[Federal Register Volume 83, Number 107 (Monday, June 4, 2018)]
[Proposed Rules]
[Pages 25617-25632]
[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,<PHONE_NUMBER>, [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.

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
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                                                                                SO2 reduction,
              State                    2000            2005            2010            2016        2000-2016 (%)

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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
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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.

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\
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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
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* 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).
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    \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.

                              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)
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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).
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* 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)
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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).
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* 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

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)
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Idaho/Pocatello.................................            2, 3       160050004             162              39
Montana/Helena..................................               1       300490004             178               2
Montana/Richland County.........................               1       300830001              33               7

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

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

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).

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

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.

                       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.

                               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

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

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

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)).

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

    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.
---------------------------------------------------------------------------

    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