petroleum Refinery, recorded 1 year of data in Cheyenne to examine 
potential population exposure near the refinery.\8\ Between March 31, 
2016, and April 3, 2017, this monitor recorded a maximum 1-hour 
SO2 concentration of 44 ppb, with a fourth highest 1-hour 
daily maximum concentration of 16.7 ppb. All of these monitoring data 
combined indicate that SO2 levels in Cheyenne, Wyoming, and 
therefore near the Frontier Petroleum Refinery, are not likely to 
exceed the 2010 SO2 NAAQS or come near the level of a NAAQS 
exceedance.
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    \8\ See Wyoming's 2016 Annual Monitoring Network Plan at pages 
50-51: http://deq.wyoming.gov/aqd/monitoring/resources/annual-network-plans/.
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    The EPA also reviewed the location of sources in neighboring states 
emitting more than 100 tpy of SO2 and located within 50 km 
of the Colorado border (see Table 4). This is because elevated levels 
of SO2, to which SO2 emitted in Colorado may have 
a downwind impact, are most likely to be found near such sources. As 
shown in Table 4, the shortest distance between any pair of these 
sources is 84 km. Given the localized range of potential 1-hour 
SO2 impacts, this indicates that there are no additional 
locations (apart from Cheyenne) in neighboring states that would 
warrant further investigation with respect to Colorado SO2 
emission sources that might contribute to problems with attainment of 
the 2010 SO2 NAAQS. The Hayden and Miami, Arizona 2010 
SO2 nonattainment areas, which Colorado reviewed as part of 
its analysis, are over 400 km from the nearest Colorado border and so 
were not included in Table 4. Colorado asserted that the significant 
distance between its border and these nonattainment areas indicates 
that it is highly unlikely that SO2 emissions generated in 
Colorado are contributing significantly to either nonattainment area in 
Arizona, and the EPA agrees with this conclusion.

                              Table 4--Neighboring State SO2 Sources Near Colorado*
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                                                                                                     Colorado
                                           2016 SO2       Distance to      Distance to nearest      source 2016
                Source                     emissions       Colorado     Colorado SO2 source (km)     emissions
                                            (tons)        border (km)                                 (tons)
----------------------------------------------------------------------------------------------------------------
San Juan Generating Station                      2,913              22  160 (Nucla Generating                439
 (Waterflow, New Mexico).                                                Station--Nucla,
                                                                         Colorado).
Four Corners Steam Electric Station              4,412              34  172 (Nucla Generating                439
 (Navajo Nation).                                                        Station--Nucla,
                                                                         Colorado).
Bonanza Power Plant (Uintah and Ouray            1,305              20  84 (Meeker Gas Plant--               210
 Reservation).                                                           Rio Blanco County,
                                                                         Colorado).
Resolute Natural Resources Company--               118              19  124 (Nucla Generating                439
 Aneth Unit (Navajo Nation).                                             Station--Nucla,
                                                                         Colorado).
Clean Harbors Env. Services (Kimball               218              17  104 (Pawnee Generating             1,493
 County, Nebraska).                                                      Station--Fort Morgan,
                                                                         Colorado).
----------------------------------------------------------------------------------------------------------------
* We have not included sources that are duplicative of those in Table 3.

    In conclusion, for interstate transport prong 1, we reviewed 
ambient SO2 monitoring data and SO2 emission 
sources both within Colorado and in neighboring states. Based on this 
analysis, we propose to determine that

[[Page 25621]]

Colorado will not significantly contribute to nonattainment of the 2010 
SO2 NAAQS in any other state, per the requirements of CAA 
section 110(a)(2)(D)(i)(I).
3. EPA's Prong 2 Evaluation
    In its prong 2 analysis, Colorado reviewed potential SO2 
impacts on the Billings, Montana area, which is currently in 
``maintenance'' status for the 2010 SO2 NAAQS, noting the 
large distance between the nearest Colorado border and the Billings 
area (520 km). The EPA interprets CAA section 110(a)(2)(D)(i)(I) prong 
2 to require an evaluation of the potential impact of a state's 
emissions on areas that are currently measuring clean data, but that 
may have issues maintaining that air quality, rather than only former 
nonattainment, and thus current maintenance, areas. Therefore, in 
addition to the analysis presented by Colorado, the EPA has also 
reviewed additional information on SO2 air quality and 
emission trends to evaluate the State's conclusion that Colorado will 
not interfere with maintenance of the 2010 SO2 NAAQS in 
downwind states. This evaluation builds on the analysis regarding 
significant contribution to nonattainment (prong 1). Specifically, 
because of the low monitored ambient concentrations of SO2 
in Colorado and neighboring states, and the large distances between 
cross-state SO2 sources, the EPA is proposing to find that 
SO2 levels in neighboring states near the Colorado border do 
not indicate any inability to maintain the SO2 NAAQS that 
could be attributed in part to sources in Colorado.
    As shown in Table 1, the statewide SO2 emissions from 
Colorado and neighboring states have decreased substantially over time, 
per our review of the EPA's emissions trends data.\9\ From 2000 to 
2016, total statewide SO2 emissions decreased by the 
following proportions: Arizona (68% decrease), Colorado (82% decrease), 
Kansas (89% decrease), Nebraska (52% decrease), New Mexico (90% 
decrease), Utah (73% decrease) and Wyoming (59% decrease). This trend 
of decreasing SO2 emissions does not by itself demonstrate 
that areas in Colorado and neighboring states will not have issues 
maintaining the 2010 SO2 NAAQS. However, as a piece of this 
weight of evidence analysis for prong 2, it provides further indication 
(when considered alongside low monitor values in neighboring states) 
that such maintenance issues are unlikely. This is because the 
geographic scope of these reductions and their large sizes strongly 
suggest that they are not transient effects from reversible causes, and 
thus these reductions suggest that there is very low likelihood that a 
strong upward trend in emissions will occur that might cause areas 
presently in attainment to violate the NAAQS.
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    \9\ Additional emissions trends data are available at: https://www.epa.gov/air-emissions-inventories/air-pollutant-emissions-trends-data.
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    As noted in Colorado's submission, any future large sources of 
SO2 emissions will be addressed by Colorado's SIP-approved 
Prevention of Significant Deterioration (PSD) program.\10\ Future minor 
sources of SO2 emissions will be addressed by Colorado's 
SIP-approved minor new source review permit program.\11\ The permitting 
regulations contained within these programs should help ensure that 
ambient concentrations of SO2 in neighboring states are not 
exceeded as a result of new facility construction or modification 
occurring in Colorado.
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    \10\ See EPA's final action of the PSD portions of Colorado's 
SIP, at 82 FR 39030, August 17, 2017.
    \11\ Id.
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    In conclusion, for interstate transport prong 2, we reviewed 
additional information about emission trends, as well as the technical 
information considered for interstate transport prong 1. We find that 
the combination of low ambient concentrations of SO2 in 
Colorado and neighboring states, the large distances between cross-
state SO2 sources, the downward trend in SO2 
emissions from Colorado and neighboring states, and state measures that 
prevent new facility construction or modification in Colorado from 
causing SO2 exceedances in downwind states, indicates no 
interference with maintenance of the 2010 SO2 NAAQS from 
Colorado. Accordingly, we propose to determine that Colorado 
SO2 emission sources will not interfere with maintenance of 
the 2010 SO2 NAAQS in any other state, per the requirements 
of CAA section 110(a)(2)(D)(i)(I).

B. Montana

1. State's Analysis
    Montana relied on existing programs to assert that SO2 
emissions from Montana will not adversely affect attainment or 
maintenance of the 2010 SO2 NAAQS in downwind states. 
Montana noted that sources within the State are subject to new source 
review and Montana Air Quality Permit (MAQP) requirements, as well as 
applicable Maximum Achievable Control Technology (MACT) and New Source 
Performance Standards (NSPS), and asserted that these requirements 
along with additional portions of Montana's SIP prevent sources within 
the State from contributing to nonattainment or interfering with 
maintenance of the 2010 SO2 NAAQS in neighboring states.
2. EPA's Prong 1 Evaluation
    The EPA proposes to find that Montana's SIP meets the interstate 
transport requirements of CAA section 110(a)(2)(D)(i)(I), prong 1 for 
the 2010 SO2 NAAQS, as discussed below. We have analyzed the 
air quality, emission sources and emission trends in Montana and 
neighboring states, i.e., Idaho, North Dakota, South Dakota and 
Wyoming. Based on that analysis, we propose to find that Montana will 
not significantly contribute to nonattainment of the 2010 
SO2 NAAQS in any other state.
    We reviewed 2014-2016 SO2 design value concentrations at 
monitors with data sufficient to produce valid 1-hour SO2 
design values for Montana and neighboring states.\12\ In Table 5, 
below, we have included monitoring data from four scenarios: (1) All of 
the monitor data from Montana; (2) the monitor with the highest 
SO2 level in each neighboring state; (3) the monitor in each 
neighboring state located closest to the Montana border; and (4) all 
monitors in each neighboring state within 50 km of the border.
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    \12\ Data retrieved from EPA's https://www.epa.gov/air-trends/air-quality-design-values#report.

                          Table 5--SO2 Monitor Values in Montana and Neighboring States
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                                                                                    Distance to      2014-2016
                   State/area                        Scenario         Site ID     Montana border   design value
                                                                                       (km)            (ppb)
----------------------------------------------------------------------------------------------------------------
Idaho/Pocatello.................................            2, 3       160050004             162              39
Montana/Helena..................................               1       300490004             178               2
Montana/Richland County.........................               1       300830001              33               7

[[Page 25622]]

 
Montana/Billings................................               1       301110066              87              53
North Dakota/Dickinson..........................               4       380070002              50               5
North Dakota/Burke County.......................               2       380130004             120              23
North Dakota/McKenzie County....................               4       380530104               5               6
North Dakota/McKenzie County....................               4       380530111               2               7
South Dakota/Sioux Falls........................               2       460990008             608               6
South Dakota/Rapid City.........................               3       461030020             118               4
Wyoming/Gillette................................               3       560050857              80              21
Wyoming/Casper..................................               2       560252601             236              25
----------------------------------------------------------------------------------------------------------------

    The EPA reviewed ambient air quality data in Montana and 
neighboring states to see whether there were any monitoring sites, 
particularly near the Montana border, with elevated SO2 
concentrations that might warrant further investigation with respect to 
interstate transport of SO2 from emission sources near any 
given monitor. The data presented in Table 5, above, show that 
Montana's network of SO2 monitors with data sufficient to 
produce valid 1-hour SO2 design values indicates that 
monitored 1-hour SO2 levels in Montana are between 2% and 
70% of the 75 ppb level of the NAAQS. There is one Montana monitor 
located within 50 km of a neighboring state's border, and this monitor 
indicates a design value at 9% of the NAAQS. Three monitors in 
neighboring states are located within 50 km of the Montana border, and 
these monitors recorded SO2 design values ranging between 6% 
and 9% of the 2010 SO2 NAAQS. Thus, these air quality data 
do not, by themselves, indicate any particular location that would 
warrant further investigation with respect to SO2 emission 
sources that might significantly contribute to nonattainment in the 
neighboring states. However, because the monitoring network is not 
necessarily designed to find all locations of high SO2 
concentrations, this observation indicates an absence of evidence of 
impact at these locations but is not sufficient evidence by itself of 
an absence of impact at all locations in the neighboring states. We 
have therefore also conducted a source-oriented analysis.
    As noted, the EPA finds that it is appropriate to examine the 
impacts of emissions from stationary sources in Montana in distances 
ranging from 0 km to 50 km from the facility, based on the ``urban 
scale'' definition contained in Appendix D to 40 CFR part 58, Section 
4.4. Therefore, we assessed point sources up to 50 km from state 
borders to evaluate trends and SO2 concentrations in area-
wide air quality, and determined that there are no such sources in 
Montana. The CHS Laurel Refinery, located 74 km north of the Wyoming 
border, is the Montana point source closest to another state's border. 
The large distances between Montana sources and the nearest neighboring 
state provide further evidence to support a conclusion that emissions 
from Montana will not contribute to problems with attainment of the 
2010 SO2 NAAQS in downwind states.
    The EPA also reviewed the location of sources in neighboring states 
emitting more than 100 tpy \13\ of SO2 and located within 50 
km of the Montana border (see Table 6). This is because elevated levels 
of SO2, to which SO2 emitted in Montana may have 
a downwind impact, are most likely to be found near such sources. As 
shown in Table 6, the shortest distance between any pair of these 
sources is 75 km. This indicates that there are no locations in 
neighboring states that would warrant further investigation with 
respect to Montana SO2 emission sources that might 
contribute to problems with attainment of the 2010 SO2 
NAAQS.
---------------------------------------------------------------------------

    \13\ We have limited our analysis to Montana sources of 
SO2 emitting at least 100 tpy, because in the absence of 
special factors, for example the presence of a nearby larger source 
or unusual physical factors, Montana sources emitting less than 100 
tpy can appropriately be presumed to not be causing or contributing 
to SO2 concentrations above the NAAQS.

                               Table 6--Neighboring State SO2 Sources Near Montana
----------------------------------------------------------------------------------------------------------------
                                           2016 SO2       Distance to      Distance to nearest    Montana source
                Source                     emissions    Montana border   Montana SO2 source (km)  2016 emissions
                                            (tons)           (km)                                     (tons)
----------------------------------------------------------------------------------------------------------------
Colony East and West Plants (Crook                 106              15  223 (Colstrip Station--            1,335
 County, Wyoming).                                                       Colstrip, Montana).
Elk Basin Gas Plant (Park County,                  641               2  75 (CHS Laurel Refinery--            272
 Wyoming).                                                               Laurel, Montana).
----------------------------------------------------------------------------------------------------------------

    In conclusion, for interstate transport prong 1, we reviewed 
ambient SO2 monitoring data and SO2 emission 
sources within Montana and in neighboring states. Based on this 
analysis, we propose to determine that Montana will not significantly 
contribute to nonattainment of the 2010 SO2 NAAQS in any 
other state, per the requirements of CAA section 110(a)(2)(D)(i)(I).
3. EPA's Prong 2 Evaluation
    The EPA has reviewed available information on SO2 air 
quality and emission trends to evaluate the state's conclusion that 
Montana will not interfere with maintenance of the 2010 SO2 
NAAQS in downwind states. The EPA notes that Montana's analysis does 
not independently address whether the SIP contains adequate provisions 
prohibiting emissions that will interfere with maintenance of the 2010 
SO2

[[Page 25623]]

NAAQS in any other state. In remanding the Clean Air Interstate Rule 
(CAIR) to the EPA in North Carolina v. EPA, the D.C. Circuit explained 
that the regulating authority must give the ``interfere with 
maintenance'' clause of section 110(a)(2)(D)(i)(I) ``independent 
significance'' by evaluating the impact of upwind state emissions on 
downwind areas that, while currently in attainment, are at risk of 
future nonattainment, considering historic variability.\14\ While 
Montana did not evaluate the potential impact of its emissions on areas 
that are currently measuring clean data, but that may have issues 
maintaining that air quality, the EPA has incorporated additional 
information into our evaluation of Montana's submission. This 
evaluation builds on the analysis regarding significant contribution to 
nonattainment (prong 1). Specifically, because of the low monitored 
ambient concentrations of SO2 in Montana and neighboring 
states, and the large distances between cross-state SO2 
sources, the EPA is proposing to find that SO2 levels in 
neighboring states near the Montana border do not indicate any 
inability to maintain the SO2 NAAQS that could be attributed 
in part to sources in Montana.
---------------------------------------------------------------------------

    \14\ 531 F.3d 896, 910-11 (D.C. Cir. 2008) (holding that the EPA 
must give ``independent significance'' to each prong of CAA section 
110(a)(2)(D)(i)(I)).
---------------------------------------------------------------------------

    As shown in Table 1, the statewide SO2 emissions from 
Montana and neighboring states have decreased substantially over time, 
per our review of the EPA's emissions trends data.\15\ From 2000 to 
2016, total statewide SO2 emissions decreased by the 
following proportions: Idaho (70% decrease), Montana (78% decrease), 
North Dakota (44% decrease), South Dakota (93% decrease) and Wyoming 
(59% decrease). This trend of decreasing SO2 emissions does 
not by itself demonstrate that areas in Montana and neighboring states 
will not have issues maintaining the 2010 SO2 NAAQS. 
However, as a piece of this weight of evidence analysis for prong 2, it 
provides further indication (when considered alongside low monitor 
values in neighboring states) that such maintenance issues are 
unlikely. This is because the geographic scope of these reductions and 
their large sizes strongly suggest that they are not transient effects 
from reversible causes, and thus these reductions suggest that there is 
very low likelihood that a strong upward trend in emissions will occur 
that might cause areas presently in attainment to violate the NAAQS.
---------------------------------------------------------------------------

    \15\ Additional emissions trends data are available at: https://www.epa.gov/air-emissions-inventories/air-pollutant-emissions-trends-data.
---------------------------------------------------------------------------

    As noted in Montana's submission, any future large sources of 
SO2 emissions will be addressed by Montana's SIP-approved 
PSD program.\16\ Future minor sources of SO2 emissions will 
be addressed by Montana's SIP-approved minor new source review permit 
program.\17\ The permitting regulations contained within these programs 
should help ensure that ambient concentrations of SO2 in 
neighboring states are not exceeded as a result of new facility 
construction or modification occurring in Montana.
---------------------------------------------------------------------------

    \16\ See EPA's final action of the PSD portions of Montana's 
SIP, at 81 FR 23180, April 20, 2016.
    \17\ Id.
---------------------------------------------------------------------------

    In conclusion, for interstate transport prong 2, the EPA has 
incorporated additional information into our evaluation of Montana's 
submission, which did not include an independent analysis of prong 2. 
In doing so, we have reviewed information about emission trends, as 
well as the technical information considered for our interstate 
transport prong 1 analysis. We find that the combination of low ambient 
concentrations of SO2 in Montana and neighboring states, the 
large distances between cross-state SO2 sources, the 
downward trend in SO2 emissions from Montana and surrounding 
states, and state measures that prevent new facility construction or 
modification in Montana from causing SO2 exceedances in 
downwind states, indicates no interference with maintenance of the 2010 
SO2 NAAQS from Montana. Accordingly, we propose to determine 
that Montana SO2 emission sources will not interfere with 
maintenance of the 2010 SO2 NAAQS in any other state, per 
the requirements of CAA section 110(a)(2)(D)(i)(I).

C. North Dakota

1. State's Analysis
    North Dakota conducted a weight of evidence analysis to examine 
whether SO2 emissions from North Dakota adversely affect 
attainment or maintenance of the 2010 SO2 NAAQS in downwind 
states. North Dakota cited the large distance between the State's 
SO2 sources and the nearest SO2 nonattainment and 
maintenance areas in downwind states, as well as the very low 
SO2 values at intervening monitors. North Dakota also noted 
that SO2 emissions within the State have been steadily 
decreasing over time, specifically noting a 35% point-source emissions 
decrease between 2002 and 2011. With regard to the interference with 
maintenance requirement, North Dakota discussed the low monitored 
ambient concentrations of SO2 in neighboring states in the 
period up to and including 2011. Based on this weight of evidence 
analysis, North Dakota concluded that emissions within the State will 
not contribute to nonattainment or interfere with maintenance of the 
2010 SO2 NAAQS in neighboring states.
2. EPA's Prong 1 Evaluation
    The EPA proposes to find that North Dakota's SIP meets the 
interstate transport requirements of CAA section 110(a)(2)(D)(i)(I), 
prong 1 for the 2010 SO2 NAAQS, as discussed below. We have 
analyzed the air quality, emission sources, and emission trends in 
North Dakota and neighboring states, i.e., Minnesota, Montana and South 
Dakota. Based on that analysis, we propose to find that North Dakota 
will not significantly contribute to nonattainment of the 2010 
SO2 NAAQS in any other state.
    To date, the only area in a state bordering North Dakota that has 
been designated nonattainment for the 2010 SO2 NAAQS is 
Billings, Montana. The EPA designated the portion of Billings 
surrounding the PPL Corette Power Plant based on a 2009-2011 monitored 
design value, concluding that this source was the key contributor to 
the NAAQS violations during that period. See 78 FR 47191 (August 5, 
2013). Following the permanent closure of the PPL Corette Plant in 
March 2015, which was accompanied by a significant decrease in 
monitored SO2 values (which indicated attainment) in the 
nonattainment area, the EPA redesignated the former Billings 2010 
SO2 nonattainment area to attainment. See 81 FR 28718 (May 
10, 2016). As shown in Table 7, below, the Billings, Montana area is 
located a large distance (343 km) from the North Dakota border, and 
recent monitoring data in the Billings area do not approach the 2010 
SO2 NAAQS. For these reasons, the EPA is proposing to find 
that emissions from North Dakota will not contribute significantly to 
nonattainment in the Billings, Montana area.
    As noted, North Dakota also referred to ambient monitor values in 
its transport analysis. We reviewed these, as well as the more recent 
2014-2016 SO2 design value concentrations at monitors with 
data sufficient to produce valid 1-hour SO2 design values 
for North Dakota and neighboring states.\18\ In Table 7, below, we have 
included

[[Page 25624]]

monitoring data from four scenarios: (1) All of the monitor data from 
North Dakota; (2) the monitor with the highest SO2 level in 
each neighboring state; (3) the monitor in each neighboring state 
located closest to the North Dakota border; and (4) all monitors in 
each neighboring state within 50 km of the border.
---------------------------------------------------------------------------

    \18\ Data retrieved from EPA's https://www.epa.gov/air-trends/air-quality-design-values#report.

                       Table 7--SO2 Monitor Values in North Dakota and Neighboring States
----------------------------------------------------------------------------------------------------------------
                                                                                    Distance to      2014-2016
                   State/Area                        Scenario         Site ID      North Dakota    Design value
                                                                                    border (km)      (ppb)\19\
----------------------------------------------------------------------------------------------------------------
Minnesota/Minneapolis-St. Paul..................               2       270370020             306              12
Minnesota/Minneapolis-St. Paul..................               3       270530954             278               5
Montana/Richland County.........................            3, 4       300830001              33               7
Montana/Billings................................               2       301110066             343              53
North Dakota/Dickinson..........................               1       380070002              50               5
North Dakota/Burke County.......................               1       380130004             121              23
North Dakota/Bismarck...........................               1       380150003              99              15
North Dakota/Fargo..............................               1       380171004               4               2
North Dakota/Dunn County........................               1       380250003             115               5
North Dakota/McKenzie County....................               1       380530002              55               6
North Dakota/McKenzie County....................               1       380530104               5               6
North Dakota/McKenzie County....................               1       380530111               2               7
North Dakota/Mercer County......................               1       380570004             150              22
North Dakota/Mercer County......................               1       380570118             159              22
North Dakota/Mercer County......................               1       380570124             160              16
North Dakota/Oliver County......................               1       380650002             139              10
South Dakota/Sioux Falls........................               2       460990008             265               6
South Dakota/Rapid City.........................               3       461030020             205               4
----------------------------------------------------------------------------------------------------------------

    The EPA reviewed ambient air quality data in North Dakota and 
neighboring states to see whether there were any monitoring sites, 
particularly near the North Dakota border, with elevated SO2 
concentrations that might warrant further investigation with respect to 
interstate transport of SO2 from emission sources near any 
given monitor. The data presented in Table 7, above, show that North 
Dakota's network of SO2 monitors with data sufficient to 
produce valid 1-hour SO2 design values indicates that 
monitored 1-hour SO2 levels in North Dakota are between 2% 
and 31% of the 75 ppb level of the NAAQS. There are four North Dakota 
monitors located within 50 km of a neighboring state's border, and 
these monitors indicate design values between 2% to 9% of the NAAQS. 
Two SO2 monitors have recently been installed in North 
Dakota to assist the state and the EPA in designating portions of North 
Dakota by 2020.\20\ These are source oriented monitors, and both the 
monitors and the source they are characterizing (the Tioga Gas Plant) 
are located over 80 km from the North Dakota border. There is one 
monitor in a neighboring state located within 50 km of the North Dakota 
border, and this monitor recorded an SO2 design value of 9% 
of the 2010 SO2 NAAQS. Thus, these air quality data do not, 
by themselves, indicate any particular location that would warrant 
further investigation with respect to SO2 emission sources 
that might significantly contribute to nonattainment in the neighboring 
states. However, because the monitoring network is not necessarily 
designed to find all locations of high SO2 concentrations, 
this observation indicates an absence of evidence of impact at these 
locations but is not sufficient evidence by itself of an absence of 
impact at all locations in the neighboring states. We have therefore 
also conducted a source-oriented analysis.
---------------------------------------------------------------------------

    \19\ Id.
    \20\ See TSD: Final Round 3 Area Designations for the 2010 1-
Hour SO2 Primary National Ambient Air Quality Standard 
for North Dakota, in http://www.regulations.gov, document ID EPA-HQ-
OAR-2017-0003-0600.
---------------------------------------------------------------------------

    As noted, the EPA finds that it is appropriate to examine the 
impacts of emissions from stationary sources in North Dakota in 
distances ranging from 0 km to 50 km from the facility, based on the 
``urban scale'' definition contained in Appendix D to 40 CFR part 58, 
Section 4.4. Therefore, we assessed North Dakota sources of 100 tpy 
\21\ or more of SO2 up to 50 km from neighboring state 
borders to evaluate trends and SO2 concentrations in area-
wide air quality in Table 8 below.
---------------------------------------------------------------------------

    \21\ We have limited our analysis to North Dakota sources of 
SO2 emitting at least 100 tpy, because in the absence of 
special factors, for example the presence of a nearby larger source 
or unusual physical factors, North Dakota sources emitting less than 
100 tpy can appropriately be presumed to not be causing or 
contributing to SO2 concentrations above the NAAQS.

                            Table 8--North Dakota SO2 Sources Near Neighboring States
----------------------------------------------------------------------------------------------------------------
                                                                                                    Neighboring
                                           2016 SO2       Distance to      Distance to nearest     state source
          North Dakota source              emissions     North Dakota     neighboring state SO2   2016 emissions
                                            (tons)        border (km)          source (km)            (tons)
----------------------------------------------------------------------------------------------------------------
Drayton Sugar Mill....................             330               2  75 (American Crystal               1,005
                                                                         Sugar--East Grand
                                                                         Forks, Minnesota).
Hillsboro Sugar Mill..................             439              15  49 (American Crystal                 875
                                                                         Sugar--Crookston,
                                                                         Minnesota).

[[Page 25625]]

 
University of North Dakota Heating                 411               2  4 (American Crystal                1,005
 Plant (Grand Forks).                                                    Sugar--East Grand
                                                                         Forks, Minnesota).
North Dakota State University Heating              123               2  4.5 km (American Crystal             373
 Plant (Fargo).                                                          Sugar--Moorhead,
                                                                         Minnesota).
Wahpeton Sugar Mill...................             227               1  44 km (Hoot Lake Plant--             940
                                                                         Fergus Falls,
                                                                         Minnesota).
Wahpeton Wet Corn Mill................             135               1  47 km (Hoot Lake Plant--             940
                                                                         Fergus Falls,
                                                                         Minnesota).
----------------------------------------------------------------------------------------------------------------

    As shown, there are six North Dakota sources within 50 kilometers 
of a cross-state source, and each neighboring state source is located 
in the State of Minnesota. The EPA has therefore assessed potential 
SO2 impacts from North Dakota on each of the four Minnesota 
areas with SO2 sources near the North Dakota border, 
specifically the Crookston, East Grand Forks, Moorhead and Fergus 
Falls, Minnesota areas.
    With regard to the Grand Forks, North Dakota, and East Grand Forks, 
Minnesota combined metropolitan area, the EPA does not have monitoring 
or modeling data to indicate transport from Grand Forks, North Dakota, 
to East Grand Forks, Minnesota. On the contrary, wind roses for three 
local meteorological stations indicate prevailing winds to be north-
south oriented as opposed to west-east that would be conducive to 
interstate transport.\22\ On this basis, the EPA is proposing to 
determine that emissions from Grand Forks, North Dakota, will not 
contribute significantly to nonattainment in East Grand Forks, 
Minnesota.\23\
---------------------------------------------------------------------------

    \22\ This wind rose data are available in a memo to the docket 
for this action, which can be found on http://www.regulations.gov.
    \23\ The EPA is aware that the University of North Dakota has 
announced plans to replace its heating plant, though this change is 
not yet federally enforceable (See http://news.prairiepublic.org/post/und-replace-its-steam-plant-wont-be-asking-state-appropriation). The EPA also notes that any changes to the current 
facility and construction of a new facility must go through the 
state's EPA-approved New Source Review program.
---------------------------------------------------------------------------

    With regard to the Crookston, Minnesota area, the EPA finds the 
distance between the Hillsboro Sugar Mill and Crookston (49 km) makes 
it very unlikely that SO2 emissions from the Hillsboro Sugar 
Mill could interact with SO2 emissions from Crookston 
American Crystal Sugar in such a way as to contribute significantly to 
nonattainment in the Crookston area.
    With regard to the Moorhead, Minnesota, and Fargo, North Dakota, 
combined metropolitan area, the EPA reviewed available monitoring data. 
There is one SO2 monitor (Site ID 380171004--See Table 7) in 
the area, on the North Dakota side of the border, located 6.5 km 
northwest of the North Dakota State University Heating Plant, and 9.5 
km northwest of the Moorhead American Crystal Sugar Mill. As shown, 
this monitor recorded a design value of 2 ppb from 2014-2016. Although 
this monitor is not sited to determine maximum impacts from either the 
Moorhead American Crystal Sugar Mill or the North Dakota State 
University Heating Plant, it does indicate that SO2 levels 
are very low (2.6% of the NAAQS) in parts of the Fargo-Moorhead 
combined metropolitan area. Additionally, wind roses for a local 
meteorological station indicates prevailing winds to be north-south 
oriented as opposed to west-east that would be conducive to interstate 
transport.\24\ For these reasons, in addition to the relatively low 
level of SO2 emissions from the North Dakota State 
University Heating Plant, the EPA is proposing to determine that 
emissions from the North Dakota State University Heating Plant will not 
contribute significantly to nonattainment in Moorhead, Minnesota.
---------------------------------------------------------------------------

    \24\ This wind rose data are available in a memo to the docket 
for this action, which can be found on http://www.regulations.gov.
---------------------------------------------------------------------------

    Finally, with regard to the Fergus Falls, Minnesota area, air 
quality modeling submitted to the EPA by the State of Minnesota for the 
Hoot Lake Plant indicates that the highest predicted 99th percentile 
daily maximum 1-hour concentration within the modeling domain is 55.8 
ppb.\25\ For this reason, the Fergus Falls area does not warrant 
further investigation with regard to potential significant contribution 
to nonattainment from North Dakota. Additionally, in our analysis of 
Minnesota's modeling in the context of designations for the 2010 
SO2 NAAQS, the EPA noted that the Wahpeton facilities' 
``modeled impact at that distance to the Hoot Lake area would be 
minimal and it's expected their impact would be represented by the 
background concentration.'' \26\ The EPA continues to support this 
conclusion with respect to an interstate transport analysis for section 
110(a)(2)(D)(i)(I).\27\
---------------------------------------------------------------------------

    \25\ See TSD: Intended Round 3 Area Designations for the 2010 1-
Hour SO2 Primary National Ambient Air Quality Standard 
for Minnesota, in http://www.regulations.gov, document ID EPA-HQ-
OAR-2017-0003-0057. This information was not changed for the final 
version of the designation, as shown at document ID EPA-HQ-OAR-2017-
0003-0618.
    \26\ Id.
    \27\ While the air quality modeling discussed here used by the 
EPA to support its final designation of the Fergus Falls area is 
also supportive of the Agency's analysis of North Dakota's 2010 
SO2 transport SIP, the designation itself or the use of 
this modeling in the specific context of that designation is not 
being re-opened through this separate proposed action.
---------------------------------------------------------------------------

    In conclusion, for interstate transport prong 1, we reviewed 
ambient SO2 monitoring data and SO2 emission 
sources both within North Dakota and in neighboring states. Based on 
this analysis, we propose to determine that North Dakota will not 
significantly contribute to nonattainment of the 2010 SO2 
NAAQS in any other state, per the requirements of CAA section 
110(a)(2)(D)(i)(I).
3. EPA's Prong 2 Evaluation
    In its prong 2 analysis, North Dakota reviewed potential 
SO2 impacts on the Minneapolis-St. Paul, Minnesota area, 
which is currently in ``maintenance'' status for the 1971 
SO2 NAAQS, noting the large distance between the North 
Dakota border and the Minneapolis-St. Paul area (255 km), as well as 
NAAQS-attaining monitoring data in eastern North Dakota and in 
Minneapolis-St. Paul. The EPA interprets CAA section 110(a)(2)(D)(i)(I) 
prong 2 to require an evaluation of the potential impact of a state's 
emissions on areas that are currently measuring clean data, but that

[[Page 25626]]

may have issues maintaining that air quality, rather than only former 
nonattainment, and thus current maintenance, areas. North Dakota also 
performed a prong 2 analysis based on the EPA's interpretation, noting 
that monitors located near North Dakota in neighboring states showed 
very low levels of SO2, indicating they should not be 
considered to have maintenance issues for this NAAQS. The EPA has 
reviewed North Dakota's analysis and other available information on 
SO2 air quality and emission trends to evaluate the State's 
conclusion that North Dakota will not interfere with maintenance of the 
2010 SO2 NAAQS in downwind states. This evaluation builds on 
the analysis regarding significant contribution to nonattainment (prong 
1). Specifically, because of the low monitored ambient concentrations 
of SO2 in North Dakota and neighboring states and our 
conclusions from our qualitative analysis of the identified sources of 
SO2 emissions, the EPA is proposing to find that 
SO2 levels in neighboring states near the North Dakota 
border do not indicate any inability to maintain the SO2 
NAAQS that could be attributed in part to sources in North Dakota.
    As shown in Table 1, the statewide SO2 emissions from 
North Dakota and neighboring states have decreased substantially over 
time, per our review of the EPA's emissions trends data.\28\ From 2000 
to 2016, total statewide SO2 emissions decreased by the 
following proportions: Minnesota (77% decrease), Montana (78% 
decrease), North Dakota (44% decrease) and South Dakota (93% decrease). 
This trend of decreasing SO2 emissions does not by itself 
demonstrate that areas in North Dakota and neighboring states will not 
have issues maintaining the 2010 SO2 NAAQS. However, as a 
piece of this weight of evidence analysis for prong 2, it provides 
further indication (when considered alongside low monitor values in 
neighboring states) that such maintenance issues are unlikely. This is 
because the geographic scope of these reductions and their large sizes 
strongly suggest that they are not transient effects from reversible 
causes, and thus these reductions suggest that there is very low 
likelihood that a strong upward trend in emissions will occur that 
might cause areas presently in attainment to violate the NAAQS.
---------------------------------------------------------------------------

    \28\ Additional emissions trends data are available at: https://www.epa.gov/air-emissions-inventories/air-pollutant-emissions-trends-data.
---------------------------------------------------------------------------

    As noted in North Dakota's submission, any future large sources of 
SO2 emissions will be addressed by North Dakota's SIP-
approved PSD program.\29\ Future minor sources of SO2 
emissions will be addressed by North Dakota's SIP-approved minor new 
source review permit program.\30\ The permitting regulations contained 
within these programs should help ensure that ambient concentrations of 
SO2 in neighboring states are not exceeded as a result of 
new facility construction or modification occurring in North Dakota.
---------------------------------------------------------------------------

    \29\ See EPA's final action of the PSD portions of North 
Dakota's SIP, at 82 FR 46681, October 6, 2017.
    \30\ Id.
---------------------------------------------------------------------------

    In conclusion, for interstate transport prong 2, we reviewed 
additional information about emission trends, as well as the technical 
information considered for interstate transport prong 1. We find that 
the combination of low ambient concentrations of SO2 in 
North Dakota and neighboring states, our conclusions from our 
qualitative analysis of the identified sources of SO2 
emissions, the downward trend in SO2 emissions from North 
Dakota and surrounding states, and state measures that prevent new 
facility construction or modification in North Dakota from causing 
SO2 exceedances in downwind states, indicates no 
interference with maintenance of the 2010 SO2 NAAQS from 
North Dakota. Accordingly, we propose to determine that North Dakota 
SO2 emission sources will not interfere with maintenance of 
the 2010 SO2 NAAQS in any other state, per the requirements 
of CAA section 110(a)(2)(D)(i)(I).

D. South Dakota

1. State's Analysis
    South Dakota conducted a weight of evidence analysis to examine 
whether SO2 emissions from South Dakota adversely affect 
attainment or maintenance of the 2010 SO2 NAAQS in downwind 
states. South Dakota provided an inventory of each SO2 
source located in a county that borders another state, including the 
emissions for each source. South Dakota provided information on 
SO2 reductions for the larger SO2 sources in this 
inventory, noting that the State's largest SO2 emissions 
source (Big Stone I) installed pollution controls between 2012 and 2015 
to reduce SO2 emissions at the facility by 80%. South Dakota 
also discussed how the State's second highest emitter (Ben French 
facility) shut down in 2012, and that the combination of reductions 
from these two facilities would result in a 75% reduction in 
SO2 emissions throughout South Dakota from 2011 to 2016. 
South Dakota noted the large distance between the State and the nearest 
nonattainment areas in downwind states. South Dakota also considered 
the predominant northwesterly wind direction in the State, asserting 
that this made it very unlikely that South Dakota sources could impact 
SO2 nonattainment in states to its west. Finally, South 
Dakota noted that its permitting programs would prevent new or modified 
sources from impacting nonattainment and maintenance areas in downwind 
states going forward. Based on this weight of evidence analysis, South 
Dakota concluded that emissions within the State will not contribute to 
nonattainment or interfere with maintenance of the 2010 SO2 
NAAQS in neighboring states.
2. EPA's Prong 1 Evaluation
    The EPA proposes to find that South Dakota's SIP meets the 
interstate transport requirements of CAA section 110(a)(2)(D)(i)(I), 
prong 1 for the 2010 SO2 NAAQS, as discussed below. We have 
analyzed the air quality, emission sources and emission trends in South 
Dakota and neighboring states, i.e., Iowa, Minnesota, Montana, 
Nebraska, North Dakota and Wyoming. Based on that analysis, we propose 
to find that South Dakota will not significantly contribute to 
nonattainment of the 2010 SO2 NAAQS in any other state.
    We reviewed 2014-2016 SO2 design value concentrations at 
monitors with data sufficient to produce valid 1-hour SO2 
design values for South Dakota and neighboring states.\31\ In Table 9, 
below, we have included monitoring data from four scenarios: (1) All of 
the monitor data from South Dakota; (2) the monitor with the highest 
SO2 level in each neighboring state; (3) the monitor in each 
neighboring state located closest to the South Dakota border; and (4) 
all monitors in each neighboring state within 50 km of the border.
---------------------------------------------------------------------------

    \31\ Data retrieved from EPA's https://www.epa.gov/air-trends/air-quality-design-values#report.

[[Page 25627]]

                       Table 9--SO2 Monitor Values in South Dakota and Neighboring States
----------------------------------------------------------------------------------------------------------------
                                                                                  Distance  from     2014-2016
                   State/Area                        Scenario         Site ID      South  Dakota   Design value
                                                                                   border  (km)     (ppb) \32\
----------------------------------------------------------------------------------------------------------------
Iowa/Muscatine..................................               2       191390020             462             113
Iowa/Sioux City.................................            3, 4       191930020              19               9
Minnesota/Minneapolis-St. Paul..................               2       270370020             270              12
Minnesota/Minneapolis-St. Paul..................               3       270530954             250               5
Montana/Richland County.........................               3       300830001             210               7
Montana/Billings................................               2       301110066             343              53
Nebraska/Omaha..................................               2       310550053             136              59
Nebraska/Omaha..................................               3       310550019             676              27
North Dakota/Burke County.......................               2       380130004             300              23
North Dakota/Bismarck...........................               3       380150003              99              15
South Dakota/Jackson County.....................               1       460710001              83               3
South Dakota/Sioux Falls........................               1       460990008              10               6
South Dakota/Rapid City.........................               1       461030020              62               4
South Dakota/Sioux City.........................               1       461270001               6               4
Wyoming/Casper..................................               2       560252601             178              25
Wyoming/Weston County...........................            3, 4       560450800              12               3
----------------------------------------------------------------------------------------------------------------

    The EPA reviewed ambient air quality data in South Dakota and 
neighboring states to determine whether there were any monitoring 
sites, particularly near the South Dakota border, with elevated 
SO2 concentrations that might warrant further investigation 
with respect to interstate transport of SO2 from emission 
sources near any given monitor. As shown, there are no violating design 
values in South Dakota or neighboring states apart from the Muscatine, 
Iowa area. In South Dakota's analysis, the State reviewed its potential 
impact on the Muscatine, Iowa 2010 SO2 nonattainment area. 
South Dakota asserted that the significant distance between its nearest 
border and the Muscatine area (shown in Table 9), as well as the low 
emissions in southeastern South Dakota indicated no SO2 
impacts to the Muscatine SO2 nonattainment area. The EPA 
agrees with South Dakota's analysis and conclusion with regard to the 
Muscatine, Iowa area. The EPA notes that during the 2014-2016 period, 
substantial reductions in SO2 emissions occurred within the 
Muscatine SO2 nonattainment area.\33\ For this reason, the 
last exceedance of the 2010 SO2 NAAQS at the violating 
monitor listed in Table 9 (site ID 191390020) occurred in June 
2015.\34\
---------------------------------------------------------------------------

    \32\ Id.
    \33\ See TSD: Final Round 3 Area Designations for the 2010 1-
Hour SO2 Primary National Ambient Air Quality Standard 
for Iowa, in http://www.regulations.gov, document ID EPA-HQ-OAR-
2017-0003-0616.
    \34\ Data retrieved from EPA's https://www.epa.gov/outdoor-air-quality-data.
---------------------------------------------------------------------------

    South Dakota also analyzed potential impacts to the Billings, 
Montana area, which was still in nonattainment status at the time of 
South Dakota's submission. As noted in the section of this notice about 
North Dakota, the EPA redesignated the former Billings 2010 
SO2 nonattainment area to attainment following the permanent 
closure of the PPL Corette Plant. See 81 FR 28718 (May 10, 2016). As 
noted by South Dakota, the Billings, Montana area is located a very 
large distance (343 km) from the nearest South Dakota border, and is 
upwind rather than downwind of South Dakota. Table 9 also shows that 
recent monitoring data in the Billings area do not approach the 2010 
SO2 NAAQS. For these reasons, the EPA agrees with South 
Dakota's conclusion that the emissions from South Dakota will not 
contribute significantly to nonattainment in the Billings, Montana 
area.
    The data presented in Table 9, above, show that South Dakota's 
network of SO2 monitors with data sufficient to produce 
valid 1-hour SO2 design values indicates that monitored 1-
hour SO2 levels in South Dakota are between 4% and 8% of the 
75 ppb level of the NAAQS. There are two South Dakota monitors located 
within 50 km of a neighboring state's border, and these monitors 
indicate design values between 5% and 8% of the NAAQS. There are two 
monitors in neighboring states located within 50 km of the South Dakota 
border, and these monitors recorded SO2 design values 
between 4% and 12% of the 2010 SO2 NAAQS. Thus, these air 
quality data do not, by themselves, indicate any particular location 
that would warrant further investigation with respect to SO2 
emission sources that might significantly contribute to nonattainment 
in the neighboring states. However, because the monitoring network is 
not necessarily designed to find all locations of high SO2 
concentrations, this observation indicates an absence of evidence of 
impact at these locations but is not sufficient evidence by itself of 
an absence of impact at all locations in the neighboring states. We 
have therefore also conducted a source-oriented analysis.
    As noted, the EPA finds that it is appropriate to examine the 
impacts of emissions from stationary sources in South Dakota in 
distances ranging from 0 km to 50 km from the facility, based on the 
``urban scale'' definition contained in Appendix D to 40 CFR part 58, 
Section 4.4. Therefore, we assessed point sources up to 50 km from 
state borders to evaluate trends and SO2 concentrations in 
area-wide air quality. The list of such sources with greater than 100 
tpy \35\ of SO2 within 50 km from state borders is provided 
in Table 10, below.
---------------------------------------------------------------------------

    \35\ We have limited our analysis to South Dakota sources of 
SO2 emitting at least 100 tpy, because in the absence of 
special factors, for example the presence of a nearby larger source 
or unusual physical factors, South Dakota sources emitting less than 
100 tpy can appropriately be presumed to not be causing or 
contributing to SO2 concentrations above the NAAQS.

[[Page 25628]]

                               Table 10--SO2 Sources Near the South Dakota Border
----------------------------------------------------------------------------------------------------------------
                                                                                                    Cross-state
                                           2016 SO2       Distance to      Distance to nearest      source 2016
                Source                     emissions     South Dakota    cross-State SO2 source      emissions
                                            (tons)        border (km)             (km)                (tons)
----------------------------------------------------------------------------------------------------------------
Big Stone Power Plant (Grant County,               827               4  113 (Wahpeton Sugar                  227
 South Dakota).                                                          Mill--Richland County,
                                                                         North Dakota).
Colony East and West Plant (Crook                  106               8  111 (GCC Dacotah--Rapid              304
 County, Wyoming).                                                       City, South Dakota).
----------------------------------------------------------------------------------------------------------------

    With regard to potential cross-state impacts from the Big Stone 
Power Plant, air quality modeling submitted to the EPA by South Dakota 
indicates that the highest predicted 99th percentile daily maximum 1-
hour concentration within the modeling domain surrounding the power 
plant is 57.88 ppb.\36\ This predicted maximum concentration, which 
includes an estimate of the background concentration, indicates that 
this source alone could not cause nonattainment in South Dakota or any 
other state. Together with the distance between Big Stone and the 
nearest cross-state source (113 km), this indicates that the Big Stone 
Power Plant will not significantly contribute to nonattainment in any 
other state. The EPA continues to support this conclusion with respect 
to an interstate transport analysis for section 110(a)(2)(D)(i)(I).\37\
---------------------------------------------------------------------------

    \36\ See TSD: Final Area Designations for the 2010 
SO2 Primary National Ambient Air Quality Standard for 
South Dakota, in http://www.regulations.gov, document ID EPA-HQ-OAR-
2014-0464-0359.
    \37\ While the air quality modeling discussed here used by the 
EPA to support its final designation of the Grant County, South 
Dakota area is also supportive of the Agency's analysis of South 
Dakota's 2010 SO2 transport SIP, the designation itself 
or the use of this modeling in the specific context of that 
designation is not being re-opened through this separate proposed 
action.
---------------------------------------------------------------------------

    The EPA also reviewed the location of sources in neighboring states 
emitting more than 100 tpy of SO2 and located within 50 km 
of the South Dakota border. This is because elevated levels of 
SO2, to which SO2 emitted in South Dakota may 
have a downwind impact, are most likely to be found near such sources. 
As shown in Table 10, the only source within this distance of the South 
Dakota border is the Colony East and West Plant. The shortest distance 
between this source and the nearest source in South Dakota, the GCC 
Dacotah facility, is 111 km. This makes it very unlikely that 
SO2 emissions from the GCC Dacotah facility could interact 
with SO2 emissions from the Colony East and West Plants in 
such a way as to contribute significantly to nonattainment in the Crook 
County, Wyoming area.
    In conclusion, for interstate transport prong 1, we reviewed 
ambient SO2 monitoring data and SO2 emission 
sources within South Dakota and in neighboring states. Based on this 
analysis, we propose to determine that South Dakota will not 
significantly contribute to nonattainment of the 2010 SO2 
NAAQS in any other state, per the requirements of CAA section 
110(a)(2)(D)(i)(I).
3. EPA's Prong 2 Evaluation
    The EPA has reviewed available information on SO2 air 
quality and emission trends to evaluate the state's conclusion that 
South Dakota will not interfere with maintenance of the 2010 
SO2 NAAQS in downwind states. The EPA notes that South 
Dakota's analysis does not independently address whether the SIP 
contains adequate provisions prohibiting emissions that will interfere 
with maintenance of the 2010 SO2 NAAQS in any other state. 
As noted, the ``interfere with maintenance'' clause of section 
110(a)(2)(D)(i)(I) must be given ``independent significance'' by 
evaluating the impact of upwind state emissions on downwind areas that, 
while currently in attainment, are at risk of future nonattainment, 
considering historic variability.\38\ While South Dakota did not 
evaluate the potential impact of its emissions on areas that are 
currently measuring clean data, but that may have issues maintaining 
that air quality, the EPA has incorporated additional information into 
our evaluation of South Dakota's submission. This evaluation builds on 
the analysis regarding significant contribution to nonattainment (prong 
1). Specifically, because of the low monitored ambient concentrations 
of SO2 in South Dakota and neighboring states, and the large 
distances between cross-state SO2 sources, the EPA is 
proposing to find that SO2 levels in neighboring states near 
the South Dakota border do not indicate any inability to maintain the 
SO2 NAAQS that could be attributed in part to sources in 
South Dakota.
---------------------------------------------------------------------------

    \38\ 531 F.3d 896, 910-11 (DC Cir. 2008) (holding that the EPA 
must give ``independent significance'' to each prong of CAA section 
110(a)(2)(D)(i)(I)).
---------------------------------------------------------------------------

    As shown in Table 1, the statewide SO2 emissions from 
South Dakota and neighboring states have decreased substantially over 
time, per our review of the EPA's emissions trends data.\39\ From 2000 
to 2016, total statewide SO2 emissions decreased by the 
following proportions: Iowa (81% decrease), Minnesota (77% decrease), 
Montana (78% decrease), Nebraska (52% decrease), North Dakota (44% 
decrease), South Dakota (93% decrease) and Wyoming (59% decrease). This 
trend of decreasing SO2 emissions does not by itself 
demonstrate that areas in South Dakota and neighboring states will not 
have issues maintaining the 2010 SO2 NAAQS. However, as a 
piece of this weight of evidence analysis for prong 2, it provides 
further indication (when considered alongside low monitor values in 
neighboring states) that such maintenance issues are unlikely. This is 
because the geographic scope of these reductions and their large sizes 
strongly suggest that they are not transient effects from reversible 
causes, and thus these reductions suggest that there is very low 
likelihood that a strong upward trend in emissions will occur that 
might cause areas presently in attainment to violate the NAAQS.
---------------------------------------------------------------------------

    \39\ Additional emissions trends data are available at: https://www.epa.gov/air-emissions-inventories/air-pollutant-emissions-trends-data.
---------------------------------------------------------------------------

    As noted in South Dakota's submission, any future large sources of 
SO2 emissions will be addressed by South Dakota's SIP-
approved PSD program.\40\ Future minor sources of SO2 
emissions will be addressed by South Dakota's SIP-approved minor new 
source review permit program.\41\ The permitting regulations contained 
within these programs should help ensure that ambient concentrations of 
SO2 in neighboring states are not exceeded as a

[[Page 25629]]

result of new facility construction or modification occurring in South 
Dakota.
---------------------------------------------------------------------------

    \40\ See EPA's final action of the PSD portions of South 
Dakota's SIP, at 82 FR 38832, August 16, 2017.
    \41\ Id.
---------------------------------------------------------------------------

    In conclusion, for interstate transport prong 2, the EPA has 
incorporated additional information into our evaluation of South 
Dakota's submission, which did not include an independent analysis of 
prong 2. In doing so, we have reviewed additional information about 
emission trends, as well as the technical information considered for 
interstate transport prong 1. We find that the combination of low 
ambient concentrations of SO2 in South Dakota and 
neighboring states, the large distances between cross-state 
SO2 sources, the downward trend in SO2 emissions 
from South Dakota and surrounding states, and state measures that 
prevent new facility construction or modification in South Dakota from 
causing SO2 exceedances in downwind states, indicates no 
interference with maintenance of the 2010 SO2 NAAQS from 
South Dakota. Accordingly, we propose to determine that South Dakota 
SO2 emission sources will not interfere with maintenance of 
the 2010 SO2 NAAQS in any other state, per the requirements 
of CAA section 110(a)(2)(D)(i)(I).

E. Wyoming

1. State's Analysis
    Wyoming conducted a weight of evidence analysis to examine whether 
SO2 emissions from Wyoming adversely affect attainment or 
maintenance of the 2010 SO2 NAAQS in downwind states. 
Wyoming primarily reviewed the potential impact of emissions from 
Wyoming on the Billings, Montana 2010 SO2 maintenance area, 
which was designated as nonattainment at the time of Wyoming's 
submittal, because Montana was the only state bordering Wyoming that 
contained a nonattainment or maintenance area for this NAAQS. Wyoming 
reviewed wind rose data from northeast Wyoming, the location in Wyoming 
with the nearest significant SO2 sources to the Billings 
area. Based on a review of this information, Wyoming concluded that 
winds in northeast Wyoming were predominantly from the north and west, 
and therefore made transport to Billings very unlikely. Wyoming also 
asserted that SO2 sources within Wyoming were all located 
much further than 50 km from the Billings area. Finally, Wyoming noted 
that no neighboring state apart from Montana contained a 2010 
SO2 nonattainment area. Based on this weight of evidence 
analysis, Wyoming concluded that emissions within the State will not 
contribute to nonattainment or interfere with maintenance of the 2010 
SO2 NAAQS in neighboring states.
2. EPA's Prong 1 Evaluation
    The EPA proposes to find that Wyoming's SIP meets the interstate 
transport requirements of CAA section 110(a)(2)(D)(i)(I), prong 1 for 
the 2010 SO2 NAAQS, as discussed below. We have analyzed the 
air quality, emission sources and emission trends in Wyoming and 
neighboring states, i.e., Colorado, Idaho, Montana, Nebraska, South 
Dakota and Utah.\42\ Based on that analysis, we propose to find that 
Wyoming will not significantly contribute to nonattainment of the 2010 
SO2 NAAQS in any other state.
---------------------------------------------------------------------------

    \42\ The EPA also analyzed potential Wyoming SO2 
transport to the Wind River Reservation in Wyoming. The Northern 
Arapaho and Eastern Shoshone Tribes have been approved by the EPA 
for treatment in a similar manner as a state (TAS) status for CAA 
Section 126 (78 FR 76829, December 19, 2013). The Tribes' TAS 
application for Section 126 demonstrates an interest in how their 
air quality is impacted by Wyoming sources outside of the 
Reservation. We determined that the only source above 100 tpy of 
SO2 within 50 km of the Wind River Reservation, the Lost 
Cabin Gas Plant, is located over 40 km downwind (see wind rose data 
in the docket for this action) from the Reservation. The area around 
this source contains a source-oriented monitor (Site ID 560130003) 
indicating a fourth highest 1-hour daily maximum below the 2010 
SO2 NAAQS in its first year of operation. Therefore, the 
available information indicates that emissions from Wyoming will not 
contribute significantly to nonattainment or interfere with 
maintenance of the 2010 SO2 NAAQS at the Wind River 
Reservation.
---------------------------------------------------------------------------

    Wyoming focused its analysis on potential impacts to the Billings, 
Montana area, which was still in nonattainment status at the time of 
Wyoming's submission. As noted, the EPA redesignated the former 
Billings 2010 SO2 nonattainment area to attainment following 
the permanent closure of the PPL Corette Plant. See 81 FR 28718 (May 
10, 2016). As asserted by Wyoming and shown in Table 11, the Billings, 
Montana area is located a large distance (87 km) from the Wyoming 
border. Further, the wind roses provided by Wyoming indicate that 
meteorology does not favor transport from Wyoming sources to the 
Billings area. Table 11 also shows that recent monitoring data in the 
Billings area do not approach the 2010 SO2 NAAQS. For these 
reasons, the EPA agrees with Wyoming's conclusion that emissions from 
Wyoming will not contribute significantly to nonattainment in the 
Billings, Montana area.
    We reviewed 2014-2016 SO2 design value concentrations at 
monitors with data sufficient to produce valid 1-hour SO2 
design values for Wyoming and neighboring states.\43\ In Table 11, 
below, we have included monitoring data from four scenarios: (1) All of 
the monitor data from Wyoming; (2) the monitor with the highest 
SO2 level in each neighboring state; (3) the monitor in each 
neighboring state located closest to the Wyoming border; and (4) all 
monitors in each neighboring state within 50 km of the Wyoming border.
---------------------------------------------------------------------------

    \43\ Data retrieved from EPA's https://www.epa.gov/air-trends/air-quality-design-values#report.

                         Table 11--SO2 Monitor Values in Wyoming and Neighboring States
----------------------------------------------------------------------------------------------------------------
                                                                                    Distance to      2014-2016
                   State/Area                        Scenario         Site ID         Wyoming      Design value
                                                                                   border  (km)     (ppb) \44\
----------------------------------------------------------------------------------------------------------------
Colorado/Denver.................................               3       080013001             127              18
Colorado/Colorado Springs.......................               2       080410015             240              52
Idaho/Pocatello.................................               2       160050004             120              39
Idaho/Caribou County............................            3, 4       160290031              45              26
Montana/Billings................................            2, 3       301110066              87              53
Nebraska/Omaha..................................               3       310550019             676              27
Nebraska/Omaha..................................               2       310550053             679              59
South Dakota/Sioux Falls........................               2       460990008             593               6
South Dakota/Rapid City.........................               3       461030020              62               4
Wyoming/Gillette................................               1       560050857              80              21
Wyoming/Cheyenne................................               1       560210100              20               9
Wyoming/Casper..................................               1       560252601             178              25

[[Page 25630]]

 
Wyoming/Rock Springs............................               1       560370300              83              21
Wyoming/Weston County...........................               1       560450800              12               3
----------------------------------------------------------------------------------------------------------------

    The EPA reviewed ambient air quality data in Wyoming and 
neighboring states to see whether there were any monitoring sites, 
particularly near the Wyoming border, with elevated SO2 
concentrations that might warrant further investigation with respect to 
interstate transport of SO2 from emission sources near any 
given monitor. The data presented in Table 11, above, show that 
Wyoming's network of SO2 monitors with data sufficient to 
produce valid 1-hour SO2 design values indicates that 
monitored 1-hour SO2 levels in Wyoming are between 4% and 
33% of the 75 ppb level of the NAAQS. There are two Wyoming monitors 
located within 50 km of the state's border, and these monitors indicate 
design values between 4% and 12% of the NAAQS. Seven SO2 
monitors have recently been installed in Wyoming to assist the State 
and the EPA in designating portions of Wyoming by 2020.\45\ These are 
source oriented monitors, and none of these monitors or the sources 
they are characterizing are located within 50 km of the Wyoming border. 
There is one monitor in a neighboring state located within 50 km of the 
Wyoming border, and this monitor recorded an SO2 design 
value of 35% of the 2010 SO2 NAAQS. Thus, these air quality 
data do not, by themselves, indicate any particular location that would 
warrant further investigation with respect to SO2 emission 
sources that might significantly contribute to nonattainment in the 
neighboring states. However, because the monitoring network is not 
necessarily designed to find all locations of high SO2 
concentrations, this observation indicates an absence of evidence of 
impact at these locations but is not sufficient evidence by itself of 
an absence of impact at all locations in the neighboring states. We 
have therefore also conducted a source-oriented analysis.
---------------------------------------------------------------------------

    \44\ Id.
    \45\ See TSD: Final Round 3 Area Designations for the 2010 1-
Hour SO2 Primary National Ambient Air Quality Standard 
for Wyoming, in http://www.regulations.gov, document ID EPA-HQ-OAR-
2017-0003-0608.
---------------------------------------------------------------------------

    As noted, the EPA finds that it is appropriate to examine the 
impacts of emissions from stationary sources in Wyoming in distances 
ranging from 0 km to 50 km from the facility, based on the ``urban 
scale'' definition contained in Appendix D to 40 CFR part 58, Section 
4.4. Therefore, we assessed point sources up to 50 km from state 
borders to evaluate trends and SO2 concentrations in area-
wide air quality. The list of sources of greater than 100 tpy \46\ of 
SO2 within 50 km from state borders is provided in Table 12 
below.
---------------------------------------------------------------------------

    \46\ We have limited our analysis to Wyoming sources of 
SO2 emitting at least 100 tpy, because in the absence of 
special factors, for example the presence of a nearby larger source 
or unusual physical factors, Wyoming sources emitting less than 100 
tpy can appropriately be presumed to not be causing or contributing 
to SO2 concentrations above the NAAQS.

                              Table 12--Wyoming SO2 Sources Near Neighboring States
----------------------------------------------------------------------------------------------------------------
                                          2016 annual                                               Neighboring
                                         SO2 emissions    Distance to      Distance to nearest     state source
            Wyoming source                  (tons)      Wyoming border    neighboring state SO2   2016 emissions
                                                             (km)              source (km)            (tons)
----------------------------------------------------------------------------------------------------------------
Carter Creek Gas Plant................             130              11  76 (Devils Slide Plant,              187
                                                                         Holcim--Morgan County,
                                                                         Utah).
Frontier Petroleum Refinery...........             311              14  35 (Rawhide Energy                   879
                                                                         Station--Larimer
                                                                         County, Colorado).
Naughton Power Plant..................         4,069.7              37  110 (Devils Slide Plant,             187
                                                                         Holcim--Morgan County,
                                                                         Utah).
Laramie Cement Plant..................             165              30  67 (Rawhide Energy                   879
                                                                         Station, Larimer
                                                                         County, Colorado).
Colony East and West Plants...........             106               8  111 km (GCC Dacotah--                304
                                                                         Rapid City, South
                                                                         Dakota).
Elk Basin Gas Plant...................             641               2  75 km (CHS Laurel                    272
                                                                         Refinery--Laurel,
                                                                         Montana).
----------------------------------------------------------------------------------------------------------------

    With regard to the Frontier Petroleum Refinery in Cheyenne, the EPA 
has assessed potential SO2 impacts from this source on the 
area near the Rawhide Energy Station, in Larimer County, Colorado.
    The EPA reviewed available monitoring data in Cheyenne, Wyoming. 
One monitor is located 6 km northeast of the Frontier Petroleum 
Refinery (Site ID 560210100--See Table 11), and recorded a 2014-2016 
SO2 design value of 9 ppb. The maximum 1-hour SO2 
value measured at this monitor from January 1, 2011 (when it began 
operation) to December 31, 2017, was 31 ppb. A second monitor not 
listed in Table 11, located 3 km east of the Frontier Petroleum 
Refinery, recorded 1 year of data in Cheyenne to examine potential 
population exposure near the Frontier Petroleum Refinery.\47\ Between 
March 31, 2016 and April 3, 2017, this monitor recorded a maximum 
SO2 concentration of 44 ppb, with a fourth highest 1-hour 
daily maximum concentration of 16.7 ppb. Although

[[Page 25631]]

these monitoring data do not provide information as to the air quality 
near the Rawhide Generating Station, they do indicate that 
SO2 levels are low near the Frontier Petroleum Refinery, and 
decrease even more at 6 km from the source. We anticipate emissions 
will continue to decrease as distance increases, resulting in very 
little SO2 impact from the Frontier Petroleum Refinery at 
the Colorado border (14 km), and even less near the Rawhide Generating 
Station (35 km). This, in combination with the relatively low level of 
emissions from the refinery (See Table 12), leads the EPA to conclude 
that SO2 transport at significant levels between Cheyenne, 
Wyoming and Larimer County, Colorado, is very unlikely.
---------------------------------------------------------------------------

    \47\ See Wyoming's 2016 Annual Monitoring Network Plan at pages 
50-51: http://deq.wyoming.gov/aqd/monitoring/resources/annual-network-plans/.
---------------------------------------------------------------------------

    With regard to the Elk Basin Gas Plant, the EPA does not have 
information at this time suggesting that the State of Montana is 
impacted by emissions from Elk Basin Gas Plant or other emissions 
activity originating in Wyoming in violation of section 
110(a)(2)(D)(i)(I). Therefore, we do not have evidence that 
demonstrates that emissions from this source will significantly 
contribute to nonattainment of the 2010 SO2 NAAQS.
    With regard to potential cross-state impacts from the Naughton 
Power Plant, air quality modeling submitted to the EPA by Wyoming 
indicates that the highest predicted 99th percentile daily maximum 1-
hour concentration within the modeling domain surrounding the power 
plant is 56.3 ppb.\48\ This predicted maximum concentration, which 
includes an estimate of the background concentration, indicates that 
this source alone could not cause nonattainment in Wyoming or any other 
state. Together with the distance between Naughton and the nearest 
cross-state source (110 km), this indicates that the Naughton Power 
Plant will not significantly contribute to nonattainment in any other 
state. The EPA continues to support this conclusion with respect to an 
interstate transport analysis for section 110(a)(2)(D)(i)(I).\49\
---------------------------------------------------------------------------

    \48\ See TSD: Final Round 3 Area Designations for the 2010 1-
Hour SO2 Primary National Ambient Air Quality Standard 
for Wyoming, in http://www.regulations.gov, document ID EPA-HQ-OAR-
2017-0003-0608, and TSD: Intended Round 3 Area Designations for the 
2010 1-Hour SO2 Primary National Ambient Air Quality 
Standard for Wyoming, at EPA-HQ-OAR-2017-0003-0033.
    \49\ While the air quality modeling discussed here used by the 
EPA to support its final designation of the Lincoln County, Wyoming 
area is also supportive of the Agency's analysis of Wyoming's 2010 
SO2 transport SIP, the designation itself or the use of 
this modeling in the specific context of that designation is not 
being re-opened through this separate proposed action.
---------------------------------------------------------------------------

    For the other sources listed in Table 12, the low levels of 
emissions and large distances between Wyoming sources within 50 km of a 
state border and the nearest SO2 source in a neighboring 
state provide further evidence to support a conclusion that emissions 
from Wyoming will not contribute to problems with attainment of the 
2010 SO2 NAAQS in downwind states.

                             Table 13--Neighboring State SO2 Sources Near Wyoming *
----------------------------------------------------------------------------------------------------------------
                                                                                                      Wyoming
                                           2016 SO2       Distance to     Distance to  nearest      source 2016
                Source                     emissions        Wyoming        Wyoming  SO2 source       emissions
                                            (tons)       border  (km)             (km)                (tons)
----------------------------------------------------------------------------------------------------------------
Clean Harbors Env. Services (Kimball               218              33  95 (Frontier Petroleum               311
 County, Nebraska).                                                      Refinery).
P4 Production Chemical Plant (Soda                 478              45  132 (Naughton Generating           4,069
 Springs, Idaho).                                                        Station).
Nu-West Industries Fertilizer Plant                364              40  134 (Naughton Generating           4,069
 (Conda, Idaho).                                                         Station).
----------------------------------------------------------------------------------------------------------------
* We have not included sources that are duplicative of those in Table 12.

    The EPA also reviewed the location of sources in neighboring states 
emitting more than 100 tpy of SO2 and located within 50 km 
of the Wyoming border (see Table 13). This is because elevated levels 
of SO2, to which SO2 emitted in Wyoming may have 
a downwind impact, are most likely to be found near such sources. As 
shown in Table 13, the shortest distance between any pair of these 
sources is within 95 km. This indicates that there are no additional 
locations in neighboring states that would warrant further 
investigation with respect to Wyoming SO2 emission sources 
that might contribute to problems with attainment of the 2010 
SO2 NAAQS.
    In conclusion, for interstate transport prong 1, we reviewed 
ambient SO2 monitoring data and SO2 emission 
sources both within Wyoming and in neighboring states. Based on this 
analysis, we propose to determine that Wyoming will not significantly 
contribute to nonattainment of the 2010 SO2 NAAQS in any 
other state, per the requirements of CAA section 110(a)(2)(D)(i)(I).
3. EPA's Prong 2 Evaluation
    The EPA has reviewed the analysis presented by Wyoming and 
additional information on SO2 air quality and emission 
trends to evaluate the State's conclusion that Wyoming will not 
interfere with maintenance of the 2010 SO2 NAAQS in downwind 
states. The EPA notes that Wyoming's analysis does not independently 
address whether the SIP contains adequate provisions prohibiting 
emissions that will interfere with maintenance of the 2010 
SO2 NAAQS in any other state. As noted, the ``interfere with 
maintenance'' clause of section 110(a)(2)(D)(i)(I) must be given 
``independent significance'' by evaluating the impact of upwind state 
emissions on downwind areas that, while currently in attainment, are at 
risk of future nonattainment, considering historic variability.\50\ 
While Wyoming did not evaluate the potential impact of its emissions on 
areas that are currently measuring clean data, but that may have issues 
maintaining that air quality, the EPA has incorporated additional 
information into our evaluation of Wyoming's submission. This 
evaluation builds on the analysis regarding significant contribution to 
nonattainment (prong 1). Specifically, because of the low monitored 
ambient concentrations of SO2 in Wyoming and neighboring 
states and the large distances between cross-state SO2 
sources, the EPA is proposing to find that SO2 levels in 
neighboring states near the Wyoming border do not indicate an inability 
to maintain the SO2 NAAQS.
---------------------------------------------------------------------------

    \50\ 531 F.3d 896, 910-11 (DC Cir. 2008) (holding that the EPA 
must give ``independent significance'' to each prong of CAA section 
110(a)(2)(D)(i)(I)).

---------------------------------------------------------------------------

[[Page 25632]]

    As shown in Table 1, the statewide SO2 emissions from 
Wyoming and neighboring states have decreased substantially over time, 
per our review of the EPA's emissions trends data.\51\ From 2000 to 
2016, total statewide SO2 emissions decreased by the 
following proportions: Colorado (82% decrease), Idaho (70% decrease), 
Montana (78% decrease), Nebraska (52% decrease), South Dakota (93% 
decrease), Utah (73% decrease) and Wyoming (59% decrease). This trend 
of decreasing SO2 emissions does not by itself demonstrate 
that areas in Wyoming and neighboring states will not have issues 
maintaining the 2010 SO2 NAAQS. However, as a piece of this 
weight of evidence analysis for prong 2, it provides further indication 
(when considered alongside low monitor values in neighboring states) 
that such maintenance issues are unlikely. This is because the 
geographic scope of these reductions and their large sizes strongly 
suggest that they are not transient effects from reversible causes, and 
thus these reductions suggest that there is very low likelihood that a 
strong upward trend in emissions will occur that might cause areas 
presently in attainment to violate the NAAQS.
---------------------------------------------------------------------------

    \51\ Additional emissions trends data are available at: https://www.epa.gov/air-emissions-inventories/air-pollutant-emissions-trends-data.
---------------------------------------------------------------------------

    As noted in Wyoming's submission, any future large sources of 
SO2 emissions will be addressed by Wyoming's SIP-approved 
PSD program.\52\ Future minor sources of SO2 emissions will 
be addressed by Wyoming's SIP-approved minor new source review permit 
program.\53\ The permitting regulations contained within these programs 
should help ensure that ambient concentrations of SO2 in 
neighboring states are not exceeded as a result of new facility 
construction or modification occurring in Wyoming.
---------------------------------------------------------------------------

    \52\ See EPA's final action of the PSD portions of Wyoming's 
SIP, at 82 FR 18992, April 25, 2017.
    \53\ Id.
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    In conclusion, for interstate transport prong 2, the EPA has 
incorporated additional information into our evaluation of Wyoming's 
submission, which did not include an independent analysis of prong 2. 
In doing so, we reviewed information about emission trends, as well as 
the technical information considered for interstate transport prong 1. 
We find that the combination of low ambient concentrations of 
SO2 in Wyoming and neighboring states, the large distances 
between cross-state SO2 sources, the downward trend in 
SO2 emissions from Wyoming and surrounding states, and state 
measures that prevent new facility construction or modification in 
Wyoming from causing SO2 exceedances in downwind states, 
indicates no interference with maintenance of the 2010 SO2 
NAAQS from Wyoming. Accordingly, we propose to determine that Wyoming 
SO2 emission sources will not interfere with maintenance of 
the 2010 SO2 NAAQS in any other state, per the requirements 
of CAA section 110(a)(2)(D)(i)(I).

IV. Proposed Action

    The EPA is proposing to approve the following submittals as meeting 
the interstate transport requirements of CAA section 110(a)(2)(D)(i)(I) 
for the 2010 SO2 NAAQS: Colorado's July 17, 2013 and 
February 16, 2018 submittals; Montana's July 15, 2013 submittal; North 
Dakota's March 7, 2013 submittal; South Dakota's December 20, 2013; and 
Wyoming's March 6, 2015 submittal. The EPA is proposing this approval 
based on our review of the information and analysis provided by each 
state, as well as additional relevant information, which indicates that 
in-state air emissions will not contribute significantly to 
nonattainment or interfere with maintenance of the 2010 SO2 
NAAQS in any other state. This action is being taken under section 110 
of the CAA.

V. Statutory and Executive Order Reviews

    Under the Clean Air Act, the Administrator is required to approve a 
SIP submission that complies with the provisions of the Act and 
applicable federal regulations. 42 U.S.C. 7410(k); 40 CFR 52.02(a). 
Thus, in reviewing SIP submissions, the EPA's role is to approve state 
choices, provided that they meet the criteria of the CAA. Accordingly, 
these proposed actions merely approve state law as meeting federal 
requirements and do not impose additional requirements beyond those 
imposed by state law. For that reason, these proposed actions:
     Are not significant regulatory actions subject to review 
by the Office of Management and Budget under Executive Orders 12866 (58 
FR 51735, October 4, 1993) and 13563 (76 FR 3821, January 21, 2011);
     are not Executive Order 13771 (82 FR 9339, February 2, 
2017) regulatory actions because SIP approvals are exempted under 
Executive Order 12866;
     do not impose an information collection burden under the 
provisions of the Paperwork Reduction Act (44 U.S.C. 3501 et seq.);
     are certified as not having a significant economic impact 
on a substantial number of small entities under the Regulatory 
Flexibility Act (5 U.S.C. 601 et seq.);
     do not contain any unfunded mandate or significantly or 
uniquely affect small governments, as described in the Unfunded 
Mandates Reform Act of 1995 (Pub. L. 104-4);
     do not have federalism implications as specified in 
Executive Order 13132 (64 FR 43255, August 10, 1999);
     are not economically significant regulatory actions based 
on health or safety risks subject to Executive Order 13045 (62 FR 
19885, April 23, 1997);
     are not significant regulatory actions subject to 
Executive Order 13211 (66 FR 28355, May 22, 2001);
     are not subject to requirements of Section 12(d) of the 
National Technology Transfer and Advancement Act of 1995 (15 U.S.C. 272 
note) because this action does not involve technical standards; and
     do not provide the EPA with the discretionary authority to 
address, as appropriate, disproportionate human health or environmental 
effects, using practicable and legally permissible methods, under 
Executive Order 12898 (59 FR 7629, February 16, 1994).
    In addition, these SIPs are not approved to apply on any Indian 
reservation land or in any other area where the EPA or an Indian tribe 
has demonstrated that a tribe has jurisdiction. In those areas of 
Indian country, the rule does not have tribal implications and will not 
impose substantial direct costs on tribal governments or preempt tribal 
law as specified by Executive Order 13175 (65 FR 67249, November 9, 
2000).

List of Subjects in 40 CFR Part 52

    Environmental protection, Air pollution control, Incorporation by 
reference, Intergovernmental relations, Nitrogen dioxide, Particulate 
Matter, Reporting and recordkeeping requirements, Sulfur dioxide, 
Volatile organic compounds.

    Authority: 42 U.S.C. 7401 et seq.

    Dated: May 29, 2018.
Douglas Benevento,
Regional Administrator, Region 8.
[FR Doc. 2018-11846 Filed 6-1-18; 8:45 am]
 BILLING CODE 6560-50-P