September 16,2015 Shaun McGrath Regional Administrator U.S. Environmental Protection Agency, Region

8 1595 Wynkoop Street Denver, CO 80202-1129 Dear Mr. McGrath: DEPARTMENT

of ENVIRONMENT and NATURAL RESOURCES JOE FOSS BUILDING 523 EAST CAPITOL PIERRE, SOUTH DAKOTA 57501-3182 denr.sd.gov 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.The consent decree does not prohibit the states from pursuing earlier EPA designations

for the remaining undesignated areas within the state. Therefore,

I am reaffirming South Dakota's original designation package and

I am again recommending EPA designate
all counties

in South Dakota, including Grant County,
as attaining the 1-hour sulfur dioxide standard (see Attachment A). These recommendations

are based on the June 2, 2011 submittal, Attachment

B which provides the technical analysis for designating

all of South Dakota's counties
in attainment, 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 99

1 h 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

# Attachment

A South Dakota Area Designations

for the 2010 1-Hour Sulfur Dioxide Standard Designated Area Designation Type Classification Type Aurora County
Attainment 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**Desi~nated** 

### Area Designation Typ_e Classification Type Lawrence County Attainment 

### Lincoln County
Attainment 

### Lyman County
Attainment 

### Marshall County
Attainment 

### McCook County
Attainment 

### McPherson County Attainment 

### 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

# Attachment

B Area Designations The agreement between the Sierra Club and the Natural Resources Defense Council (hereinafter referred

to as the Sierra Club) and EPA, which was approved

in the March
2, 2015, consent decree, attempts

to require states to use modeling

to demonstrate attainment status of areas around large sources of sulfur dioxide emissions. The agreement classifies large sources based on actual sulfur 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 Bl 

# 3Y. D. V.l

. S tlDk a e



# -

- 

# - 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-1Site County

99

1 h Percentile

3- Year Design Value Attainment Badlands Jackson 2012

- 2.6 parts per billion

5 parts per billion 2013

- 9.1 parts per billion 2014

- 1.8 parts per billion Wind Cave Custer 2008- 3.0 parts per billion

6 parts per billion 2009

- 10.0 parts per billion 2010- 5.0 parts per billion 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 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 99

1 h 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 th e 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 99

1 h 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 east
e rn half of South Dakota. The SD School Site represents South Dakota's lar g est populated area in th e Sioux Falls Metropolitan Statistical Area (MSA). The Credit Union Site represents South Dakota

' s second lar g est 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.industrialareas.Basedonthecurrentmonitoringdatathroughoutthestatewhichreflectsthepotentialhighestsulfurdioxideconcentrationsinthestate,SouthDakotacontinuestoattainthe1-hoursulfurdioxidestandardineverycountyinthestate.FigureB-1-DesignValuesComparedtothe1-HourSulfurDioxideStandard807060s:::§50m40...30G)c..II)20tCIS10c..0c:::::J3-YearAverage-Standard6&;6IIr--1II-SOSchoolBadlandsUC#1..A96nrlr--1CreditUnionWindCaveBigStoneIISites2.0BigStoneCoal-FiredPowerPlantAccordingtotheconsentdecree,thefirstroundofdesignationsinvolvestwogroupsofareas:1)Areasthathavenewlymonitoredviolationsofthe2010sulfurdioxidestandard;and2)AreasthatcontainanystationarysourcethataccordingtoEPA'sAirMarketDatabase:•Emittedmorethan16,000tonsofsulfurdioxidein2012;•Emittedmorethan2,600tonsofsulfurdioxideandhadanemissionrateofatleast0.45poundsofsulfurdioxidepermillionBtusin2012;and•FacilityhasnotbeenannouncedforretirementasofMarch2,2015.TheBigStonecoal-firedpowerplantemitted12,290tonsofsulfurdioxidein2012basedoncontinuousemissionmonitoring.Incalendaryear2012,theBigStonecoal-firedpowerplantproduced30,221,559millionBtus.Therefore,theBigStonePowerPlant'semissionratewas0.81poundsofsulfurdioxidepermillionBtusin2012.TheBigStonePowerPlantmeetsthecriteriaforroundonedesignationestablishedintheconsentdecreeandistheonlyfacilityinSouthDakotathatmeetsthosecriteria.InSouthDakota'soriginalsubmittal,SouthDakotaprovidedEPAtheambientairqualitydataforsulfurdioxidecollectedin2001and2002.Inthattimeframe,12consecutivemonthsofdatawascollectedandthehighest99thpercentilewas14partsperbillionor19percentofthe1-hoursulfurdioxidestandard.ThemonitoringsitelocationwasbasedonmodelingandrepresentedthehighestsulfurdioxideconcentrationaroundtheBigStonecoal-firedpowerplant.Althoughthereislessthanthreeyearofdataavailabletocomparetothestandard,thisisagoodindicationthattheBigStoneB-3.coal- fired power plant is not causing

a violation
of the 1-hour sulfur dioxide standard

. Keep in mind the ambient air quality data was collected while the Big Stone coal-fued power plant was emitting

at 0.70 and 0.66 pounds per million Btus in 2001 and 2002, respectively. As part of South Dakota's Regional Haze Program submitted

to EPA on January 21,2011, and amended September 19,2011,

a sulfur dioxide emission limit of0.09 pounds per million Btus, which _ includes periods of startup

, shutdown

, and malfunctions,
was promulgated

in the Administrative Rules 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 dio x ide standard

. EPA further states
it does not believe

it would be appropriate

to designate areas as attainment without B - 4appropriaterefineddispersionmodelingandwhereavailable,airqualitymonitoringdataindicatingnoviolationsofthe!-hoursulfurdioxidestandard.DENRagreesmodelingmaybeusedasatoolbystatesbutdisagreesitistheonlyorbesttooltodemonstrateattainmentforthefollowingreasons:I.DENRrecentlyusedAERMODtomodeltheimpactsofanexistingcoal-firedpowerplantusingsulfurdioxideemissionsbeingreportedtoEPAinaccordancewiththeAcidRainProgram.AreceptorwasplacedontwoambientairqualitymonitoringsitesinSouthDakotatocomparehourlymonitoringdatatothehourlymodelingdata.Thiscomparisonassumednoothersulfurdioxideemittingsourceswereimpactingthemonitors.Inreality,ifthemodelwasaccurate,themodelingresultswouldbelessthanthemonitoringresults.ThecomparisonindicatedAERMODhasthepotentialtoover-predicttheconcentrationsofsulfurdioxidebygreaterthanafactoroftwo(seeAppendixDforanalysis).2.Inaccordancewith40CFRPrui51,AppendixW,whichisthefederalguidelineforairqualitymodeling,uncertaintiesandaccuracyofthemodelsarediscussed.Asnotedinsection9.1.2,StudiesofModelAccuracy,"(I)Modelsaremorereliableforestimatinglongertime-averagedconcentrationsthanforestimatingshort-termconcentrations atspecificlocations;and(2)themodelsarereasonablyreliableinestimatingthemagnitudeofhighestconcentrationsoccurringsometime,somewherewithinanarea.Forexrunple,errorsinhighestestimatedconcentrationsof±I0to40percentarefoundtobetypical,i.e.,certainlywellwithintheoftenquotedfactorof-twoaccuracythathaslongbeenrecognizedforthesemodels."EPA'sguidelineagreesthatairqualitymodelingcanover-predicttheconcentrationsofsulfurdioxidebygreaterthanafactoroftwo.3.Theoptiontoonlyusemodelingbythetimelineestablishedintheconsentdecreeisalsocontrru·ytoEPA'spreviousdecisions,courtcases,andruleasidentifiedbelow:a.Onpage26382oftheFederalRegister,Vol.43,No.118,June19,1972,EPAstatesinthepreambletothe1977PSDrules,"...EPAintendsthatmonitoringshouldgenerallyfocusonobtainingdatanecessmyforrequiredreviewagainstNAAQS.Althoughtheincrementconsumptionmustofnecessitybetrackedthroughtheuseofmodeling,EPAdoesnotintendthattherebeno"realworld"checksontheaccuracyofmodeling."b.InAlabamaPowerCo.v.CostleC.A.D.C.1979,theU.S.CourtofAppeals,DistrictofColumbiaCircuitstates,"Wediscernfromthestatuteatechnology-forcingobjective.Congressintendedthatmonitoringwouldimposeacertaindisciplineontheuseofmodelingtechniques,whichwouldbetheprincipaldevicerelieduponfortheprojectionoftheimpactonairqualityofemissionsfromaregulatedsource.Thisprojectsthattheemploymentofmodelingtechniquesbeheldtoearthbyacontinualprocessofconfirmationandreassessment,aprocessthatenhancesconfidenceinmodeling,asameansforrealisticprojectionofairquality."c.ThisisfurtheremphasizedbyEPA'scurrentrulesunderthePreventionofSignificantDeteriorationprogrrun.Inaccordancewith40CFR§52.21(m)(2),iftheAdministratorbelievesitisnecessmy,theowneroroperatorshallconductrunbientairqualitymonitoring,"...todeterminetheeffectemissionsfromthestationarysourceormodificationmayhave,orarehaving,onairqualityinanyarea."EvenafteraPSDsourcehasdemonstrateditcanconstructandoperatem1dnotcauseaviolationoftheNationalAmbientAirQualitystandardorPSDincrementusingmodeling,EPAmayrequireasourcetoconductmonitoringtoensurethemodelingprovidedrealisticresultsandnoviolationswilloccur.B-5Historically, both Congress and EPA intended for monitoring

to be the real determination



### if an area is attaining the National Ambient Air Quality Standards. DENR believes monitoring provides the reality check both Congress and EPA believe are necessary for states to demonstrate

an area is attaining
or not attaining the standard and current air quality models are not the best tool for demonstrating attainment. Therefore

, if EPA does not agree that the evidence provided

in this package demonstrates
that Grant 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 Su

l fur 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 99
t h 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

# Figure B-2- South Dakota Counties with Sulfur Dioxide Data 

# 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 compar

i son

. 

# 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

2 1

39

47 9 Minnehaha Sioux Falls Water Reclamation Facility

13

11

12 B-7#CountyFacility20092012201410CodingtonGlacialLakesEnergy111212Top10Total=13,28412,75514,480TotalfromTitleVSources13,323 12,82114,542SulfurdioxideemissionsinSouthDakotahavefluctuatedupanddownsinceDENRsubmitteditsoriginalproposaltoEPA.Itisnotablethatthetop10sulfurdioxideemittersinSouthDakotaonaveragerepresentapproximately99.6%ofthetotalsulfurdioxideemissionsinSouthDakotafromfacilitiespermittedundertheTitleVairqualityoperatingpermitprogram.TheBigStonecoal-firedpowerplantistheonlysourceinSouthDakotathatmeetsthedefinitionintheconsentdecreeofalargesourceofsulfurdioxideemissions.Thecoal-firedpowerplantatBlackHillsCorporation'sBenFrenchstationwasshutdownonMarch20,2014andisbeingdecommissioned.TableB-3providesalistofthetop10mostpopulatedcountiesinthestateandthelargestcitywithinthecountybasedonthe2010Census.ThelargestcityinSouthDakotaisSiouxFallswithapopulationof153,888.ThelargestofthethreeMetropolitanStatisticalAreas(MSAs)inthestateincludesthecityofSiouxFallsandincludesthecountiesofMinnehaha,Lincoln,McCook,andTurner.ThecombinedpopulationfortheSiouxFallsMSAis228,261.The10mostpopulatedareasinSouthDakotahavesourcesofsulfurdioxideemissions,butnonehavealargesourceofsulfurdioxideasdefinedintheconsentdecree.blB 3HthPl.C.ShDka e--enzgJestopuatzonountzesmoutaotaNumberCountyPopulationLargestCityPopulation1Minnehaha169,468SiouxFalls153,8882Pennington100,948RapidCity67,9563Lincoln44,828SiouxFalls153,8884Brown36,531Aberdeen26,0915Brookings31,965Brookings22,0566Codington27,227Watertown21,4827Meade25,434Sturgis6,6278Lawrence24,097Spearfish10,4949Yankton22,438Yankton14,45410Davison19,504Mitchell15,254Basedonthepopulationandsulfurdioxideemissions,MinnehahaandPenningtonCountyhavesourcesofsulfurdioxideemissionsandthehighestpopulationsinthestate.Bothoftheseareashaveambientairmonitoringdataforsulfurdioxidethatdemonstratestheseareasareattainingthe1-hoursulfurdioxidestandard.ThesulfurdioxideconcentrationsobservedinSiouxFallsandRapidCitywouldbegreaterthanwhatonewouldexperienceintheothercountiesidentifiedTableB-2,exceptforGrantCounty,becauseoflowersulfurdioxideemissionsandpopulation.Therefore,DENRisrecommendingthosecountiesbedesignatedasattainingthe1-hoursulfurdioxidestandard.OtherthanGrantCounty,theremainingcountiesinSouthDakotanotlistedinTableB-2haveverylowsulfurdioxideemissions(seeAppendixEoftheJune2,2011submittal).ThesulfurdioxideconcentrationsintheseareaswouldbesimilartotheconcentrationsSouthDakotaisexperiencingatitsBadlands,WindCave,andUnionCountymonitoringsitesdependingonwhatruralareaoneisB-8located. DENR believes these counties should be designated

as attaining the 1-hour sulfur dioxide standard and the state should not be required

to do further characterization through data collection

or modeling. Grant County has the highest sulfur dioxide emissions

in South Dakota and are generated

by the Big Stone

I coal-fired power plant. DENR does not believe modeling

is necessary

at this site since modeling was used to determine the location
of highest concentrations,

an ambient air monitor was 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 S TA TE S BNVIR O Nlllm'rAL P R O T ECTION AQ BUCY A I R OUALITY SY STEM 

# PRB!.IMIUAR Y DB S IGN VALUB 

# REPORT



# Des i gn Value Year• 2010 Repo r t D a t e: S e p . 4 , 2 0 1 5



# P o llutant • Sulfur dioxide ( 4240 1 ) S tand a rd U ni ts • Parte per bil li o n ( OOB) NAA Q S Stand a rd • S02 1 -h o u r 2010 

# REPORT RXCLUDES MRASUREMBNTS WITH REGIONALLY CONCURRED EVENT FLAGS. St a t i s t i c •

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# UNITED STATES ENVIRONMBNTAL PROTECTION AGENC Y AIR QUALITY SYSTI!M PRELIMI!IARY DESIGN VALUE REPORT



# Design Value Year: 2010 Report Date: Sop

. 4, 2015



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### Attachment

D AERMOD Modeling Accuracy This analysis was taken from DENR's section 4.4.1 -Modeling Accuracy
of the Statement
of Basis for Hyperion Energy Center's extension request for it Prevention

of Significant Deterioration

air quality permit #28.070 1-PSD. 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-1away from the center tend to over or under-predict. Values equal to -0

. 67 are equivalent

tooverpredictions

by a factor of two while values equal to +0.67 are equivalent

to under-predictions

by a factor of two. As the graph indicates, AERMOD over-predicts
the concentrations

of sulfur dioxide with five of the eight scenarios indicating the model would over-predict
the concentrations greater than

a factor of two. 

### Figure 4-1-Screening
Test t: 0 **~** CIS **·~** Q Qlen 

# - 0 II) .!!! 

### m

2



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# """ 

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# / lo....er-prea 

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j Impacts



# I

-2 Bias of Average 

### 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 Hour ly

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