1 

Draft Technical Support Document 

South Dakota 

Area Designations for the 2010 SO2 Primary National Ambient Air Quality Standard 

Summary 

Pursuant to section 107(d) of the Clean Air Act (CAA), the U.S. Environmental Protection 

Agency (EPA, or the Agency) must designate areas as either “unclassifiable,” “attainment,” or 

“nonattainment” for the 2010 one-hour sulfur dioxide (SO2) primary national ambient air quality 

standard (NAAQS). The CAA defines a nonattainment area as one that does not meet the 

NAAQS or that contributes to a violation in a nearby area. An attainment area is defined as any 

area other than a nonattainment area that meets the NAAQS. Unclassifiable areas are defined as 

those that cannot be classified on the basis of available information as meeting or not meeting the 

NAAQS. 

South Dakota submitted updated recommendations on September 16, 2015, ahead of a July 2, 

2016, deadline for the EPA to designate certain areas established by the U.S. District Court for 

the Northern District of California. This deadline is the first of three deadlines established by the 

court for the EPA to complete area designations for the 2010 SO2 NAAQS. Table 1 below lists 

South Dakota’s recommendations and identifies the counties or portions of counties in South 

Dakota that the EPA intends to designate by July 2, 2016 based on an assessment and 

characterization of air quality through ambient air quality data, air dispersion modeling, other 

evidence and supporting information, or a combination of the above. 

Table 1. South Dakota’s Recommended and EPA’s Intended Designations 

Area South Dakota’s 

Recommended 

Area Definition1

South 

Dakota’s 

Recommended 

Designation 

EPA’s Intended 

Area Definition 

EPA’s Intended 

Designation 

Grant County, 

South Dakota 

Grant County, 

South Dakota 

Attainment Grant County, 

South Dakota 

Unclassifiable 

Background 

On June 3, 2010, the EPA revised the primary (health based) SO2 NAAQS by establishing a new 

one-hour standard at a level of 75 parts per billion (ppb) which is attained when the three-year 

average of the 99th percentile of one-hour daily maximum concentrations does not exceed 75 

ppb. This NAAQS was published in the Federal Register on June 22, 2010 (75 FR 35520) and is 

codified at 40 CFR 50.17. The EPA determined this is the level necessary to protect public health 

with an adequate margin of safety, especially for children, the elderly and those with asthma. 

These groups are particularly susceptible to the health effects associated with breathing SO2. The 

two prior primary standards of 140 ppb evaluated over 24 hours, and 30 ppb evaluated over an 

 

1

 South Dakota recommended attainment for every county in the State, but EPA is only addressing the 

recommendation for the recommendation for Grant County at this time. 
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entire year, codified at 40 CFR 50.4, remain applicable.2 However, the EPA is not currently 

designating areas on the basis of either of these two primary standards. Similarly, the secondary 

standard for SO2, set at 500 ppb evaluated over 3 hours has not been revised, and the EPA is also 

not currently designating areas on the basis of the secondary standard. 

General Approach and Schedule 

Section 107(d) of the Clean Air Act requires that not later than one year after promulgation of a 

new or revised NAAQS, state governors must submit their recommendations for designations 

and boundaries to EPA. Section 107(d) also requires the EPA to provide notification to states no 

less than 120 days prior to promulgating an initial area designation that is a modification of a 

state’s recommendation. If a state does not submit designation recommendations, the EPA will 

promulgate the designations that it deems appropriate. If a state or tribe disagrees with the EPA’s 

intended designations, they are given an opportunity within the 120 day period to demonstrate 

why any proposed modification is inappropriate. 

On August 5, 2013, the EPA published a final rule establishing air quality designations for 29 

areas in the United States for the 2010 SO2 NAAQS, based on recorded air quality monitoring 

data from 2009 - 2011 showing violations of the NAAQS (78 FR 47191). In that rulemaking, the 

EPA committed to address, in separate future actions, the designations for all other areas for 

which the Agency was not yet prepared to issue designations. 

Following the initial August 5, 2013 designations, three lawsuits were filed against the EPA in 

different U.S. District Courts, alleging the Agency had failed to perform a nondiscretionary duty 

under the CAA by not designating all portions of the country by the June 2013 deadline. In an 

effort intended to resolve the litigation in one of those cases, plaintiffs Sierra Club and the 

Natural Resources Defense Council and the EPA filed a proposed consent decree with the U.S. 

District Court for the Northern District of California. On March 2, 2015, the court entered the 

consent decree and issued an enforceable order for the EPA to complete the area designations 

according to the court-ordered schedule. 

According to the court-ordered schedule, the EPA must complete the remaining designations by 

three specific deadlines. By no later than July 2, 2016 (16 months from the court’s order), the 

EPA must designate two groups of areas: (1) areas that have newly monitored violations of the 

2010 SO2 NAAQS and (2) areas that contain any stationary sources that had not been announced 

as of March 2, 2015 for retirement and that according to the EPA’s Air Markets Database 

emitted in 2012 either (i) more than 16,000 tons of SO2 or (ii) more than 2,600 tons of SO2 with 

an annual average emission rate of at least 0.45 pounds of SO2 per one million British thermal 

units (lbs SO2/mmBTU). Specifically, a stationary source with a coal-fired unit that as of January 

1, 2010 had a capacity of over 5 megawatts and otherwise meets the emissions criteria, is 

 

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 40 CFR 50.4(e) provides that the two prior primary NAAQS will no longer apply to an area one year after its 

designation under the 2010 NAAQS, except that for areas designated nonattainment under the prior NAAQS as of 

August 22, 2010, and areas not meeting the requirements of a SIP Call under the prior NAAQS, the prior NAAQS 

will apply until that area submits and EPA approves a SIP providing for attainment of the 2010 NAAQS. Grant 

County, South Dakota was neither designated nonattainment under the prior NAAQS as of August 22, 2010, nor 

failed to meet the requirements of a SIP Call under the prior NAAQS, so this exception does not apply. 
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excluded from the July 2, 2016 deadline if it had announced through a company public 

announcement, public utilities commission filing, consent decree, public legal settlement, final 

state or federal permit filing, or other similar means of communication, by March 2, 2015, that it 

will cease burning coal at that unit. 

The last two deadlines for completing remaining designations are December 31, 2017, and 

December 31, 2020. The EPA has separately promulgated requirements for states and other air 

agencies to provide additional monitoring or modeling information on a timetable consistent with 

these designation deadlines. We expect this information to become available in time to help 

inform these subsequent designations. These requirements were promulgated on August 21, 2015 

(80 FR 51052), in a rule known as the SO2 Data Requirements Rule (DRR). 

 

Updated designations guidance was issued by the EPA through a March 20, 2015 memorandum 

from Stephen D. Page, Director, U.S. EPA, Office of Air Quality Planning and Standards, to Air 

Division Directors, U.S. EPA Regions I-X. This memorandum supersedes earlier designation 

guidance for the 2010 SO2 NAAQS, issued on March 24, 2011, and it identifies factors that the 

EPA intends to evaluate in determining whether areas are in violation of the 2010 SO2 NAAQS. 

The guidance also contains the factors the EPA intends to evaluate in determining the boundaries 

for all remaining areas in the country, consistent with the court’s order and schedule. These 

factors include: 1) Air quality characterization via ambient monitoring or dispersion modeling 

results; 2) Emissions-related data; 3) Meteorology; 4) Geography and topography; and 5) 

Jurisdictional boundaries. This guidance was supplemented by two technical assistance 

documents intended to assist states and other interested parties in their efforts to characterize air 

quality through air dispersion modeling or ambient air quality monitoring for sources that emit 

SO2. Notably, the EPA released its most recent versions of documents titled, “SO2 NAAQS 

Designations Modeling Technical Assistance Document” (Modeling TAD) and “SO2 NAAQS 

Designations Source-Oriented Monitoring Technical Assistance Document” (Monitoring TAD) 

in December 2013.

Based on ambient air quality data collected between 2012 and 2014, no violations of the 2010 

SO2 NAAQS have been recorded in any undesignated part of the State.3

 However, there is 1 

source in the State meeting the emissions criteria of the consent decree for which the EPA must 

complete designations by July 2, 2016. In this draft technical support document, the EPA 

discusses its review and technical analysis of South Dakota’s updated recommendation for the 

area that we must designate. The EPA also discusses any intended modifications from the State’s 

recommendation based on all available data before us. 

 

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 For designations based on ambient air quality monitoring data that violates the 2010 SO2 NAAQS, the consent 

decree directs the EPA to evaluate data collected between 2013 and 2015. Absent complete, quality assured and 

certified data for 2015, the analyses of applicable areas for the EPA’s intended designations will be informed by data 

collected between 2012 and 2014. States with monitors that have recorded a violation of the 2010 SO2 NAAQS 

during these years have the option of submitting complete, quality assured and certified data for calendar year 2015 

by April 19, 2016to the EPA for evaluation. If after our review, the ambient air quality data for the area indicates 

that no violation of the NAAQS occurred between 2013 and 2015, the consent decree does not obligate the EPA to 

complete the designation. Instead, we may designate the area and all other previously undesignated areas in the State 

on a schedule consistent with the prescribed timing of the court order, i.e., by December 31, 2017, or December 31, 

2020. 
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The following are definitions of important terms used in this document: 

1) 2010 SO2 NAAQS – The primary NAAQS for SO2 promulgated in 2010. This NAAQS is 

75 ppb, based on the three year average of the 99th percentile of the annual distribution 

of daily maximum one-hour average concentrations. See 40 CFR 50.17. 

2) Design Value - a statistic computed according to the data handling procedures of the 

NAAQS (in 40 CFR part 50 Appendix T) that, by comparison to the level of the NAAQS, 

indicates whether the area is violating the NAAQS. 

3) Designated nonattainment area – an area which the EPA has determined has violated the 

2010 SO2 NAAQS or contributed to a violation in a nearby area. A nonattainment 

designation reflects considerations of state recommendations and all of the information 

discussed in this document. The EPA’s decision is based on all available information 

including the most recent 3 years of air quality monitoring data, available modeling 

analysis, and any other relevant information. 

4) Designated unclassifiable area – an area which the EPA cannot determine based on all 

available information whether or not it meets the 2010 SO2 NAAQS. 

5) Designated unclassifiable/attainment area – an area which the EPA has determined to 

have sufficient evidence to find either is attaining or is likely to be attaining the NAAQS. 

The EPA’s decision is based on all available information including the most recent 3 

years of air quality monitoring data, available modeling analysis, and any other relevant 

information. 

6) Modeled violation – a violation based on air dispersion modeling. 

7) Recommended attainment area – an area a state or tribe has recommended that the EPA 

designate as attainment. 

8) Recommended nonattainment area – an area a state or tribe has recommended that the 

EPA designate as nonattainment. 

9) Recommended unclassifiable area – an area a state or tribe has recommended that the 

EPA designate as unclassifiable. 

10) Recommended unclassifiable/attainment area – an area a state or tribe has recommended 

that the EPA designate as unclassifiable/attainment. 

11) Violating monitor – an ambient air monitor meeting all methods, quality assurance and 

siting criteria and requirements whose valid design value exceeds 75 ppb, based on data 

analysis conducted in accordance with Appendix T of 40 CFR part 50. 

 
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Technical Analysis for the Grant County, South Dakota Area 

Introduction 

The Grant County area contains a stationary source that according to the EPA’s Air Markets 

Database emitted in 2012 either more than 16,000 tons of SO2 or more than 2,600 tons of SO2

and had an annual average emission rate of at least 0.45 pounds of SO2 per one million British 

thermal units (lbs SO2/mmBTU). As of March 2, 2015, this stationary source had not met the 

specific requirements for being “announced for retirement.” Specifically, in 2012, the Big Stone 

Plant emitted 12,290 tons of SO2, and had an emissions rate of 0.81 SO2/mmBTU. Pursuant to 

the March 2, 2015 court-ordered schedule, the EPA must designate the area surrounding the 

facility by July 2, 2016. 

In its submission, South Dakota recommended that the area surrounding the Big Stone Plant, 

specifically the entirety of Grant County, be designated as attainment based on an assessment 

and characterization of air quality from the facility. This assessment and characterization was 

performed using historic ambient monitoring from November 2001 to October 2002. After 

careful review of the State’s assessment, supporting documentation, and all available data, the 

EPA does not agree with the State’s recommendation for the area, and intends to designate the 

area as unclassifiable. Specifically, based on available information we are not able to determine 

whether the area is meeting or not meeting the NAAQS, and our intended unclassifiable area 

consists of Grant County. 

The Big Stone Plant is located in Eastern South Dakota in the northeastern portion of Grant 

County. As seen in Figure 1 below, the facility (indicated by a red star) is located in the northeast 

corner of the county. Also included in the figure are nearby emitters of SO2 (none), the State’s 

recommended area for the attainment designation, and the EPA’s intended unclassifiable 

designation for the area, both of which consist of the entire county as shown in blue. 

Figure 1. The EPA’s intended designation for Grant County, South Dakota 
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 *Blue indicates unclassifiable 

 

The discussion and analysis that follows below will reference the State’s use of the Modeling 

TAD, the EPA’s assessment of the State’s modeling in accordance with the Modeling TAD, and 

the factors for evaluation contained in the EPA’s March 20, 2015 guidance, as appropriate. 

Detailed Assessment 

Air Quality Data 

This factor considers the SO2 air quality monitoring data in the area surrounding the Big Stone 

Plant. The facility is located in Grant County; however, there are no ambient air quality monitors 

located in this county. The State included historic monitoring data from 2001 and 2002 in its 

recommendation for the closest neighboring county, i.e., Roberts County. The table below shows 

information related to the monitor(s) located in this county, which was provided by the State. 
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Table 2: Available Air Quality Data for the Area Closest to Big Stone Power Plant 

County State 

Recommendation

Air Quality 

(Systems) 

Monitor ID 

Monitor 

Location 

Distance to 

Big Stone 

Power Plant 

SO2 Max 

Value in ppb

Roberts Attainment 46-109-4003 482nd Ave, 

Big Stone 

City, South 

Dakota 

4 miles 28 ppb 

(2001-2002, 

both partial 

years) 

EPA does not consider the historic monitoring data presented by South Dakota as appropriate for 

comparison to the NAAQS. The data are not complete, recent or shown to be in the area of 

maximum concentration. EPA does not consider this historic monitor as sufficient to rule out that 

an exceedance of the 2010 SO2 NAAQS may occur in the immediate vicinity of the facility. 

South Dakota also included the design values for every other SO2 monitor in the State. This was 

provided as justification for the State’s recommended attainment designation for every county in 

South Dakota. At this time, EPA is not reviewing the designations information for areas in the 

State apart from the area surrounding the Big Stone Power Plant, which must be designated by 

July 2, 2016. 

Emissions and Emissions-Related Data 

Evidence of SO2 emissions from the source meeting the emissions criteria of the March 2, 2015 

consent decree, i.e., Big Stone Power Plant, is an important factor for determining whether the 

immediate area is experiencing elevated levels of SO2 concentrations. Other considerations for 

this factor include county level SO2 emissions data and data for sources located within 50 km. 

As part of its recommendation, South Dakota included the annual emissions from the top 

emitters of SO2 in the State of South Dakota. South Dakota obtained the data for these sources 

and their emissions from the yearly facility reports that are submitted to the State. South Dakota 

included emissions information from sources across the State rather than only emissions 

somewhat nearby the Big Stone Power Plant because the State recommended attainment for the 

entire State rather than making a designation just for the area near Big Stone. As previously 

noted, Big Stone emitted 12289.66 of SO2 in 2012. According to the 2011 NEI, there are no 

other sources within 100 km of Big Stone that emit at or above 100 tpy of SO2. 
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Emissions Controls 

The EPA recognizes that control strategies implemented after the release of the 2011 NEI may 

not be reflected, or may warrant further discussion. In its designation recommendation, the State 

indicated that SO2 pollution controls required as part of the State’s regional haze plan (approved 

by EPA April 26, 2012, 77 FR 24845) would be installed and operational at the Big Stone 

facility by fall of 2015.4 The regional haze SIP requires the installation of semi-dry flue gas 

desulfurization (FGD), which was estimated to reduce SO2 levels at the facility by 90%, for an 

estimated 1,880 tons/year of SO2 from the facility. 

Meteorology (Weather & Transport Patterns) 

Evidence of source-receptor relationships between specific emissions sources and high SO2 

concentrations in the surrounding area is another important factor in determining the appropriate 

extent of the EPA’s intended unclassifiable area. The State did not include any meteorological 

information in its designation recommendation. 

Geography and Topography (Mountain Ranges or Other Air Basin Boundaries) 

The Big Stone Power Plant is located in flat terrain, and is about 3 kilometers southwest of Big 

Stone Lake, which forms a portion of the South Dakota/Minnesota border. 

Jurisdictional Boundaries 

Once the geographic area associated with the immediate area surrounding the Big Stone Power 

Plant and any nearby areas which may potentially be contributing to elevated levels of SO2

around the facility are determined, existing jurisdictional boundaries are considered for the 

purpose of informing our intended unclassifiable area. Specific attention will be given to clearly 

defined legal boundaries. 

The EPA believes that our intended unclassifiable area, consisting of Grant County, South 

Dakota, are comprised of clearly defined legal boundaries, and we find these boundaries to be a 

suitably clear basis for defining our intended unclassifiable area. Although South Dakota 

requested in its September 16, 2015 recommendation letter that each county in the State be 

designated attainment for the 2010 SO2 NAAQS, EPA is not proposing to designate any areas

outside of Grant County at this time. This is because no other South Dakota counties contain 

sources subject to the July 2, 2016 designation deadline. 

Other Relevant Information 

 

4

 The Big Stone FGD system was fully operational on December 29, 2015. 
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The EPA did not receive any additional information about the area in the immediate vicinity of 

Big Stone Power Plant. 

Conclusion 

After careful evaluation of the State’s recommendation and supporting information, as well as all 

available relevant information, the EPA intends to designate the area around Big Stone Power 

Plant as unclassifiable for the 2010 SO2 NAAQS. The boundaries of our intended unclassifiable 

area are the same as those of Grant County, South Dakota. 

EPA intends to make this designation based on a lack of technical information, including data 

from ambient air quality monitors or air dispersion modeling sufficient to indicate whether or not 

the area around Big Stone Power Plant is attaining the 2010 SO2 NAAQS. At this time, our 

intended designations for the State only apply to Grant County, South Dakota. Consistent with 

the conditions in the March 2, 2015 court-ordered schedule, the EPA will evaluate and designate 

all remaining undesignated areas in South Dakota by either December 31, 2017 or December 31, 

2020.