Shaun McGrath 

Regional Administrator 

U.S. Environmental Protection Agency, Region 8 

1595 Wynkoop Street 

Denver, CO 80202-1129 

Dear Mr. McGrath: 

On June 2, 2011, South Dakota submitted its timely designation letter tor EPA's new !-hour 

sulfur dioxide standard of75 parts per billion, which EPA promulgated on June 2, 2010. After 

reviewing South Dakota's sulfur dioxide air emissions inventory, ambient air monitoring data, 

and other factors, South Dakota proposed designating each county in South Dakota as attaining 

the new !-hour sulfur dioxide standard. 

EPA failed to act on South Dakota's proposed designation within the timeframe specified in the 

Clean Air Act. Therefore, South Dakota joined North Dakota along with other states in a lawsuit 

requiring EPA to act on each state's proposed designation letters. EPA was also sued by the 

Sierra Club and the Natural Resources Defense Council. EPA decided to settle with the Sierra 

Club and the Natural Resources Defense Council and proposed a consent decree, which was 

accepted by the U.S. District Court for the Northern District of California on March 2, 2015. 

On March 20,2015, EPA notified South Dakota of the requirements in the consent decree. Of 

the three deadlines in the consent decree that EPA was ordered to meet, South Dakota is 

impacted by the first round and third round. In the first round, EPA is required to promulgate 

final sulfur dioxide area designations for areas containing large sources of sulfur dioxide 

pollution by no later than July 2, 2016. In South Dakota, this involves the Big Stone coal-fired 

power plant located in Grant County. In the third round, EPA is required to address all 

remaining undesignated areas by December 31, 2020. The consent decree does not render 

invalid or obsolete the state's proposed designation that EPA failed to act on originally. 

However, if states wish, EPA is requesting that states submit any additional information and/or 

updates for their recommendations for the first round by September 18, 2015. 

On January 18, 2011, Governor Daugaard submitted a letter to EPA designating the Secretary of 

the Department of Environment and Natural Resources as his designee for submitting 

designations and other matters that involves South Dakota's Air Quality Program. In that 

capacity, 1 have the authority to submit proposed designations for South Dakota. 
Attachment C which provides a copy of the Air Quality System AMP480 and AMP450 reports, 

and Attachment D which provides an analysis of modeling accuracy. The Air Quality System 

AMP 480 and AMP450 reports show the Design Values and yearly 991h percentile concentrations 

for calendar years 2012 to 2014 for current ambient air monitoring sites, 2008 to 2010 for the 

Wind Cave Site and the 12 months of data collected near the Big Stone Power Plant in Roberts 

County. 

Thank you for the opportunity to propose designations for the 2010 1-hour sulfur dioxide 

standard and I look forward to your timely concurrence. If you have questions, please contact 

Brian Gustafson at 605-773-3151 . 

Steven M. Pirner, PE 

Secretary 

Attachments 

cc: Governor Dennis Daugaard 

Attorney General Marty Jackley 

Matt Konenkamp, Governor's Office 

Monica Morales, EPA Region 8 
Beadle County Attainment 

Bennett County Attainment 

Bon Homme County Attainment 

Brookings County Attainment 

Brown County Attainment 

Brule County Attainment 

Buffalo County Attainment 

Butte County Attainment 

Campbell County Attainment 

Charles County Attainment 

Clark County Attainment 

Clay_ County Attainment 

Codington County Attainment 

Corson Conn!)' Attainment 

Custer County Attainment 

Davison County Attainment 

Day County Attainment 

Deuel County Attainment 

Dewey County Attainment 

Douglas County Attainment 

Edmunds County Attainment 

Fall River County Attainment 

Faulk County Attainment 

Grant County Attainment 

Gregory County Attainment 

Haakon County Attainment 

Hamlin County Attainment 

Hand County Attainment 

Hanson County Attainment 

Harding County Attainment 

Hughes County Attainment 

Hutchinson County Attainment 

Hyde County Attainment 

Jackson County Attainment 

Jerauld County Attainment 

Jones County Attainment 

Kingsbury County Attainment 

Lake County Attainment 

A-I 
Meade County Attainment 

Mellette County Attainment 

Miner County Attainment 

Minnehaha County Attainment 

Moody County Attainment 

Pennin~ton County Attainment 

Perkins County_ Attainment 

Potter County Attainment 

Roberts County Attainment 

Sanborn County Attainment 

Shannon County Attainment 

Spink County Attainment 

Stanley County Attainment 

Sully County Attainment 

Todd County Attainment 

Tripp County Attainment 

Turner County Attainment 

Union County Attainment 

Walworth County Attainment 

Yankton County Attainment 

Ziebach County Attainment 

A-2 
g

dioxide emissions in 2012, without consideration of state and federal regulations that a majority of 

these large sources are required to meet in the near future to comply with the federal Regional Haze 

Program. 

DENR believes the demonstration package South Dakota submitted in June 2011, shows South 

Dakota is attaining the 1-hour sulfur dioxide standard and request that EPA use that demonstration 

package for area designations in South Dakota. In addition, DENR is providing updated data which 

supports South Dakota's area designation of every county in South Dakota, including Grant County, 

as attaining the !-hour sulfur dioxide standard. 

1.0 Air Monitoring 

In South Dakota's submittal in June 2011, all sulfur dioxide monitoring in South Dakota 

demonstrated that the !-hour sulfur dioxide standard was being attained. For that demonstration 

package, DENR used sulfur dioxide monitoring data throughout the state for 2008 through2010, 

except for the Big Stone II project. For the Big Stone II project, DENR used the 12 months of 

continuous data collected in Roberts County from November 200 I through October 2002. 

In support of South Dakota's demonstration that every county in South Dakota is attaining the 1-hour 

sulfur dioxide standard, DENR updated the data using 2012 through 2014 sulfur dioxide 

concentrations throughout the state including new data from the Credit Union Site in Rapid City. The 

sulfur dioxide data for the Wind Cave Site will not change from the original submittal because the 

sulfur dioxide monitor was moved to the Credit Union Site in Rapid City in 2011. In addition, since 

there is no new data around the Big Stone coal-fired power plant and the consent decree does not 

allow states the time to gather additional ambient air quality data, DENR will use the data collected 

for the Big Stone II project to demonstrate even the area around the Big Stone coal-fired power plant 

is attaining the !-hour sulfur dioxide standard. 

The sulfur dioxide data collected throughout the state continues to demonstrate that sulfur dioxide !-

hour concentrations in South Dakota are low statewide. Table B-1 displays the three year calculated 

design value concentration for each site using the most current three years of complete data, except as 

specified above. 

I:bl a e Bl 3Y. D. V.l . S tlDk - - - ear eSif!ll a ues 111 OU I a ota 

Site County 99'h Percentile 3-Y car Dcsi2n Value Attainment 

SD School Minnehaha 2012 - 5.5 parts per billion 6 parts per billion Yes 

2013-3.3 parts per billion 

2014- 10.5 parts per billion 

B-1 
UC#1 Union 2012 - 9.2 parts per billion 6 parts per billion 

2013-5.9 parts per billion 

2014 - 4.1 parts per billion 

Credit Pennington 2012- 9.6 parts per billion 9 parts per billion 

Union 2013 - 9.4 parts per billion 

2014 - 7.2 parts per billion 

Big Stone II Roberts 20011 2002 - 14 parts per billion 1 

I \lh -The 99 percenttle for the Btg Stone II proJect ts based on the 12 months of data collected from 

November 2001 through October 2002; and 

2

- Not comparable to the standard because there is only 12 months of data. 

Yes 

Yes 

2 

The sulfur dioxide 1-hour concentrations collected in the state demonstrate there were no 

concentrations exceeding the 1-hour sulfur dioxide standard. The highest design value concentration 

was recorded at the Credit Union Site in Rapid City at 9 parts per billion or 12% of the 1-hour sulfur 

dioxide standard. The Badlands Site has the lowest design value concentrations at 5 parts per billion 

or 7% of the 1-hour sulfur dioxide standard. Even if it is assumed the Big Stone II sulfur dioxide 

concentration of 14 parts per billion was the 3-year average of the 991h percentile, sulfur dioxide 

concentrations throughout the state are less than 19% of the 1-hour sulfur dioxide standard. 

The concentrations in South Dakota are low for several reasons. First, the state's populatipn and sulfur 

dioxide emissions from point and area sources are low. Second, all but five sources with Title V air 

quality operating permits in the state emit sulfur dioxide emissions less than 100 tons per year. 

Finally, sulfur dioxide emissions from area sources are low because of the move to ultra-low sulfur 

fuels in 2010. 

Figure B-1 provides a graph comparison of the design value concentrations from Table B-1 for each 

site compared to the 1-hour sulfur dioxide standard. Although the Big Stone II Site does not have 

three years of data, the 991h percentile for the 12 months of testing is provided for comparison 

purposes. 

The Badlands Site represents rural areas consisting mainly of rangeland area representing the counties 

with low population levels in the western half of South Dakota. The. Union County UC # 1 Site 

represents the low population areas with mainly farming activities in the eastern half of South Dakota. 

The SD School Site represents South Dakota's largest populated area in the Sioux Falls Metropolitan 

Statistical Area (MSA). The Credit Union Site represents South Dakota's second largest populated 

area in the Rapid City MSA and represents testing near industrial sources. The Big Stone II site 

represents sulfur dioxide concentrations near South Dakota's largest emitting sulfur dioxide source. 

DENR believes that its sulfur dioxide monitoring network represents sulfur dioxide concentrations 

each county in South Dakota would experience since the monitors are located in rural, urban and 

B-2 
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Sites 

2.0 Big Stone Coal-Fired Power Plant 

According to the consent decree, the first round of designations involves two groups of areas: 

1) Areas that have newly monitored violations of the 2010 sulfur dioxide standard; and 

2) Areas that contain any stationary source that according to EPA's Air Market Database: 

• Emitted more than 16,000 tons of sulfur dioxide in 20 12; 

• Emitted more than 2,600 tons of sulfur dioxide and had an emission rate of at least 

0.45 pounds of sulfur dioxide per million Btus in 2012; and 

• Facility has not been announced for retirement as of March 2, 2015. 

The Big Stone coal-fired power plant emitted 12,290 tons of sulfur dioxide in 2012 based on 

continuous emission monitoring. In calendar year 2012, the Big Stone coal-fired power plant 

produced 30,221,559 million Btus. Therefore, the Big Stone Power Plant's emission rate was 0.81 

pounds of sulfur dioxide per million Btus in 2012. The Big Stone Power Plant meets the criteria for 

round one designation established in the consent decree and is the only facility in South Dakota that 

meets those criteria. 

In South Dakota's original submittal, South Dakota provided EPA the ambient air quality data for 

sulfur dioxide collected in 2001 and 2002. In that time frame, 12 consecutive months of data was 

collected and the highest 99th percentile was 14 parts per billion or 19 percent of the 1-hour sulfur 

dioxide standard. The monitoring site location was based on modeling and represented the highest 

sulfur dioxide concentration around the Big Stone coal-fired power plant. Although there is less than 

three year of data available to compare to the standard, this is a good indication that the Big Stone 

B-3 . 
of South Dakota, section 74:36:21:06 for Unit #1 at the Big Stone coal-fired power plant. In addition, 

Unit # 1 also has to meet a 505 pounds of sulfur dioxide per hour emission limit which is applicable at 

all-times including periods of startup, shutdown, and malfunctions. The air emission rates in section 

74:36:21:06 were submitted as part of South Dakota's state implementation plan. EPA approved 

South Dakota's Regional Haze Program as part of South Dakota's state implementation plan on April 

26,2012. According to the Administrative Rules of South Dakota, section 74:36:21:07, the Big Stone 

coal-fired power plant is required to install the air pollution controls and meet the sulfur dioxide 

emission rate within five years of EPA's approval or by April26, 2017. However, the air pollution 

controls required under EPA's approved state implementation plan will be operational in the fourth 

quarter of2015. 

If the Sierra Club and EPA in their negotiations or the judge would have considered federally 

enforceable limits in place at the time the consent decree was established, the Big Stone coal-fired 

power plant would not be considered a large emitting source. The sulfur dioxide emission rates that 

the Big Stone coal-fired power plant has to meet will ensure sulfur dioxide emissions will stay below 

the thresholds established in the consent decree for a large sulfur dioxide emitting source. 

At this lower sulfur dioxide emission rate, DENR anticipates sulfur dioxide concentrations at the 

highest concentration points identified by any type of modeling will record sulfur dioxide 

concentrations lower than the levels observed in 2001 and 2002. In 2001 and 2002, the sulfur dioxide 

emission rates based on continuous emission monitoring systems were 0.70 and 0.66 pounds per 

million Btus, respectively. The emission limit of 0.09 pounds of sulfur dioxide per million Btus 

represents an 86% reduction in sulfur dioxide emissions from the emission rate occurring in 2002. 

Therefore, the monitoring data collected in 2001 and 2002 overestimates the sulfur dioxide 

concentrations Grant County will experience once the federal Regional Haze Program limits are 

implemented. Based on this information, DENR believes Grant County is attaining the 1-hour sulfur 

dioxide standard and will continue to attain the standard after the air pollution control equipment 

under the Regional Haze Program is operational. Therefore, EPA should designate Grant County as 

attaining the 1-hour sulfur dioxide standard. 

3.0 Air Modeling 

Because of the timeline agreed upon by the Sierra Club and EPA and established in the consent 

decree, states are being pushed to use modeling to determine if areas around large sources of sulfur 

dioxide emissions are attaining the 1-hour sulfur dioxide standard. EPA's Memorandum from Stephen 

D. Page to Regional Air Division Directors, I-X, dated March 24, 2011 , indicates EPA may initially 

designate an area attaining the standard if it is clear it meets the 1-hour sulfur dioxide standard. EPA 

further states it does not believe it would be appropriate to designate areas as attainment without 

B-4 
to compare hourly monitoring data to the hourly modeling data. This comparison assumed no 

other sulfur dioxide emitting sources were impacting the monitors. In reality, if the model was 

accurate, the modeling results would be less than the monitoring results. The comparison 

indicated AERMOD has the potential to over-predict the concentrations of sulfur dioxide by 

greater than a factor of two (see Appendix D for analysis). 

2. In accordance with 40 CFR Prui 51, Appendix W, which is the federal guideline for air quality 

modeling, uncertainties and accuracy of the models are discussed. As noted in section 9 .1.2, 

Studies of Model Accuracy, "(I) Models are more reliable for estimating longer time-averaged 

concentrations than for estimating short-term concentrations at specific locations; and (2) the 

models are reasonably reliable in estimating the magnitude of highest concentrations occurring 

sometime, somewhere within an area. For exrunple, errors in highest estimated concentrations 

of± I 0 to 40 percent are found to be typical, i.e., certainly well within the often quoted factorof-two accuracy that has long been recognized for these models." EPA's guideline agrees that 

air quality modeling can over-predict the concentrations of sulfur dioxide by greater than a 

factor of two. 

3. The option to only use modeling by the time line established in the consent decree is also 

contrru·y to EPA's previous decisions, court cases, and rule as identified below: 

a. On page 26382 of the Federal Register, Vol. 43, No. 118, June 19, 1972, EPA states in the 

preamble to the 1977 PSD rules, " ... EPA intends that monitoring should generally focus 

on obtaining data necessmy for required review against NAAQS. Although the increment 

consumption must of necessity be tracked through the use of modeling, EPA does not 

intend that there be no "real world" checks on the accuracy of modeling." 

b. In Alabama Power Co. v. Costle C.A.D.C. 1979, the U.S. Court of Appeals, District of 

Columbia Circuit states, "We discern from the statute a technology-forcing objective. 

Congress intended that monitoring would impose a certain discipline on the use of 

modeling techniques, which would be the principal device relied upon for the projection of 

the impact on air quality of emissions from a regulated source. This projects that the 

employment of modeling techniques be held to earth by a continual process of 

confirmation and reassessment, a process that enhances confidence in modeling, as a 

means for realistic projection of air quality." 

c. This is further emphasized by EPA's current rules under the Prevention of Significant 

Deterioration progrrun. In accordance with 40 CFR §52.21(m)(2), if the Administrator 

believes it is necessmy, the owner or operator shall conduct runbient air quality monitoring, 

" ... to determine the effect emissions from the stationary source or modification may have, 

or are having, on air quality in any area." Even after a PSD source has demonstrated it can 

construct and operate m1d not cause a violation of the National Ambient Air Quality 

standard or PSD increment using modeling, EPA may require a source to conduct 

monitoring to ensure the modeling provided realistic results and no violations will occur. 

B-5 
County is attaining the 1-hour sulfur dioxide standard, EPA should provide South Dakota with the 

option to monitor in Grant County to ensure the most accurate data can be used in designating Grant 

County. 

4.0 Sulfur Dioxide Monitoring Network in South Dakota 

The first sampling effort in South Dakota to collect hourly sulfur dioxide data was near the Big Stone 

coal-fired power plant. The monitoring location for sulfur dioxide was based on modeling indicating 

the area of highest concentration located near the facility and still in South Dakota. A continuous 12-

month period of air monitoring was completed in the years of2001 and 2002 as part of a Prevention 

of Significant Deterioration permit application. Sulfur dioxide levels were low with a 99th percentile 

1-hour average concentration level of 14 parts per billion. 

In 2002, DENR expanded its sulfur dioxide monitoring network and began collecting hourly sulfur 

dioxide data at the Hilltop Site in Sioux Falls. The monitor was later moved to the SD School Site 

and continues today. In 2005, two more locations were added at the Badlands and Wind Cave 

National Parks. In 2009, two more locations were added in Union County. In 2011, the sulfur 

dioxide analyzer was moved from Wind Cave Site to Rapid City Credit Union Site. In 2013, the 

sulfur dioxide analyzer at the Union County UC #2 site was shutdown after an analysis demonstrated 

that the sulfur dioxide concentrations recorded at the Union County UC #1 sites were comparable. 

The current sampling network includes sites in several counties around the state with goals of high 

concentration, population, source impact, background, and regional transport. See Figure B-2 for a 

map of the state showing the counties with sulfur dioxide air monitoring data. 

B-6 
Years of Data: 3 Years 1 Year D No Sampling 

5.0 South Dakota's Population and Sulfur Dioxide Emissions 

In the original submittal, DENR provided the top 10 sulfur dioxide emitters in South Dakota for 

calendar year 2009. In this submittal, DENR is reviewing those top ten emitters and comparing them 

to calendar year 2012 and 2014: Calendar year 2012 was chosen because that is the year the Sierra 

Club and EPA chose in the consent decree for identifying large sources of sulfur dioxide emissions 

and 2014 was chosen because that is the most recent year. Table B-2 provides the comparison. 

Table B-2- 2009 Top 10 Sulfur Dioxide Emitters in South Dakota (tons per year) 

# County Facility 2009 2012 2014 

1 Grant Otter Tail Power Company - Big Stone I 11,651 12,290 13,885 

2 Pennington Black Hills Corporation- Ben French 823 12 0 

3 Pennington GCCDacotah 285 152 304 

4 Brookings South Dakota State University 183 99 59 

5 Minnehaha John Morrell & Company 170 39 101 

6 Brookings Valero Renewable Fuels Company 85 59 19 

7 Spink Redfield Energy 42 42 41 

8 Turner Great Plains Ethanol 21 39 47 

9 Minnehaha Sioux Falls Water Reclamation Facility 13 11 12 

B-7 
average represent approximately 99.6% of the total sulfur dioxide emissions in South Dakota from 

facilities permitted under the Title V air quality operating permit program. The Big Stone coal-fired 

power plant is the only source in South Dakota that meets the definition in the consent decree of a 

large source of sulfur dioxide emissions. The coal-fired power plant at Black Hills Corporation's Ben 

French station was shut down on March 20, 2014 and is being decommissioned. 

Table B-3 provides a list of the top 1 0 most populated counties in the state and the largest city within 

the county based on the 2010 Census. The largest city in South Dakota is Sioux Falls with a 

population of 153,888. The largest of the three Metropolitan Statistical Areas (MSAs) in the state 

includes the city of Sioux Falls and includes the counties of Minnehaha, Lincoln, McCook, and 

Turner. The combined population for the Sioux Falls MSA is 228,261. The 10 most populated areas 

in South Dakota have sources of sulfur dioxide emissions, but none have a large source of sulfur 

dioxide as defined in the consent decree. 

a e bl B 3 - Ht h P l . C . S hD k - en zgJ est opu atzon ountzes m out a ota 

Number County Population Largest City Population 

1 Minnehaha 169,468 Sioux Falls 153,888 

2 Pennington 100,948 Rapid City 67,956 

3 Lincoln 44,828 Sioux Falls 153,888 

4 Brown 36,531 Aberdeen 26,091 

5 Brookings 31,965 Brookings 22,056 

6 Codington 27,227 Watertown 21,482 

7 Meade 25,434 Sturgis 6,627 

8 Lawrence 24,097 Spearfish 10,494 

9 Yankton 22,438 Yankton 14,454 

10 Davison 19,504 Mitchell 15,254 

Based on the population and sulfur dioxide emissions, Minnehaha and Pennington County have 

sources of sulfur dioxide emissions and the highest populations in the state. Both of these areas have 

ambient air monitoring data for sulfur dioxide that demonstrates these areas are attaining the 1-hour 

sulfur dioxide standard. The sulfur dioxide concentrations observed in Sioux Falls and Rapid City 

would be greater than what one would experience in the other counties identified Table B-2, except 

for Grant County, because of lower sulfur dioxide emissions and population. Therefore, DENR is 

recommending those counties be designated as attaining the 1-hour sulfur dioxide standard. 

Other than Grant County, the remaining counties in South Dakota not listed in Table B-2 have very 

low sulfur dioxide emissions (see Appendix E of the June 2, 2011 submittal). The sulfur dioxide 

concentrations in these areas would be similar to the concentrations South Dakota is experiencing at 

its Badlands, Wind Cave, and Union County monitoring sites depending on what rural area one is 

B-8 
located at the modeling site in Roberts County, and recorded concentrations just under 19% of the 1-

hour sulfur dioxide standard. In addition, sulfur dioxide emissions will be lowered by approximately 

86% from the Big Stone I coal-fired power plant once the control equipment required by the Regional 

Haze Program is installed and operational in the fourth quruter of2015. Therefore, DENRbelieves 

there is enough evidence to designate Grant County as attaining the 1-hour sulfur dioxide standard 

without modeling. 

B-9 
Attachment C 

Air Quality System Reports 

AMP480 

UNITED STATES BNVIRONlllm'rAL PROTECTION AQBUCY 

AI R OUALITY SYSTEM 

PRB!.IMIUARY DBSIGN VALUB REPORT 

Desi gn Value Year• 2010 

Repor t Da t e: Sep . 4 , 2015 

Po llutant• Sulfur dioxide (42401 ) 

S tanda rd Units• Parte per billio n (OOB) 

NAAQS Standa rd • S02 1 -hou r 2010 

REPORT RXCLUDES MRASUREMBNTS WITH REGIONALLY CONCURRED EVENT FLAGS. 

Sta t i s t i c • ~smual 99ch Perc e n cile 

Site I D 

~6 03) 0132 

o-on-ooo1 

09~ 0008 

1:) 7- 0001 

~6·1~7 000~ 

S TREET ADDRESS 

I·:! :ID C:JWE l>ATIOt!iU. PAP.l:, SO 

BADLJUIDS PO BOY. 6 :!EADQUAAT 

:!001 E St!1 St 

31986 ~75th Ave 

31307 473rd Av e 

Level• 75 . 

2010 

Comp. 99th caru. 

Qrtra Percentile Eval 

~ 5 

4 9 

4 

~ 12 

4 :.·· 

State Nama • South Dakota 

2009 2008 

I I Coap. 99th c ert• Comp . 99th cart• 

1 Qrtr a Percenti le Rval Ortr • Per centile Bval 

l 10 . !I 

4 s ~ s ~ 

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~ :. Cotr.pl't ed d;;sign -n.lu-:s a l:e a s naps hot o f t h9 data at ~ tine ;:he report :<a.; n:n I cay not b& all data f o r ea ) . 

Y~r 

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3. Annual Values ~: meeting ccr.;plete 

- C-1 
UNITED STATES ENVIRONMBNTAL PROTECTION AGENCY 

AIR QUALITY SYSTI!M 

PRELIMI!IARY DESIGN VALUE REPORT 

Design Value Year: 2010 

Report Date: Sop. 4, 2015 

Pollutant: Sulfur dioxide(42401 ) 

Standard Units: Parts per billion (008 ) 

NAAQS Standard: S02 1-hour 2010 

REPORT EXCLUDES HBASURBMRNTS WITH REGIONALLY CONCURIUID BVENT FLAGS . 

Statistic : Annual 99th Percent i le Level: 75 State Name : South Dakota 

Site ID STREET ADDRESS 

2010 2009 2008 3-Year I 

COUip. 99th ceru. I Comp. 99th cart" COI!lp. 99th Cert" Dea1gn Val1d 

Ortra Percent1la Eval j Qrtrs Perce.ntila Eval Ortra Parcant1le Bvd Value Ind. 

~6-033-0132 l'liNO CAVE NATrONAL PAP.:.:, SO ~ s 3 J.O ~ N € t: 

Notes: l. computed si~l vAlues are a snapshot of th~ data at the time the r eport was rwl (may not c s a ll d~ta for yaar ). 

2. some Pl~2. s 24-hour DVs for inco:':lplet .. data that are 1\Urked invalid b<1re ll'.ay be nurl:wd valid 2n til& Of!:icial 1·epcrt due to <?.ddition;ol an .. lysh. 

3. Ar.nual Values not meeting compl~~9nes s criteria a re o~ ked •!th an asteria}: ( ••• ) . 

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C-2 
4.4.1 Model Accuracy 

The modeling analysis indicates Hyperion will not cause or contribute to a violation of the new 1-hour 

sulfur dioxide National Ambient Air Quality Standard; but the analysis does show the modeled 

concentration plus the background monitoring concentration is within 2 percent of the new 1-hour 

sulfur dioxide National Ambient Air Quality Standard. Therefore, DENR considered several factors 

involving how realistic the models predict the concentration and what facility or facilities is 

contributing to the modeling concentration. 

The high-modeled sulfur dioxide concentrations are located in the southeast comer of the modeling 

domain. The facility contributing to the high-modeled concentration for sulfur dioxide (greater than 

90% contribution) is the MidAmerican George Neal facilities in Iowa. During these periods when 

MidAmerican George Neal is the majority contributor, Hyperion's contribution is less than 1%. 

In accordance with 40 CFR Part 51, Appendix W, uncertainties and accuracy ofthe models are 

discussed. As noted in section 9 .1.2, Studies of Model Accuracy, "( 1) Models are more reliable for 

estimating longer time-averaged concentrations than for estimating short-term concentrations at 

specific locations; and (2) the models are reasonably reliable in estimating the magnitude of highest 

concentrations occurring sometime, somewhere within an area. For example, errors in highest 

estimated concentrations of± 1 0 to 40 percent are found to be typical, i.e., certainly well within the 

often quoted factor-of-two accuracy that has long been recognized for these models. However, 

estimates of concentrations that occur at a specific time and site are poorly correlated with actually 

observed concentrations and are much less reliable." 

To determine if the AERMOD is correctly quantifying the sulfur dioxide concentration, DENR 

conducted a screening test to determine if AERMOD would meet a minimum operational performance 

for the 1-hour sulfur dioxide National Ambient Air Quality Standard. DENR used the fractional bias 

procedure identified in EPA's Protocol for Determining the 'Best Performing Model (EPA- 454/R-92-

025). DENR modeled MidAmerican George Neal facilities actual emissions from March 31, 2009 

through March 31, 2010, at two receptor points in South Dakota using the two meteorological data 

sets for Union County. The two receptor points used were the location of the two sulfur dioxide 

monitoring stations currently being operated by DENR in Union County. The modeled results were 

then compared to the monitored results using the screening approach specified in EPA's protocol. 

Since the new 1-hour sulfur dioxide National Ambient Air Quality standard represents a new form of 

a standard, DENR compared both the highest 25 hourly readings and the highest 25 readings 

representing the form of the standard (e.g., highest 25 daily 1-hour values). 

Figure 4-1 shows a graphical representation of the screening test. As identified in the protocol 

"Models that plot close to the center (0,0) are relatively free from bias, while models that plot further 

D-1 
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e UCSF #1 Standard + UCSF #1 Hourly e UCSF #2 Standard + UCSF #2 Hourly 

• UCSC #1 Standard + UCSC #1 Hourly • UCSC #2 Standard + UCSC #2 Hourly 

2 

Based on the analysis, the model is over-predicting the impact MidAmerican George Neal will have 

on the sulfur dioxide concentrations in the modeling domain. If it is over-predicting MidAmerican 

George Neal it is also likely over-predicting all of the sources in the area including Hyperion. Since 

the approved models are inherently conservative, DENR believes actual monitoring data will be lower 

then what is being predicted by the models. 

D-2